Subtidal Gastropods Research Articles

Genetic adaptation despite high gene flow in a range-expanding population.

Signals of natural selection can be quickly eroded in high gene flow systems, curtailing efforts to understand how and when genetic adaptation occurs in the ocean. This long-standing, unresolved topic in ecology and evolution has renewed importance because changing environmental conditions are driving range expansions that may necessitate rapid evolutionary responses. One example occurs in Kellet's whelk (Kelletia kelletii), a common subtidal gastropod with an ~40- to 60-day pelagic larval duration that expanded their biogeographic range northwards in the 1970s by over 300 km. To test for genetic adaptation, we performed a series of experimental crosses with Kellet's whelk adults collected from their historical (HxH) and recently expanded range (ExE), and conducted RNA-Seq on offspring that we reared in a common garden environment. We identified 2770 differentially expressed genes (DEGs) between 54 offspring samples with either only historical range (HxH offspring) or expanded range (ExE offspring) ancestry. Using SNPs called directly from the DEGs, we assigned samples of known origin back to their range of origin with unprecedented accuracy for a marine species (92.6% and 94.5% for HxH and ExE offspring, respectively). The SNP with the highest predictive importance occurred on triosephosphate isomerase (TPI), an essential metabolic enzyme involved in cold stress response. TPI was significantly upregulated and contained a non-synonymous mutation in the expanded range. Our findings pave the way for accurately identifying patterns of dispersal, gene flow and population connectivity in the ocean by demonstrating that experimental transcriptomics can reveal mechanisms for how marine organisms respond to changing environmental conditions.

Genomic DNA extraction optimization and validation for genome sequencing using the marine gastropod Kellet’s whelk

Next-generation sequencing technologies, such as Nanopore MinION, Illumina Hiseq and Novaseq, and PacBio Sequel II, hold immense potential for advancing genomic research on non-model organisms, including the vast majority of marine species. However, application of these technologies to marine invertebrate species is often impeded by challenges in extracting and purifying their genomic DNA due to high polysaccharide content and other secondary metabolites. In this study, we help resolve this issue by developing and testing DNA extraction protocols for Kellet’s whelk (Kelletia kelletii), a subtidal gastropod with ecological and commercial importance, by comparing four DNA extraction methods commonly used in marine invertebrate studies. In our comparison of extraction methods, the Salting Out protocol was the least expensive, produced the highest DNA yields, produced consistent high DNA quality, and had low toxicity. We validated the protocol using an independent set of tissue samples, then applied it to extract high-molecular-weight (HMW) DNA from over three thousand Kellet’s whelk tissue samples. The protocol demonstrated scalability and, with added clean-up, suitability for RAD-seq, GT-seq, as well as whole genome sequencing using both long read (ONT MinION) and short read (Illumina NovaSeq) sequencing platforms. Our findings offer a robust and versatile DNA extraction and clean-up protocol for supporting genomic research on non-model marine organisms, to help mediate the under-representation of invertebrates in genomic studies.

Increased Thermal Sensitivity of a Tropical Marine Gastropod Under Combined CO2 and Temperature Stress

The ability of an organism to alter its physiology in response to environmental conditions offers a short-term defense mechanism in the face of weather extremes resulting from climate change. These often manifest as multiple, interacting drivers, especially pH and temperature. In particular, decreased pH can impose constraints on the biological mechanisms which define thermal limits by throwing off energetic equilibrium and diminishing physiological functions (e.g., in many marine ectotherms). For many species, however, we do not have a detailed understanding of these interactive effects, especially on short-term acclimation responses. Here, we investigated the metabolic plasticity of a tropical subtidal gastropod (Trochus maculatus) to increased levels of CO2 (700 ppm) and heating (+3°C), measuring metabolic performance (Q10 coefficient) and thermal sensitivity [temperature of maximum metabolic rate (TMMR), and upper lethal temperature (ULT)]. Individuals demonstrated metabolic acclimation in response to the stressors, with TMMR increasing by +4.1°C under higher temperatures, +2.7°C under elevated CO2, and +4.4°C under the combined stressors. In contrast, the ULT only increased marginally in response to heating (+0.3°C), but decreased by −2.3°C under CO2, and −8.7°C under combined stressors. Therefore, although phenotypic plasticity is evident with metabolic acclimation, acute lethal temperature limits seem to be less flexible during short-term acclimation.

RAD sequencing of common whelk, Buccinum undatum, reveals fine-scale population structuring in Europe and cryptic speciation within the North Atlantic.

Buccinum undatum is a subtidal gastropod that exhibits clear spatial variation in several phenotypic shell traits (color, shape, and thickness) across its North Atlantic distribution. Studies of spatial phenotypic variation exist for the species; however, population genetic studies have thus far relied on a limited set of mitochondrial and microsatellite markers. Here, we greatly expand on previous work by characterizing population genetic structure in B. undatum across the North Atlantic from SNP variation obtained by RAD sequencing. There was a high degree of genetic differentiation between Canadian and European populations (Iceland, Faroe Islands, and England) consistent with the divergence of populations in allopatry (F ST > 0.57 for all pairwise comparisons). In addition, B. undatum populations within Iceland, the Faroe Islands, and England are typified by weak but significant genetic structuring following an isolation‐by‐distance model. Finally, we established a significant correlation between genetic structuring in Iceland and two phenotypic traits: shell shape and color frequency. The works detailed here enhance our understanding of genetic structuring in B. undatum and establish the species as an intriguing model for future genome‐wide association studies.

Survived but not safe: Marine heatwave hinders metabolism in two gastropod survivors

Marine heatwaves (MHWs) are an emerging threat to marine organisms that have increased in frequency and magnitude in the past decade. These extreme heating events can have differential impacts on organisms with some experiencing mortality while others survive. Here, we experimentally exposed two species of subtidal gastropod (Trochus sacellum and Astralium haematragum) to two realistic intensities of MHW to test the ability of different species to physiologically cope with extreme heating events. Extreme MHW conditions caused 100% mortality in both species within five days. While both species survived under moderate MHW conditions they showed evidence of nonadaptive metabolic depression. Both species demonstrated an inability to upregulate their metabolic rates at the higher temperatures following exposure to a MHW (i.e. reduced temperature of maximum metabolic rate; TMMR), suggesting a lack of molecular protective responses and ongoing physiological damage. Therefore, the physiological damage endured by heatwave survivors may lessen their ability to cope with subsequent stress until fully recovered. Repairing this damage may have serious repercussions for the rate of recovery of these normally resilient species and their ability to maintain their ecological functions post MHW, especially under the predicted increasing frequency, duration and magnitude of MHWs.

Morphological variation in genetically divergent populations of the common whelk, Buccinum undatum (Gastropoda: Buccinidae), across the North Atlantic

Abstract The variation in shelled marine gastropod morphology across small spatial scales can reflect restricted population connectivity, resulting in evolution or plastic responses to environmental heterogeneity. The common whelk, Buccinum undatum, is a subtidal gastropod, ubiquitous in the North Atlantic, that exhibits considerable spatial variation in shell morphology and colour. Given that species delimitation in shelled marine gastropods is often based on shell characteristics, such morphological variation can lead to taxonomic confusion. Phylogeographical analysis based on mitochondrial DNA and microsatellites suggested cryptic species composed of Western and Eastern North Atlantic common whelk populations, the separation of which dates to the onset of the Pleistocene glaciation ~2.1 Mya. Divergence within the Eastern North Atlantic is more recent and characterized by isolation by distance. In the present study, phenotypic variation in shell morphology across the North Atlantic range is analysed and compared with molecular divergence. The morphological variation of B. undatum populations reflected the pattern observed for the molecular markers only for certain comparisons of populations and might, in other cases, reflect larger constraints on the morphological variation and, possibly, the impact of environmental influences.

Revised phylogeography of the common whelk Buccinum undatum (Gastropoda: Buccinidae) across the North Atlantic

AbstractThe purpose of this study was to revisit the mitochondrial genetic divergence of North Atlantic populations of the subtidal gastropod Buccinum undatum, because previous mitochondrial DNA (mtDNA) analysis by Pálsson et al. (2014) included 16S ribosomal RNA sequences that were incorrectly assigned to the species. In the present study, population mtDNA variation is now assessed using COI sequences obtained from previous research (Pálsson et al., 2014) and, to increase the geographical cover of the study, data from recently assembled transcriptomes of 96 Icelandic whelks (Jónsson et al., 2019) and GenBank are also used. To estimate the mtDNA divergence in B. undatum across the North Atlantic, two species screening indices were used: automatic barcode gap discovey (ABGD; Puillandre et al., 2012a) and species screening threshold index (SSTI; Witt et al., 2006). Phylogenetic reconstruction revealed monophyletic Eastern and Western North Atlantic whelk lineages, which diverged early in the Pleistocene glaciation (2.1 Mya), followed by a subsequent divergence event between Greenlandic and Canadian populations at 1.3 Mya. Species screening indices, ABGD and SSTI, indicated cryptic speciation or allopatric divergence. Genetic distances between populations from the two continents were similar to or greater than interspecific genetic distances across several North Pacific and North Atlantic Buccinum species.

Trophic and growth baseline of dominant subtidal gastropods in contrasting subtropical marine environments

Using 13C/12C, 15N/14N and 18O/16O isotopes, the trophic relationship and growth estimation were analyzed in gastropods Nassarius siquijorensis, Murex trapa and Turritella bacillum and their potential food sources and predators in summer and winter from estuarine and oceanic environments in subtropical Hong Kong. Results of δ13C and δ15N values and isotopic mixing model revealed N. siquijorensis and M. trapa were one trophic level higher than T. bacillum, in which its main food source was particulate organic matter (POM) whereas N. siquijorensis largely consumed POM and polychaetes and M. trapa also preyed on other gastropods. Crabs were the major predator of gastropods. Organisms collected from oceanic waters were more 13C enriched than from estuarine waters, reflecting different carbon food sources from marine or terrestrial origin. The δ18O profile from shell carbonate suggested these gastropods were one to two years old. T. bacillum exhibited faster summer growth than the other two species.

Heatwaves\xa0diminish the survival of a subtidal gastropod through reduction in energy budget and depletion of energy reserves

Extreme climatic events, such as heatwaves, are predicted to be more prevalent in future due to global climate change. The devastating impacts of heatwaves on the survival of marine organisms may be further intensified by ocean acidification. Here, we tested the hypothesis that prolonged exposure to heatwave temperatures (24 °C, +3 °C summer seawater temperature) would diminish energy budget, body condition and ultimately survival of a subtidal gastropod (Thalotia conica) by pushing close to its critical thermal maximum (CTmax). We also tested whether ocean acidification (pCO2: 1000 ppm) affects energy budget, CTmax and hence survival of this gastropod. Following the 8-week experimental period, mortality was markedly higher at 24 °C irrespective of pCO2 level, probably attributed to energy deficit (negative scope for growth) and concomitant depletion of energy reserves (reduced organ weight to flesh weight ratio). CTmax of T. conica appeared at 27 °C and was unaffected by ocean acidification. Our findings imply that prolonged exposure to heatwaves can compromise the survival of marine organisms below CTmax via disruption in energy homeostasis, which possibly explains their mass mortality in the past heatwave events. Therefore, heatwaves would have more profound effects than ocean acidification on future marine ecosystems.

Impacts of Near-Future Ocean Acidification and Warming on the Shell Mechanical and Geochemical Properties of Gastropods from Intertidal to Subtidal Zones.

Many marine organisms produce calcareous shells as the key structure for defense, but the functionality of shells may be compromised by ocean acidification and warming. Nevertheless, calcifying organisms may adaptively modify their shell properties in response to these impacts. Here, we examined how reduced pH and elevated temperature affect shell mechanical and geochemical properties of common grazing gastropods from intertidal to subtidal zones. Given the greater environmental fluctuations in the intertidal zone, we hypothesized that intertidal gastropods would exhibit more plastic responses in shell properties than subtidal gastropods. Overall, three out of five subtidal gastropods produced softer shells at elevated temperature, while intertidal gastropods maintained their shell hardness at both elevated pCO2 (i.e., reduced pH) and temperature. Regardless of pH and temperature, degree of crystallization was maintained (except one subtidal gastropod) and carbonate polymorph remained unchanged in all tested species. One intertidal gastropod produced less soluble shells (e.g., higher calcite/aragonite) in response to reduced pH. In contrast, subtidal gastropods produced only aragonite which has higher solubility than calcite. Overall, subtidal gastropods are expected to be more susceptible than intertidal gastropods to shell dissolution and physical damage under future seawater conditions. The increased vulnerability to shell dissolution and predation could have serious repercussions for their survival and ecological contributions in the future subtidal environment.

Comparisons of the metabolic responses of two subtidal nassariid gastropods to hypoxia and re-oxygenation

Changes in the levels of carbohydrate, lipid, protein and anaerobic metabolites (succinate, lactate, acetate, fumarate and propionate), upon exposure to hypoxia (1.5mg O2l−1) and after reoxygenation in subtidal gastropods Nassarius siquijorensis and N. conoidalis, were compared. A significant decrease of the glycogen content was observed under hypoxia in N. conoidalis but not in N. siquijorensis. A greater increase in the concentrations of anaerobic metabolites was observed in N. conoidalis under hypoxia, and their levels did not return to baseline after returning to normoxia for 24h. In contrast, a lower rate of accumulation of the metabolites was observed in N. siquijorensis, and complete recovery from anaerobic metabolism was observed after reoxygenation. The results lend further support to the role of hypoxia in governing the different distributional patterns between the two subtidal gastropods in Hong Kong waters.

Divergence and molecular variation in common whelkBuccinum undatum(Gastropoda: Buccinidae) in Iceland: a trans-Atlantic comparison

The dispersal and history of species affects their genetic population structure at both small and large geographical scales. The common whelk, Buccinum undatum, is a widespread subtidal gastropod in the North Atlantic that has no planktonic larvae and has thus limited dispersal capacity. The snail, which has been harvested by humans for centuries, is highly variable in morphology. To evaluate the population structure in the rich fishing grounds in western Iceland and its divergence from samples across the Atlantic, genetic patterns based on sequence variation in two mitochondrial (mt)DNA genes (COI and 16S) and five microsatellites were studied and compared with variation in populations from both sides of the Atlantic. Significant differences in allele and haplotype frequencies were found among samples separated by short distances along the coast of Iceland. Partition of the variation showed larger variance among samples obtained from distant regions than from neighbouring sites and genetic distances were correlated with geographical distance among populations in Europe. Phylogeographic patterns in mtDNA reveal different monophyletic lineages on both sides of the Atlantic, which predate the onset of the Ice Age and which may constitute cryptic species. Similar micro- and macrogeographical patterns were observed for the mtDNA and microsatellite markers, despite high frequencies of null alleles. Bayesian skyline reconstructions of the demographic history and mismatch distributions suggest that, although sizes of some populations were unaffected by Ice Age glaciations, others show signs of expansion after the Last Glacial Maximum. These phylogeographical patterns are consistent with patterns expected for low dispersal species that have survived in allopatric glacial refugial populations on both sides of the Atlantic and in deep-sea refugia within each continent. The observed genetic structure has implications for conservation and sustainable management of the harvested populations. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 145–159.

Life cycles, molecular phylogeny and historical biogeography of the ‘pygmaeus’ microphallids (Digenea: Microphallidae): widespread parasites of marine and coastal birds in the Holarctic

The 'pygmaeus' microphallids (MPG) are a closely related group of 6 digenean (Platyhelminthes: Trematoda) Microphallus species that share a derived 2-host life cycle in which metacercariae develop inside daughter sporocysts in the intermediate host (intertidal and subtidal gastropods, mostly of the genus Littorina) and are infective to marine birds (ducks, gulls and waders). Here we investigate MPG transmission patterns in coastal ecosystems and their diversification with respect to historical events, host switching and host-parasite co-evolution. Species phylogenies and phylogeographical reconstructions are estimated on the basis of 28S, ITS1 and ITS2 rDNA data and we use a combination of analyses to test the robustness and stability of the results, and the likelihood of alternative biogeographical scenarios. Results demonstrate that speciation within the MPG was not associated with co-speciation with either the first intermediate or final hosts, but rather by host-switching events coincident with glacial cycles in the Northern Hemisphere during the late Pliocene/Pleistocene. These resulted in the expansion of Pacific biota into the Arctic-North Atlantic and periodic isolation of Atlantic and Pacific populations. Thus we hypothesize that contemporary species of MPG and their host associations resulted from fragmentation of populations in regional refugia during stadials, and their subsequent range expansion from refugial centres during interstadials.

Scales of variation of gastropod densities over multiple spatial scales: comparison of common and rare species

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 354:147-160 (2008) - DOI: https://doi.org/10.3354/meps07205 Scales of variation of gastropod densities over multiple spatial scales: comparison of common and rare species M. G. Chapman*, A. J. Underwood Centre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories A11, University of Sydney, New South Wales 2006, Australia *Email: gee@bio.usyd.edu.au ABSTRACT: Quantifying spatial variation of populations of animals and plants is fundamental to understanding the relative importance of different ecological processes that influence them. Small-scale patterns in marine invertebrate densities are often thought to be determined by organisms’ responses to very localized cues, e.g. quality of habitat or food. In contrast, large-scale patterns have often been attributed to much larger-scale processes, such as availability of larvae, wave-action or upwelling. However, if features of habitat (which primarily influence populations at a small scale) vary over larger spatial scales, population variation over these larger scales may be due entirely to local responses to habitat occurring at a small scale. This study used artificial units of habitat (AUHs) to standardize features of habitat, in order to measure rates of colonization of subtidal gastropods at scales of 20 cm to 4 km. Patterns were compared for 8 common and 6 rare species, in addition to the assemblage of gastropods. Many of the species that colonized these AUHs were found in similar densities to those in natural habitats. Despite the standardisation of habitat, all species showed small-scale differentiation among AUHs only 20 cm apart, again mimicking patterns in natural patches of habitat. This was most probably due to choices of whether to colonize or not, rather than passive colonization forced by local environmental factors, because different species showed different patterns of colonization among AUHs. There was little additional variation in numbers across scales from 1 m to 4 km. The rare species showed greater small-scale and less large-scale variation than did the common species, but species in each group did not show similar patterns among individual AUHs. Where there are infrequent sample units that have large numbers of individuals (or species), they affect means at larger scales, causing larger-scale variation. Understanding small-scale responses to habitat must underpin studies of broad-scale patterns, so that changes in local processes having influences over large scales can be differentiated from broad-scale processes. This also has large implications for assessments of environmental impacts and monitoring of biodiversity, two of the most important tasks for ecologists in the immediate future. KEY WORDS: Artificial habitat · Gastropoda · Patchiness · Spatial scales · Variation Full text in pdf format PreviousNextCite this article as: Chapman MG, Underwood AJ (2008) Scales of variation of gastropod densities over multiple spatial scales: comparison of common and rare species. Mar Ecol Prog Ser 354:147-160. https://doi.org/10.3354/meps07205 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 354. Online publication date: February 07, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Genetic population structure across a range of geographic scales in the commercially exploited marine gastropod Buccinum undatum

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 317:157-169 (2006) - doi:10.3354/meps317157 Genetic population structure across a range of geographic scales in the commercially exploited marine gastropod Buccinum undatum D. Weetman1,*, L. Hauser1,2, M. K. Bayes3, J. R. Ellis4, P. W. Shaw3 1Department of Biological Sciences, University of Hull, Hull HU6 7RX, UK 2School of Aquatic and Fishery Sciences, University of Washington, 1122 NE Boat Street, Box 355020, Seattle, Washington 98195-5020, USA 3School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK 4Centre for Environmental Fisheries and Aquaculture Sciences, (CEFAS) Lowestoft Laboratory, Lowestoft, Suffolk NR33 0HT, UK *Email: dweetman@hotmail.co.uk ABSTRACT: Marine invertebrates are exploited increasingly as food resources worldwide, but knowledge of genetic population structure is limited for most commercially valuable taxa. We investigated genetic structure in the widely distributed, subtidal gastropod Buccinum undatum by screening samples from 28 locations at 5 microsatellite loci. Multidimensional scaling and cluster analysis were applied to study macrogeographic structure, and revealed 4 well-supported groupings of populations: Canada, Iceland, Swedish Skaggerak, and a cluster containing most samples from the European continental shelf (hereafter ‘Shelf’). A fifth cluster comprising all 5 samples from the Solent (UK) relied on exceptional differentiation of just 2 alleles, which is consistent with significant evidence for recent bottlenecks in this area. Within the Shelf cluster most pairwise tests of differentiation were significant, although the global FST of 0.014 was very low for a direct-developing species. Our data suggest that use of highly polymorphic markers caused a relatively minor downward bias to FST, although historical connectivity of populations that are not in migration-drift equilibrium might be more important. However, significant isolation by distance among British North Sea coast samples (FST = 0.010) is consistent with approach to equilibrium and suggests recent gene flow, probably between semi-continuous populations. At a microgeographic scale, we found migration to be consistently higher from inshore to offshore within 3 separate areas; a factor that may underpin the lower diversity and greater differentiation observed for bay and inlet populations. Such populations might serve as important sources of genetic diversity, but are likely to be particularly vulnerable to exploitation. KEY WORDS: Fishery genetics · Whelk · Microsatellites · Isolation by distance · Asymmetric migration · Bottleneck Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 317. Online publication date: July 18, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

A comparison of the ecology of shallow subtidal gastropods between western Indian Ocean and Caribbean coral reefs

I studied the ecology of snails inhabiting shallow wave-protected subtidal coral reef seascapes on the down-welling eastern edge of the American (Florida Keys and Belize) and African (Kenya and Madagascar) continents. Snail population abundance (CPUE), body sizes, modes of feeding, numbers of species, and species diversity were compared between regions and ocean basins. The Caribbean region had a third the number of species above 50 h or 5,000 individuals of sampling and higher dominance. When analyzed as numbers of individuals, the Caribbean community had a greater abundance of herbivorous and detritivorous individuals compared to the western Indian Ocean region. When analyzed as a percentage of species, there were no trophic differences, with two thirds of the species being carnivorous and one third herbivorous or detritivorous in both ocean basins. Body sizes were also not different in analyses based on either species or population abundance, but the Caribbean sites had higher variation in body size than the western Indian Ocean. The trophic, dominance, and species richness data suggest that the Caribbean assemblage has been influenced by environmental stress, but body-size data do not reflect this pattern unless stress increases variation rather than decreases body size. Recent environmental conditions may account for differences in the population dominance of trophic categories, but differences in species richness has required many millions of years of differential species origination and extinction.

Adaptations to Physical Stresses in the Intertidal Zone: The Egg Capsules of Neogastropod Molluscs

The encapsulation of eggs within benthic egg capsules or gelatinous egg masses is a common phenomenon among many marine invertebrate groups, yet the functional significance of many aspects of these egg coverings remains unexplored. In this paper I review what is known about the effectiveness of neogastropod egg capsules in protecting embryos from physical stresses associated with the marine intertidal environment. Egg capsules spawned by intertidal neogastropod molluscs can provide embryos with significant protection from desiccation, osmotic stress, and ultraviolet (UV) radiation, relative to embryos devoid of such coverings. Despite this, capsules desiccate rapidly in air, are highly permeable to small solute molecules, and are not impervious to incident UV radiation. Egg capsules of intertidal gastropods are also substantially more permeable to water molecules than the well studied egg cases and egg shells of insects and terrestrial vertebrates and may be no more effective in protecting embryos from such physical stresses than the capsules of exclusively subtidal gastropods. Hence, capsular cases appear to be poorly adapted to protecting embryos from environmental stresses associated with periodic exposure to air. The degree to which the encapsulated embryos of intertidal neogastropods are protected from environmental stresses thus may be more reflective of adult spawning site selection and tolerances of encapsulated embryos to these stresses, than properties of the capsular case, per se. Clearly, however, there is much still to be learned about the protective nature of capsule walls and the tremendous diversity of egg coverings that exists within the Gastropoda.

Effect of Copper on Activity and Feeding in the Subtidal Prosobranch Babylonia lutosa (Lamarck) (Gastropoda: Buccinidae)

Individuals of the subtidal gastropod Babylonia lutosa were exposed to three sublethal concentrations of copper (0.02, 0.06 and 0.20 ppm) for four weeks. The percentage of normally active individuals (body extended and moving with the foot attached) decreased and the percentage of snails retracted (body extended, foot not attached, unable to move) and in a distressed stage (body retracted within the shell) increased with increase in copper concentration. Exposure to copper also decreased significantly the ability of an individual to move, and to burrow, but the effect was only observed in week 1, individuals were fully recovered in week 2. Feeding, however, was not affected, both in terms of the percentage number of individuals which fed, and the time spent feeding. Results were discussed with respect to the potential of using this animal as a bioassay tool in monitoring the effect of environmental perturbations on the subtidal community in Hong Kong.

Seaweed detritus versus benthic diatoms as important food resources for two dominant subtidal gastropods

We assess the distribution, abundance, and recruitment of Margarites costalis (Gould) and Lacuna marmorata Dall within a shallow subtidal gastropod assemblage in the southern Strait of Georgia (British Columbia, Canada) in relation to season and depth, and discuss them in terms of food availability. During the summer, the fast rate of formation and favourable C: N ratio of seaweed detritus suggest that it may be an important and timely food resource for juveniles of these two dominant species. Although it is often difficult to determine the biogenic origin of stomach contents and the degree of partitioning of food resources, we argue for greater consideration of the rôle of detritus formed from seaweeds. We think future research on trophodynamic relationships in seaweed communities in this region should focus on the potential importance of seaweed detritus for fauna recruiting or developing within the seaweed zone during the summer months.

Freezing tolerance in intertidal invertebrates (a review)

1. 1. Freezing tolerance in intertidal invertebrates is restricted to species in the supra- or eulittoral zone of temperate and arctic regions. 2. 2. The cold resistance of the animals seems mainly to be based on tolerance to tissue ice formation. Freezing injury apparently occurs when a critical amount of water is frozen. 3. 3. An inverse relationship between acclimation temperature and cold-hardiness is demonstrated in a variety of invertebrates. Freezing tolerance also depends on freezing rates, increased rates reducing cold-hardiness. Time spent at subfreezing temperatures is important for survival of littoral molluscs. 4. 4. Invertebrates found naturally in seawater of high salinity are more cold-tolerant than specimens inhabitating brackish waters. In molluscs, the cold tolerance can be increased by acclimating the animals to higher salinities. This is probably based on increased concentrations of intracellular solutes such as amino acids. The acclimation process seems to be time-dependent. Improved freezing resistance can most effectively be obtained by acclimating the animals to low temperature in conjunction with high salinity. 5. 5. Many intertidal invertebrates exhibit seasonal fluctuations in freezing tolerance, in some species due to an endogeneous circannual rhythm. 6. 6. Thermal hysteresis factors are to some extent present in marine littoral invertebrates. 7. 7. The differences in lethal temperatures among bivalve species seem to be due to differences in the freezable fraction of tissue water. Some intertidal gastropod species are more tolerant to freezing than subtidal gastropods. This may be due to a higher tolerance to tissue ice in the intertidal species.