Small Gastropods Research Articles

Seasonal variation in carbon sources of ­mesograzers and small predators in an eelgrass community: stable isotope and fatty acid analyses

We assessed the seasonal change in trophic interactions at the mesograzer and small predator level in a temperate eelgrass system. This was done through stable isotope (δ13C and δ15N) and fatty acid analyses of eelgrass Zostera marina, attached epiphytes, sand microflora, the red alga Delesseria sanguinea, and the 4 most common mesograzer and predator species. Sampling took place monthly in the western part of the Baltic Sea during a whole year (April 2002 to February 2003). The trophic importance of microalgae was corroborated in all studied species. Red algae were consumed to a lesser extent and eelgrass was of minor importance. The degree of dependence on the main carbon sources was species-specific and varied with time. The small gastropod, Rissoa membranacea, mostly grazed epiphytes (73% on average), whereas the fractions of epiphytes and sand microflora showed a more balanced pattern in the other mesograzer species. Stable carbon data and fatty acid composition strongly suggested that epiphytic algae were the primary source of organic matter for mesograzers in late spring and autumn. In summer, sand microflora were of greater importance as carbon sources, except for the amphipod Gammarus oceanicus for which red algae was the most important. Stable nitrogen values indicated that the degree of carnivory was size-dependent in both omnivorous crustacean species studied; larger individuals generally occupied a higher trophic position than did smaller ones. Furthermore, the isopod Idotea baltica was more herbivorous in summer than in the other seasons. Our results confirm the importance of species-specific and temporal variability for the effect of mesograzers in eelgrass systems. The significance of mesograzers as determinants of food-web structure via the reduction of epiphytes is further corroborated by our study, and the observed high plasticity of mesograzers concerning food sources may promote the stability of eelgrass food webs despite strong seasonal variations in the biomass of primary producers.

HYDROBIA AS "JONAH IN THE WHALE": SHELL REPAIR AFTER PASSING THROUGH THE DIGESTIVE TRACT OF SHELDUCKS ALIVE

In the Wadden Sea, shell repair frequency in the small gastropod Hydrobia ulvae varied from 2.8% to 11.2%. On tidal flats of the Mok, a small bay on the island of Texel, The Netherlands, in the Wadden Sea, higher repair frequencies varying from 11.8% to 41.8% were measured. The shelduck, Tadorna tadorna, a predator of Hydrobia, occurs here in densities far above average densities for the Wadden Sea. Shelducks ingest their prey whole and crush the shells of H. ulvae internally. Live specimens of H. ulvae were collected from shelduck feces. Those with intact operculum and only a damaged outer aperture rim of the shell were kept in aquaria and repaired their shell rapidly. This indicates that predators that ingest shelled prey can also leave repair scars on shells. Such scars, however, are indistinguishable from those resulting from failed predation by predators using such pre-ingestive shell breakage as decapod crustaceans.

Reduction of flatfish habitat as a consequence of the proliferation of an invasive mollusc

Coastal bays provide habitats for juveniles and adults of many marine species. Mont Saint-Michel Bay (MSMB, France) hosts a highly diversified fish community and constitutes one of the most important nursery grounds for many commercially exploited marine species, such as sea bass, flatfish, clupeids and rays in the English Channel. Besides, MSMB also suffers from the massive invasion of an exotic mollusc, the American slipper-limpet ( Crepidula fornicata, L.). This species arrived four decades ago and now represents the main filter-feeder biomass in the bay (150 Mt), an order of magnitude larger than local farmed and natural shellfishes. Recent analyses underlined the impact of this small gastropod on the trophic structure of this bay and its negative influence on juvenile sole densities in the nursery grounds. The present study uses a geostatistical approach to explore the effect of the extension of the slipper-limpet on flatfish (common sole Solea solea, L.; plaice Pleuronectes platessa, L.; brill Scophthalmus rhombus, L. and flounder Platichthys flesus, L.) spatial distribution. Data collected during survey of the MSMB at the end of the 1970s and three decades later have been used to build interpolated maps of (1) slipper-limpet and (2) flatfish spatial distributions. Slipper-limpets were concentrated in a small area, in the western part of the MSMB, in the 1970s while today they occupy half of the bay. This rapid proliferation led to the decrease of available surface for flatfishes, which previously occupied the whole bay and are now restricted to its eastern part. The present study highlighted that the negative influence on fish habitat in MSMB is apparently more related to changes in the substratum than to trophic interactions. This invasion has possible consequences on flatfish population renewal at a large scale and may also damage other benthic or demersal species, such as rays.

Nassarius wesselinghi, new name for the homonymous N. reductus Vermeij and Wesselingh, 2002 (Gastropoda: Nassariidae)

Vermeij and Wesselingh (2002: 267, figs. 3 and 4) named a small freshwater gastropod from the Pebas Formation (middle Miocene) of Peruvian Amazonia as ?Nassarius reductus new species. This name turns out to be a secondary homonym of Nassa reticulata var. reducta Dollfus, 1926 (p. 103-106), a taxon from the Pliocene of Albania. The genus name Nassa as used by Dollfus is a misused variant on the correct genus name Nassarius Duméril, 1806. Although the Peruvian species is clearly not a Nassarius in the strict sense (restricted to the marine Indo-West Pacific region) a global phylogenetic analysis of Nassariidae is needed before its proper allocation can be determined, as already noted by Vermeij and Wesselingh (2002). We here rename the Peruvian fossil as Nassarius (s.l.) wesselinghi in honor of Frank P. Wesselingh who discovered the species. We thank Richard E. Petit for pointing out Dollfus's monograph to us.

A meso-predator release of stickleback promotes recruitment of macroalgae in the Baltic Sea

In the Baltic Sea, increased populations of the three-spined stickleback are invading the coastal zone in summer, following declines in large predatory fish such as cod, pike, and perch. Here, we explore the consequences of such a meso-predator release on a near-natural scale, by manipulating stickleback densities in four large 600 m 2 enclosures: two ‘removal’ and two ‘addition’ enclosures. Higher densities of stickleback resulted in a three times higher recruitment of ephemeral green macroalgae. At the same time we found higher abundances of the dominating invertebrate grazers with lower stickleback densities: higher numbers of both amphipods and smaller gastropods were found in one stickleback ‘removal’ enclosure and higher numbers of large gastropods occurred in the other ‘removal’ enclosure. Grazer abundances also depended on the macrophyte species that dominated the enclosures. Nutrient enrichment had no statistically significant effect on algal recruitment, although the mean number of algal recruits was almost doubled under enriched conditions. Our results indicate that a meso-predator release of stickleback may dramatically shift coastal food web constitution towards increased abundances of ephemeral macroalgae through a trophic cascade.

How do nutrient conditions and species identity influence the impact of mesograzers in eelgrass-epiphyte systems?

Coastal eutrophication is thought to cause excessive growth of epiphytes in eelgrass beds, threatening the health and survival of these ecologically and economically valuable ecosystems worldwide. Mesograzers, small crustacean and gastropod grazers, have the potential to prevent seagrass loss by grazing preferentially and efficiently on epiphytes. We tested the impact of three mesograzers on epiphyte biomass and eelgrass productivity under threefold enriched nutrient concentrations in experimental indoor mesocosm systems under summer conditions. We compared the results with earlier identical experiments that were performed under ambient nutrient supply. The isopod Idotea baltica, the periwinkle Littorina littorea, and the small gastropod Rissoa membranacea significantly reduced epiphyte load under high nutrient supply with Rissoa being the most efficient grazer, but only high densities of Littorina and Rissoa had a significant positive effect on eelgrass productivity. Although all mesograzers increased epiphyte ingestion with higher nutrient load, most likely as a functional response to the quantitatively and qualitatively better food supply, the promotion of eelgrass growth by Idotea and Rissoa was diminished compared to the study performed under ambient nutrient supply. Littorina maintained the level of its positive impact on eelgrass productivity regardless of nutrient concentrations.

Possibility of direct utilization of seagrass and algae as main food resources by small gastropod, Lacuna decorata, in a subarctic lagoon, Hichirippu, eastern Hokkaido, Japan with stable isotope evidences of carbon and nitrogen

The small gastropod, Lacuna decorata Adams, living on macrophytobenthos or surface sediment, is one of the most dominant species of macrozoobenthos in Hichirippu lagoon covered with seagrass and macroalgae, eastern Hokkaido, Japan. We measured the standing stocks of primary producers and macrozoobenthos, and determined the stable carbon and nitrogen isotope ratios of the primary producers and L. decorata. With these results, we identify the main food items for L. decorata and discuss the feeding strategy of the small gastropod. This gastropod occupied about 64% in density and about 25% in biomass of the macrozoobenthos at all six sampling stations in the lagoon. It occurred densely on the surface of the sediment with dense patches of benthic microalgae (BMA), which contained extremely high levels of Chl.-a between 84 to 226 mg m �2 throughout the period of this study. Nevertheless, the stable isotope sig- natures of carbon and nitrogen of this gastropod clearly show the direct utilization of organic matter derived from sea- grass, Zostera japonica, in the areas where the seagrass luxuriated. However, it shows also a flexible feeding strategy in food preference. It fed green algae such as Ulva pertusa and Urospora wormskioldii in the areas where the seagrass grew scarcely.

Trophic interactions in Zostera marina beds along the Swedish coast

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 369:63-76 (2008) - DOI: https://doi.org/10.3354/meps07646 Trophic interactions in Zostera marina beds along the Swedish coast Therese Jephson1,*, Per Nyström2, Per-Olav Moksnes3, Susanne P. Baden4 1Department of Ecology, Marine Biology, Campus Helsingborg, Lund University, Box 882, 25108 Helsingborg, Sweden 2Department of Ecology, Limnology, Ecology Building, Lund University, 22362 Lund, Sweden 3Department of Marine Ecology, University of Gothenburg, Box 461, 40530 Göteborg, Sweden 4Department of Marine Ecology, University of Gothenburg, The Sven Loven Centre for Marine Sciences—Kristineberg, 45034 Fiskebäckskil, Sweden *Email: therese.jephson@limnol.lu.se ABSTRACT: We compared eelgrass Zostera marina communities in 3 regions in Sweden believed to be affected by eutrophication and overfishing, to determine whether bottom-up or top-down processes control the biomass of epiphytic macroalgae and grazers. Nitrogen and carbon isotope signatures were analyzed to explore the food webs and to identify the grazing species feeding on filamentous algae and/or eelgrass. Mixing model (IsoSource version 1.3.1) analysis of the isotope signatures indicated that the amphipods Gammarus locusta and Microdeutopus gryllotalpa fed primarily on filamentous algae and that only 2 small gastropod species consumed eelgrass. Moreover, the grass shrimp Palaemon elegans and P. adspersus were ca. 1 trophic level above amphipods and algae, but according to the mixing model played different trophic roles in the different areas. The highest biomass of filamentous algae was found in the west coast beds housing grazers with the lowest biomass and mean size (predominantly G. locusta and M. gryllotalpa, 0.5 to 3 mm). In contrast, the Baltic Sea beds had low algal biomass, but the grazers (mostly G. locusta and Idotea baltica) had higher biomass and were significantly larger (mean size ca. 10 mm). An overall negative correlation was found between grazer biomass and biomass of filamentous algae. The significantly smaller grazers and absence of isopod grazers on the west coast may be due to substantial consumption by small predatory fish. This supports the suggestions that, in Swedish eelgrass beds, grazers are top-down controlled, and overexploitation of large predators and eutrophication play an important role in the recent increases in algal biomass. KEY WORDS: Seagrass · Eelgrass · Zostera · Stable isotopes · Mixing model · Trophic interactions Full text in pdf format PreviousNextCite this article as: Jephson T, Nyström P, Moksnes PO, Baden SP (2008) Trophic interactions in Zostera marina beds along the Swedish coast. Mar Ecol Prog Ser 369:63-76. https://doi.org/10.3354/meps07646 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 369. Online publication date: October 13, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Late Permian (Lopingian) gastropods from the Qubuerga Formation at the Qubu section in the Mt. Everest (Qomolangma) Region, Southern Tibet (Xizang), China

Late Permian gastropod fauna in the Mt. Everest (Qomolangma) region, southern Tibet (Xizang), China is poorly known. This paper describes a small gastropod fauna collected by one of the authors (SSZ) from the upper part of the Qubuerga Formation at the Qubu section. The section is located at about 30 km north of Mt. Everest (Fig. 1). Although a limited number of specimens is available, the fauna is more diverse than all previously reported gastropod faunas (e.g., Yu, 1975) from southern Tibet. Description of the gastropod fauna in the Mt. Everest region adds significant data for understanding the distribution of gastropods during the Late Permian and the paleobiogeographic relationship between the Himalayan and Tethyan regions.

Tetrodotoxin – Distribution and Accumulation in Aquatic Organisms, and Cases of Human Intoxication

Many pufferfish of the family Tetraodontidae possess a potent neurotoxin, tetrodotoxin (TTX). In marine pufferfish species, toxicity is generally high in the liver and ovary, whereas in brackish water and freshwater species, toxicity is higher in the skin. In 1964, the toxin of the California newt was identified as TTX as well, and since then TTX has been detected in a variety of other organisms. TTX is produced primarily by marine bacteria, and pufferfish accumulate TTX via the food chain that begins with these bacteria. Consequently, pufferfish become non-toxic when they are fed TTX-free diets in an environment in which the invasion of TTX-bearing organisms is completely shut off. Although some researchers claim that the TTX of amphibians is endogenous, we believe that it also has an exogenous origin, i.e., from organisms consumed as food. TTX-bearing animals are equipped with a high tolerance to TTX, and thus retain or accumulate TTX possibly as a biologic defense substance. There have been many cases of human intoxication due to the ingestion of TTX-bearing pufferfish, mainly in Japan, China, and Taiwan, and several victims have died. Several cases of TTX intoxication due to the ingestion of small gastropods, including some lethal cases, were recently reported in China and Taiwan, revealing a serious public health issue.

Tetrodotoxin--distribution and accumulation in aquatic organisms, and cases of human intoxication.

Many pufferfish of the family Tetraodontidae possess a potent neurotoxin, tetrodotoxin (TTX). In marine pufferfish species, toxicity is generally high in the liver and ovary, whereas in brackish water and freshwater species, toxicity is higher in the skin. In 1964, the toxin of the California newt was identified as TTX as well, and since then TTX has been detected in a variety of other organisms. TTX is produced primarily by marine bacteria, and pufferfish accumulate TTX via the food chain that begins with these bacteria. Consequently, pufferfish become non-toxic when they are fed TTX-free diets in an environment in which the invasion of TTX-bearing organisms is completely shut off. Although some researchers claim that the TTX of amphibians is endogenous, we believe that it also has an exogenous origin, i.e., from organisms consumed as food. TTX-bearing animals are equipped with a high tolerance to TTX, and thus retain or accumulate TTX possibly as a biologic defense substance. There have been many cases of human intoxication due to the ingestion of TTX-bearing pufferfish, mainly in Japan, China, and Taiwan, and several victims have died. Several cases of TTX intoxication due to the ingestion of small gastropods, including some lethal cases, were recently reported in China and Taiwan, revealing a serious public health issue.

Differences in feeding ecology among three co-occurring species of wrasse (Teleostei: Labridae) on rocky reefs of temperate Australia

The foraging behaviours and dietary compositions of three co-occurring labrids (Ophthalmolepis lineolatus, Notolabrus gymnogenis and Pictilabrus laticlavius), which are conspicuous on rocky reefs in temperate south-eastern Australia, were investigated between 2003 and 2005. SCUBA observations at two locations showed that the feeding intensity, and hence the associated effects of these fishes on rocky reef invertebrate prey, was temporally consistent. Relative differences in the contributions of ingested prey and use of different feeding microhabitats demonstrated that the feeding ecology differed significantly among the three species. Thus, O. lineolatus fed on proportionately higher volumes of polychaetes, polyplacophorans, marginellid gastropods (especially Austroginella sp.), bivalves and echinoids, which were sighted opportunistically in a wide selection of microhabitats, but particularly in sand/rubble. Ambush hunting was used regularly by smaller N. gymnogenis and all sizes of P. laticlavius to forage on amphipods, small decapods and small gastropods at algal bases or fronds and Diopatra dentata tubes. Amphipods were similarly important in the diet of smaller O. lineolatus. Larger N. gymnogenis foraged opportunistically over an increased reef area and made greater use of microhabitats that offered minimal prey refuge (e.g. sand/rubble, bare rock/steel) from which common prey, in particular decapods, were obtained. The significant intra- and inter-specific differences in dietary compositions, allied with differences in the use of feeding microhabitats, would facilitate co-occurrence of these three conspicuous species and contribute to maintaining high richness of labrid species in reef systems. Echinoids were regularly consumed by each species but they made a moderate contribution to the diet of only O. lineolatus, which suggests that only one of the three labrids is likely to play a significant role in regulation of echinoid densities in these rocky reef habitats. However, the broad diets and diverse forging strategies employed by these labrid species imply that they have a system-wide influence on invertebrate prey on rocky reefs.

Ecology of Camptoceras lineatum Blanford (Pulmonata : Basommatophora) studied in Deepar wetland of Assam, India and a note on its embryonic development

Camptoceras lineatum Blanford is a small freshwater gastropod mollusc in the family Planorbidae. During a limnological investigation in Deepar wetland of Assam, India, the species was recorded for the first time from this region. Some of the ecological aspects of the species studied during June to November, 2005 are presented here. The animal was cultured ex situ and some notes on its embryonic development is presented.

The contents of labile sulfides in the bottom sediments of the central part of the Sea of Azov: Impact on benthic communities

At 17 stations in the central part of the Sea of Azov, the contents of labile (acid-soluble) sulfides were determined in the upper layer (0–3 cm) of the bottom sediments. At 14 of these stations, the contents of sulfides in the oozy silts were over 300 mg S/dm3 of wet sediment; at seven of the stations, the contents of sulfide were as high as 420–720 mg S/dm3 of wet ooze, or 0.1–0.2% of dry weight. At the other three stations, where neutral matter such as shells and sand prevailed in the samples, the contents of sulfides varied from 80 to 110 mg S/dm3. At these stations, a high density and species diversity of the benthic fauna was retained. At other stations with labile sulfide contents over 200–300 mg S/dm3, the benthos biomass decreased by one or two orders of magnitude. At most of them, it was below 3 g/m2 and the small gastropod Hydrobia tolerant to sulfides dominated. The data obtained show that, in the central part of the Sea of Azov, reduced sediments with high contents of labile sulfides migrate towards the bottom surface, which conforms to the high intensity of the hydrogen sulfide formation process caused by the bacterial sulfate reduction. The study considers the environmental effects of the sulfide contamination of the upper layer of the bottom sediments in the Sea of Azov as a key factor causing the recurrent hypoxy in the near-bottom layers of the water, the suffocation occurrence, and the progressive depletion of the benthic and pelagic fauna.

Comparisons between the influences of habitat, body size and season on the dietary composition of the sparid Acanthopagrus latus in a large marine embayment

Seasonal samples from Shark Bay on the west coast of Australia were used to determine (1) the habitats occupied by the juveniles and adults of Acanthopagrus latus in this large subtropical marine embayment and (2) the extent to which the dietary composition of this sparid is influenced by habitat type, body length and season. Sampling was undertaken in two habitat types in which A. latus was known to be abundant, namely mangrove ( Avicennia marina) creeks and nearby rocky areas, the latter comprising sandstone boulders and/or limestone reefs. The mean total length ±95% CLs of A. latus was far lower in mangrove creeks, 126 ± 6.1 mm, than in rocky areas, 313 ± 4.7 mm. As A. latus attains maturity at ca. 245 mm, the juveniles of this species typically occupy mangrove areas and then, with increasing body size, move to nearshore rocky areas, where they become adults. The species composition of the food ingested by juvenile A. latus in mangrove creeks differed markedly from that of large juveniles and adults in rocky areas. Based on analyses of data for both habitat types combined, this difference was far greater than that between size classes and season, which was negligible. There were indications, however, that, overall within each habitat, the dietary composition did change seasonally, although not with body size. Acanthopagrus latus fed predominantly on mangrove material, sesarmid crabs and small gastropods in mangrove habitats, and mainly on Brachidontes ustulatus in rocky areas, where this mytilid bivalve is very abundant. The mangrove material, which contributed nearly 40% of its overall dietary volume in mangrove creeks, consisted mainly of lateral root primordia. This apparently unique food source for a teleost is presumably ingested through subsurface nipping, which would be facilitated by the mouth and dentitional characteristics of sparids. The almost total lack of correspondence in the dietary compositions of fish in the length class that was well represented in both mangrove and rocky areas illustrates the extent to which this sparid is capable of opportunistic feeding behaviour.

Mesoherbivores reduce net growth and induce chemical resistance in natural seaweed populations.

Herbivory on marine macroalgae (seaweeds) in temperate areas is often dominated by relatively small gastropods and crustaceans (mesoherbivores). The effects of these herbivores on the performance of adult seaweeds have so far been almost exclusively investigated under artificial laboratory conditions. Furthermore, several recent laboratory studies with mesoherbivores indicate that inducible chemical resistance may be as common in seaweeds as in vascular plants. However, in order to further explore and test the possible ecological significance of induced chemical resistance in temperate seaweeds, data are needed that address this issue in natural populations. We investigated the effect of grazing by littorinid herbivorous snails (Littorina spp.) on the individual net growth of the brown seaweed Ascophyllum nodosum in natural field populations. Furthermore, the capacity for induced resistance in the seaweeds was assessed by removing herbivores and assaying for relaxation of defences. We found that ambient densities of gastropod herbivores significantly reduced net growth by 45% in natural field populations of A. nodosum. Seaweeds previously exposed to grazing in the field were less consumed by gastropod herbivores in feeding bioassays. Furthermore, the concentration of phlorotannins (polyphenolics), which have been shown to deter gastropod herbivores, was higher in the seaweeds that were exposed to gastropod herbivores in the field. This field study corroborates earlier laboratory experiments and demonstrates that it is important to make sure that the lack of experimental field data on marine mesoherbivory does not lead to rash conclusions about the lack of significant effects of these herbivores on seaweed performance. The results strongly suggest that gastropods exert a significant selection pressure on the evolution of defensive traits in the seaweeds, and that brown seaweeds can respond to attacks by natural densities of these herbivores through increased chemical resistance to further grazing.

Causes of rarity and range restriction of an endangered, endemic limpet, Siphonaria compressa

The pulmonate limpet Siphonaria compressa is South Africa's most endangered marine mollusc. It is endemic to just two localities: Langebaan Lagoon on the west coast, and Knysna Estuary on the south coast, and occurs only on the eelgrass Zostera capensis. In Langebaan Lagoon, eelgrass has fluctuated substantially over the last 34 years, and S. compressa has twice approached extinction. S. compressa is largely confined to the lower edge of the eelgrass beds there, being replaced higher up by another small gastropod, Assiminea globulus. We explored the physical and biological factors underlying the limpet's narrow habitat, using field observations, translocations, caging and transplant experiments. Abundance of S. compressa was positively correlated with Z. capensis cover and negatively correlated with shore height. When moved to the upper portions of the eelgrass bed, S. compressa had lower rates of persistence and survival than in the lowest zone. The lower limit of zonation for S. compressa was set indirectly by bioturbation by the sandprawn Callianassa kraussi, which excluded eelgrass from intertidal sandbanks. Transplants of eelgrass into the sandbanks proliferated provided C. kraussi was experimentally eliminated, and supported densities of S. compressa 20-fold greater than in control eelgrass beds, suggesting that high-shore eelgrass beds to which S. compressa is normally confined are suboptimal for the limpet. A. globulus showed patterns opposite to those of S. compressa: its persistence and survival were greatest in the upper zone and it actively avoided the lower sections of these beds and never colonised eelgrass transplanted into sandflats lower on the shore. There was no evidence that competition between S. compressa and A. globulus influenced the zonation or abundance of either species. Rarity of S. compressa and its endangered status seem dictated by its extremely narrow and temporally changeable habitat-range, which is defined by physical stress in the high-shore and bioturbation by C. kraussi in the low-shore. Fluctuations in eelgrass abundance and limitation of S. compressa to just two localities add substantially to the risks of extinction for this embattled stenotypic limpet.

Land hermit crab (Coenobita clypeatus) densities and patterns of gastropod shell use on small Bahamian islands

AbstractAim To examine patterns of abundance, density, size and shell use in land hermit crabs, Coenobita clypeatus (Herbst), occurring on three groups of small islands, and to determine how these variables change among islands.Location Small islands in the Central Exuma Cays and near Great Exuma, Bahamas.Methods Land hermit crabs were captured in baited pitfall traps and were separately attracted to baits. A mark–recapture technique was used in conjunction with some pitfall traps monitored for three consecutive days. The size of each crab and the type of adopted gastropod shell were recorded, along with physical island variables such as total island area, vegetated area, island perimeter, elevation and distance to the nearest mainland island.Results Relative abundances, densities and sizes of crabs differed significantly among the three island groups. Densities of land hermit crabs were as high as 46 m−2 of vegetated island area. In simple and multiple linear regressions, the only variable that was a significant predictor of the abundance of hermit crabs was the perimeter to area ratio of the island. Patterns of gastropod shell use varied significantly among the island groups, and the vast majority of adopted shells originated from gastropod species that inhabit the high intertidal and supratidal shorelines of the islands.Main conclusions Although densities of land hermit crabs varied, they were relatively high on many islands, and land hermit crabs may play an important role in these insular food webs. Patterns of shell use may be strongly restricted by island geomorphology: irregular shorelines provide relatively more habitat for the gastropod species that account for the majority of adopted shells and the steep sides of the islands prevent the accumulation of marine gastropod shells. The size of adult hermit crabs appears to be limited by the relatively small gastropod shells available, while the abundance of hermit crabs may be limited by the number of shells available.

PREDATOR–PREY INTERACTIONS BETWEEN LEPSIELLA (BEDEVA) PAIVAE (GASTROPODA: MURICIDAE) AND KATELYSIA SCALARINA (BIVALVIA: VENERIDAE) IN PRINCESS ROYAL HARBOUR, WESTERN AUSTRALIA

On a sandy beach at Shoal Bay in Princess Royal Harbour, Albany, southwestern Western Australia, lives a small muricid gastropod that feeds virtually monotonically on the overwhelmingly dominant resident bivalve Katelysia scalarina. Lepsiella paivae lives buried in the sand and attacks its prey within it. Because of its small size (,13 mm shell height), bivalve prey is also small and this study demonstrates a preference for K. scalarina of 5 mm shell length, i.e. juveniles. Laboratory experiments also suggested a possible preference for attack of the right valve. Lepsiella paivae can and does, however, attack larger prey (up to 15 mm shell length), but cannot consume them completely. A second visit to Princess Royal Harbour in the Austral winter, when there was no juvenile K. scalarina present, showed L. paivae to be attacking at the sand surface, also by drilling, the small (,4 mm) gastropod Hydrococcus brazieri (Hydrococcidae). SEM studies of experimentally determined drill holes of L. paivae show them to be of variable form, some straight sided, others bevelled (like a naticid) and ,500 mm in diameter. On this sheltered Southern Ocean beach, therefore, L. paivae has specialized to attack juvenile bivalves by burrowing after them. It can, however, attack other species opportunistically on the sand surface when seasonally favoured juvenile bivalve prey are not present.

Comparative Ecology of the Gobies Nes longus and Ctenogobius saepepallens, Both Symbiotic with the Snapping Shrimp Alpheus floridanus

We re-examined the symbiotic association of the western Atlantic gobiid fishes Nes longus and Ctenogobius saepepallens with the snapping shrimp Alpheus floridanus on the basis of a critical literature review and new data. Our research confirms that N. longus interacts closely with the shrimp and is dependent on it for the cover provided by the burrow that the shrimp constructs; the goby serves as the sentinel at the burrow entrance. Ctenogobius saepepallens is often seen occupying a burrow of the alpheid, and the shrimp will leave the burrow to deposit sediment with the goby at the entrance, even pushing the goby aside at times. However, the shrimp does not make contact with the goby with its antennae, nor does the goby communicate with caudal fin fluttering at the approach of danger. We suggest that their relationship is a first step in an evolutionary process that may lead to the very close mutualistic association exhibited by N. longus and the alpheid, as well as Indo-Pacific shrimp gobies of 13 different genera and their alpheid partners. Nes longus remains close to the burrow entrance; it feeds mainly on small gastropods, decapod crustaceans, ostracods, and isopods. By contrast, C. saepepallens makes longer excusions from the shelter of the burrow; its diet is dominated by benthic copepods, followed by ostracods and lesser amounts of foraminiferans, isopods, and decapod crustaceans. By virture of its greater mobility, it can be more selective in its prey.