Regeneration of lost siphon tissues in the tellinacean bivalve Nuttallia olivacea

The inhalant siphon of the tellinacean bivalve Nuttallia olivacea is an important prey item for juvenile stone flounder Platichthys bicoloratus in estuaries in Japan. We examined quantitative siphon regeneration of N. olivacea in rearing experiments of siphon-removed bivalves (> 30 mm shell length) both in the laboratory and in their natural habitat. Under laboratory conditions, siphon-removed bivalves regenerated lost tissues quantitatively at 15 and 25 °C 1 mo after siphon removal, although regeneration was incomplete. A 3-mo caging experiment in the field showed that great regeneration occurred in siphon-removed bivalves. However, the siphon weight of removed bivalves was significantly smaller than that of non-amputated bivalves, suggesting the incomplete regeneration. In a 1-mo caging experiment, bivalves that had approximately 15% of their siphons amputated were selected at some intervals to illustrate the quantitative regeneration process. Estimated daily siphon production was remarkably high only a few days after amputation. It decreased greatly thereafter, but regeneration was not completed within 30 d. These results indicate that bivalves regenerate siphons rapidly just after losing siphon tissues and then regeneration is slowed down before it is completed.     

Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis

In a floodplain lake of the Amazon River near the city of Iquitos, northeastern Peru, a one-year monitoring experiment was conducted during which water samples and living bivalves (Anodontites trapesialis) were collected with the aim to investigate seasonal δ¹⁸O variation in and fractionation between bivalve aragonite and host water. Both host water and molluscan growth increments show more than 8‰ seasonal variation in δ¹⁸O. In the floodplain lake under study the δ¹⁸O variation of the water is controlled by contrasting dry and wet season evaporation-precipitation regimes. Molluscan δ¹⁸O appears to be in equilibrium with the host water. Although an approximately 4.0‰ offset occurs, δ¹³C records of water and bivalves are in good agreement, suggesting that both δ¹⁸O and δ¹³C of the shells of freshwater bivalve A. trapesialis are good recorders of (palaeo-)environmental conditions. The δ¹³C of Dissolved Inorganic Carbon (DIC) is governed by plant growth and/or by changes in aquatic chemistry, affecting the DIC pool.     

Do Zebra Mussels Grow Faster on Live Unionids than on Inanimate Substrate? A Study with Field Enclosures

The zebra mussel Dreissena polymorpha has invaded numerous freshwaters in Europe and North America and can foul many types of solid substrates, including unionid bivalves. In field experiments we compared growth rates of dreissenids on live specimens of the freshwater bivalve Anodonta cygnea to growth rates of dreissenids on stones. Dreissena density in the study lake was about 1000 m^–2 in most places, Anodonta density approximately 1 m^–2 and about 50% of the Anodonta were infested with 10–30 Dreissena . In summer/autumn small dreissenids generally grew faster on live Anodonta than on stones. Similar trends were observed for spring, but differences of growth increments between dreissenids on live Anodonta and stones were usually not significant. Dreissenids settled down or moved towards the ingestion/egestion siphons of Anodonta and ingestion siphons of dreissenids were directed towards siphons of Anodonta . These results suggest that dreissenids can use the food provided by the filter current of the large Anodonta . (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Movements and burrowing activity in the Antarctic bivalve molluscs<Emphasis Type="Italic"> Laternula elliptica</Emphasis> and<Emphasis Type="Italic"> Yoldia eightsi</Emphasis>

Burrowing was investigated in two Antarctic infaunal bivalve molluscs, Laternula elliptica and Yoldia eightsi, representing amongst the least and most active members of the class Bivalvia in the Southern Ocean. Burrowing rate was expressed via the Burrowing Rate Index (BRI=[³wet weight/time to bury]×10⁴), and produced values of 0.1–10.6 for L. elliptica and 8.8–49.8 for Y. eightsi. These compare with values ranging from 3 to 2,000 for N. American bivalves (mean=222, SE=42.6, n=81), and 200 to 2,200 for Hong Kong bivalves (mean=1,140, SE=346, n=6). Values for the Antarctic species are, therefore, low compared to warmer-water bivalves, and the values below 1 for large L. elliptica are the lowest on record by around ×5. There is no compensation of burrowing activity for low temperature in these species. The relative BRI values for L. elliptica and Y. eightsi reflect the differences in their mode of life, with the former being large, sedentary and suspension-feeding, and the latter being smaller, mobile, ploughing through the sediment and feeding on sediment-surface organic matter. Burrowing in L. elliptica is unexpected, because other members of the Laternulidae do not burrow. This ability is most probably a response to the regular disturbance of sediments in Antarctica by ice, and the strong selective advantage to being able to resume a protected position after disturbance. The burrowing cycle in L. elliptica is composed of three main phases: (1) foot extension and sediment penetration; (2) foot dilation to form an anchor; (3) the drawing down of the shell by contraction of the pedal retractor muscles. Burrowing in Y. eightsi also has three phases: (1) foot extension and penetration of the sediment (digging); (2) rocking movements in the upright position; (3) shell anchorage. In excess of burrowing activity, L. elliptica exhibits a unique suite of movements when exposed at the surface. These comprise levering, where the tips of the siphons are pressed against the sediment to lift the shell from the substratum, looping, where the siphons are extended and rotated and, in the process, translocate the whole animal across the sediment, and jetting, where water is ejected forcibly through the siphons while their tips are directed towards the sediment, lifting part or all of the animal clear of the substratum. In the field, following exhumation by icebergs, these activities serve to place the animal in a favourable position for reburial, which is a clear advantage in disturbed polar environments where predatory nemerteans and asteroids are abundant.     

A Novel Filtering Mutualism between a Sponge Host and Its Endosymbiotic Bivalves

Sponges, porous filter-feeding organisms consisting of vast canal systems, provide unique substrates for diverse symbiotic organisms. The Spongia (Spongia) sp. massive sponge is obligately inhabited by the host-specific endosymbiotic bivalve Vulsella vulsella, which benefits from this symbiosis by receiving protection from predators. However, whether the host sponge gains any benefit from this association is unclear. Considering that the bivalves exhale filtered water into the sponge body rather than the ambient environment, the sponge is hypothesized to utilize water exhaled by the bivalves to circulate water around its body more efficiently. We tested this hypothesis by observing the sponge aquiferous structure and comparing the pumping rates of sponges and bivalves. Observations of water currents and the sponge aquiferous structure revealed that the sponge had a unique canal system enabling it to inhale water exhaled from bivalves, indicating that the host sponge adapted morphologically to receive water from the bivalves. In addition, the volume of water circulating in the sponge body was dramatically increased by the water exhaled from bivalves. Therefore, this sponge-bivalve association can be regarded as a novel mutualism in which two filter-feeding symbionts promote mutual filtering rates. This symbiotic association should be called a “filtering mutualism”.     

Recent and historical discharge of a large European river system - oxygen isotopic composition of river water and skeletal aragonite of Unionidae in the Rhine

Seasonal variations in the oxygen isotopic composition of Rhine River water were analyzed in detail and compared with the oxygen isotopic record from recent and historical specimens of freshwater bivalves (Unionidae). The purpose of this study was to investigate the potential use of these aragonitic bivalves as proxy recorders for varying amounts and sources of discharge, and thereby infer climate change. Seasonal variations in the river water δ¹⁸O are on the order of 1-2‰. During summer, Alpine melt-water contributes significantly to the total discharge, resulting in average values of −10 to −10.5‰, whereas the non-Alpine contribution is higher during winter, as indicated by mean δ¹⁸O values of −8.5 to −9‰. The basic pattern of the modern seasonal variation of δ¹⁸O of river water can be described by a numerical mass balance approximation of the various contributions from the Alpine and non-Alpine catchments with their average δ¹⁸O composition. The δ¹⁸O of growth increments in the prismatic shell layer of Anodonta corresponds perfectly to what is predicted by known fractionation of ¹⁸O between water and aragonite. Shell growth is restricted to water temperatures above 8-10°C, so variations in δ¹⁸O and the river water temperature are faithfully recorded by relatively large growth increments during summer. The distinctive isotopic signatures of individual flood events during summer and autumn are also recorded in the shells.     

Morphological and Genetic Diversity of the Wood-Boring Xylophaga (Mollusca,Bivalvia): New Species and Records from Deep-Sea Iberian Canyons

Deep-sea bivalves of the Xylophagaidae, a poorly known group, are obligate wood-borers. Deployment of wood in three submarine canyons off the Iberian coast, the Blanes and La Fonera Canyons (Mediterranean Sea) and the Avilés Canyon (Cantabric Sea, Bay of Biscay), lead to the discovery of four xylophagaid species in our samples. Xylophaga dorsalis (the dominant species), X. atlantica, X. cf. anselli and the new species X. brava, were identified on the basis of morphological data, and supported by a phylogenetic reconstruction based on the nuclear genes 18S rDNA and 28S rDNA and including several genus of Xylophagaidae. Genetic divergence between species of Xylophaga varied between genes, ranging from 0.5 to 4.0% for the 18SrDNA and from 4.1 to 16.6% for the 28SrDNA. Xylophaga brava sp. nov. appeared to be restricted to the Mediterranean and morphologically resembled the closely related X. cf. anselli from the Cantabrian Sea. However, they clearly diverged in two well-supported clades. Low levels of intraspecific variability and higher interspecific divergence between species also supported the existence of these two different species. Morphologically they differ in the number of cirri at the siphon openings, in the shape of the posterior shell and in the size of prodissoconch II. The new species is characterized by having weak, poorly mineralized mesoplax and siphons united throughout, covered by a periostracal, non-calcified tube; distinct proximal and distal siphons, the former translucent and soft, the latter muscular, with concentric rings. Xylophaga atlantica, previously known only from the western Atlantic, is reported for the first time in the Mediterranean Sea. Whether its presence in the Mediterranean indicates its natural distribution or reflects its recent introduction is unknown. Although xylophagaids have been previously reported to recruit heavily to wood deposited on the seabed, these four species colonized wood suspended 30 m above the seafloor.     

Mate competition during pseudocopulation in shipworms

Shipworms are predominantly wood-eating bivalves that play fundamental roles in biodegradation, niche creation and nutrient cycling across a range of marine ecosystems. Shipworms remain confined to the wood they colonize as larvae; however, continual feeding and rapid growth to large sizes degrade both food source and habitat. This unique lifestyle has led to the evolution of a stunning diversity of reproductive strategies, from broadcast spawning to spermcasting, larval brooding and extreme sexual size dimorphism with male dwarfism. Some species also engage in pseudocopulation, a form of direct fertilization where groups of neighbouring individuals simultaneously inseminate one another via their siphons—the only part of the animal extending beyond the burrow. Among the Bivalvia, this exceptionally rare behaviour is unique to shipworms and remains infrequently observed and poorly understood. Herein, we document pseudocopulation with video footage in the giant feathery shipworm (Bankia setacea) and novel competitive behaviours, including siphon wrestling, mate guarding and the removal of a rival''s spermatozoa from the siphons of a recipient. As successful sperm transfer is likely greater for larger individuals with longer siphons, we suggest that these competitive behaviours are a factor selecting for rapid growth and large size in species that engage in pseudocopulation.     

Carbon and nitrogen stable isotope ratios of macroinvertebrates in the littoral zone of Lake Biwa as indicators of anthropogenic activities in the watershed

Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) of macroinvertebrates inhabiting littoral zones of lakes can serve as useful indicators of material loading from the watershed. We collected snails (Semisulcospira spp.) and bivalves (Unio douglasiae biwae Kobelt) from 29 littoral sites in Lake Biwa near the mouths of river tributaries with various human population density (HPD) and land-use patterns. The δ¹³C and δ¹⁵N signatures were determined for three potential food sources: particulate organic matter in the pelagic zone (PPOM), riverine particulate organic matter from tributaries (RPOM) and epilithic organic matter in the littoral zone (EOM). The stable isotope mixing model revealed that snails relied mainly on EOM, and bivalves on PPOM and RPOM. Multiple regression analysis showed that intersite variation in δ¹⁵N for snails was best explained by HPD, while variation in δ¹⁵N of EOM and nitrate was explained to a lesser extent by HPD. Comparison with isotope signatures of their food sources and riverine nutrients revealed that snails assimilated anthropogenic nitrogen from wastewater in the watershed. Our results also showed that the δ¹³C value of bivalves was marginally related to the fraction of paddy fields in the watersheds. In conclusion, the isotope signatures of macroinvertebrates inhabiting the littoral zone can be useful indicators of anthropogenic impacts from the watershed.     

Plastron respiration in bugs and beetles

In the naucorid bug, Alphelocheirus, the plastron hairs are twice as thick and nearly twice as dense (ca. 4 × 10⁶/mm²) as they had been thought to be by previous workers. From experiments and calculations it seems clear that when the plastron is subjected to excess pressures it is wetted long before there is any question of the collapse of the hair pile itself. The plastron of Aphelocheirus is thus like the plastrons of other insects in that it is wetted long before the structures supporting the air film collapse.A plastron has been independently evolved in at least five subfamilies of the Naucoridae. A plastron is here recorded for the first time in bugs of the family Helotrephidae.It has been claimed that the plastron-bearing elmid beetles are unable to fly. Many, if not most, of these beetles fly after they emerge from their pupal cells. However, once they have begun to live under water they cease to be able to fly: the flight muscles degenerate, and this degeneration seems to be irreversible.The structure of the plastron scales of several kinds of weevils is described. The resistance of the plastron of the rice water weevil, Lissorhoptrus, to wetting at excess pressures is examined. An explanation is advanced to account for the fact that weevils and other plastron-bearing beetles that live in still waters can often swim whereas those, like elmids and dryopids, that live in running waters cannot swim.     

Stable isotope records (O, C) of Jurassic aragonitic shells from England and NW Poland: palaeoecologic and environmental implications

Shells of fully marine Middle to Upper Jurassic molluscs from England and north-western Poland were analysed with respect to their stable isotope (δ¹⁸O, δ¹³C) compositions, and palaeoecological and environmental life conditions of these molluscs were inferred from them. Light microscopical and SEM inspection and an analysis of the minor element content (Fe, Mn, Mg, Sr) suggest rather unaltered isotope signals. The δ¹⁸O and δ¹³C values show a characteristic distribution among three groups of co-occurring organisms. Benthic (adult) bivalves generally preserved higher δ¹⁸O and δ¹³C values than ammonites, whereas planktic bivalve larvae tend to possess the lowest δ¹⁸O but higher δ¹³C than adult bivalves. As this distribution pattern is found in numerous horizons and sections of Bathonian to Kimmeridgian age in NW Poland and England, it is thought to reflect real palaeoenvironmental parameters. All observations can be incorporated in a single model that assumes (i) seasonally induced temperature stratification of the water column, (ii) a correlation between phytoplankton blooms and reproduction season of planktic-planktotrophic bivalves, and (iii) insignificant vital effects with respect to the δ¹³C in bivalves, but strong biological control in ammonites. In addition, the δ¹⁸O evolution suggests that the Late Bajocian to Middle/Late Bathonian and Early Oxfordian to Late Kimmeridgian were considerably warmer than the latest Bathonian to Late Callovian time interval. The oxygen isotopic records from other European regions indicate a similar pattern of long-term palaeotemperature evolution. The comparatively high water temperatures during the Callovian to Oxfordian of the Isle of Skye (NW Scotland) are enigmatic, however. In the Early Oxfordian, sea surface and bottom temperatures began to rise in continental Europe and England. These changes coincide with a south-westward drift of the West European crustal plate, but a causal relationship remains to be demonstrated.     

Multi-species okadaic acid contamination and human poisoning during a massive bloom of Dinophysis acuminata complex in southern Brazil

On June 2016, a major bloom of Dinophysis acuminata complex was noticed over the coast of Paraná State (PR), southern Brazil, an area unprotected by any official monitoring program. Here we report the results of an extensive sampling effort that ultimately led PR authorities to issue the first State shellfish-harvesting ban due to multi-species okadaic acid (OA) contamination. During its peak, the bloom covered an area of 201 km² (∼2.0–3.5 × 54.0 km), attaining unprecedentedly high cell densities along the shallow (<15 m) continental shelf (mean 2.2 × 10⁵, maximum 2.1 × 10⁶ cells L⁻¹) and adjacent sandy beaches (mean 2.8 × 10⁵, maximum 5.2 × 10⁶ cells L⁻¹). Only OA was detected in suspension (max. 188 ng L⁻¹). Toxin levels measured in bivalves were several times greater than the regulatory limit of 160 ng g⁻¹, reaching up to 3600 ng g⁻¹ in Crassostrea gasar, by far the highest OA concentrations ever reported in oysters worldwide, 7700 ng g⁻¹ in brown mussels, Perna perna, and lower levels in clams, Anomalocardia brasiliana, and mangrove mussels, Mytella spp. Nine cases of human intoxication were officially reported and five people were hospitalized with typical symptoms of Diarrhetic Shellfish Poisoning linked to the consumption of contaminated bivalves. All bivalves quickly converted most of the OA into its esterified form, DTX-3, and eliminated the toxins only a few weeks following the bloom, with C. gasar being the slowest-detoxifying species. Lower OA levels were accumulated in zooplankton, gastropods and several novel toxin vectors, including benthic organisms such as sand dollars Mellita quinquiesperforata and the ghost-shrimp Callichirus major, which may act as a good indicator of the presence of toxins in sandy beaches, and pelagic fish species that can serve as potential alternative sources of OA to humans (Chaetodipterus faber and Mugil liza). Monitoring toxin contamination in seafood other than bivalves is thus recommended to ensure comprehensive human health protection during massive Dinophysis blooms. Additionally, since OA was also present at low concentrations in the liver of Guiana dolphins Sotalia guianensis and penguins Spheniscus magellanicus, exposure to biotoxins should be considered in conservation actions involving threatened and near-threatened marine organisms in this region.     

Prey-species selection by the anemone predator Aeolidiapapillosa (L.): The influence of ingestive conditioning and previous dietary history,and a test for switching behaviour

Interactions of two tubicolous, deposit-feeding polychaetes, Clymenella torquata (Leidy) and Polydora ligni Webster, with a filter-feeding bivalve, Gemma gemma (Totten), were examined to test the functional group hypothesis that small, filter-feeding bivalves with large young should attain high densities with tube-builders of any trophic mode because the large young escape predation by tube-builders. The prediction was not supported by Polydora and Gemma, since the two species have not been reported to coexist in high densities in nature. In laboratory experiments, Polydora reduced recruitment and adult survival of Gemma relative to a control. Polydora preyed on juvenile clams and reduced the time which adult clams had their siphons in the feeding position, relative to a control.For Clymenella and Gemma, the functional group hypothesis made the correct prediction (i.e. the two species are reported to coexist in high densities in nature), but for the wrong reason. The size of Gemma's offspring is not related to the intensity of Clymenella predation. Clymenella does not ingest Gemma since the worm does not usually feed from the zone occupied by the clam. In contrast to Polydora, Clymenella did not reduce the time which clams fully extend and open their siphons relative to a control. In laboratory experiments Clymenella increased adult bivalve growth, survival after one year, and recruit size after one month. Clam life history characteristics were similar in the Clymenella and Clymenella plus Polydora treatments because the number of surviving Polydora was reduced by a negative interaction with Clymenella.The tube-builders had different effects on the filter-feeder, Gemma, because these tube-building species differ in feeding position, diet and behavior. Thus, the functional group hypothesis was too general to predict and explain the species interactions and distributions.     

Spatial structure of a subtidal macrobenthic community in the Bay of Veys (western Bay of Seine, English Channel)

The spatial distribution of the muddy fine sand community from the Bay of Veys (western English Channel) were investigated during spring and autumn 1997. A grid of 55 and 54 sites was sampled in March and October, respectively, using two replicates per site of a Hamon grab (0.25 m²) for macrofauna collection and an additional one for sediment analysis. A total of 172 species were sampled with a dominance of polychaetes, followed by crustaceans and bivalves. The species richness and abundance show low temporal changes despite higher values in October than in March. In March, the mean abundance was 165 ind. 0.5 m⁻²; in October, the mean abundance was 212 ind. 0.5 m⁻². Four assemblages from the Abra alba-Pectinaria koreni community were identified corresponding to a bathymetric and sedimentary gradient from muddy fine sands with high levels of fine particles in shallow water to fine sands in deeper water. The discussion focuses on factors prevailing on the spatial structure of sandy communities in the English Channel.     

亚热带养殖海湾皱瘤海鞘生物沉积的现场研究

皱瘤海鞘是亚热带海域分布最广和数量最多的附着生物种类之一.2012年1月-7月在典型的亚热带养殖海湾-大亚湾大鹏澳海域,利用沉积物捕集器现场测定了皱瘤海鞘(Styela plicata)的生物沉积速率,并测定了生物沉积物中有机物(OM)、总碳(TC)、总氮(TN)、有机碳(OC)和有机氮(ON)含量.结果显示:皱瘤海鞘的生物沉积速率变化范围为每天145.5-1011.8m/个,平均每天516.0mg/个,海鞘的生物沉积速率变化范围为每天154.8-1065.8 mg/g干重,平均每大463.3 mg/g干重.海鞘生物沉积物中OM、TC、OC、TN和ON含量分别为14.38％、10.80％、2.87％、3.06％和0.86％,高于自然沉积物中的含量,分别为13.39％、7.36％、2.32％、2.29％和0.67％.其中TC和ON含量要显著高于自然沉积物(P＜0.05).皱瘤海鞘的OM、TC、OC、TN和ON的生物沉积速率分别为每天74.20,55.73,14.80,15.79和4.43 mg/个.实验期间附着在浮筏养殖设施和养殖牡蛎壳上的海鞘密度变化范围为54.9-222.1个/m2,平均147.5个/m2,养殖海域单位面积的海鞘生物沉积速率平均为每天76.1 g/m2,是自然沉积速率(平均每天62.7 g/m2)的1.21倍,其中OM、TC、OC、TN和ON的平均沉积速率分别为每天10.94,8.21,2.18,2.32和0.65 g/m2.据此可推算,大鹏澳筏式牡蛎养殖海区(约103 hm2)皱瘤海鞘的年生物沉积物负荷为29000 t,其中OM,TC,OC,TN和ON分别4100,3100,820,870和240 t.研究结果说明,海鞘等附着生物在大规模浅海贝类养殖中对养殖生态环境的影响也不容忽视.     

Alternative food sources of native and non-native bivalves in a subtropical eutrophic lake

The aim of this study was to evaluate the consumption of alternative food sources (water column and sediment) and the potential competition under different conditions between Diplodon parallelopipedon (native) and Corbicula fluminea (non-native). We evaluated filtration and ingestion rates of water column at 6 and 24 h, held with and without organic matter and different phytoplankton types: needle-shaped green algae (Ankistrodesmus sp.) and filamentous cyanobacteria (Planktothrix agardhii) dominated communities. Our results confirmed higher filtration and ingestion rates per biomass unit for C. fluminea in the presence of sediment without organic matter. However, when we compared the filtration of bivalves held in sediment with organic matter, D. parallelopipedon rates were not significantly different from C. fluminea values. Moreover, in the presence of filamentous cyanobacteria, only C. fluminea filtration and ingestion rates decreased significantly. Our experimental results and previous field evidence concur that C. fluminea were not able to outcompete the native bivalves in organic matter rich sediment and cyanobacteria blooms conditions. The differential responses to the eutrophication process between exotic and native bivalves, apparently favoring the later, might buffer the potential competition interactions allowing their coexistence.     

Can postlarval bivalves select sediment type during settlement? A field test with Macoma balthica (L.) and Cerastoderma edule (L.)

Many soft-bottom benthic invertebrates display sediment-associated patterns of dispersion. Habitat selection experiments have shown that the larvae of some species can choose to settle in favourable habitats, and this process could establish patterns observed in the field. However, many soft-bottom infauna, including the bivalves Cerastoderma edule and Macoma balthica, show postlarval relocation. Such movements could obviate patterns established at initial settlement, and active habitat selection in these postlarval stages may be responsible for adult distributions. We investigated sediment selection in postlarval stages of these bivalves on an intertidal mudflat using a field survey and experiment. Our 1225 m² grid survey showed a strong spatial correlation between the smallest individuals of both species, which showed no correlation with sediment characteristics. Larger size groups showed increasingly strong associations with sediment type: C. edule with sandy sediments and M. balthica with muddy. In our sediment choice experiment, the numbers of bivalves recruited into muddy, sandy and muddy depression (to detect small-scale hydrodynamic effects) treatments after 2 days in the field were recorded. There were no significant treatment effects in either C. edule or M. balthica. All bivalves were too large to be initial settlers; they were thus relocating individuals. Neither the survey nor the experiment provided any evidence that relocating individuals of these two species could choose the sediment in which they alight. The sediment-related patterns observed in adults of these species must therefore be due to other mechanisms, possibly postsettlement predation by Carcinus maenas and/or an ability to leave less-preferred sediments in a series of migrations until the preferred habitat is found.     

Behaviour controls post-settlement dispersal by the juvenile bivalves Austrovenus stutchburyi and Macomona liliana

Post-settlement dispersal is a key process affecting the population dynamics of many soft sediment benthic invertebrates. Despite its importance, few studies have quantified those factors that influence juvenile dispersal. In a laboratory flume, we examined the effects of three flow velocities and two substrate types on the dispersal of two common intertidal bivalves: the deposit-feeding wedge shell Macomona liliana and the suspension-feeding cockle Austrovenus stutchburyi. Juveniles of three size classes (<2, 2-4 and 4-8 mm shell length) were added to cores of defaunated natural sediment or glass beads. We recorded the number of bivalves remaining in cores versus those recovered downstream either on the flume floor, in bedload traps or in a plankton net at the end of the working section of the flume after 48 h at three freestream velocities (4.8, 11.0 and 16.6 cm s⁻¹). At flow speeds of 4.8 cm s⁻¹, <5% of individuals were recovered outside the cores for both species. At higher flows, the dispersal mode (crawling, bedload or in suspension) and frequency of dispersal differed markedly between species. Austrovenus dispersed primarily by crawling in the low flow treatments. The frequency of dispersal increased substantially (2-6×) between 11.0 and 16.6 cm s⁻¹, and most Austrovenus were found in bedload traps at the highest flow. At the highest flow, twice as many Austrovenus individuals left the glass bead treatment as from the natural sediment. The number of dispersing Macomona also increased with increasing flow from the natural sediment, but numbers dispersing from glass beads were similar for the two higher flows (11.0 and 16.6 cm s⁻¹). Macomona dispersal mode was associated with size; smaller size classes were collected in the bedload traps and the plankton net in approximately equal proportions, while only a small proportion of the largest size class were collected in the plankton net. In contrast to flume dispersal experiments with live bivalves, most dead Austrovenus did not move at the highest flow speed, while most dead Macomona were transported at the highest flow speed. The live-dead comparisons, in conjunction with our primary experimental results, imply that there are active behavioural components to both water column and bedload transport. Our research emphasises that both species actively influence post-settlement transport in response to both substrate type and flow regime, and that bedload transport, often categorised as a passive transport process, is also greatly influenced by active behavioural processes.     

Late quaternary variations in oxygen and carbon isotopic compositions in Canadian Arctic marine bivalves

The oxygen and carbon isotopic composition of arctic marine bivalves Mya truncata, Hiatella arctica, and Mytilus edulis are reported on samples from raised marine deposits in Hudson Bay and eastern Baffin Island. The shells range in age from modern, through the Holocene, to “old” marine units. During the Holocene the ¹⁸O/¹⁶O ratio in shells rose to a maximum about 3,500 B.P. which coincides in time with the period of maximum growth rates of bivalves, maximum size and maximum faunal diversity. The change is interpreted to indicate that about 3,500 years ago arctic waters may have reached a salinity ≈ 1–2% greater than present. Comparison of Holocene shell-carbonate isotopic compositions with those from the “old” marine shells (that are characteristically extremely thick) suggests that during the early Wisconsin advance on eastern Baffin Island, surface and near-surface waters were more saline than at present. This may have been related to low meltwater discharge. Paradoxically, positive values of ¹⁸O/¹⁶O and ¹³O/¹²C in marine shells occurred during the Holocene marine optimum and during the early Wisconsin ice advance.     

Distribution and some biological features of Lycodes raridens (Zoarcidae) in the western part of the Bering Sea

Based on a bottom trawl survey, it is shown that in Karaginskii and Olyutorskii bays of the western part of the Bering Sea, Lycodes raridens is a typically elittoral species, since it was recorded at depths of 52–120 m, and the majority of individuals of this species were found in the range of depths 50–100 m. The maximum concentrations of Lycodes raridens were 139 ind./km² or 98 kg/km², whereas the average values of the distribution density of this species were 7.8 ind.km² or 5.4 kg/km². In Karaginskii and Olyutorskii bays, Lycodes raridens is represented by individuals with a length of 16–86 cm, weight of 34–4510 g, at the age of 1+ to 7+, although fish with a length of 30–50 cm, weight of 100–1000 g, and age of 3+–4+ dominate. The data obtained suggest that in the autumn or autumn-winter period, the study species spawns in the western part of the Bering Sea. The dominant food of Lycodes raridens here are amphipods, polychaetes, bivalves, and decapods; by occurrence and the relative content in the stomach (% of weight), amphipods (as a rule, Melita spp.) and bivalves (Yoldia spp. and Macoma sp.) dominate. Lycodes raridens is a benthophage in which, at the early stage of the life cycle, gammarids prevail in food; with growth, its diet becomes more diverse due to bivalves, polychaetes, and decapods.