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Nabq as “A Managed Resource Protected Area” or as “Multiple Use Management Area” is subjected to human activities that influence its biodiversity. Therefore, many field trips from January 2016 to January 2017 were carried out to detect the main activities in Nabq protectorate and their impacts on faunal macro-benthos biodiversity. Four stations representing two major habitats (mangrove and rocky shore) were chosen depending on anthropogenic activities. A total of 112 macro-benthos taxa were recorded in Nabq dominated with Planaxis salcatus, Nerita spp., Barbatia trapezina and Ophiocoma scolopendrina. The anthropogenic activities don't only affect the presence and absence of species, but also influence on the dominance status of species in the investigated stations. El-Rowayseaa and El-Dagal were unaffected stations showing more abundance and diversification than El-Gharkana and El-Monkateaa which were affected mangrove and rocky shore stations, respectively. In conclusion, anthropogenic activities are the main cause of recent changes to the marine biodiversity in Nabq Protected Area and their continuation may cause habitat damage. Keywords: Macro-benthos, Biodiversity, Human activities, Ecotourism, Fishing, Protected areas

When Mermod (1953) depicted the types of Theodoxus baeticus of Lamarck’s collection his photo did not display a pseudo-apophysis (peg). This led recent authors believe, that Th. baeticus is a synonym of Th. fluviatilis and Theodoxus meridionalis is the valid species (Alba et al. 2016: 55). In this paper I studied the syntypes of Th. baeticus as well as the syntypes of Th. meridionalis and demonstrated that in Th. baeticus a pseudo-apophysis exists and the opercula of Th. baeticus and Th. meridionalis are similar. Thus Th. baeticus is a good species and Th. meridionalis is a younger synonym of it. Therefore I can conclude that three Theodoxus spp., i.e., Th. fluviatilis, Th. baeticus and Th. valentinus occur in the Iberian Peninsula.

A chronological relationship between the annual profiles of oxidative stress markers, the key regulator of stress physiology has been sought in a terrestrial mollusc (Nerita articulata) under natur...

Macrobenthic algae and grazers like gastropods, amphipods etc. eventually form a key component of intensified grazing ecosystem in rocky intertidal area. Grazing gastropods suppresses the abundance of canopy forming algal species, thus lead to a reduced algal diversity in the long term. We conducted feeding trials with single and mixed diet during a 28-Days indoor experiment to examine the grazing intensity among green, red and brown algae namely Ulva lactuta, Enteromorpha linza, Gelidium and Padina tetrastromatica following a gut content analysis of experimental gastropods such as Euchelus asper and Nerita oryzarum. Feeding experiments revealed that Euchelus asper consumed 70% Enteromorpha, 10% Ulva and about 10% Gelidium whereas, Nerita oryzarum consumed 45% Ulva and 10% Enteromorpha during single diet experiment. Euchelus asper showed its preference towards all experimental diet where as Nerita preferred Ulva to a greater extends than Enteromorpha and rest other combined diet. Our results showed a higher grazing pressure exerted by Euchelus asper might pose a threat for algal culture and form competition in animals of same trophic level intern affecting their population. Similar experiment may be useful to control mass canopy forming algal growth and habitat studies for grazing animals will also be attributed.

Describing temporal variation in the foraging behaviour of intertidal gastropods is important to gain an understanding of the environmental conditions that constrain their activity patterns. Foraging behaviour in the tropical gastropod Nerita yoldi was mainly deter- mined by the tidal cycle: individuals foraged when they were awash, or emersed when the rock was wet, migrating downshore to forage and upshore when they returned to refuges. Apart from tidal influences, foraging in N. yoldii also varied with season and tidal state. Indi- viduals were generally more active in summer than in winter and on neap than spring tides; although small-scale daily variation existed. Such temporal variation may be caused by several interacting environmental variables, especially air temperature, pressure and irradi- ance, which influence physical conditions as well as individuals' metabolism. Individual for- aging patterns were also highly labile between seasons: most individuals would stop and remain stationary on the open rock surface in winter, but returned to refuges after foraging in summer, a pattern probably driven by physical stress. The foraging behaviour of N. yoldii, therefore, is predictable on a coarse scale, in terms of when they are active, but highly labile in duration and foraging range, which indicates the importance of local, daily variation in environmental factors controlling foraging on shores that have strong temporal changes in abiotic conditions.

Comparative analysis may help elucidate how interacting influences have shaped regional phylogeography. The coasts of southeastern Australia potentially offer an opportunity for such analysis by comparison of southeastern Tasmania and Bass Strait. The latter has been extensively studied but there are fewer phylogeographic studies in southeastern Tasmania and these have mostly shown little evidence of genetic structure. The possibility that snails of the Austrocochlea constricta s.l complex have such structure was investigated here using nuclear and mitochondrial DNA sequences. Similarly to the few known examples of geographically-structured taxa in the region, especially the gastropod genus Nerita, most differentiation in A. constricta s.l. occurs between localities east and west of the Tasman Peninsula. In contrast, Nerita and A. constricta s. l. differ in their phylogeographic structure around Bass Strait. Possible explanations of inter-regional phylogeographic differences include the interaction of breeding season and current variability, whether the geographically partitioned genetic lineages represent intra-specific or species-level divergence and the decoupling of landbridge formation and sea surface temperature at the beginning and end of glacial maxima.

This is the first report of the mitochondrial gene order and almost-complete DNA sequence of a representative of the Neritimorpha, the highest-ranking gastropod clade lacking such data. Mitochondrial gene order in Nerita is largely plesiomorphic. Its only difference from the cephalopod Octopus vulgaris is a tRNA transposition shared by Vetigastropoda and Caenogastropoda. Genome arrangements were not informative enough to resolve the evolutionary relationships of Neritimorpha, Vetigastropoda and Caenogastropoda. The sister-group taxon of Neritimorpha varied in sequence-based analyses. Some suggested that Neritimorpha is the sister group of Caenogastropoda plus Heterobranchia and some that Neritimorpha and Caenogastropoda are sister groups. No analysis significantly supported the hypothesis that Vetigastroda is more closely related to Caenogastropoda than is Neritimorpha.

The present work aims to assess the spatial distribution, analyze shell utilization, shell fitness and determine the effect of coexistence of two hermit crabs Calcinus latens and Clibanarius signatus on used shell resources in various habitats on the Red Sea Coast. Also, to determine the choice of shells and investigate the shell species preference of C. latens and C. signatus in the laboratory. The hermit crabs C. latens and C. signatus were found to occupy shells of 39 gastropod species. The most commonly occupied gastropod shells are those belonging to genera Strombus, Nerita, Cerithium and Planaxis. The results showed that crab individuals utilized mainly the shell with elongate aperture. Laboratory experiments showed that two crab species preferred shells of Strombus followed by Cerithium and Nerita when offered shells of nearly similar size (optimal). Crab individuals showed a significant preference for optimal sized shells when given suboptimal shells as an alternative choice. Also, the hermit crabs avoid damaged shells when given a choice of optimal sized damaged shell and optimal sized intact one. In addition, two hermit crab species chose shells of smaller than optimal size when given a choice of damaged optimal sized shells and smaller intact ones. On the other hand, field observations showed that most crab individuals lived in adequate sized shells. The present data conclude that shell selection by hermit crabs C. latens and C. signatus depends mostly on shell internal volume, shell quality and shell aperture size than other factors, because they provide a maximum protection for hermit crabs.

Marine biogeographers have long recognized broad east‐west differentiation of marine communities across southern Australia, but few studies have explicitly assessed the site of disjunction or the factors potentially underlying this biodiversity. A recent, detailed phylogeographical and distributional study of the dispersive gastropod genus Nerita revealed an abrupt shift in species abundance across mainland Australia, apparently correlated with the site of an historical vicariant barrier: the Bassian Isthmus. In the current study I provide an independent assessment of this vicariant hypothesis by morphologically analysing over 3000 intertidal Nerita specimens from eight coastal sites around Tasmania. Consistent with the Bassian Isthmus hypothesis, the study reveals a dramatic east‐west disjunction across north-east Tasmania. A survey of marine biogeographical literature reveals several additional examples supporting the east‐west species turnover detected in Nerita . I discuss the role of contemporary oceanographic factors in preserving the biogeographical signature of marine vicariance, even in highly dispersive taxa. Based on recent marine connectivity data, the east‐west disjunction in Nerita taxa can be interpreted as an historic vicariant pattern perpetuated by contemporary oceanographic conditions. The results of this study emphasize the potential importance of considering relative abundance data ‐ rather than just species range data ‐ in marine biogeographical analyses. As the observed disjunction is likely to have broad implications for Australia’s marine biodiversity, it is imperative that conservation biologists incorporate such data in the design of marine protected areas.

Marine species with ranges that span the Indo-Australian Archipelago (IAA) exhibit a range of phylogeographical patterns, most of which are interpreted in the context of vicariance between Indian and Pacific Ocean populations during Pliocene and Pleistocene low sea-level stands. However, patterns often vary among ecologically similar taxa, sometimes even within genera. This study compares phylogeographical patterns in two species of highly dispersive neritid gastropod, Nerita albicilla and Nerita plicata, with nearly sympatric ranges that span the Indo-Pacific. Mitochondrial COI sequences from >1000 individuals from 97 sites reveal similar phylogenies in both species (two divergent clades differing by 3.2% and 2.3%, for N. albicilla and N. plicata, respectively). However, despite ecological similarity and congeneric status, the two species exhibit phylogeographical discordance. N. albicilla has maintained reciprocal monophyly of Indian and Pacific Ocean populations, while N. plicata is panmictic between oceans, but displays a genetic cline in the Central Pacific. Although this difference might be explained by qualitatively different demographic histories, parameter estimates from three coalescent models indicate that both species have high levels of gene flow between demes (2Nem>75), and share a common history of population expansion that is likely associated with cyclical flooding of continental shelves and island lagoons following low sea-level stands. Results indicate that ecologically similar, codistributed species may respond very differently to shared environmental processes, suggesting that relatively minor differences in traits such as pelagic larval duration or microhabitat association may profoundly impact phylogeographical structure.

Cobbled shores in subtropical Japan are small and disturbed by sand transported from terrestrial runoff. How benthic organisms live in such a harsh environment has been seldom studied. On a cobbled shore at Ishigaki Island, the present study investigated the life-history traits of the gastropod, Nerita squamulata Le Guillou 1841. Seasonal quadrat samplings showed that the neritid density decreased when and where sand covered cobbles, indicating impact of sand on the neritid. Monthly frequency histograms of the shell width suggested that the neritid lives less than 2 years and recruits as individuals of 1-4 mm shell width in both May-August and February. With the possible short longevity and frequent recruitments, the neritid population may quickly recover after the disturbance by sand.

In Hong Kong, low-shore rock pools support a patchy mosaic of different successional stages of crustose coralline algae. Echinoid and molluscan grazers are abundant in these low-shore rock pools and spatial and temporal variation in grazing pressure is thought to play an important role in structuring these assemblages. Direct records of graze marks using a modified wax-disc method were used to monitor spatial (different pools) and temporal (periods, 3 or 4 days each; spring vs. neap tides; summer vs. winter) variations in grazing pressure. Radula marks of Chlorostoma argyrostoma, Lunella coronata and Nerita albicilla were common in all pools. Grazing pressure was highly variable with tidal state and season but also within these two factors. Lunella and Nerita exerted highest grazing pressure at neap tides during summer, whilst Chlorostoma exerted highest grazing pressure in winter. Although overall patterns of temporal variation were similar and consistent in all pools, there was between pool variation in grazing pressure and individual species showed different patterns. Within the seasonal and tidal variations, large period-to-period differences were found for all species, illustrating small-scale temporal variation. Fine-scale spatial and temporal patterns of variation in grazing pressure of different species, therefore, contribute to the highly dynamic nature of these low-shore rock pools.

Colour-infrared (CIR) imagery was evaluated for its usefulness to test hypotheses about grazing in the intertidal gastropods Nerita atramentosa (Reeve) and Bembicium nanum (Lamarck). Tiles of natural sandstone were placed onto the shore to accumulate micro-algae for a period of 6 weeks. Then, in the laboratory, three replicate tiles were assigned into each of 6 treatments (Nerita and Bembicum at densities of 3 or 6 animals, plus a control and a control for artifacts due to cages). CIR images were acquired before and three times after the animals were placed on the tiles. NIR/red ratio images, which are used to estimate micro-algal biomass, were generated from calibrated CIR data. Nerita at the greater density were more effective at removing microalgae than at the smaller density. The latter were more effective than Bembicium at either density, which did not significantly decrease algal biomass from that in controls. These data are consistent with previous field observations and indicate that CIR imagery is an effective method for testing hypotheses about grazing by intertidal gastropods. D 2005 Elsevier B.V. All rights reserved.

Shellfish collection by humans is a form of selective predation, with the larger individuals of a few target species being taken, and can have direct and indirect effects. We investigated the indirect effects of collection by experimental removals of two target species, Cellana tramorerica (Sowerby) and Nerita atramentosa (Reeve) from a rocky shore in south eastern Australia. Removals of single species were compared with the removal of both species and frequent removals were compared against once-off removal events. The removal of Cellana resulted in an increase in micro-algal abundance. This increase was greater where Cellana were frequently removed than where once-off removal of the species occurred. The removal of Cellana also promoted the immigration of adult Cellana, but the overall population was dominated by juveniles. Immigration of juvenile Cellana was high in all cases and was not affected by removals of either target species. The removal of Nerita, whether once-off or frequent, resulted in a decrease in micro-algae, but did not affect the migration of other species. The combined removal of Cellana and Nerita did not affect microalgal abundance, a result that was successfully predicted from the summed effect of the single species removals. In addition, the combined removal had no effect on the abundance of other species. The single and combined removal of these two gastropod species did not influence the species richness at this study site.

Terrestrial hermit crabs (Coenobita compressus (H. Milne Edwards)) on Isabel island, Mexico, prefer Nerita (Ritena) scabricosta (Lanark 1822) over other species of shells. Nerita scabricosta, the lightest species of shell, has the highest internal volume/weight ratio (IV/W ratio) of all species available at the island. Heavier shells (low IV/W ratio) are more costly to carry and may restrain growth. We hypothesized that crabs will search for shells with high IV/W ratios to save energy, and predicted that in every individual shell-exchange crabs will prefer a shell with a higher IV/W ratio. Observations of spontaneous aggressive shell-exchange interactions in nature between crabs carrying preferred shells (with high IV/W ratio) and crabs using less preferred species of shells (low IV/W ratio) support this idea. By inducing individual shell exchanges in the field, we experimentally confirmed the preference for shells with high IV/W ratio, and we also showed that crabs prefer larger shells over those they were bearing. Moving to a larger but not heavier shell may facilitate growth though saving energy that would otherwise be spent in locomotion. High growth rates seem to be advantageous because size is a correlate of fertility, since large males apparently have more access to females, and large females produce larger clutches, thus indicating the possible reproductive benefits for preferred light and large shells.

This dataset records the occurrence and inventory of molluscan fauna on Gueishan Island, the only active volcanic island in Taiwan, based on the literature survey and field investigation conducted between 2011 and 2012. The literature review involved seven studies published from 1934 to 2003, which collectively reported 112 species from 61 genera and 37 families of Mollusca on Gueishan Island. Through our field investigation, we identified 34 species from 28 genera and 23 families. Fourteen of these species were new records on Gueishan Island: Liolophura japonica, Lottia luchuana, Nerita costata, Nerita rumphii, Diplommatina suganikeiensis, Littoraria undulata, Solenomphala taiwanensis, Assiminea sp., Siphonaria laciniosa, Laevapex nipponica, Carychium hachijoensis, Succinea erythrophana, Zaptyx crassilamellata, and Allopeas pyrgula. In Total, there are 126 species from 71 genera and 45 families of Mollusca on Gueishan Island. These data have been published through GBIF [http://taibif.org.tw/ipt/resource.do?r=gueishan_island] and integrated into the Taiwan Malacofauna Database (http://shell.sinica.edu.tw/).

Five gastropod taxa of the family Neritidae in the Ypresian and Bartonian (Eocene) of the Paris and Basse-Loire Basins, are introduced: Cuisenerita gen. nov., Tomostoma angusta sp. nov., Nerita gouetensis sp. nov., Clithon barbei sp. nov., Neritodryas guillioui sp. nov. and Cuisenerita tuberosa sp. nov. New combinations introduced are: Neritodryas dutemplei (Deshayes, 1864) and Neritodryas globosa (J. de C. Sowerby, 1823). This is the earliest record of Neritodryas and the first record for this genus from Europe.

Aim In this study, I examined the relative contributions of geography and ecology to species diversification within the genus Nerita, a prominent clade of marine snails that is widely distributed across the tropics and intertidal habitats. Specifically, I tested whether geographical patterns of speciation correspond primarily to allopatric or sympatric models, and whether habitat transitions have played a major role in species diversification. Location Indo-West Pacific, eastern Pacific, Atlantic, tropical marine intertidal. Methods I used a previously reconstructed molecular phylogeny of Nerita as a framework to assess the relative importance of geographical and ecological factors in species diversification. To evaluate whether recently diverged clades exhibit patterns consistent with allopatric or sympatric speciation, I mapped the geographical distribution of each species onto the species-level phylogeny, and examined the relationship between range overlap and time since divergence using age‐range correlation analyses. To determine the relative contribution of habitat transitions to divergence, I traced shifts in intertidal substrate affinity and vertical zonation across the phylogeny using parsimony, and implemented randomization tests to evaluate the resulting patterns of ecological change. Results Within the majority of Nerita clades examined, age‐range correlation analysis yielded a low intercept and a positive slope, similar to that expected under allopatric speciation. Approximately 75% of sister species pairs have maintained allopatric distributions; whereas more distantly related sister taxa often exhibited complete or nearly complete geographical overlap. In contrast, only 19% of sister species occupy distinct habitats. For both substrate and zonation, habitat transitions failed to concentrate towards either the tips or the root of the phylogeny. Instead, habitat shifts have occurred throughout the history of Nerita, with a general transition from the lower and mid-littoral towards the upper and supra-littoral zones, and multiple independent shifts from hard (rock) to softer substrates (mangrove, mud and sand). Main conclusions Both geography and ecology appear to have influenced diversification in Nerita, but to different extents. Geography seems to play a principal role, with allopatric speciation driving the majority of Nerita divergences. Habitat transitions appear insignificant in shaping the early and recent history of speciation, and promoting successive diversification in Nerita; however, shifts may have been important for respective divergences (i.e. those that correspond to the transitions) and enhancing diversity throughout the clade.

Rates of radular activity were measured in four species of Australian gastropods. The species were Austrocochlea constricta, Bembicium nanum, Cellana tramoserica, and Nerita atramentosa. Work was done in Botany Bay at Cape Banks, NSW, Australia from September 1989 to June 1990. A hydrophone was used to record radular raspings of snails in situ. Averages based on all observations showed Cellana had the fastest rate with 80 rasps/min, and was followed by Austrocochlea (71 rasps/min), Bembicium (57), and Nerita (39). Faster rates were associated with warmer water temperature and smaller body sizes in all species except Bembicium. The potential for these differences in radular activity affecting competitive interactions is discussed.

The concentrations of Cd, Cu, Pb, Zn, Ni and Fe were determined in the gastropod Nerita lineata collected from 15 sites in the west intertidal area of Peninsular Malaysia (shell length 11.5–33.8 mm). The results of the present study showed that metal concentrations in the shells, opercula and soft tissues of N. lineata were distributed differently at different sampling sites. The mean concentrations (μg/g dry weight) of Cd, Cu, Fe, Ni, Pb and Zn in the samples were 2.99, 6.38, 35.05, 23.34, 48.22 and 16.59 in the opercula, 3.15, 5.59, 49.78, 24.18, 48.86 and 7.86 in the shells and 1.03, 2.65, 566.63, 5.85, 92.72 and 92.75 in the soft tissues. All the populations of N. lineata showed similar trends in the accumulation of Cd, Ni and Pb which decreased as follows: shell > operculum > soft tissue, Cu and Zn deceased in the order: soft tissue > operculum > shell and Fe in the order: soft tissue >shell > operculum. In general, higher concentrations of metals were recorded in samples collected from the stations closed to the anthropogenic sites. In particular, snail samples collected from KSAyam accumulated higher Pb concentrations when compared to other sites in the opercula, shells and soft tissues. This may indicate high bioavailability and contamination via Pb of the study site. The snail N. lineata is therefore suggested as a potential biomonitor of bioavailability and contamination of heavy metal in general and Pb in particular for the tropical intertidal area of Peninsular Malaysia.

To determine how historical processes, namely speciation, extinction, and dispersal, have contributed to regional species diversity patterns across the marine tropics, we examined the biogeographical history of a circumtropical genus of intertidal gastropods. A species-level phylogeny of Nerita, representing approximately 87% of extant species, was developed from 1608 bp of mitochondrial (COI and 16S) and nuclear (ATPSα) markers. Phylogenetic relationships generally corresponded to prior classifications; however, comprehensive sampling revealed a number of previously undetected ESUs. Using the resulting tree as a framework, we combined geographical distributions and fossil evidence to reconstruct ancestral ranges, produce a time-calibrated chronogram, and estimate diversification rates. Analyses revealed two monophyletic eastern Pacific + Atlantic (EPA) clades, each of which likely split from an Indo-West Pacific (IWP) sister clade prior to an early Miocene Tethys Seaway closure. More recent diversification throughout the IWP appears to have been driven by both vicariance and dispersal events; EPA diversity has been further shaped by speciation across the Central American Seaway prior to its closure and dispersal across the Atlantic. Despite the latter, inter-regional dispersal has been rare, and likely contributes little to regional diversity patterns. Similarly, infrequent transitions into temperate regions combined with reduced diversification rates may explain low diversity in West and South Pacific clades. Since origination, Nerita diversification appears remarkably constant, with the exception of a lag in the late Eocene–early Oligocene and elevated rates in the late Oligocene–early Miocene. However, a comparison among regions suggested that IWP clades have experienced, on average, higher rates of speciation. Fossil evidence indicates that the EPA likely witnessed greater extinction relative to the IWP. We propose that regional differences in species diversity in Nerita have been largely shaped by differential rates of speciation and extinction.

Phylogeographical disjunctions in high-dispersal marine taxa are variously ascribed to palaeogeographical conditions or contemporary ecological factors. Associated biogeographical studies, however, seldom incorporate the sampling design required to confidently discriminate among such competing hypotheses. In the current study, over 7800 gastropod specimens were examined for operculum colour, and 129 specimens genetically, to test ecological and historical biogeographical hypotheses relating to biogeographical disjunction in the Southern Hemisphere, and to southern Australia in particular. Mitochondrial DNA sequence analysis of the high-dispersal intertidal gastropod Nerita atramentosa in southern Australia (88 specimens; 18 localities) revealed an east-west phylogeographical split involving two highly divergent clades (26.0 +/- 1.9%) exhibiting minimal geographical overlap in the southeast. The eastern clade of Nerita atramentosa is also widespread in northern New Zealand (43 specimens, 10 localities), but no significant genetic differentiation is explained by the Tasman Sea, a 2000-km-wide oceanic barrier. Spatial genetic structure was not detected within either clade, consistent with the species' dispersive planktotrophic phase lasting for 5-6 months. Digital analysis of operculum colouration revealed substantial differences between eastern (tan) and western (black) specimens. Genetic analysis and visual inspection of 88 Australian specimens revealed a completely nonrandom association between mtDNA data and operculum colouration. Independent examination of a further 7822 specimens from 14 sites in southern Australia revealed both colour morphs at all localities, but reinforced the phylogeographical data by indicating a marked turnover in colour morph abundance associated with a palaeogeographical barrier: Wilsons Promontory. This sharp biogeographical disjunction is in marked contrast to the species' high dispersal abilities. The genetic similarity of Nerita morio (Easter Island) and the eastern Australian + New Zealand lineage (1.1 +/- 0.3%) provides further evidence of long-distance dispersal in southern Nerita. Phylogenetic relationships of nine species (four genera) of Neritidae, an almost exclusively tropical gastropod family, are consistent with the hypothesis that southern temperate black nerites comprise a monophyletic radiation.

Systematic mapping and morphosedimentary analysis of Fuerteventura and Lanzarote Islands supported by laboratory techniques (U-series mostly by TIMS, 14C analysis and allo-isoleucine measurements on biogenic carbonates from raised marine deposits, paleomagnetic and a few K/Ar measurements on volcanic formations related to marine deposits) provide a basis for constraining the age of Late Cainozoic marine units. The most complete sequences of raised marine terraces are found at similar elevations in both islands. They include up to 12 marine terraces (Episodes) at elevations between 0 m and 70 m above mean sea level (asl). At least six terraces should be of Quaternary age, and more recent than 1.2 Myr. Throughout the whole marine sequence with the exception of the Holocene terrace, the warm fauna assemblage is characterized by the presence of Ostrea virleti, Nerita emiliana, and Strombus (S. cf. coronatus-S. cf. bubonius). However, there is a major change, highlighted by the disappearance of the first two species, below the 8-10 m terrace, that could possibly correspond to MIS 11. K-Ar measurements allow an estimate for mean uplift rate of 1.7 cm/ka during the last million years. The present elevation of the Last Interglacial deposits (about 1 and 2 m asl) shows discontinuous vertical movements with possibly a reverse trend since MIS 9 in eastern Canary Islands., Financial support from: Spanish Dirección General de Enseñanza Superior (DGES) Projects PB98-0514, PB98-0265, BTE2000-1311, Areces Project “Registro geológico y cambios climáticos” (1997–2000), NSERC-Canada and FCAR Funds of Quebec. This is a contribution to IGCP Project 437 (Coastal environmental change during sea-level highstands) and to the INQUA Sea Level Change and Neotectonic Commission.

Our understanding of the physical factors driving fine-scale structuring of marine biodiversity remains incomplete. Recent studies have hypothesised that oceanography and coastal geometry interact to influence marine biogeographic structure on small spatial scales. The coastal waters of eastern Tasmania, located at the oceanographic interface between two major boundary current systems (the East Australia Current (EAC) and the Leeuwin Current (LC)) represent an informative system for assessing this hypothesis. Parallel biogeographic and oceanographic analyses, focusing on the relative abundance of two widespread, larval-dispersed Nerita gastropods, suggest that the relative influences of the EAC and LC at this interface are modulated by coastal topographical variation. Specifically, east-facing coastalsitesaredominatedbytheEAC-derivedN.melanotragus,whereassouth-facingbaysaredominatedbyLC-derived N.atramentosa.Thesecombinedoceanographicandbiologicaldataimplythatcoastaltopographyandhydrodynamicscan combine to influence the local distributions and abundances of planktotrophic-developing taxa at coastal convergence zones. Additional keywords: boundary current, climate change, connectivity, conservation, dispersal, invasion, marine invertebrate, marine protected areas (MPAs), recruitment.

This paper reports the gill morphology of seven common gastropods from Moreton Bay, southeastern Queensland, to test the level of difference in gill structure between major taxa. The investigated species include representatives of the clades Patellogastropoda, Neritimorpha and Vetigastropda as well as representatives of the more ‘advanced’ Caenogastropoda. Examination by SEM and LM revealed that the external gill structure of the investigated caenogastropods (including Planaxis sulcatus, Littoraria articulata, Bembicium auratum and Morula marginalba) shows basic uniformity. The gill filaments are composed of a clearly defined ridge and an extended sheet of non-ciliated cells. The gill filaments of these species differ in the shape of the filaments (corrugated, triangular or rounded) and the presence of secretory cells. The gills of the limpet Patelloida mimula and neritid Nerita chameleon, representatives of the clades Patellogastropoda and Neritimorpha respectively, are both triangularly shaped, but differ by the presence of paddle shaped cilia in the former species and secretory cells in the latter. The gill morphology of the vetigastropod trochid Austrocochlea constricta, characterized by blade shaped filaments covered with nodules and a striped pattern of ciliated cells, showed the least similarities with the other investigated species.

Members of the genus Nerita are abundant components of the intertidal fauna in many parts of the world and yet Nerita taxonomy remains unsettled. Here, the relationships among black-shelled Nerita populations from Australia, New Zealand, Norfolk Island, Lord Howe Island, the Kermadec Islands and Easter Island are discussed. Four species are recognised: N. atramentosa Reeve, 1855 from the southern half of Australia; N. melanotragus E.A. Smith, 1884 from eastern Australia, northern New Zealand, Lord Howe Island, Norfolk Island and the Kermadec Islands; N. morio (G. B. Sowerby I, 1833) from Easter Island and the Austral Islands; and N. lirellata Rehder, 1980 from Easter Island alone. These species are of great importance in studies of intertidal community structure and yet two of them have been consistently confused in the ecological and taxonomic literature. Moreover, the relationships among the species are not at all as implied by recent subgeneric classifications; it is argued that all four species should be placed in the subgenus Lisanerita Krijnen, 2002. The superficially similar N. picea Récluz, 1841 is not closely related. An accurate taxonomy of the genus will almost certainly require considerable genetic analysis. The nomenclature for each species is herein established by complete synonymies, and lectotypes for both N. atramentosa and N. melanotragus are selected.

Significant differences remain between gastropod phylogenetic hypotheses based on morphological and molecular datasets. We collected additional data from three gene segments (28S rDNA expansion region D1 (36 taxa plus two from GenBank), cytochrome c oxidase subunit 1 (35 species plus one from Genbank) and small nuclear RNA U2 (24 species)). These were combined with data available for the same species for histone H3 and two other segments of 28S rDNA. Analyses of these data using cladistic, maximum likelihood or Bayesian methodologies were conducted in an attempt to resolve some of the differences between current hypotheses of gastropod relationships based on morphological and molecular data. The results were of particular interest in four areas. (1) Patellogastropoda in most analyses are included in a derived clade with some Vetigastropoda. In an analysis with Nautilus as the sole outgroup, transversions weighted threefold as costly as transitions and, with third codon position data ignored, Patellogastropoda are excluded from an otherwise monophyletic Gastropoda. (2) Cocculiniformia was never monophyletic in our analyses, although this possibility is not statistically rejected. (3) Nerita, the only representative of Neritopsina in this dataset, is placed anomalously in most analyses, but is, in a few cases, shown as a sister-group to the Apogastropoda, in accord with some morphological hypotheses. (4) Heterobranchia is rarely monophyletic in our analyses owing to the variable placement of the architectonicoid Philippea. This genus, even judged by the high levels of divergence within Heterobranchia, has undergone extreme rates of substitution. The Euthyneura is invariably monophyletic and nearly always included in a clade with the valvatoidean Cornirostra as its sister-group.

Populations of the intertidal hermit crab Calcinus tibicen were observed in the laboratory and reproductive behaviors recorded. Of the 218 interactions, 68 resulted in copulation(s). Male and female sizes were positively correlated. Male size affected copulation success in a non-linear fashion. In particular, the largest males did not obtain any copulations. This was largely a consequence of the shell species occupied by large individuals; males in Nerita sp and Cittarium pica shells were unsuccessful in courtship. The ability to execute precopulatory rotation of the female was negatively affected by certain shell types. Repeated pairings of individuals suggested some level of individual recognition within the reproductively active population.

The patterns of dispersion of Nerita atramentosa, Bembicium nanum and Cellana tramoserica within two major substrata, the encrusting alga Peyssonelia gunniana and bare rock were analysed from quadrat samples taken at low tide, over 1 year. Distances to the nearest members of each species from random points were analysed to determine spatial patterns of dispersion. The frequencies of nearest neighbours of each species to each animal in the sampled area were analysed to determine the degree of spatial segregation and symmetry in the three species. Nerita atramentosa and Bembicium nanum showed significant aggregated (clumped) dispersion in both substrata. Cellana tramoserica, however, was dispersed at random on Peyssonelia, and regularly dispersed on bare rock. There was no correlation between index of dispersion and density, or time of year, in any species. No useful tests for segregation and symmetry between Cellana and the other two species could be made because of the difference in dispersion of this species. Nerita and Bembicium were significantly segregated on both substrata, indicating that the microhabitat requirements of the two species are different. The two species were symmetrical on both substrata, indicating that there is no tendency for members of one species to be more isolated than members of the other species. Possible explanations of the differences in spatial pattern shown by the three species are discussed, and experimentally testable hypotheses are proposed to clarify the importance of physical and biological factors in the spatial patterns of distribution of the three species.

Populations of three coexisting intertidal gastropods, Nerita atramentosa Reeve, Bembicium nanum (Lamarck) and Cellana tramoserica (Sowerby), were sampled from a shore in Botany Bay, New South Wales, from July 1972 to September 1973. The recruitment and growth rates of each species were analysed from size frequency distributions. Mortality of each age cohort, and longevity, were estimated from analyses of the densities of Nerita atramentosa and Bembicium nanum . Nerita atramentosa showed no significant mortality during the first two years on the shore, but high mortality (at an instantaneous rate of 0.084 deaths/individual/month) after reaching the age of reproductive maturity, which was 20 months from settlement and at a mean shell-length of 13.5 mm. No growth could be detected after reproductive maturity was reached. Longevity of N. atramentosa was estimated as 3–5.5 years. Bembicium nanum juveniles showed higher mortality (at an instantaneous rate of 0.233 deaths/individual/month) than reproductively mature animals (0.060 deaths/individual/month). Reproductive maturity was reached at a mean shell-breadth of 11.0mm, i . e ., about ten months after settlement on the shore. Longevity was estimated as from 4–8 years. Cellana tramoserica showed different growth patterns depending on the time of settlement. The three species showed different patterns of growth and life history relating to variable recruitment (which was demonstrated in all three species) and different rates of mortality of age cohorts.

Oligocene-Miocene rocks of the Torquay Basin contain at least 55 different species of calcareous nannofossils. These species allow zonation of an offshore subsurface section (Nerita No. 1 Well) and two onshore sections (near Torquay). The Angahook Formation and the base of the Jan Juc Formation in the Nerita No. 1 Well belong to Martini's (1971) NP 22 calcareous nannofossil Zone (early Oligocene). The NP 23 Zone could not be identified with certainty, but Zones NP 24/25 (late Oligocene) to NN 2 (early Miocene) were recognized in the Jan Juc Formation. The Oligocene-Miocene boundary is present between 122 m and 135 m below the sea floor at this site. The Puebla Formation, exposed at the sea floor, contains NN 5 spanning the early-middle Miocene boundary. Onshore along the Torquay coast, NP 24/25 (late Oligocene) is present in the Angahook Formation; the Jan Juc Formation at Bird Rock also contains NP 24/25, except the uppermost 2.5 m which belongs to NN 1. The NN 2 Zone begins at the contact between the Ja...

The archaeogastropod Nerita scabricosta is an abundant and extremely mobile herbivore which occurs from above mean high water through the mid—intertidal zone of exposed rocky areas along the Pacific coast of Panama. It is the principal prey of Purpura pansa, a patchily distributed, high intertidal neogastropod. Both snails exhibit activity patterns and habitat preference which appear to have evolved in response to heavy predation pressure by fish and to harsh physical factors. These behavioral mechanisms restrict movement and limit foraging time for both species, but also result in a cyclic overlap of their distributions. Potential interaction between the two gastropods is thus limited to well defined, discrete periods. Purpura elicits both an escape and an avoidance response in Nerita. The intensity of predation on the nerite by Purpura is lessened by these responses and Purpura appears unable directly to control the abundance of Nerita. However, it is experimentally shown that in a heterogeneous environment, Nerita's distribution is still more restricted and its local abundance further reduced by avoidance responses to temporally stable aggregations of Purpura. These aggregations, centered on specific fish—safe microhabitat, are thus hypothesized to result in the creation of local patches of intertidal substrate which are free from herbivory or disturbance effects by Nerita scabricosta. Evidence from field observations and manipulations is presented to support the hypothesis that in this complex, four trophic level community, a predator—prey interaction between two gastropods can limit potential resource monopolization and affect primary space occupancy.

The oxygen consumption of three species of Malaysian mangrove gastropods was measured in air and sea water at the temperatures commonly recorded in the mangrove. The experiments in air were carried out after the animals had regained fluid lost from the mantle due to handling. Fluid loss can have considerable effects on rate of oxygen consumption. Nerita arliculata (Gould) was found throughout the mangrove and experiences from 50 to 92% aerial exposure. It has a gill and a ratio of aerial to aquatic respiration rates of 2.7 at 28°C. 50% of the animals can survive underwater for 72 h at 28°C. The other two species, Cerithidea obtusa (Lamarck) and Cassidula aurisfelis (Brugiere) experience over 95% aerial exposure, have their mantle cavities modified as lungs and have air : water respiration rate ratios of 5.5 and 6.0, respectively. 50% can survive from 48 to 36 h underwater at 28°C. Acclimated animals have Q10's of about 1.6 in air and 1.4 in water. The respiratory physiology of the snails is compared with that of rocky shore species.

Intraspecific competition for food within and between age-classes of the intertidal grazing gastropod Nerita atramentosa Reeve was investigated using experimental cages on a rock platform in Botany Bay, New South Wales. Adults (mean length 21.0 mm) and juveniles (mean length 12.36 mm) were confined separately and together, at densities of 10, 20 and 50 per cage. The lowest density was the same as the mean density of Nerita on the shore. Cages were maintained for 146 days from November, 1973 to April, 1974. Each animal was numbered and its growth rate recorded over the period of the experiment. Dead animals were replaced by others of the same size to maintain densities, but replacements were not included in analyses. Proportional mortality of each size-class in each cage, growth rates and wet tissue weights at the end of the experiment were analysed. Adult animals did not grow. Growth rate of juveniles decreased with increasing density. Mortality of juveniles was not significantly different at different densities, but mortality of adults increased linearly as density increased. Wet tissue weights of adults and juveniles decreased with increasing density. In adults there was a decrease in weight at densities of 20 and 50 per cage. In juveniles, tissue weight increased with growth, from the start of the experiment except at densities of 50/cage where tissue weight remained constant and growth was absent. For all three factors, there were no differences between the effects of increasing density by additional members of the same age-class or by members of the other age-class. Adults and juveniles have, therefore, identical effects on the growth rates of juveniles, mortality of adults and wet tissue weights of both age-classes. Population density regulation in Nerita is possible, since high densities (e.g., due to increased juvenile recruitment) will cause reduced growth of juveniles and increased mortality of adults, until the densities are lowered. This effect of increased density is compared with intraspecific competition reported in other intertidal gastropods.

The densities of populations of Nerita atramentosa Reeve, Austrocochlea constricta Lamarck, Bembiciuin nanum (Lamarck), and Cellana tramoserica (Sowerby) were recorded in 0.25 m2 quadrats on landward and seaward halves of a sheltered and a moderately exposed rock platform from June 1972 to June 1973. The data have been subjected to variance and multiple regression analysis. Differences between densities of each species on the two shores and in the two areas of each shore are discussed with respect to the regressions on littoral height. Densities of Nerita on the sheltered shore, Austrocochlea and Bembicium on both shores, and Cellana, on the exposed shore, increased with height. The density of Cellana on the sheltered shore decreased as height increased. The density differences in the landward and seaward areas on the two shores were attributable to the effect of height for Nerita and Austrocochlea. Height was the major variable affecting the density of these two species on the sheltered shore. Bembicium and Cellana densities on both shores, and Austrocochlea density on the exposed shore, are not primarily determined by height. On the sheltered shore, Nerita and Cellana increased in density with increasing cover of water in rock pools. On the exposed shore, Bembicium and Cellana became sparser with increasing water cover. Bembicium on both shores increased in density with increasing cover of the alga Peyssonelia, as did Cellana on the exposed shore. Cellana and Austrocochlea tended to increase in density with Nerita on the sheltered shore. Cellana tended to be sparse where Bembicium was dense on the exposed shore. The biological effects of substrata and other species on the densities of each species remain unexplained. Significant time effects were found for variations in the densities of Nerita and Cellana on the sheltered shore and of Austrocochlea and Cellana on the exposed shore. The seasonal change in density of Nerita was parabolic with a minimum in summer, and is consistent with the known period of recruitment of this species. The lack of significant time effects on density of Austrocochlea on the sheltered shore is consistent with the known continuous recruitment of this species. The declining density of Austrocochlea on the moderately exposed shore was found, by size-frequency analysis, to be due to reduced recruitment compared with surrounding shores. The density of Cellana on both shores showed seasonal changes, with a maximum in summer, which is consistent with the known period of recruitment in Cellana. The lack of seasonal changes in density of Bembicium is briefly discussed. This type of multifactor analysis identifies biological interactions affecting variation in density of these species. It also corroborates the results of other ecological investigations and determines the relative importance of a variety of physical and biological factors in the distribution of density of intertidal gastropods.

The reproductive behavior of two species of diogenid hermit crabs was studied in Hawaii. In the shell generalist, Clibanarius zebra, male reproductive success varied little with size, although the largest males were less successful in obtaining copulations than were medium-large males. Male and female size were positively correlated, in successful pairs, thus larger males had the potential to fertilize more eggs when they were successful in obtaining a copulation. Female fecundity in C. zebra was not affected by species of gastropod shell inhabited once female size was taken into account. Male copulatory success was very strongly influenced by the species of gastropod shell inhabited. Males in Trochus or Nerita shells had greatly reduced reproductive success compared to males in Turbo or Nassarius shells. This result was due both to (1) males in Trochus especially dropping and otherwise poorly handling females during precopulatory behavior and (2) females not responding to precopulatory behavior patterns executed by males in Trochus and Nerita. Transferring males from “good” to “bad” shapes of shells and vice versa showed that male success was a function of shell type inhabited and not some correlated feature of the crabs. In the shell specialist, Calcinus seurati, which is found primarily in Nerita shells as an adult, males in Nerita shells were quite successful in obtaining copulations.

The rates of growth of two common intertidal gastropods, Nerita atramentosa Reeve and Bembicium nanum (Lamarck), were measured during ≈ 3 months in cages at four heights on a shore at Cape Banks, New South Wales. Replicate cages at each level were used to keep snails at constant densities throughout the experiments. Amounts of food on the substratum in each cage were measured as the concentration of chlorophyll in samples of the rock, which is known to be a reliable estimator of number of algal cells present. Rates of feeding were measured as the number of radular scrapes per min in snails removed to the laboratory, and the sizes of radulae were measured at the end of the experiments. Nerita did not increase in shell-length at the two higher levels on the shore, but grew at the two lower levels. Rate of growth was positively correlated with the amount of chlorophyll in the substratum, and was not apparently affected by the greater period of submersion (and concomitant time available for feeding) towards the lower levels on the shore. There was no difference in rates of feeding at the four heights. Radulae of snails from the lower levels were longer than those from higher on the shore, but this was attributable to the greater overall size of snails from lower levels (i.e. there was no significant difference in relative size of radula from one height on the shore to another). Bembicium increased in width of shell at all levels on the shore, but grew more quickly at all three lower levels than at the top of the shore. There was no difference in rate of feeding, nor in relative size of the radula from one level to another on the shore. Rate of growth was not significantly correlated with the amount of food available, nor with the period of submersion, nor the product of these two variables. The simplest explanation for these results was that sufficient food was available for maximal growth of Bembicium at the three lower levels, but not at the highest level. Thus, only where the amount of food was inadequate, at the highest level, was rate of growth reduced. These results are discussed with reference to the difficulties of determining the relative importance of influences on the growth of intertidal snails, because of inherent confounding of the period of submersion with different amounts of food at different heights on the shore.

Population energy budgets estimated on the assumption of steady state conditions for Nerita tessellataGmelin, N. versicolorGmelin, and N. peloronta L. on Barbados, W. Indies, are presented. Large differences in population structure, and hence energetics, occurred at different localities along the beach. Relatively high proportions (81 to 88%) of the assimilated energy were lost via metabolism. Assimilation efficiencies ranged from 39 to 43%, net growth efficiencies from 5 to 13%, and ecological efficiencies from 3 to 7%. For each species, production (P), energy flow (A) and total energy consumption (C) were expressed as functions of animal size, in order to facilitate gross estimations of the energy components for other populations for which data on size-frequency and density are available. Respiration studies of all three species in the laboratory failed to detect differences between respiration rates in air or under seawater.

The complex reproductive systems of the mangrove-dwelling Nerita birmanica are described and related to those of other neritids. Gonad condition in both sexes and numbers of spermatophores in males and females varied slightly during a one-year survey. It is concluded that a slight tendency for larger number of “full” ovaries with big oocytes to occur in collections taken shortly before full and new moon does not represent a significant lunar cycle of gonad activity since almost all gonads bore gametes in all stages of development at all times of year. It appears merely that ovulation may be held up during relatively rainless periods after which substantial rain is followed by reduced ovaries lacking large oocytes. In brief, breeding appears to be continous with but slight variation which does not consistently follow observable environmental factors.

The shell selection behaviour of Coenobita scaevola Forskal has been examined in a natural population by statistical methods and by selection experiments using shells whose weights had been artificially changed. In a natural population of C. scaevola there was a definite weight index (weightof the shell/weight of the animal) for the shells of each species of snail. The weight index for one population ranged between 0.7 and 13.0, with a maximum (46% of all values) between 1.3 and 1.9. The volume index (volume of the shell/weight of the animal) ranged from 0.7 to 4.0. The maximum (59%) for the shells of all snail species lay between 1.0–1.3. Small specimens of C. scaevola tolerate greater differences in the weight and volume indexes of their host shells than large specimens. A comparison between weight index and volume index shows that shell selection depends on volume. This is probably also true for a small hermit crab, Clibanarius signatus Heller, of the tidal zone. Selection experiments on the weight index showed maximum value for Galeodes of about 1, for Turbo at about 2, and for Nerita between 2 and 4. Selection experiments on the volume index showed that, independent of the species of the occupied snail shell, there was a maximum between 1.0 and 1.5. In selection experiments between shells of different snail species but of equal volumes the selection was made in regard to their weight indexes. At a weight index lower than 1.0 only Natica was preferred, between indexes 1.0 and 1.5 Natica and Galeodes were similar, while at a weight index of 2.0 or more Turbo was chosen. The shells of those snail species which are generally used by Coenobita scaevola are found in the same frequency in the snail fauna of a place as they are occupied by C. scaevola. With the exception of a few species of aquatic hermit crabs which are characterized by flat thoraxes and which live in shells with a long and narrow opening, all other water and land hermit crabs seem to have no relation to the shells of a given snail species. C. scaevola e.g., lives in the shells of 29 snail species. Broken shells or shells with a fouling of calcareous algae are better tolerated by water hermit crabs than by land hermit crabs. Land hermit crabs also live in fossil and subfossil shells.

Nerita undata, N. plicata, N. polita, N. albicilla and N. textilis are common intertidal gastropods on Aldabra Atoll. Each species prefers a different level on the shore and/or different degrees of exposure to wave action. Patterns of zonation, population size-frequency structure and biomass are given. N. undata exhibits 2 distinct ecophenotypes. Normal types occur on expsed to moderately sheltered shores but are replaced by ‘type B’ in extremely sheltered conditions. Differences in shell morphology and population structure are also noted. Foraging by all species was confined to hours of darkness, generally at low tide. Activity of N. polita was induced slightly on overcast days. N. undata and N. textilis are cryptically coloured and often visible during the day. N. plicata is conspicuous, but is protected by a strong shell. N. polita and N. albicilla are often brightly and variably coloured, but both are concealed during daylight. No Nerita were found infected with trematodes, suggesting that the degree of infection was extremely low. Habitats of the Aldabran Nerita are compared with 3 species on Barbados, West Indies.

Previous investigations have shown that competition for space among sessile organisms in rocky intertidal communities is often reduced by predation or harsh environmental factors. Grazing gastropods are unlikely to compete for space, but at high densities might compete for food, unless some factor, such as predation, reduced the densities. The coexisting species of herbivourous gastropods on rock-platforms in New South Wales are not, however, subject to high levels of predation on shores sheltered from waveaction. In this study, three of these species were caged at densities from the natural, to 4 times the natural density on the shore, in different combinations of species, to investigate their competitive interactions. All three species, Nerita atramentosa, Bembicium nanum and the limpet Cellana tramoserica, showed increased mortality and reduced weight at increased density, over 100- or 200-day periods. The effect of high density on Cellana was greater than on Nerita or on Bembicium. In addition, Nerita was competitively superior to the other two species. Cellana, at high densities, adversely affected Bembicium. Nerita was not affected by high densities of either of the other two species, and Cellana was not affected by Bembicium.Under entirely natural conditions, the effects of increased density of Cellana on the mortality and tissue-weight of Bembicium could not occur, because of the high rate of mortality of Cellana when at increased density of its own species. The other effects of increased density of snails would, however, occur. Thus, there can be regulation of numbers of snails because of high densities of their own or other species on the shore.The supply of benthic, microalgal food is proposed as the limiting resource for which the species compete. Hypotheses are proposed to account for the mechanisms by which the three species acquire different amounts of the resource. These are based on aspects of the feeding biology of the snails.The continued coexistence, in intertidal regions, of Cellana, which shows the highest rate of mortality due to members of its own species, with the competitively dominant Nerita, which increases the mortality of Cellana, is apparently due to three factors. These are: (i) the presence of subtidal refuges for breeding populations of Cellana, (ii) regular spatial dispersion of Cellana which would probably decrease intraspecific competition for food, and (iii) the high variability in space and time of recruitment of planktonic larvae of both Cellana and Nerita. This last factor makes it unlikely that high densities of Nerita could occur on all shores in every year. Cellana could always recolonize any area where its density had previously been reduced, and thus, cannot be excluded by competitive interactions. Competitive interactions at high densities of gastropods can therefore cause reductions in the number of each species present on a shore. They cannot, however, lead to exclusion of any species or alter the composition of the community of herbivores on the shore.The difference between competition for space by sessile intertidal organisms, and competition for food by gastropods which graze on microalgae is discussed with respect to the structure of intertidal communities. In the former case, competitively dominant species must be dislodged from the resource, space, by disturbance (e.g. predation or hazards) before recolonization by inferior competitors can occur. In the latter situation, the food resource is renewable without further perturbation of the community, and the competitively dominant species could never consume it completely over a wide enough area, and for a sufficient length of time, to eliminate totally another species.

Field experiments were used to determine the effect of a common intertidal snail (Nerita funiculata) on the use of space for foraging by the hermit crab Clibanarius digueti. Removals of Nerita resulted in an increased density of foraging Clibanarius, while additions of the gastropod had the opposite effect. The observed negative effect of the gastropod on individual hermit crabs appears to be food-related. Field surveys, however, suggested that the hermit crab population is limited by shell number, rather than food. Because Nerita contributes to the shell resource, its effect on the hermit crab population is positive. Nerita, therefore, has a negative effect on the distribution of foraging hermit crabs, but a positive effect on their abundance. Such decouplings of distribution and abundance effects are rare.

The patterns of dispersion of Nerita atramentosa, Austrocochlea constricta, Bembicium nanum and Cellana tramoserica are analysed from quadrat samples taken between June 1972 and June 1973. In each quadrat, the proportion of animals in each of two substrata, the encrusting alga Peyssonelia gunniana and small pools, was recorded, with the proportional area of cover of each substratum, height on the shore, density of the species and the time of sampling. Multiple regression and covariance analysis showed that, in all cases, there was a linear relationship between proportion in a substratum and proportional area of substratum. No other variables were significant. Juvenile Nerita and Austrocochlea showed preferences for both substrata. Adult Nerita and all Bembicium were dispersed at random with respect to both substrata. Adult Austrocochlea were randomly dispersed with respect to Peyssonelia, but showed a preference for water. Cellana showed a preference for water and against Peyssonelia. These microhabitat preferences are discussed in relation to previously described effects of substrata on the densities of these four species.