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Snapshot assessments of species, which live for a number of years, are often misleading if they are conducted at an inappropriately short temporal scale. Densities of the trochid gastropods Steromphala umbilicalis (da Costa, 1778) and S. cineraria (Linnaeus, 1758) fluctuated widely between 1996 and 2014 at ten rocky-shore sites within Lough Hyne, southwest Ireland. Between 2014 and 2019, the abundance of S. cineraria decreased to levels not seen in the previous two decades. This significant reduction was possibly in response to extreme dissolved oxygen fluctuations related to eutrophication of the semi-enclosed lough. Steromphala umbilicalis showed low density in 2018 and 2019, but this was within the limits of interannual fluctuations. The Lusitanian trochid Phorcus lineatus (da Costa, 1778) was first recorded by us in the lough in the early 2000s, just as it was establishing a flourishing population with densities of c. 6 snails/0.25 m2. Although this native trochid survived the cold winters of 2009–2012, the population suffered high adult mortality and little recruitment. Between 2013 and 2019, the population increased and the population density stabilized. The increase in P. lineatus reflects its position high on the shore, where air temperatures have risen over the last two decades. It may, therefore, be a sentinel species to detect the effects of climate change. By contrast, the decrease in S. cineraria was related to its shallow subtidal habitat where hypoxia and superoxia have recently become marked in the lough. This trochid, therefore, offers potential as an indicator of eutrophication-associated stress. These temporal variations in trochid species emphasize the need for longer term assessments when estimating the importance of any one species to the ecosystem.

Recent declines in coastal water quality in SW Ireland have led to changes in the abundance of algal-associated bryozoans and a hydroid. These common North Atlantic species offer considerable potential as indicator species for reduced oxygen conditions due to habitat degradation. Annual monitoring for more than two decades (1994–2016) at 10 rocky shore sites within Lough Hyne Marine Reserve, SW Ireland revealed a significant reduction in abundance of the ctenostome bryozoan, Flustrellidra hispida, through time, coincident with the development of daily, extreme, shallow water oxygen fluctuations (hyperoxia and hypoxia) due to eutrophication. In contrast, the ctenostome, Alcyonidium species-complex, increased significantly during the two-decade period. The thecate hydroid, Dynamena pumila, did not show any significant decadal-scale pattern but temporal variability in abundance increased since 2010. Abundances of F. hispida and D. pumila were positively correlated with estimates of water flow. F. hispida inhabited a variety of algal hosts but appeared most sensitive (of the taxa surveyed) to variation in habitat quality, especially experimentally reduced levels of dissolved oxygen ( −1 ). Such sensitivity indicates F. hispida could be useful as an indicator of environmental change, whereas D. pumila was more robust in response to reduced oxygen with 38.6% of the hydroid zooids being able to survive under hypoxic conditions as compared to only 0.2% of the bryozoan zooids. Given the widespread European distribution of these species, they offer the opportunity to act as important bioindicators of water quality and hence environmental degradation in north temperate intertidal systems.

Long-term cyclical changes in rocky shore community structure were documented over two decades at sheltered shores in Lough Hyne Marine Reserve, County Cork, Ireland. Three peaks of abundance were recorded for the limpet, Patella vulgata (1990-5, 2002-6 and 2010-14) with oscillations that varied in amplitude and frequency among sites. The cover of the fucoid Fucus vesiculosus varied inversely with limpet abundance and showed the strongest negative correlation with a lag time of 0–2 years. The species complex Fucus spiralis/guiryi showed a weaker correlation with a lag time of 1–2 years. Two other fucoid species showed no such negative correlations despite their close proximity to limpets within the lough's compressed tidal range. There was no relationship between overall barnacle cover (dominated by Austrominius modestus ) and the limpet-fucoid cycles, suggesting that the shelter provided by A. modestus for algae to escape from limpet grazing pressure may not be necessary for these cycles to occur on wave-sheltered shores.

The benthos of Lough Hyne (Europe’s first marine reserve) in SW Ireland has changed dramatically in the past decade with declining invertebrate communities and proliferating ephemeral macroalgae. In summer 2011 and 2012, we measured dissolved oxygen (DO) levels on shallow subtidal rocky shores of the lough and recorded both benthic hypoxia (

A twenty-five year (1994–2018) survey at 10 rocky sites within Lough Hyne Marine Reserve (SW Ireland) showed that, despite considerable spatial and temporal patchiness, three barnacle species (Chthamalus stellatus, Semibalanus balanoides, and Austrominius modestus) declined in overall abundance in the early 2000s, particularly at three sites. There was no clear response of A. modestus or C. stellatus to the cold winters of 2010 and 2011. In 2004, however, S. balanoides declined suddenly, but showed a slight recovery after the cold winter of 2010 to maintain sparse populations in the lough, but remained abundant outside the lough. A fourth species, Chthamalus montagui, showed a different pattern, with wide fluctuations in abundance but no overall decline. Changes in the Atlantic Multidecadal Oscillation may have influenced the overall composition of barnacle populations. Rising maximum air temperatures, resulting in high body temperatures recorded for A. modestus even during mild days, are suggested to have impacted the barnacle populations as a result of the prolonged aerial exposure times caused by the lough's asymmetric tidal pattern.

Alcyonacean octocorals are anthozoans which are found in many coastal benthic habitats, where they can be sensitive to environmental and/or anthropogenic stress. As part of a two-decade monitoring study of Lough Hyne (Europe’s first marine reserve and Ireland’s only one), we documented benthic communities at rocky-shore sites. As a fully marine, semi-enclosed, tidal ‘lake’ connected to the Atlantic Ocean via tidal rapids, Lough Hyne has long been noted for its high species and habitat diversity. One of the noteworthy guilds we report here was the alcyonacean octocorals: (1) the soft coral Alcyonium hibernicum under shallow subtidal rocks at monitoring sites in the lough from 2002 to 2015 and (2) the first known records (2013 to present) of the red soft coral A. glomeratum inside the lough (above the rapids). Furthermore, in August/September 2014 and 2015, we rediscovered the stoloniferous octocoral Sarcodictyon catenatum, last reported in the lough in the 1930s. We documented the distribution and abundance of these species in shallow subtidal areas of the lough as a baseline in the face of rapidly degrading conditions due to extreme oxygen fluctuations from eutrophication.

Two-decade-long monitoring studies at Europe's first statutory marine reserve—Lough Hyne in SW Ireland—indicate that benthic communities are rapidly changing. Populations of the ecologically important purple urchin ( Paracentrotus lividus ) have fluctuated widely, most recently with a population boom in the late 1990s, followed by a mass mortality that persists to the present day. Eight general hypotheses have been proposed to account for the urchin decline including cold temperature limiting reproduction, ephemeral algal exudates disrupting urchin fertilization, low larval availability (due to over-harvesting and/or episodic recruitment), high mortality of settlers and juveniles due to hypoxia, hyperoxia or predation (a trophic cascade hypothesis), and increased mortality due to pathogens (stress hypothesis). The cold-temperature and the trophic cascade hypotheses appear unlikely. The remaining hypotheses, however, all seem to play a role, to some degree, in driving the urchin decline. Ulvoid exudates, for example, significantly reduced urchin fertilization and few larvae were found in plankton tows (2012–2015), indicating low larval availability in summer. Whilst settling urchins regularly recruited under shallow-subtidal rocks until 2011, no settlers were found in these habitats from 2011 to 2014 or in field experiments (2012–2018) using various settlement substrata. Seawater quality was poor in shallow areas of the lough with extreme oxygen fluctuations (diel-cycling hypoxia), and 1-day experimental exposures to DO values −1 were lethal to most juvenile urchins. Multiple increases of the predatory spiny starfish ( Marthasterias glacialis ) population in recent decades may also have contributed to the demise of the coexisting juvenile urchins. Finally, urchins of all sizes were seen suffering from dropped spines, tissue necrosis, or white-coloured infection, suggestive of stress-related pathogen mortality. There was a paucity of broken tests, indicating limited predation by large crustaceans; the large number of adult urchins ‘missing’ and few P. lividus tests on the north shore points to possible urchin removal by poachers and/or starfish predation. While these ecological, environmental, and anthropogenic processes occur on many open coast rocky shores, many are exacerbated by the semi-enclosed nature of this fully marine sea lough due to its limited flushing. Multiple factors, including low larval availability and rapidly expanding starfish populations, coupled with degraded habitat quality (ephemeral algal mats and extreme oxygen fluctuations), indicate that the purple urchin populations will not recover without an improvement in the water quality of Lough Hyne Marine Reserve, the restocking of urchins and protection from poaching.

Distributions of brown seaweeds (Phaeophyceae: Fucales, Laminariales and Tilopteridales) were surveyed in 2011 in Europe’s first marine reserve, Lough Hyne in SW Ireland, and compared with distributions from three historical surveys (1930, 1955 and 1980). The most salient phycological differences were the incursion of the low intertidal and shallow subtidal species Fucus serratus, Himanthalia elongata and Saccharina latissima into the north basin of the marine reserve after the recent mass mortality of the purple urchin (Paracentrotus lividus). Monitoring surveys (1990–2012) at ten sites in the lough indicated that populations of S. latissima peaked in 1994, abruptly crashed in 1996 and then slowly recovered. As well as documenting the expansion of kelp, our annual, whole-lough snorkel surveys (2004–2012) demonstrated the recent proliferation of the introduced Sargassum muticum and native Cystoseira foeniculacea. Although the causal role of top-down factors (fishing ban and/or urchin population crash), bottom-up factors (nutrient enhancement), climatic variables (warming vs. episodic cooling) and pathogens (urchin and algal) is being widely debated, decadal-scale and inter-annual changes are clearly detectable and most consistent with a release from herbivory within the marine protected area.

Surveys of non-indigenous species (NIS) typically concentrate on the establishment and expansion phases of population dynamics. Our study, in contrast, investigated the status of an invader that has declined to low levels throughout much of the British Isles. The green macroalga Codium fragile ssp. fragile (Suringar) Hariot appeared in the British Isles in the 1800s and probably entered Lough Hyne, County Cork, Ireland in the 1930s. It proliferated on Irish shores in the 1940s to 1970s, but has declined to low levels in the region. Broad-scale surveys in County Cork, SW Ireland (2002-2005) and extensive surveys (2001-2011) within Lough Hyne indicate that the introduced alga has declined substantially since earlier decades. These results are consistent with other studies on NE Atlantic shores indicating a long-term decline.

At Lough Hyne Marine Reserve in SW Ireland, shallow subtidal, under-rock biodiversity was investigated to assess (i) any deleterious effects of scientific sampling and (ii) quantitative baseline community patterns. Comparisons were made between 10 sites with annual rock-turning disturbance and 10 with multi-decadal (historical) disturbance. At each site, shallow subtidal rocks (N = 1289 total) were lifted, organisms recorded, and rocks replaced in their original position. Biodiversity indices were calculated to evaluate how diversity varied with location within the lough, frequency of sampling disturbance, degree of hypoxia/anoxia, dissolved oxygen (DO) concentration, and number of rocks turned. The richness of solitary invertebrates surveyed in situ averaged 21 taxa per site with significantly more in the South Basin (near the lough's connection to the ocean) than in the North Basin. The Shannon-Wiener Index did not differ significantly with variables investigated. However, evenness was higher at annually disturbed sites than at historical ones where anemones with algal symbionts often dominated. Several sites were hypoxic to anoxic under the shallow subtidal rocks. Cup corals were most abundant in the South Basin; DO was a crucial explanatory variable of these sensitive species. Solitary ascidians were most abundant at South-Basin annual sites with DO levels being a highly significant explanatory variable.

Changes in the distributional ranges of marine indicator species have been used to support hypotheses of climate change on NE Atlantic shores. One such indicator species, Osilinus lineatus (da Costa, 1778) (a trochid gastropod), recently colonized the Lough Hyne Marine Reserve in SW Ireland and spread around the lough shores in the last two decades. The incursion and local spread of O. lineatus was documented during annual intertidal community surveys of the lough from 1994 to 2010. This species is presently an abundant member of the lough's biota. Although the tidal rapids connecting the lough to the open ocean may have acted as a partial barrier to incursion, substantial populations of the trochid have been noted within the lough from 2003 to the present. Climate change may have enabled this primarily Lusitanian species to penetrate and fl ourish in the lough. However, based on environmental variables measured and other species surveyed, changes in the semi-enclosed lough may be more complex.

Long-term annual monitoring of rocky intertidal and shallow subtidal shores of an Irish sea lough (1990–2010) documented major regime shifts in the past decade. When population densities of the purple urchin Paracentrotus lividus plummeted in Lough Hyne Marine Reserve in southwestern Ireland, the warm-water fucalean alga Cystoseira foeniculacea and ephemeral algae proliferated shortly after. We discuss the possible influences of release from herbivory and climate change on this algal proliferation, which blanketed the benthos. Smothering of the benthos led to high levels of shallow subtidal anoxia. Furthermore, the invasive fucalean alga Sargassum muticum has made repeated incursions into the reserve. Although being reduced by persistent eradication efforts (2003–2011), Sargassum is spreading within the lough. Limited seawater flushing and propagule dispersal within the lough and eutrophication in coastal waters may have contributed to community-level changes. Whether the regime change is cyclical (contingent on re-establishment of urchins within the lough and continued eradication of S. muticum) or irreversible (the result of critical transitions) remains unclear.

The north-eastern Atlantic sacoglossan gastropod Elysia viridis was studied on littoral and sublittoral shores of the British Isles from 2001 to 2007 to evaluate its potential role in the known decline of the invasive green alga Codium fragile ssp. fragile. Across its European range, this sacoglossan associates with eight genera of algal hosts within three algal orders and two divisions; these hosts include native and introduced macroalgae. The ‘specialist’ herbivore was investigated primarily within the rocky shore community at Lough Hyne Marine Reserve, County Cork, south-west Ireland, with comparative information from surveys of 95 other sites in the British Isles. In Lough Hyne, E. viridis associated with the green algae Codium, Cladophora and Chaetomorpha as well as the red alga Griffithsia. The sacoglossan associated with Codium, Cladophora and Chaetomorpha as well as the red alga Halurus in Devon, Isle of Wight and the Channel Islands. Recruitment of E. viridis to Codium spp. in the lough was substantially higher than in most areas of the British Isles with 100% of the Codium hosts attacked during annual September surveys. The strong autumn pulse of sacoglossan recruitment, coupled with the asynchronous population dynamics compared to other shores, indicates retention of planktotrophic larvae within the lough. The previously reported decline in sacoglossan density with tidal level was hypothesized to be caused by sublittoral predators. Although our experiments failed to demonstrate an effect by large predators, the foraging of the abundant fingerling fish and small crabs could not be tested conclusively. Field experiments were consistent with the interpretation of intense sacoglossan herbivory in certain areas of the lough. However, even if the sacoglossans contribute to the local decline of C. fragile in the lough, they cannot account for the regional decline of the alga on the north-eastern Atlantic shores.

The under-rock guild of nemerteans from Lough Hyne, south-west Ireland was recorded during a long-term benthic survey of the marine reserve. From 2004 to 2014, common nemerteans were recorded annually in late summer for ten historical monitoring sites (each 10 m of shoreline); more comprehensive surveys were completed in 2012 (4126 low-intertidal to shallow-subtidal rocks were turned) and 2014 (1289 rocks turned) with supplementary information in 2015. The bootlace worm,Lineus longissimus,was the most abundant nemertean species. It was significantly more abundant at the extremely shallow East Castle site than the other nine sites; peak populations (across all ten sites) occurred in 2012. The Lusitanian nemerteanParadrepanophorus crassuswas abundant after 2007. To date, 13 nemertean species were recorded with numerous new records for the lough, includingTubulanus annulatus, Micrura purpurea, Ramphogordius sanguineus, Micrella rufaandEmplectonema gracile. Three additional nemertean species were previously recorded, indicating that at least 16 species occur in the lough. Given the low densities and paucity of comprehensive earlier records, we are not able to ascertain definitely whether these species are new incursions or if sparsely distributed residents have increased.

Preliminary observations of limpet activity at Lough Hyne, in south-west Ireland, showed that individuals on steep slopes were primarily active at night, when emersed; while those on near-horizontal rocks were often active during daytime submersion. Observations of limpet populations over an 11 d period of limpet populations on a near-vertical and a near-horizontal site, only 45 m apart, confirmed that animals on the near-vertical site were active on nocturnal low tides, whilst those on the near-horizontal site were active on daytime high waters. A short-term survey at ten sites, which had limpets on both extremes of slope (i.e. either near-vertical or near-horizontal), showed that limpets on near-horizontal surfaces were, on average, more active at daytime high waters than those on near-vertical faces. In 1996 and 1997 surveys of activity at daytime high, and nocturnal low waters were conducted at sites (14–15) with varying rock slopes (∼3–87°). In all cases, limpets on more steep slopes were active at nocturnal emersion whilst animals on more gentle slopes were active on daytime submersion periods. In most cases these trends were significant and explained between 22–40% and 37–44% of the variation in activity with site in 1996 and 1997 respectively. Analysis of the head orientation of limpets on their home scars showed that animals orientated in a down shore direction at all sites (1997 data) suggesting that limpets do perceive and respond to slope. Whilst slope does appear to influence the timing of limpets' activity (and especially on very steep or gently sloping sites) it does not account for a large degree of the variation in activity and, on sites with slopes between 30 and 60°, is likely to work in combination with other factors.

The warm-water nemertean species Paradrepanophorus crassus, described from Mediterranean shores, was first recorded in Lough Hyne, County Cork, Ireland in 1931 by Renouf. This large nemertean that forms membranous tubes under low intertidal to shallow subtidal rocks has increased in frequency, particularly from 2009 to 2012 with peak values of 0.26 specimens per metre of shoreline. Eggs were noted within the membranous tube of one specimen collected in June 2012. Furthermore, based on digital photographs of specimens, the striped nemertean species Punnettia splendida was also noted in September 2011; this is a new species record for Lough Hyne.

Data from a series of rocky shore transects, within the enclosed basin of the Rance estuary and immediately outside the barrage, describe the distribution and zonation patterns of the intertidal biota, after more than a decade of its regular operation as a tidal power station. In the absence of quantitative pre-barrage information, the distribution of species has been compared with published information on their former extent of penetration into the estuary.

Activity of Patella vulgata was monitored in the asymmetrical tidal regime of an Irish sealough, Lough Hyne, and also outside the lough in a normal tidal regime. An automated method was used, allowing continuous records to be made over two weeks. Most activity occurred at night while the limpets were emersed. Rainfall depressed activity. There was little activity during daytime emersion, contrary to the results of previous studies in which low-shore limpets foraged diurnally as well as nocturnally. Timing of activity in relation to tidal coverage was similar inside the lough and outside.Activity of high-shore limpets within the lough was greater at spring tides than at neaps, but that of low-shore limpets was greater at neaps. Outside the lough, both high-shore and low-shore limpets showed greater activity at spring tides. Reasons for the differences are discussed.Using Maximum Entropy Spectral Analysis, three periodicities underlying limpet activity were indicated. These were at h, 124 h and 8–2 h. When limpet activity was simulated by adding three sine waves of appropriate periodicity, rhythms very similar to those recorded from the shore were produced.

Temperate reefs are superb tractable systems for testing hypotheses in ecology and evolutionary biology. Accordingly there is a rich history of research stretching back over 100 years, which has made major contributions to general ecological and evolutionary theory as well as providing better understanding of how littoral systems work by linking pattern with process. A brief resumé of the history of temperate reef ecology is provided to celebrate this rich heritage. As a community, temperate reef ecologists generally do well designed experiments and test well formulated hypotheses. Increasingly large datasets are being collected, collated and subjected to complex meta-analyses and used for modelling. These datasets do not happen spontaneously – the burgeoning subject of macroecology is possible only because of the efforts of dedicated natural historians whether it be observing birds, butterflies, or barnacles. High-quality natural history and old-fashioned field craft enable surveys or experiments to be stratified (i.e. replicates are replicates and not a random bit of rock) and lead to the generation of more insightful hypotheses. Modern molecular approaches have led to the discovery of cryptic species and provided phylogeographical insights, but natural history is still required to identify species in the field. We advocate a blend of modern approaches with old school skills and a fondness for temperate reefs in all their splendour.

The surface cohesive strength of intertidal sediments at six sites on two estuaries throughout the period of a single diurnal subaerial exposure was investigated. Stations at different tidal heights were examined at each site. Hourly measurements were made using a cohesive strength meter which provides an ordinal measure of sediment stability. The sediment was most easily eroded immediately after the emersion of the mudflat and there was little difference between the initial stability of the mud at different sites or stations. As the subaerial exposure continued the sediment of mid-shore and high-shore stations began to increase in stability. No significant change was found at low-shore stations before the return of the tide. The surface density of diatom populations was also intermittently measured at each site. The greatest increase in sediment stability occurred at high-shore stations and this was most extreme where there were also dense populations of epipelic diatoms present at the surface of the sediment. This supports laboratory evidence that the extracellular products of diatom populations of sufficient density increase the stability of intertidal sediments.

Activity patterns of limpets were studied at two adjacent sites in an Irish sea lough, Lough Hyne, in order to relate timing of activity to physical and biological influences. Activity was suppressed during heavy rainfall, and osmotic stress from dilution appears to have led to increased mortality in transplanted limpets. Activity increased as tides progressed from neaps to springs, and for limpets low on the shore it was enhanced by wave action. It is suggested that lack of activity in calm water may reduce predation pressure from crabs, which caused high mortality in transplanted limpets. Either low relative humidity or dryness of the rock diminished activity of low-water limpets. Differences in feeding activity between low-water limpets and high-water limpets may relate to food supply, since more food was available low on the shore, and the guts of low-water limpets contained more diatoms than those of high-water individuals. Differences between the two sites are at present unexplained but may relate to differences in micro habitats.

Haloes of encrusting algae, mostly Hildenbrandia, and cyanobacteria were found on the home scars under the shell of Patella vulgata, beneath the pallial cavity. The presence and area of halo varied between sites. Area generally increased with limpet size and at one site was negatively associated with slope of the shore. It is suggested that the algae provide a resource and may, therefore, play a role in the foraging ecology of the limpet.

Both Hydrobia ulvae (Pennant) and Potamopyrgus jenkinsi Smith climb vertically in the laboratory but stop or crawl horizontally if presented with a sharp salinity drop. 50% of Hydrobia ulvae respond to a difference of 18–26% from the original solution. 50% of Potamopyrgus jenkinsi respond to a difference of 53–55%. In both species, the response is impaired at high temperatures, and in Hydrobia ulvae it is also impaired at low temperatures. From 9–22 °C no temperature influence is evident. From acclimation experiments it is suggested that the snails respond not to the gradient across the interface, but to the gradient between body fluids and the upper solution presented. Snails respond to non-ionic gradients when these are presented either as density gradients or as solutions separated by a semi-permeable membrane, and it is argued that they detect osmotic pressure directly. The sense organs are probably located in the tentacles.

Diatom assemblages on the surface of fine estuarine sediments have, for the first time, been examined undisturbed by a novel application of low-temperature SEM. The technique allows the visualization of surface diatoms and determination of the composition and orientation of the exposed assemblages at any one time. In a laboratory experiment, the total number of diatoms counted was similar to that obtained using the standard lens tissue technique. Although identification was more accurate using the light microscope, many different species could be readily distinguished with low-temperature SEM. An index of similarity analysis suggested a difference in the composition of the assemblages measured by the two techniques. This difference was considered to be due to the selectivity of the lens tissue technique. Further avenues of research in this subject using low-temperature SEM are briefly outlined.

The foraging movements of limpets Patella vulgata L. were studied on a vertical rock face at Lough Hyne, County Cork, Ireland. Rhythms on two adjacent grids were similar, but showed some differences in timing. Distance moved and speed of movement were related to limpet size. Feeding excursions were more regular and prolonged on spring tides than on neaps. Limpets high on the shore were active ony at night, at low water, when they moved vertically up the rock face. They fed in habitats unlike those surrounding their homes, mainly on rock that was either bare, or covered by lichens. Limpets near mid-tide level did not move preferentially in any one direction, and they appeared to show no choice of feeding area. Limpets low on the shore were active near the times of low water both at night and during the day. They tended to move horizontally and, at least on some occasions, they chose areas of low barnacle density in which to feed. The feeding patterns seen at different tidal heights are discussed in relation to growth rates.

Growth and survival of veligers of Amphibola crenata (Gmelin) were followed in a range of salinities from 4 to 35%. In salinities of 18%. and below, growth was slowed and a large proportion of veligers died. In 4 to 12%., no larvae survived to metamorphosis, but these low salinities were tolerated for short periods. Newly post-metamorphic snails and juveniles collected from the field survived well even at 4%., showing that greater tolerance of low external salinities developed at metamorphosis. Examination of the physiology of adult snails suggested that this may be due to the development of osmoregulating mechanisms, because the adults maintain the blood hyperosmotic to the medium in 4 and 9%. The kidney, however, does not seem to be involved in this regulation because urine is isosmotic with the blood. An extra-renal mechanism of salt uptake is, therefore, postulated.

Although many physiological characteristics are very labile, it is proposed that some are stable enough to provide evidence concerning the routes by which terrestrial animals moved on to land. Osmotic tolerance and osmoregulatory ability are investigated first. Marine littoral invertebrates tolerate wide osmotic changes but osmoregulate little. Freshwater invertebrates osmoregulate well, but over a narrow range. Terrestrial prosobranchs of the family Pomatiasidae, which have wide tolerance, may have originated directly from the sea. The Cyclophoridae, which are intolerant, may have moved to land from fresh water. Isopods and talitrid amphipods may all have had direct marine origins with no freshwater phase. Secondly, terrestrial animals of marine origin may have blood with higher osmotic pressures than those passing through fresh water. In prosobranchs, isopods and amphipods, the evidence agrees with the conclusions suggested from character one. Thirdly, the production of hypo-osmotic urine in terrestrial animals may be evidence of freshwater origin. The evidence again supports the initial suggestions about prosobranchs and amphipods. It is concluded that because there is little selection pressure acting on these characters on land, theycanbe used as evidence of ancestry. Possible use of further characters is discussed.