Search

Quantifying sessile marine invertebrate recruitment often requires destructive sampling or extrapolation from artificial substrata, the latter introducing the danger of artifacts. We measured intertidal mussel recruitment into mussel beds and into brushes at three-month intervals for five years across 3,200 km of southern Africa and determined substrata effects on recruitment rate. Recruitment into mussel beds showed a strong, coast-wide gradient, with high recruitment on the West coast, diminishing on the South coast, and increasing slightly on the East coast. At scales of 10 s of km, brushes reflected natural temporal recruitment variability, with a strong significant linear correlation between recruitment into brushes and into mussel beds. However, the relationship became semi-logarithmic when comparing among locations at a scale of 100 s of km. Artificial substrata thus reflect local natural settlement well but may be a poor indicator of it when spatial scales are large, particularly when mussel bed topography is complex, or localities have very different recruitment densities.

The invasive mussel Mytilus galloprovincialis and the indigenous mussel Perna perna coexist intertidally on the south coast of South Africa through partial vertical habitat segregation: M. galloprovincialis dominates the upper shore and P. perna the lower shore. Recruitment patterns can explain the zonation of P. perna, but not the invasive species. We examined the role of post-recruitment interactions by measuring spatial and temporal differences in adult growth and mortality rates of the two species. Specifically, we tested the hypothesis that interspecific differences in growth and mortality reflect adult distribution patterns. The two study locations, Plettenberg Bay and Tsitsikamma, are 70 km apart with two sites (separated by 300–400 m) per location, each divided into three vertical zones. Growth was measured seasonally using different marking methods in 2001 and 2003. Cumulative adult mortality was measured through summer in 2003/2004. Both species generally grew more slowly upshore, but they showed different effects of season. For P. perna, growth was significantly reduced in winter in the low zone, but unaffected by season in the high zone. For M. galloprovincialis, growth was either unaffected by season or increased in winter, even in the high zone. Thus, growth of P. perna and M. galloprovincialis was reduced under cool winter and warm summer temperatures, respectively; and while growth was more similar between species in summer, M. galloprovincialis grew much faster than P. perna in winter. Mortality of P. perna increased upshore. For M. galloprovincialis, mortality was not zone-dependent and was significantly greater than for P. perna on the low-shore and (generally) across the shore in Tsitsikamma. Both species had higher growth and mortality rates in Plettenberg Bay than in Tsitsikamma. Thus, P. perna seems able to maintain spatial dominance on the low-shore and at certain sites because of higher mortality of M. galloprovincialis. We conclude that seasonality in growth of the two species reflects their biogeographic affinities and that coexistence is possible through pre-recruitment effects that limit the vertical distribution of P. perna and post-recruitment effects that limit M. galloprovincialis.

Top-down effects on intertidal communities concern species interactions such as competition and predation, and occur on small-scales. In contrast, bottom-up factors concern the supply of resources such as food and recruits. These are generally understood to be large-scale effects that are oceanographically controlled. Here we examined the effects of small-scale bottom-up factors on the growth of intertidal mussels to test the hypothesis that such factors can also operate on small (cm m) spatial scales. L-shaped aluminium baffles were used to reduce flow around mussels on very small (cm) scales in before/after experiments run at different sites in 2004 and 2006. Each experiment had two phases, each phase lasting approximately 100 days. At the start of phase one, mussels were individually marked at the growing edge of the shell in 10 control and 10 treatment patches before applying the treatment. At the start of phase 2, the same individuals were marked again at the new growing edge and baffles were placed around treatment patches. After phase one, there was no significant difference in growth rates between treatments in either year. In both years, applying the treatment reduced growth rates by approximately 30% compared to control patches. Condition index at the end of phase two showed the reverse and was lower for control mussels. Untreated control patches also showed a significant temporal effect (i.e. differences in growth between the two phases) during 2004 but not 2006. The results clearly show that bottom-up factors associated with food-supply operate at very small, within-shore scales, strongly influencing growth rates and possibly competitive abilities. They also indicate that bottom-up and top-down effects can interact across multiple spatial scales.

Retracing the origins of invasive species is a first critical step in identifying potential mechanisms of introduction, implementation of management strategies and forecasting the spread of the invader. Mytilus galloprovincialis is an intertidal mussel that is widely distributed in many temperate and subtropical regions. It is invasive worldwide and the most successful invasive marine species in southern Africa. Previous studies have examined genetic relationships between a few South African populations from the south-western coast and other worldwide populations, presenting evidence of a north-eastern Atlantic origin of the invasion. Here, a combination of nuclear (Me15/16 PCR-based) and mitochondrial (16S restriction fragment-length polymorphism; 16S RFLP) DNA assays was applied to infer the origin of this strong invader across its entire southern African distribution (South Africa and Namibia). The 16S RFLP confirmed the northern hemisphere as being the likely sole source of invasion. Additionally, the frequencies of haplotypes at the 16S marker and alleles at the Me15/16 locus point to north-eastern Atlantic shores as the most likely origin throughout the Namibian and South African distribution of the species.

Environmental heterogeneity challenges both indigenous species and invaders and can play a defining role in the dynamics of their interactions. We compare bay and open coast habitats to show how environmental heterogeneity and seasonality affect survival and physiological performances of invasive (Mytilus galloprovincialis) and indigenous (Perna perna) intertidal mussels. P. perna had significantly higher attachment strength than M. galloprovincialis. Attachment was strongly correlated with hydrodynamic stress and was lower for both species in bays. Both species had a major spawning event when wave action was weakest. In bays, there was no correlation between gonad index (GI) and attachment strength for either species, but on the open coast GI was negatively correlated with attachment. In bays, maximum GI of M. galloprovincialis was 64% higher than for P. perna, while on the open coast values did not differ between the two. Thus, on the open coast, both species invest more energy in attachment but P. perna can accommodate energetic demands of increased byssal production without altering gonad production, while M. galloprovincialis cannot. Mortality was significantly correlated to sand stress, while the correlation with wave action was very weak in bays and non-significant on the open coast probably because sand stress peaked during periods of low wave action. The success of the invader and thus the outcomes of its interaction with the indigenous species are governed by habitat-to-habitat variability. In this case the invasive species is likely to prove a weaker competitor on the more stressful and energetically demanding open coast.

The interactions between invasive exotic and indigenous species can have profound harmful effects on the recipient community; however, not all such interactions are negative. Facilitation is increasingly recognised as important in shaping natural communities and is believed to vary under different conditions. Earlier studies have shown that the indigenous intertidal mussel Perna perna initially facilitates survival of the invasive Mytilus galloprovincialis in the low mussel zone by providing protection against waves, but later excludes M. galloprovincialis through interference competition for space. Here, we examined interactions between these species in the mid and upper mussel zones, moving mussels to experimental plots in different combinations of densities and species. Mussels were left on the shore for more than a year and treatment effects on mortality, shell length and condition were compared. In the high zone, treatment had no effects and P. perna showed greater mortality than M. galloprovincialis, indicating that its exclusion from the high shore is due to emersion stress. In the mid zone, treatment had no significant effects on M. galloprovincialis, but multiple comparisons among treatments involving P. perna showed that facilitation occurred. P. perna survived better at higher densities, but survived even better when mixed with the physiologically more tolerant M. galloprovincialis. Length data indicated both inter- and intraspecific competition for P. perna in the mid zone. Whereas facilitation occurs strongly in the low zone (P. perna facilitates M. galloprovincialis) and weakly in the mid zone (M. galloprovincialis facilitates P. perna), the lack of facilitation in the high zone suggests that the probability of facilitation is not linearly linked to increasing physical stress. Instead it is likely to be hump shaped: relatively unimportant under conditions that are benign for a particular species, significant under more severe conditions, and overridden by physical stress under very harsh conditions.

Investigations into trophic ecology and aquatic food web resolution are increasingly accomplished through stable isotope analysis. The incorporation of dietary and metabolic changes over time results in variations in isotope signatures and turnover rates of producers and consumers at tissue, individual, population and species levels. Consequently, the elucidation of trophic relationships in aquatic systems depends on establishing standard isotope values and tissue turnover rates for the level in question. This study investigated the effect of diet and food quality on isotopic signatures of four mussel tissues: adductor muscle, gonad, gill and mantle tissue from the brown mussel Perna perna. In the laboratory, mussels were fed one of the two isotopically distinct diets for 3 months. Although not all results were significant, overall δ13C ratios in adductor, mantle and gill tissues gradually approached food source signatures in both diets. PERMANOVA analyses revealed significant changes over time in tissue δ13C (mantle and gill) with both diets and in δ15N (all tissues) and C:N ratios (mantle and gill) for one diet only. The percentage of replaced carbon isotopes were calculated for the 3 month period and differed among tissues and between diets. The tissue with the highest and lowest amount of replaced isotopes over 81 days were mantle tissue on the kelp diet (33.89%) and adductor tissue on the fish food diet (4.14%), respectively. Percentages could not be calculated for any tissue in either diet for δ15N due to the lack of significant change in tissue nitrogen. Fractionation patterns in tissues for both diets can be linked to nutritional stress, suggesting that consumer isotopic signatures are strongly dependent on food quality, which can significantly affect the degree of isotopic enrichment within a trophic level.

Macroalgae contribute to intertidal food webs primarily as detritus, with unclear implications for food web studies using stable isotope analysis. We examined differences in the thallus parts of two South African rhodophytes (Gelidium pristoides and Hypnea spicifera) and changes in overall δ13C, δ15N signatures and C:N ratios during degradation in both the field and laboratory. We hypothesized that both degrading macroalgal tissue and macroalgal-derived suspended particulate material (SPM) would show negligible changes in δ13C, but enriched δ15N signatures and lower C:N ratios relative to healthy plants. Only C:N laboratory ratios conformed to predictions, with both species of macroalgae showing decomposition related changes in δ13C and significant depletions in δ15N in both the field and laboratory. In the laboratory, algal tissue and SPM from each species behaved similarly (though some effects were non-significant) but with differing strengths. Gelidium pristoides δ13C increased and C:N ratios decreased over time in tissue and SPM; δ15N became depleted only in SPM. Hypnea spicifera, δ13C, δ15N and C:N ratios all decreased during degradation in both SPM and algae. Over 60 days in the field, δ13C was depleted in both species (1–2‰) and in naturally senescent Gelidium pristoides fronds. δ15N was depleted in Hypnea spicifera (approx. 1‰), while C:N ratios of both species were unaffected. The two species differed in δ13C, δ15N and C:N after degradation, but only in C:N beforehand. We suggest isotope changes in the laboratory mainly reflect microbial effects, while in the field these are combined with leaching due to constant water replenishment and agitation. Differences between these two species in the isotope responses to degradation highlight the difficulty of linking the signature of SPM to its multiple sources.

Recent phylogeographic research has indicated that biodiversity in the sea may be considerably greater than previously thought. However, the majority of phylogeographic studies on marine invertebrates have exclusively used a single locus (mitochondrial DNA), and it is questionable whether the phylogroups identified can be considered distinct species. We tested whether the mtDNA phylogroups of the southern African sandprawn Callianassa kraussi Stebbing (Decapoda: Thalassinidea) are also recovered using nuclear sequence data. Four mtDNA phylogroups were recovered that were each associated with one of South Africa’s four major biogeographic provinces. Three of these were poorly differentiated, but the fourth (tropical) group was highly distinct. The nuclear phylogeny recovered two major clades, one present in the tropical region and the other in the remainder of South Africa. Congruence between mitochondrial and nuclear DNA indicates that the species comprises two Evolutionarily Significant Units sensu Moritz (1994). In conjunction with physiological data from C. kraussi and morphological, ecological and physiological data from other species, this result supports the notion that at least some of the mtDNA phylogroups of coastal invertebrates whose distributions are limited by biogeographic disjunctions can indeed be considered to be cryptic species.

The mussel Mytilus galloprovincialis is highly invasive worldwide, but displays varying degrees of local and regional coexistence with indigenous mussels through spatial habitat segregation. We investigated the roles of settlement, post-settlement mortality, juvenile growth and recruitment in partial habitat segregation between the invasive M. galloprovincialis and the indigenous mussel Perna perna on the south coast of South Africa. We used two study locations, Plettenberg Bay and Tsitsikamma, 70 km apart, with two sites (separated by 300–400 m) per location, each divided into three vertical zones. There were no significant effects in Tsitsikamma, where daily settlement and monthly recruitment were significantly lower than in Plettenberg Bay. In Plettenberg Bay, settlement (primary and secondary) and recruitment of both species decreased upshore. Post-settlement mortality was measured over two consecutive 6-day periods during a spring tide and a neap tide. For both species mortality was low on the low-shore. High-shore mortality was consistently low for M. galloprovincialis, but increased dramatically for P. perna during spring tide. No data were obtained for growth of P. perna, but juvenile M. galloprovincialis grew more slowly farther upshore. P. perna recruited mainly in spring and summer, with a peak in summer far greater than for M. galloprovincialis. Recruitment of M. galloprovincialis was more protracted, continuing through autumn and winter. Thus local coexistence is due to a combination of pre- and post-recruitment factors differing in importance for each species. P. perna is excluded from the high-shore by recruitment failure (low settlement, high mortality). High survival and slow growth in juveniles may allow large densities of M. galloprovincialis to accumulate there, despite low settlement rates. With no differences between species in settlement or mortality on the low-shore, exclusion of M. galloprovincialis from that zone is likely to be by post-recruitment processes, possibly strengthened by periodic heavy recruitments of P. perna. At larger scales, larval retention and protracted recruitment contribute to the success of M. galloprovincialis at Plettenberg Bay, while recruitment limitation may explain why M. galloprovincialis is less successful at other sites.

Coastline topography has important effects on nearshore oceanography, larval trans- port, settlement and the adult distribution of benthic organisms. The resultant physical regime also influences interactions between invasive and indigenous species. Such interactions can alter inter- tidal communities dramatically, including the local extinction and replacement of native species. We examined the effect of bays and their associated headlands on the distribution of indigenous (Perna perna) and invasive (Mytilus galloprovincialis) mussels along 500 km of the south coast of South Africa. Within this single biogeographic region, mussel cover was estimated at 22 sites across 4 bays and the intervening open coast. Given that mussel biomass is greater at intermediate levels of wave exposure and that wave exposure is strongly dependent upon coastline topography, we hypothesised that mussel cover would be greater in bays, and that bays would specifically favour M. galloprovin- cialis which is more easily disturbed by strong waves. The 2 species show partial vertical separation into 3 zones within the lower eulittoral zone. Both species had significantly greater cover within bays. There was, however, an interaction between bay and zone for P. perna, and the effect of bays was strongest within the preferred zones of each species. Although the overall effect of bay was stronger for M. galloprovincialis than for P. perna, this resulted from the strong spatial structure identified for the M. galloprovincialis distribution using semivariogram analysis. Overall findings illustrate how coastline topography and local processes operate in synchrony to affect the dynamics of invasive and indigenous intertidal species.

Scales of spatial and temporal variation in the settlement of early and late plantigrades of the mussel Perna perna to artificial substrata were measured on the south coast of South Africa. One study examined daily settlement over one month at three sites of similar tidal height and aspect. The sites were separated by 100′s of m with 5 replicated artificial settlement substrata approximately 1 m apart within each site. Early (

Ecosystem dynamics driven by top-down controls have been well documented in rocky intertidal communities, while the effects of bottom-up influences are comparatively poorly understood. We hypothesized that large-scale signatures of the physical environment may be identifiable along the South African coastline as it is subject to two very different current systems (Benguela and Agulhas Currents) that profoundly influence primary production and thus both food type and availability. Through stable isotope analysis, we examined biogeographic patterns in multiple trophic levels at four sites along a 1400-km stretch of South African coastline and investigated the dietary role of macroalgal-derived organic carbon in rocky intertidal communities. The general positioning of trophic groups was comparable across all sites, with animals from the same trophic levels grouping together and with a δ 15 N fractionation of 1–2‰ between levels. The species found at all sites demonstrated east–west δ 15 N enrichment, presumably reflecting a biogeographic shift in nitrogen sources linked to upwelling on the west coast. Filter-feeders gave particularly clear results. Using discriminant analysis, mussels could be categorized into four geographic groups based on carbon and nitrogen signatures: east coast, southeast coast, south-west coast and west coast. Barnacles and polychaetes showed similar geographic groupings to mussels, but with shifts in actual values (1‰ depletion in δ 13 C and 3‰ enrichment in δ 15 N relative to mussels). This suggests that fractionation varies between species within a trophic level. IsoSource models showed that Ulva sp. made large contributions to the diets of two microalgal grazers ( Siphonaria capensis and Scutellastra granularis ) and this dietary dependence increased when moving from west to east coast, along the shoreline. Additionally, IsoSource models determined that relative to phytoplankton, macroalgae accounted for upwards of 60% of suspended particulate matter sampled from the shore (SPM; δ 13 C and δ 15 N) at three out of four sites and linear mixing models showed over 40% (all sites) and 50% (three sites) contribution of nearshore δ 13 C and δ 15 N, respectively, to the diet of all sampled filter feeders, inferring heavy dependence on macroalgal carbon. Numerous processes influence the stable isotope composition of algae, obscuring direct links between macroalgae and their consumers. In light of this, the clarity of the biogeographic patterns of filter feeders is remarkable and demonstrates a very strong signature of the physical environment in the intertidal community.

Understanding population connectivity is fundamental] to ecology, and, for sedentary organisms, connectivity is achieved through larval dispersal. We tested whether coastal topography influences genetic structure in Perna Perna mussels by comparing populations inside bays and on the open coast. Higher hydrodynamic stress on the open coast produces higher mortality and thus genetic turnover. Populations on the open coast had fewer private haplotypes and less genetic endemism than those inside bays. Gene flow analysis showed that bays act as Source populations, with greater migration rates Out. of bays than into them. Differences in genetic structure on scales of 10s of kilometres show that coastal configuration strongly affects selection, larval dispersal and haplotype diversity, Rhodes University; National Research Foundation of South Africa [2069119]; Claude Harris Leon postdoctoral research fellowship Foundation

Intertidal population dynamics are driven by a complex series of processes, including larval supply and the possibility of larval predation by benthic animals such as filter-feeders. We hypothesised that cannibalism by adults could play a major role in the population connectivity of mussel populations by removing larvae as they attempt to settle in the adult habitat. Specifically, we tested hypotheses that consumption of mussel larvae by adults removes a significant proportion of potential settlers and is influenced by both settlement intensity and tidal state (flooding or ebbing). Predation of mussel larvae by adult mussels was investigated on incoming and ebbing tides during four spring tides by analysing the gut contents of adult Perna perna and Mytilus galloprovincialis collected from the low intertidal mussel zone between October 2005 and January 2006. Consumption rates were then compared with estimates of successful settler densities on natural beds. The results showed that mortality of competent mussel larvae through adult ingestion removes up to 77% a of potential settlers. Rates of larval consumption were highest during months of intense settlement, suggesting that mussels feed opportunistically, filtering a relatively fixed volume of water and removing particles, including larvae, in proportion to their densities in the water. Rates of larviphagy were also higher during receding than incoming tides. We suggest that this is due to changes in larval density or, more probably, in adult filtration efficiency that are related to the state of the tide. Despite significant effects of both tidal state and settlement intensity on rates of larval ingestion, neither had a significant effect on the proportion of potential settlers removed. During settlement more than half of all potential settlers are lost through cannibalism, with potentially serious consequences for population maintenance. The results highlight the paradoxical nature of the evolution of settlement mechanisms in mussels, which must balance the advantages of settlement in habitats favourable to adults against the consumption of larvae by adults.

Temporal changes in hydrography affect suspended particulate matter (SPM) composition and distribution in coastal systems, potentially influencing the diets of suspension feeders. Temporal variation in SPM and in the diet of the mussel Perna perna, were investigated using stable isotope analysis. The δ13C and δ15 N ratios of SPM, mussels and macroalgae were determined monthly, with SPM samples collected along a 10 km onshore–offshore transect, over 14 months at Kenton-on-Sea, on the south coast of South Africa. Clear nearshore (0 km) to offshore (10 km) carbon depletion gradients were seen in SPM during all months and extended for 50 km offshore on one occasion. Carbon enrichment of coastal SPM in winter (June–August 2004 and May 2005) indicated temporal changes in the nearshore detrital pool, presumably reflecting changes in macroalgal detritus, linked to local changes in coastal hydrography and algal seasonality. Nitrogen patterns were less clear, with SPM enrichment seen between July and October 2004 from 0 to 10 km. Nearshore SPM demonstrated cyclical patterns in carbon over 24-h periods that correlated closely with tidal cycles and mussel carbon signatures, sampled monthly, demonstrated fluctuations that could not be correlated to seasonal or monthly changes in SPM. Macroalgae showed extreme variability in isotopic signatures, with no discernable patterns. IsoSource mixing models indicated over 50% reliance of mussel tissue on nearshore carbon, highlighting the importance of nearshore SPM in mussel diet. Overall, carbon variation in SPM at both large and small temporal scales can be related to hydrographic processes, but is masked in mussels by long-term isotope integration.

Growth of the abalone, Haliotis midae, was investigated at Port Alfred, on the south coast of South Africa, using both new and established techniques. A new method for aging animals is described, which makes use of shell autofluorescence under UV light to visualise internal growth bands. The deposition of growth bands was validated using measurements from shells of known age and, at one site, comparing growth estimates to those from cohort analysis undertaken at the same site. The new technique is far less time consuming and labour intensive than previously described methods; it is also non-destructive and proved to have potential for the reliable and rapid assessment of growth in large-scale studies. Growth of H. midae was also investigated at nine other sites, incorporating the full distribution range of the species. Systematic geographic variation in growth was observed along the South African coastline. Statistically significant differences existed among sites in growth rates for animals

The ability of an invasive species to spread in a new locality depends on its interaction with the indigenous community and on variation in time and space in the environment. The Mediterranean mussel Mytilus galloprovincialis invaded the South African coast 30 years ago and it now competes and coexists with the indigenous mussel Perna perna. The two species show different tolerances to wave and sand stress, two of the main environmental factors affecting this intertidal community. P. perna is more resistant to hydrodynamic stress than M. galloprovincialis, while the invasive species is less vulnerable to sand action. Our results show that mortality rates of the two species over a period of 6 months had different timing. The indigenous species had higher mortality than M. galloprovincialis during periods of high sand accumulation in mussel beds, while the pattern reversed during winter, when wave action was high. A negative correlation between sand accumulation and attachment strength of the two mussels showed that sand not only affects mussel mortality through scouring and burial, but also weakens their attachment strength, subjecting them to a higher risk of dislodgement. Here we underline the importance of variations in time and space of environmental stress in regulating the interaction between invasive and indigenous species, and how these variations can create new competitive balances.

Many South African populations of the brown mussel Perna perna have been depleted through over-exploitation by subsistence harvesters. This is problematic because recovery after disturbance is very slow, partly because recruits are largely associated with adult mussels. However, unlike large recruits of 3.5–10 mm that exhibit spatial structure related to that of adults, a very high proportion of settlers and small recruits (0–3.5 mm) occur on foliose algae. We tested the hypothesis that recruits on algae move to adult mussel beds after a period of growth, with the null hypothesis that they die at a certain size. We conducted an indirect field study comparing the ratios of large to small recruits in 100% cover mussel patches at locations with high and low algal cover. A second laboratory experiment analysed whether the size of recruits on algae affects their active movement behaviour in response to nearby mussel patches. Large/small recruit ratios were slightly, but not significantly greater in high than low algal cover locations. Both small (2–2.5 mm) and medium (4.5–5.5 mm) recruits remained on algae and moved very short distances throughout the laboratory experiment, while larger recruits (9–10 mm) moved significantly further distances and more often into mussel patches. The results suggest that very large recruits are able to migrate actively to nearby mussel patches, indicating ontogenetic shifts in this behaviour. However, the absence of a significant difference in ratios between field locations with high and low algal cover suggests many large recruits are accidentally dislodged from the algae and presumably die. Thus settlement of P. perna onto algae is likely to be wasted, with consequences for sustainable management of the mussel resource.

Recent genetic studies have shown that most widely distributed, passively dispersing invertebrates in southern Africa have regional intraspecific units that are associated with the three main marine biogeographic provinces (cool-temperate, warm-temperate and subtropical). The caridean shrimp Palaemon peringueyi also occurs in all threeprovinces, but the fact that it can disperse both actively and passively (i.e. larval drifting, adult walking/swimming and potential adult rafting by means of floating objects) suggests that the amount of gene flow between regions may be too high for evolutionary divergence to have taken place. Samples of P. peringueyi were collected throughout South Africa and an intraspecific phylogeny was reconstructed using mitochondrial COI and 16S rRNA sequences. Three major clades were recovered, which were broadly associated with the three biogeographic regions. This suggests that, even though P. peringueyi can disperse actively, the fact that neither larvae nor adults are strong swi...

Specimens of four sympatric intertidal limpet species (Siphonaria dayi, S. tenuicostulata, S. anneae and S. nigerrima ) were collected from four localities on the east coast of South Africa and southern Mozambique. Their phylogenetic relationships were investigated using sequences of the mitochondrial COI gene and the intron-containing nuclear ATPSb gene. Two closely related lineages were recovered, which grouped specimens on the basis of geography rather than morphology. One lineage was associated with the subtropical coastline of South Africa’s east coast and the other with the tropical coastline of northeastern South Africa and southern Mozambique. This genetic discontinuity coincides with a biogeographic boundary located in the vicinity of Cape St Lucia. Combined genetic diversity of the four species was lower than that of three other southern African congeners, and fell within the range determined for single southern African marine mollusc species. We suggest that the four limpet species are in fact different morphotypes of a single species.

The amount of genetic structure in marine invertebrates is often thought to be negatively correlated with larval duration. However, larval retention may increase genetic structure in species with long-lived planktonic larvae, and rafting provides a means of dispersal for species that lack a larval dispersal phase. We compared genetic structure, demographic histories and levels of gene flow of regional lineages (in most cases defined by biogeographic region) of five southern African coastal invertebrates with three main types of larval development: (1) dispersal by long-lived planktonic larvae (mudprawn Upogebia africana and brown mussel Perna perna), (2) abbreviated larval development (crown crab Hymenosoma orbiculare) and (3) direct development (estuarine isopod Exosphaeroma hylecoetes and estuarine cumacean Iphinoe truncata). We hypothesized that H. orbiculare, having abbreviated larval development, would employ a strategy of larval retention, resulting in genetic structure comparable to that of the direct developers rather than the planktonic dispersers. However, regional population structure was significantly lower in all species with planktonic larvae, including H. orbiculare, than in the direct developers. Moreover, nested clade analysis identified demographic histories resulting from low levels of gene flow (isolation by distance and allopatric fragmentation) in the direct developers only, and migration rates were significantly higher in all three species having planktonic larvae than in the direct developers. We conclude that the amount of genetic structure within marine biogeographic regions strongly depends on the presence or absence of free-swimming larvae. Whether such larvae are primarily exported or retained, whether they have long or short larval duration, and whether or not they are capable of active dispersal seems to have little effect on connectivity among populations.

Settlement is a major determinant of intertidal populations. However, the energy costs of lost larvae are very high. Accordingly, arrival and attachment on suitable substrata are essential requirements for species’ survival. On the intertidal, the presence of cues left by adult or juvenile conspecifics could be vital for the successful establishment of larvae arriving on the shore. Two mussel species, the indigenous Perna perna and the invasive Mytilus galloprovincialis, co-occur on the lower eulittoral zone on the south coast of South Africa. P. perna dominates the low and M. galloprovincialis the high mussel zones, with co-existence in the mid mussel zone. This study tested the hypothesis of settlement selectivity for conspecifics in these two mussel species, to understand whether the final adult distribution of mussels on the shores is determined by active behavioural and chemical mechanisms. Preferential selection by larvae for conspecifics was tested in the field during the peak settlement period in 2004 in natural mussel beds across zones and through manipulative experiments in the mid-zone where the species co-exist. On natural beds, settlement was determined by counts of settlers attached over 48 h onto artificial collectors. Collectors were placed on beds of P. perna and M. galloprovincialis present at both high- and low-adult densities, as well as in mixed beds. On such natural beds, settlers of both species consistently favored low-zone P. perna beds. Settlement patterns over 24 h onto experimentally created mussel patches consisting of P. perna, M. galloprovincialis or the two species combined beds, set in the mixed zone, did not conform with the results of the natural beds study: settlers of both species settled with no discrimination among different patches. The results indicate that mussels, which are sedentary, lack attraction to conspecifics at settlement. This highlights the importance of tidal height in setting settlement rates, and of post-settlement events in shaping populations of these broadcast spawners.

Genetic structure of sedentary marine organisms with planktonic larvae can be in- fluenced by oceanographic transport, larval behaviour and local selection. We analysed the pop- ulation genetic structure (based on mtDNA) of the invasive mussel Mytilus galloprovincialis and the indigenous mussel Perna perna along the southern African coastline. Low genetic divergence of M. galloprovincialis confirms its recent arrival in South Africa. In contrast, the genetic structure of P. perna revealed strong divergence on the south-east coast, forming a western and an eastern lin- eage. The distribution of the 2 lineages is extraordinary. They overlap for ca. 200 km on the south- east coast, and the western lineage includes animals occurring on either side of a 1000 km break in distribution across the Benguela upwelling system. In cluster analyses, animals on the south coast grouped with others 1000s of km to the west, rather than with those only 200 km to the east. This genetic disjunction may be caused by the south-flowing Agulhas Current preventing larval dispersal, or by different selective forces acting on local populations. M. galloprovincialis spread eastward along the south coast for 15 yr, but its range extension has virtually ceased in the region of genetic disjunction in P. perna, again indicating an oceanographic barrier to larval dispersal or selection dri- ven by sharp gradients in environmental conditions. The results suggest that local selection can pro- duce genetic structure opposite to that predicted by oceanographic data and that determining the population structure of indigenous species with similar larval dispersal can help us understand domain expansion of invading species.

Although small copepods are one of the main dietary sources for many commercially important fish, their role in the pelagic trophic dynamics has traditionally been underestimated due to the methodology commonly used in plankton sampling. Temporal variation in abundance of adults and nauplii of small copepods (particularly Oithona plumifera ) in nearshore waters on the south coast of South Africa was investigated fortnightly over 14 months at site (km) and location (100 m) scales. Sampling was within Oithona spp. was consistently the most abundant. Taxon richness was 7–19 on each sampling occasion. There was strong temporal variation ( Oithona varied between 0 and 2300 m −3 ), but much of this was short-term variability (e.g. between consecutive sampling sessions), with no seasonality or other long-term discernable patterns. There were periods of consistently low numbers, but very high numbers often followed samples with low abundances. Nor was there spatial structure at the location scale, though numbers differed between sites. Despite considerable variability at the location scale within sites, Kenton consistently showed higher densities than High Rocks. Separate analyses, with Bonferroni adjustment, showed that this difference was significant on eight out of 21 occasions for Oithona , six for other pelagic copepods and three for nauplii. This suggests that hydrodynamics favour aggregation of plankton at Kenton. A high degree of short-term variability, with a tendency for aggregation of small zooplankton at certain sites has implications for both pelagic processes and food-web links between the benthic and pelagic environments.

We compared the behavioural responses of an indigenous South African mussel (Perna perna) and an invasive mussel (Mytilus galloprovincialis) to the risk of predation. Both species were subjected to the specific risk of predation by the native rock lobster Jasus lalandii and the general risk of predation simulated by the presence of damaged conspecifics. We hypothesised that, because M. galloprovincialis evolved in allopatry from the rock lobster, P. perna would respond more strongly to J. lalandii, but that there would be less difference between species in their response to damaged con- specifics. The results confirmed our initial hypotheses. M. galloprovincialis crawled farther under both predation treatments and generally aggregated more than P. perna. P. perna has a larger foot, and our results therefore do not reflect morphological differences between the species but imply greater sensitivity in M. galloprovincialis to chemical cues. Crawling distance was not enhanced under predation threat relative to the control; therefore, increased clumping due to more rapid ran- dom movement cannot explain our results, indicating that chemical attraction among individuals is important. P. perna clumping behaviour was significantly greater when exposed to local lobster efflu- ent or damaged conspecifics, whereas M. galloprovincialis clumped more when exposed to a general threat of predation but exhibited naive behaviour to local predator effluent. This lack of responsive- ness to a native predator could be due not only to the recent arrival of the European mussel, but also to the relatively low selective effects of predation in South Africa.

The indigenous mussel Perna perna and the invasive mussel Mytilus galloprovincialis coexist on the low shore of the South African south coast, with partial habitat partitioning. P. perna dominates the lower mussel zone, where hydrodynamic stress is higher than in the high mussel zone, where M. galloprovincialis dominates. Attachment strength and gonad maturation are highly demanding processes in terms of energy, the former allowing mussels to withstand wave forces, while the latter contributes to recruitment rates. During an 18 mo study, both biological parameters fluctuated seasonally. The invasive and the indigenous species spawned in different periods; gamete maturation was positively correlated with sea surface temperature only for P. perna. For both species, attachment strength was negatively correlated with sea surface temperature, but increased in response to wave action through the production of more and thicker threads. Peaks in attachment strength coincided with periods of relatively low gamete production for both species, suggesting that mussels cannot afford to invest simultaneously in both processes. P. perna attachment strength was always significantly higher than that of M. galloprovincialis , while the latter had a greater reproductive output. We suggest that, although high gamete production improves the ability of M. galloprovincialis to colonise free space, it may energetically limit its capacity to invade wave exposed shores.

Recruitment and population structure of Perna perna in low shore mussel beds were investigated over 15 months at six sites along the south coast of South Africa. Initial, subjective classification of sites as wave exposed or wave sheltered (three of each) was confirmed using the dissolution of cement blocks to measure average water flux and dynamometers for maximum wave force. Recruitment occurred throughout the year, but recruit (1–5 mm) densities were significantly higher from January to April 1996 on both shore types. Recruit densities were positively correlated with adult (>15 mm) densities for both shore types (P 0.05). Thus wave exposure dramatically affects density, recruitment and mussel size, but not recruitment timing or biomass where there is 100% cover, and mediates a three-way interaction among food supply, larval supply and intraspecific competition for space. In contrast to shores with saturation recruitment, mussel biomass here appears to be limited by recruit supply and constraints of food, especially on sheltered shores, while density is regulated through intraspecific competition for space primarily on exposed shores and at small spatial scales.

Understanding species distributions and patterns of coexistence is a basic aim of ecology and fundamental to understanding not only how communities have come to be as they are but also how they will change in the future. This is particularly important when trying to predict biological responses to rapid and extreme changes in environmental conditions. Our perception of community dynamics depends on the temporal and the spatial scales at which we make our observations and the taxonomic resolution we use. Thus, observations and empirical approaches that allow for variability in time and space of multiple, interacting drivers are essential to counteract the assumption that simple patterns of species’ arrangement are driven by similarly simple processes. By altering the structure of resident communities, invasive species allow us to observe the development of new distributional equilibria and identify how variation in time and space of biotic and abiotic factors determines patterns of coexistence with native species. Here, we use the example of an invasive and an indigenous intertidal mussel to achieve this. We identify rather simple patterns of distribution of an exceptionally successful marine invasive species, the mussel Mytilus galloprovincialis, in its distribution along the coast of South Africa, examining relevant processes at multiple scales. In particular, we draw together research on its coexistence with an indigenous mussel, Perna perna. Over the last 40 years, Mytilus galloprovincialis has spread along thousands of kilometres of the southern African coast. Mytilus galloprovincialis has now reached at least a temporary equilibrium in its eastern limit and in its pattern of coexistence with the native mussel Perna perna. We synthesize a comprehensive body of literature on these two species that addresses processes occurring from centimetres to thousands of kilometres and from minutes to evolutionary timescales to develop a more robust understanding of the relative contributions of the deterministic and stochastic processes that structure the dynamics of coexistence. The review shows that the development and maintenance of simple distributional patterns emerge from and are maintained by variations in multiple biotic and abiotic interactions that occur at different and sometimes-nested scales. These include species-specic effects of environmental factors, such as not only high temperatures, wave action, desiccation, and sand scour or sand burial but also biological effects such as predation, parasitism, larval supply, and recruitment. Importantly, abiotic conditions can modulate species interactions, including interference and exploitation competition, so that species-specic responsesto environmental conditions are important. Viewed across the environmental gradient offered by the intertidal landscape, direct interactions include examples of initial facilitation followed by later competitive exclusion. Although in a state of ux at small scales, the present patterns of distribution and coexistence have been relatively stable at larger scales for decades and reect place-specic balances among the interacting factors, with different factors assuming prominence in different places. Detailed observation and experimentation are necessary to avoid assuming that species’ arrangements emerge from a single driver or a few simple drivers, and this has clear implications for attempts to predict species distributions under conditions of climate change. In addition, the same pattern can emerge for different reasons. For example, recruitment of both mussel species is weak in the high mussel zone, but for different reasons. For Perna perna, this is because of poor survival of few settlers, while for Mytilus galloprovincialis, it reects good survival of few settlers. With so many interacting factors achieving uctuating local balances, the surprise is not that the pattern is so clear, but that there is any pattern at all.

Consumers and prey diversity, their interactions, and subsequent effects on ecosystem function are important for ecological processes but not well understood in high diversity ecosystems such as coral reefs. Consequently, we tested the potential for diversity-effects with a series of surveys and experiments evaluating the influence of browsing herbivores on macroalgae in Kenya’s fringing reef ecosystem. We surveyed sites and undertook experiments in reefs subject to three levels of human fishing influence: open access fished reefs, small and recently established community-managed marine reserves, and larger, older government-managed marine reserves. Older marine reserves had a greater overall diversity of herbivores and browsers but this was not clearly associated with reduced macroalgal diversity or abundance. Experiments studying succession on hard substrata also found no effects of consumer diversity. Instead, overall browser abundance of either sea urchins or fishes was correlated with declines in macroalgal cover. An exception was that the absence of a key fish browser genus, Naso, which was correlated with the persistence of Sargassum in a marine reserve. Algal selectivity assays showed that macroalgae were consumed at variable rates, a product of strong species-specific feeding and low overlap in the selectivity of browsing fishes. We conclude that the effects of browser and herbivore diversity are less than the influences of key species, whose impacts emerge in different contexts that are influenced by fisheries management. Consequently, identifying key herbivore species and managing to protect them may assist protecting reef functions.

Background Intraspecific variability is seen as a central component of biodiversity. We investigated genetic differentiation, contemporary patterns of demographic connectivity and intraspecific variation of adaptive behavioural traits in two lineages of an intertidal mussel (Perna perna) across a tropical/subtropical biogeographic transition. Results Microsatellite analyses revealed clear genetic differentiation between western (temperate) and eastern (subtropical/tropical) populations, confirming divergence previously detected with mitochondrial (COI) and nuclear (ITS) markers. Gene flow between regions was predominantly east-to-west and was only moderate, with higher heterozygote deficiency where the two lineages co-occur. This can be explained by differential selection and/or oceanographic dynamics acting as a barrier to larval dispersal. Common garden experiments showed that gaping (periodic closure and opening of the shell) and attachment to the substratum differed significantly between the two lineages. Western individuals gaped more and attached less strongly to the substratum than eastern ones. Conclusions These behavioural differences are consistent with the geographic and intertidal distributions of each lineage along sharp environmental clines, indicating their strong adaptive significance. We highlight the functional role of diversity below the species level in evolutionary trends and the need to understand this when predicting biodiversity responses to environmental change. Electronic supplementary material The online version of this article (doi:10.1186/s12862-015-0366-5) contains supplementary material, which is available to authorized users.

Several studies suggest invertebrate recruitment is affected by coastal configuration, implying effects on population structure. Mesoscale (1 to 100 km) variability in spawning and recruit- ment of intertidal mussels was examined at 12 sites on the South African south coast. There were 3 shore types: Algoa Bay shores (sandstone) and open coast shores (sandstone or dune rock). Timing of spawning and recruitment did not differ among shore types, but intensity did. All shores had contin- uous spawning and trickle recruitment, with peak periods. Spawning at each shore peaked 3 to 4 times per year with poor synchrony among sites, even sites only 3 to 4 km apart. Spawning corre- lated negatively with air and sea temperature, being most likely in winter/spring. Individual gamete output was greater in Algoa Bay. Recruitment was poorly pulsed and much lower than in most boreal studies; 1 to 2 juvenile cohorts appeared on each shore each year, with no synchrony among shores. Each cohort recruited over 2 to 3 mo. Subsequent mortality and growth were density independent, which suggested that pre-settlement factors are important in population regulation. New cohort den- sities correlated with spawning intensity, and were significantly greater in Algoa Bay. Spawning intensity, new cohort density, growth and mortality accounted for 76% of the variation in recruitment into adult populations (multiple regression). Differences in spawning and recruitment were associ- ated with differences in population size structure and adult longevity. Mesoscale differences in recruitment and a spawning output/new cohort density correlation can be accounted for by limited dispersal of larvae on scales of tens of kilometres, with open coast and Algoa Bay populations acting as semi-discrete elements of a metapopulation.

The mussel Mytilus galloprovincialis is invasive worldwide, has displaced indigenous species on the west coast of South Africa and now threatens Perna perna on the south coast. We tested the hypothesis that Mytilus will replace Perna by examining changes in their distribution on shores where they co-exist. Total cover, adult density, recruit density, recruit/adult correlations and mean maximum lengths of both species were measured in 2001 at two contrasting sites (Plettenberg Bay and Tsitsikamma) 70 km apart, each including two locations 100 m apart. Cover and density were measured again in 2004. Total mussel abundance was significantly lower in Tsitsikamma, and recruit density was only 17% that of Plettenberg Bay. Abundance and cover increased upshore for Mytilus, but decreased for Perna, giving Mytilus higher adult and recruit density and total cover than Perna in the upper zones. Low shore densities of recruits and adults were similar between species but cover was lower for Mytilus, reflecting its smaller size, and presumably slower growth or higher mortality there. Thus, mechanisms excluding species differed among zones. Recruitment limitation delays invasion at Tsitsikamma and excludes Perna from the high shore, while Mytilus is excluded from the low shore by post-recruitment effects. Recruitment limitation also shapes population structure. Recruit/adult correlations were significant only where adult densities were low, and this effect was species-specific. Thus, at low densities, larvae settle or survive better near adult conspecifics. After 3 years, these patterns remained strongly evident, suggesting Mytilus will not eliminate Perna and that co-existence is possible through partial habitat segregation driven by recruitment limitation of Perna on the high shore and post-settlement effects on Mytilus on the low shore.

The potential of introduced species to become invasive is often linked to their ability to colonise disturbed habitats rapidly. We studied the effects of major disturbance by severe storms on the indigenous mussel Perna perna and the invasive mussel Mytilus galloprovincialis in sympatric intertidal populations on the south coast of South Africa. At the study sites, these species dominate different shore levels and co-exist in the mid mussel zone. We tested the hypotheses that in the mid- zone P. perna would suffer less dislodgment than M. galloprovincialis, because of its greater tenacity, while M. galloprovincialis would respond with a higher re-colonisation rate. We estimated the per- cent cover of the 2 mussels in the mid-zone from photographs, once before severe storms and 3 times afterwards. M. galloprovincialis showed faster re-colonisation and 3 times more cover than P. perna 1 and 1.5 yr after the storms (when populations had recovered). Storm-driven dislodgment in the mid- zone was highest for the species that initially dominated at each site, conforming to the concept of compensatory mortality. This resulted in similar cover of the 2 species immediately after the storms. Thus, the storm wave forces exceeded the tenacity even of P. perna, while the higher recruitment rate of M. galloprovincialis can explain its greater colonisation ability. We predict that, because of its weaker attachment strength, M. galloprovincialis will be largely excluded from open coast sites where wave action is generally stronger, but that its greater capacity for exploitation competition through re-colonisation will allow it to outcompete P. perna in more sheltered areas (especially in bays) that are periodically disturbed by storms.

There are broad differences in oceanography and primary production around the south- ern African coast that are likely to give rise to major differences in trophic pathways. Stable isotope ratios provide integrated information on trophic relationships, yet there has been limited research on geographic variation in isotopic composition of marine consumers and their food. In this study, δ 13 C and δ 15 N of suspended particulate matter (SPM), intertidal mussels and common macroalgae along the southern African coastline were explored. Nearshore-offshore isotope trends as well as biogeo- graphic and temporal patterns in isotopic ratios of mussel tissue, macroalgae and SPM were investi- gated at 12 sites along the coast from Namibia to the Mozambique border. SPM exhibited overall trends of nearshore 13 C depletion from south-west to north-east along the coastline and from nearshore (0 km) to offshore (10 km) waters, in both cases suggesting a shift from a nearshore signa- ture strongly influenced by macroalgal detritus to one more representative of oceanic phytoplankton. With one exception it was possible, using discriminant analysis, to categorize mussel populations into 4 geographic groups, on the basis of both carbon and nitrogen signatures: the east coast, the south- east coast, the south-west coast and the west coast. Macroalgae showed no consistent biogeographic trends and need to be examined in greater detail to relate nearshore SPM values to living macroalgal signatures. A linear mixing model indicated that mussels along the entire coastline generally demon- strated more than 50% dependence on nearshore carbon and nitrogen, emphasizing the importance of nearshore primary production to intertidal consumers.

The South African coastline comprises 3 main biogeographic provinces: (1) the cool- temperate west coast, (2) the warm-temperate south coast, and (3) the subtropical east coast. The boundaries between these regions are defined by changes in species compositions and hydrological conditions. It is possible that these affect phylogeographic patterns of coastal organisms differently, depending on the species' ecologies and modes of dispersal. In the present study, genealogies of 3 estuarine crustaceans, each characterized by a different mode of passive dispersal and present in more than one biogeographic province, were reconstructed using mtDNA COI sequences, and the impacts of biogeographic boundaries on their phylogeographic patterns were compared. The species were (mode of dispersal in brackets): (1) the mudprawn Upogebia africana (planktonic larvae), (2) the isopod Exosphaeroma hylecoetes (adult rafting), and (3) the cumacean Iphinoe truncata (adult drift- ing). Two major mtDNA lineages with slightly overlapping distributions were identified in U. africana (the species with the highest dispersal potential). The other 2 species had 3 mtDNA lineages each, which were characterized by strict geographic segregation. Phylogeographic breaks in U. africana and E. hylecoetes coincided with biogeographic boundaries, whereas the phylogeographic patterns identified in I. truncata may reflect persistent palaeogeographic patterns. Ecological factors and modes of dispersal are likely to have played a role in both cladogenesis of the different lineages and in the establishment of their present-day distribution patterns.

On the south coast of South Africa, the invasive alien mussel Mytilus galloprovincialis shows partial habitat segregation with the indigenous mussel Perna perna. P. perna predominates in the lower mussel zone and M. galloprovincialis in the upper zone, with mixed beds where the two overlap. We examined competitive interactions between these species by translocating mussels into small plots at high densities. Treatments involved different combinations of species and densities placed in each zone. Mortality was monitored regularly and at the end of each experiment, growth and condition index were measured. The experiment was attempted three times. The first two attempts were disrupted by wave action, especially winter storms, but provided information on species-specific effects of wave action. In experiment one, wave induced mortality decreased from a mean for both species of approximately 90% on the low shore to ca. 50% on the high shore, and was 15–30% lower for P. perna than M. galloprovincialis in each zone. In experiment two, M. galloprovincialis mortality was not affected by zone (Kruskal–Wallis test, P > 0.05), but was higher than P. perna mortality in the low zone (P

Population dynamics of many intertidal organisms are strongly affected by the abundance and distribution of larvae arriving on the shore. In particular, not only absolute numbers of settlers but also the degree of synchronisation of settlement can have a strong influence on whether density-dependent or density-independent processes shape adult shape populations. Temporal variation in rates of settlement and recruitment of the mussel Perna perna on the south coast of South Africa was investigated using a nested spatial design at different temporal scales. Variability in settlement at spring tides was examined at two temporal scales: lunar (to investigate the effect of state of the moon on settlement) and tidal (to investigate the influence of state of the tide on mussel settlement). Recruitment over neap tides was examined at one temporal scale, fortnight (to investigate the effect of date on mussel recruitment). Strong temporal variation was evident for both settlement and recruitment, but not at all time scales. Distinct peaks of settler/recruit abundance were observed during the lunar and neap tide studies. Recruitment intensity differed over the course of the year, and pulsing of recruitment was generally synchronised among locations. However, the strength of pulsing differed dramatically among locations, giving a significant interaction between fortnight and location. The finest temporal scale, investigated in the tidal study, did not reveal a significant effect of the state of the tide on settlement. The state of the moon (new or full) was not significant as a main factor (p = 0.052), although generally more settlers arrived on the shore during new moon. Phase of the moon appeared to have an effect on settler abundances, but only when and where densities were high.

The ability of a mussel to withstand wave-generated hydrodynamic stress depends mainly on its byssal attachment strength. This study investigated causes and consequences of different attachment strengths of the two dominant mussels species on the South African south coast, the invasive Mytilus galloprovincialis and the indigenous Perna perna, which dominate the upper and the lower areas of the lower balanoid zone, respectively and co-exist in the middle area. Attachment strength of P. perna was significantly higher than that of M. galloprovincialis. Likewise solitary mussels were more strongly attached than mussels living within mussel beds (bed mussels), and in both cases this can be explained by more and thicker byssal threads. Having a wider shell, M. galloprovincialis is also subjected to higher hydrodynamic loads than P. perna. Attachment strength of both species increased from higher to lower shore, in response to a gradient of stronger wave action. The morphological features of the invasive species and its higher mortality rates during winter storms help to explain the exclusion of M. galloprovincialis from the low shore. The results are discussed in the context of the evolutionary strategy of the alien mussel, which directs most of its energy to fast growth and high reproductive output, apparently at the cost of reduced attachment strength. This raises the prediction that its invasive impact will be more pronounced at sites subject to strong but not extreme wave action.

Marine reserves are commonly used to conserve living resources, but their effectiveness where policing is difficult is unclear. We compared monthly population density and size structure data collected over 20 months for two rarely and two commonly exploited intertidal limpets inside and outside reserves in South Africa. Densities were greater inside reserves, but significantly so in very few months. Unexpectedly, no significant reserve effect was detected for the territorial Scutellastra longicosta, shown by surveys to be favoured by harvesters. Reserve effects for the rarely exploited S. granularis probably result from indirect effects of higher barnacle cover in reserves where trampling is reduced, while interviews indicated that Cellana capensis is not targeted but large individuals are sometimes misidentified and hence are harvested outside the reserve. There were few reserve effects on mean or mean maximum size. The results indicate a gradient of exploitation among species and sites. One non-reserve site was more heavily exploited than the other, while one reserve experienced more poaching than the other. The effectiveness of marine reserves thus differed between reserves and among species. Exploited limpets were generally larger and at higher densities in reserves, but these effects were rarely statistically significant, indicating that reserve effects are weak compared to natural variability, probably reflecting ineffective policing.Keywords: exploited sites, marine protected areas, poaching, rocky intertidal, subsistence harvestingAfrican Journal of Marine Science 2014, 36(3): 303–311

The subtidal whelk Burnupena papyracea co-occurs with a voracious predator, the rock lobster Jasus lalandii, in situations where other potential prey are largely eliminated. The survival of B. papyracea has been ascribed to a symbiotic bryozoan, Alcyonidium nodosum, which characteristically encrusts the shells of this mollusc and deters feeding by J. lalandii. Although B. papyracea shells encrusted with A. nodosum were found to be stronger than non-encrusted shells, this bryozoan-induced physical defence was insufficient to deter predation by J. lalandii. However, laboratory bioassays using individual J. lalandii suggested a chemical basis for feeding deterrence originating from A. nodosum and highlighted the advantages of using ecologically relevant bioassays to monitor chemically mediated interactions between species in the marine environment. The nature of the chemical compound(s) responsible for the acquired chemical defence passively imparted by A. nodosum to B. papyracea is unknown. Keywords: Alcyonidium nodosum, bryozoan, Burnupena papyracea, chemical defence, Jasus lalandii, predation, rock lobster, whelkAfrican Journal of Marine Science 2005, 27(3): 549–556

Periodical sand inundation influences diversity and distribution of intertidal species throughout the world. This study investigates the effect of sand stress on survival and on habitat segregation of the two dominant mussel species living in South Africa, the invasive Mytilus galloprovincialis and the indigenous Perna perna. P. perna occupies a lower intertidal zone which, monthly surveys over 1.5 years showed, is covered by sand for longer periods than the higher M. galloprovincialis zone. Despite this, when buried under sand, P. perna mortality rates were significantly higher than those of M. galloprovincialis in both laboratory and in field experiments. Under anoxic condition, P. perna mortality rates were still significantly higher than those for M. galloprovincialis, but both species died later than when exposed to sand burial, underlining the importance of the physical action of sand on mussel internal organs. When buried, both species accumulate sediments within the shell valves while still alive, but the quantities are much greater for P. perna. This suggests that P. perna gills are more severely damaged by sand abrasion and could explain its higher mortality rates. M. galloprovincialis has longer labial palps than P. perna, indicating a higher particle sorting ability and consequently explaining its lower mortality rates when exposed to sand in suspension. Habitat segregation is often explained by physiological tolerances, but in this case, such explanations fail. Although sand stress strongly affects the survival of the two species, it does not explain their vertical zonation. Contrary to our expectations, the species that is less well adapted to cope with sand stress maintains dominance in a habitat where such stress is high.

Sampling of recruitment-associated variables of Perna perna was done approximately monthly for 14 months at intertidal locations 500 m apart, nested within sites 25 km apart. Paired with intertidal locations were nearshore locations, 600 m to sea. Sampling assessed spawning, densities of larvae in the water column and densities of late plantigrades and juveniles on the shore. Major events in each variable were synchronous over larger scales (10s of kilometres) while subsidiary events were synchronised at smaller scales, varying within sites (100s of metres) or even within locations (metres). This suggests that the processes driving major events operated over large scales while processes operating at much more local scales drove less intense, more localised events. A major spawning event occurred at all locations in May–June 1998. Weaker spawning events occurred at different times in different locations. Larvae were found on 80% of sampling occasions, densities peaking in January–March 1998 and 1999 at all locations. Plantigrades and juveniles showed less clear patterns, with considerable residual variation. There was no sign of strong coupling among variables with few significant direct or cross correlations. The major sources of variability shifted from time to space as one progressed from spawning, to plantigrade density to juvenile density. For spawning, time was the most important source (58%) of heterogeneity and space accounted for little (8%) of the total variance. For larvae and late plantigrades, time was still the most important source of variability (41% and 33%, respectively), but space was a much more substantial component. For juveniles, small-scale (residual) spatial variability dominated total variability (75%). This strongly suggests the importance of hydrography and its effects on variation in delivery of larvae to the intertidal from offshore. These findings also indicate greater spatial heterogeneity as recruits age, reflecting small-scale variations in larval delivery and the increasing importance of post-settlement mortality.

We examined the spatial heterogeneity in three sessile rocky shore organisms, the mussel Perna perna, the barnacle Octomeris angulosa (Sowerby) and the red alga Gelidium pristoides (Turn.) at a range of continuous local scales along horizontal transects within mid- and upper mussel beds of South African shores. We also examined the relationships between variability of organisms and topographic features (rock depressions, slope, aspect), and between mussel, barnacle and algal variability over the same scales. To estimate spatial heterogeneity, we analyzed scaling properties of semivariograms using a fractal approach. Relationships between different variables at the different scales were examined by cross-semivariograms. Spatial dependence of P. perna variability increased with spatial dependence of topographic variability, so that scaling regions of mussel and topographic distributions corresponded well. This relationship often improved with larger local scales (mussel cover increased with depressions, steeper slope and aspect towards waves), while at smaller spatial scales, variability in mussel cover was less well explained by variability in topography. The variability of the barnacle O. angulosa exhibited spatial dependence, even on topographically unstructured shores. In contrast, the distribution of the alga G. pristoides revealed high fractal dimensions, showing spatial independence on topographically unstructured shores. Algae also showed a very strong negative relationship with mussels at most local scales, and a negative relationship with barnacles in upper zones, especially at larger local scales. Barnacles may show clear spatial dependence because of hydrodynamics (at larger local scales) and the need to find a future mate in close proximity (at smaller local scales), while algae may show a strong negative relationship with mussels because of competition for space. D 2004 Elsevier B.V. All rights reserved.

A fine time-scale study was undertaken on the effects of wave exposure, tidal height and substratum type on mussel recruit densities on the south coast of South Africa. Prior to sampling, an exposed and a sheltered shore were identified at each of 2 sites (Diaz Cross and High Rocks) 7 km apart. Each shore was divided into 3 shore levels (termed zones) within which 3 substrata (adult mus- sels, coralline and non-coralline macroalgae) were sampled. Destructive sampling of early (

The current status of marine alien species along the South African coast is reviewed and the ecological and economic impacts of these invasions are discussed. In all, 10 confirmed extant alien and 22 cryptogenic species are recorded from the region. All 10 alien species support well-established populations and the majority of these remain restricted in distribution to sheltered bays, estuaries and harbours. Only one species, the Mediterranean mussel Mytilus galloprovincialis, has spread extensively along the coast and caused significant ecological impacts. These include the competitive displacement of indigenous species and a dramatic increase in intertidal mussel biomass. These changes have also increased available habitat for many infaunal species and resulted in enhanced food supply for intertidal predators. Considerable economic benefits have also resulted from this invasion because M. galloprovincialis forms the basis of the South African mussel culture industry.

To test the assumption that there is no spatial structure in small-scale recruitment vari- ability of rocky shore mussels, we examined spatial dependence in the distribution of density of recruits (late plantigrades: 0.5 to 3.5 mm; larger recruits: 3.5 to 10 mm) and adults of the brown mus- sel Perna perna within local scales (30 lags ranging between 0.35 and 10.5 m) in mid- and upper mus- sel beds. Spatial heterogeneity was estimated by analyzing scaling properties of semivariograms using a fractal approach. Relationships between density of mussel recruits and adults and biomass of the red alga Gelidium pristoides at the different scales were examined by cross-semivariograms. We found that the distribution of adults showed spatial dependence at all transects, often with higher spatial heterogeneity (higher fractal dimension, D) at smaller scales (1st scaling region). The distrib- ution of larger recruits exhibited spatial dependence at all transects, revealing a spatial structure, which was related to that of adults. In contrast, the distribution of late plantigrades showed mainly spatial independence (random pattern; 1.97 < D ≤ 2). Densities of both size classes of recruits were positively related to those of adults at all transects and scales, but the relationship was stronger for larger recruits than late plantigrades, explaining why there was clearer spatial structure of larger recruits. The relationship with algae was mainly negative for larger recruits, while it tended to be positive at many scales for late plantigrades. Thus, both adult mussels and G. pristoides are suitable habitats for plantigrades, while mussels are the main habitat for larger recruits. This may mean that recruits on algae either die or migrate to mussel clumps at a certain size. This study highlights the importance of recruit size when analyzing recruitment patchiness of mussels, and has implications for sustainable management of P. perna.

Kelp beds are thought to provide a refuge for a variety of marine organisms by damping waves, changing hydrodynamic flow, offering substrata for epiphytic species, and altering the abundances of predators and prey. At the sub-Antarctic Prince Edward Islands, a typical flow-through system, kelp beds have the potential of acting as a prime habitat for a variety of pelagic species that would otherwise be swept away and lost from the island ecosystem. The current study investigates the effects of kelp beds on the abundance and composition of zooplankton and microphytoplankton, with the main aim of determining whether kelp beds act as a repository for larval stages in general, and for the swimming prawn, Nauticaris marionis, in particular. Salinity gradients created by freshwater run-off indicated that kelp beds at Marion Island retain surface water and restrict water exchange with the open ocean. Both the abundance and diversity of zooplankton were greatest within the kelp beds. This was partially due to the high abundance of kelp-associated species, but also to the apparent retention of naupliar larvae within the kelp canopy. Unlike the larvae of ostracods and copepods, caridean zoeae were rare, indicating that kelp beds are not a prime habitat for larvae of N. marionis. The larval retention mechanism for this species, therefore, remains unknown. Apart from retaining water and certain species of zooplankton, chlorophyll a/phaeopigment ratios confirmed the existence of a substantial detrital pool within the kelp beds. This retention of detritus may explain the observed importance of kelp-derived matter in the diet of many nearshore animals. In contrast to the zooplankton, kelp beds seemed to have little effect on the abundance and composition of microphytoplankton, and chlorophyll a values were low at all stations.

Mesozooplankton community structure was investigated in four consecutive years from 1996 to 1999, during late austral summer (April/May), in the vicinity of the Prince Edward Islands. Sampling was carried out using a Bongo net with a mouth area of 0.25 m 2 , and fitted with 300 μm mesh. Surveys were of high resolution, with net tows being separated by ±10–20 km. Zooplankton species composition demonstrated a high degree of similarity between net tows, containing a mix of sub-Antarctic and Antarctic species. However, variation in species abundance levels showed that within each year of study the region was characterised by the presence of different zooplankton communities. These communities differed by as much as an order of magnitude in total zooplankton density. Analysis of physical variables indicated that zooplankton communities were associated with different water masses, identifiable by their physical characteristics. Sea temperature was the most important physical variable, accounting for as much as 69% of the variation in community structure. Communities were often not spatially discrete, and many stations demonstrated a high degree of similarity despite wide spatial separation. The heterogeneity in the biological and oceanographic environment extended to a small scale, on the order of 10 km. This is indicative of the oceanographic complexity of the region. The complex oceanography associated with shallow topography, including seamounts, islands, and frontal zones, indicates that small-scale spatial heterogeneity in zooplankton communities may be an important factor to consider when modelling these ecosystems.