Increasing Shell Length Research Articles

Predation on Bivalves on a Sandy Beach in Soma, Japan

Predation may be an important cause of early postsettlement mortality of bivalves, but little is known of in situ predation processes on bivalves by multiple predators on sandy beaches. To address this issue, bivalve shells in the guts of demersal fishes and benthic invertebrates were counted and measured monthly from April to November 2009 in Soma, Japan. For the Sakhalin surf clams Pseudocardium sachalinense and Raeta pulchella, the frequency of occurrence and number of shells in guts of predators per transect peaked around May–June, when the length of shells in guts tended to be smaller than in later months. These two indices decreased to 0 in July for P. sachalinense and to a low level in August for R. pulchella, which was associated with increasing shell length (ShL) in guts. For the bivalve Mactra crossei, both occurrence and number of shells in guts of predators per transect increased around November in some of the predators. The largest ShL of P. sachalinense in guts tended to be similar among the predators and was 1.1–1.5 mm. The largest ShL of R. pulchella and M. crossei tended to vary among the predators, and were 5.6–17.8 mm and 1.0–7.2 mm, respectively. The spiny sand sea star Luidia quinaria and red tongue sole Cynoglossus joyneri had a high frequency of occurrence and number of shells per gut per transect. These results indicate that predation on P. sachalinense and R. pulchella was most intense in the early postsettlement period of May–June in Soma, Japan. The size of the bivalve shell may have provided protection, but was dependent upon on the predator.

Fecundity of the invasive marine gastropodCrepidula fornicatanear the current northern extreme of its range

AbstractThe calyptraeid gastropodCrepidula fornicatais native to the eastern coast of the United States but has now become an extremely successful invader along much of the European coastline. As the northern limit of its spread is thought to be determined by an inability of adults to tolerate prolonged exposure to low winter temperatures, this study sought to compare the fecundity of females collected from two sites along the Norwegian coastline with that of females collected from Rhode Island, USA. Few other studies have compared the fecundities of marine invertebrates from invasive populations with those found in native populations. For both populations studied, fecundities increased with increasing shell length. However, contrary to expectations, size‐related fecundities were significantly higher for Norwegian females than for Rhode Island females, with Norwegian females producing larger egg capsules and a greater number of embryos per capsule, but not a greater number of egg capsules per brood. Current evidence suggests that at the northern extreme of its invaded range, the fecundity ofC. fornicatais increased rather than compromised.

Temporal trends in condition and reproduction of quagga mussels (Dreissena rostriformis bugensis) in southern Lake Michigan

Currently little is known about the condition and spawning patterns of quagga mussels (Dreissena rostriformis bugensis) in deeper, consistently cold regions of the Great Lakes. This lack of information limits our ability to predict the future expansion of D. r. bugensis in the offshore regions of the Great Lakes and other large, deep lakes. We collected quagga mussels on a monthly basis in 2013 (April–September) at three established sites along a depth transect (25, 45, and 93 m) in southern Lake Michigan and calculated a condition index (CI), a ratio of dry soft tissue weight to internal shell capacity, to assess condition, and a gametogenic index to assess reproductive activity. We also measured size–frequency, density, and biomass in March, July, and October. Condition was consistently highest at 25 m, population biomass and average shell length were highest at 45 m, and density was highest at 93 m. At all three depths, CI decreased with increasing shell length suggesting that food availability may be limiting for larger individuals. Mussels at 45 m spawned earliest with over 50% spent by July while mussels at 93 m began spawning in August. Mussels at 25 m had not yet spawned by September. Long-term trends in density indicate that D. r. bugensis populations continued to expand at the 93 m site but may be stabilizing at 25 m and 45 m. Consistent with density trends, CI at 25 m and 45 m was lower than in 2004 and 2008.

Effect of Body Size on Accumulation of Po<sup>210 </sup>in Mussel, <i>Perna viridis</i>, at Samut Sakhon Province

Levels of 210Po in mussel (Perna viridis) collected from green mussel farming area at the Samut Sakhon province during the period of 2012 are presented. Parameters including total tissues wet weight and dry weight, maximum length, and the physiological performance of mussels using Condition Index were measured. The type of fluctuations observed with 210Po concentrations are interpreted as a seasonal effect which peaks of high concentrations in wet season and low concentrations in dry season. Analysis of 210Po activities in mussel (activity per individual) revealed the lower activities in smaller sized mussels (2.0-2.5 cm) and the higher in the larger sized group (6.5-7.0 cm). In the contrary, 210Po concentrations decreased with increasing shell length, or with increasing body size. The 210Po body burdens (activity per mussel) varied from 2.413 to 17.183 mBq while the 210Po concentrations were ranged from 2.511 to 4.474 Bq.Kg-1 ww. The results indicated that larger mussels were transfer lower 210Po concentrations to consumer on a weight basis. It is concluded that the apparent seasonal fluctuation of radionuclide concentration were mostly caused by mussel body size fluctuation and not by radionuclide body burden fluctuation.

Population dynamics of Megapitaria squalida (Bivalvia: Veneridae) at Magdalena Bay, Baja California Sur, Mexico

The population dynamics of an intertidal population of Megapitaria squalida was studied from September 2002 to February 2003 in Bahia Magdalena, Mexico. To obtain information about the artisanal and recreational fishery of M. squalida, local fishermen at different parts of the bay were interviewed. Clam densities were determined at one unexploited and two exploited sites along perpendicular transects from the shore line. Average clam density was 2.01+/-1.41 ind/m2, densities did not differ significantly between the three sampling sites. The length-total mass relation was Mtot = 0.0001*SL3.1644 (r2 = 0.9804, n = 92) and the length-tissue mass relation was Mtis = 0.0002*SL2.7144 (r2 = 0.945, n = 92). Von Bertalanffy-growth parameters were determined using marked individuals. Strong interindividual and also seasonal differences in growth were observed. Average annual values were: K = 0.655, Linfinity = 83 mm. Since M. squalida can reach a shell length of more than 135 mm in deeper waters, it is necessary to consider intertidal and subtidal populations separately. Average total mortality (Z) was 1.61 y(-1). No fishing mortality was calculated because total mortality Z at the unexploited control site was higher than Z at the exploited sites. It appears that M. squalida migrates into deeper water with increasing shell length, as average shell length increased at lower tidal levels and marked specimens showed a clear migration towards deeper waters. As yet no overexploitation could be found in the study area.

The symbiosis between the gastropod Strombus gigas and the dinoflagellate Symbiodinium: An ontogenic journey from mutualism to parasitism

Dinoflagellates in the genus Symbiodinium form mutualistic symbioses not only with corals and other cnidarians but also with sponges, bivalves, and gastropods, including the commercially important queen conch, Strombus gigas (Linnaeus 1758). The eggs of S. gigas are aposymbiotic, but acquisition of Symbiodinium at the larval stage significantly enhances survival and growth, demonstrating the mutualistic nature of this symbiosis at the larval stage. The objective of the present study was to investigate the symbiotic association between adult S. gigas and Symbiodinium. S. gigas were collected along the Mexican Caribbean coast. Symbiodinium did not occur in the muscle tissue or gonads, the latter supporting the lack of symbionts in S. gigas' eggs. Symbiodinium cells were morphologically intact in all of the anatomical structures in which they were found that included, in order of abundance, the digestive gland and other parts of the digestive tract, gills, in connective tissue surrounding the mantle and foot, as well as in the nephridium and proboscis. The symbionts underwent mitosis in the mantle, stomach and digestive gland. The healthy condition of the Symbiodinium, evident in their morphology and mitotic index, was surprising given the low light habitat in which they were found in adult S. gigas. The shell of S. gigas is a strong light attenuator; a maximum of 21% of incident radiation passed through the conch shell and decreased exponentially with increasing shell length to a low of 0.1%. The maximal photochemical efficiencies (Fv/Fm) of the symbionts were low, suggesting that their photosynthetic activity would be suboptimal in the reduced light environment in which they are found. The large population of Symbiodinium may be maintained by the symbionts acquiring nutrients heterotrophically from the host. Consequently, in S. gigas, a mutualistic symbiont at the larval life stage may confer a net cost at the adult life stage.

Imperfect Recapture: A Potential Source of Bias in Freshwater Mussel Studies

Assessing the population status and dynamics of species is an important component of monitoring efforts aimed at improving understanding of relationships between freshwater mussels (Bivalvia:Unionidae) and their environment. Most freshwater mussel population assessments are conducted using raw count (density, abundance) or presence/absence data but relatively few studies account for potential biases associated with incomplete detection of individuals or species during sampling. We conducted a capture-mark-recapture study over 7 y to assess survival and recapture probabilities of three federally endangered freshwater mussels in a small southeastern U.S. stream. Although similar numbers of mussels were collected among sampling occasions, only a small proportion of the individuals present within the sampling site were collected on any given occasion. Mean apparent survival was 0.81 and did not vary among species or among years. Modeling results indicated that recapture probabilities varied among species, through time, and among individuals as a function of shell length. Recapture probabilities for Pleurobema pyriforme and Medionidus penicillatus increased with increased shell length, whereas recapture probabilities for Hamiota subangulata decreased with increasing shell length. Under the generally accepted assumption of constant recapture among sampling occasions, apparent survival estimates ranged from 0.89 to 0.99 among the three species and decreased for H. subangulata with increased shell length. Our study demonstrates that failure to account for incomplete recapture of freshwater mussels could bias inferences regarding the status and trajectory of mussel populations, which may in turn result in the implementation of ineffective management and conservation strategies.

Variations in eastern oyster (Crassostrea virginica) sex-ratios from three Virginia estuaries: protandry, growth and demographics

Oyster population reproductive capacity and dynamics are controlled at the most basic level by the observed sex-ratios. Since oysters are sequential, protandric hermaphrodites the population sex-ratio is related to the demographics (shell length, age, and biomass). Oysters were collected from June through to August 2008 at twelve bars in the James, Rappahannock and Great Wicomico Rivers, Virginia, USA. Bars were aggregated into five groups on the basis of similar age–length relationships. Sex-ratios (fraction female), age–length, and biomass–length relationships were determined for each group. The fraction female increased within increasing shell length, age, and biomass at all sites. Simultaneous hermaphrodites were rarely observed. Group specific differences in shell length (SL, mm) and age (yr) for the timing of the protandric shift were observed with the earliest shift from male to female occurring at ~60 mm SL and ~1.6 yr. The proportion of females observed in the larger or older individuals was at least 70–80%. Sex-ratios from summer 2008 were used to develop sex–length, sex–age, and sex–biomass keys that were applied to autumn-survey data from 2006, 2007, 2008 and 2009. In these years, sex-ratios by shell length and age were strongly biased towards males while the sex-ratio by biomass was strongly biased towards females. Disease mortality compounds natural and fishing mortality resulting in age/size specific cropping yielding truncated population demographics and an earlier protandric shift in populations on the extremes of the range examined. Regardless of location, market (>76 mm SL) oysters are predominantly female.

Calcein and manganese experiments for marking the shell of the common cockle (Cerastoderma edule): tidal rhythm validation of increments formation

This work focuses on investigating the potential of calcein and manganese as growth markers of the common cockle (Cerastoderma edule) in the bay of Somme (France). Recapture of shells, previously marked using a chemical marking and then bred in natural conditions, was performed in order to determine the shell growth patterns. Calcein marking has shown a fluorescent increment in shells after only 30 min immersion time at 150 mg L-1, but also for shells immersed 3 h at 50 mg L-1. Likewise, manganese shell marking was revealed under cathodoluminescence for shells immersed 1 h at 120 mg L-1 as well as for shells which spent 4 h at 90 mg L-1. A numerical analysis performed on each marked cockles has revealed 23 micro-increments between the mark and the ventral edge of the valves, corresponding to the 23 tides that occurred during the 12 days at liberty post marking. The periodicity of increment formation is thus validated for a tidal frequency. The growth rates of C. edule, ranged from 11.67 to 19.94 μm d-1, decreased significantly with increasing shell length. This preliminary study gives a clue to the understanding of cockle growth and could be used in shellfish production for cockle age monitoring, but also for chemical analysis to learn more about biomineralization process of this species.

Size‐specific growth patterns and estimated longevity of the unionid mussel (Pronodularia japanensis)

AbstractDespite the dwindling populations and an urgent need for conservation of unionid freshwater mussels in Japan, there are gaps in our understating of their fundamental ecology. This study examined size‐dependent annual growth rates, elucidated size‐specific intra‐annual growth patterns, and estimated age and longevity of P. japanensis individuals for two locally isolated populations in agricultural drainage channels. Annual growth rates of P. japanensis were strongly size‐dependent, with growth rates being exponentially decelerated with increasing shell length. Irrespective of sizes, individuals ceased to grow in winter when water temperature fell below 10°C. Intra‐annual growth patterns were weakly explained by the changes in water temperature and differed among size classes; juveniles (<25 mm) maximized growth rate in May whereas the growth rates were the highest in June or July for larger individuals. Only adult individuals exhibited growth cessation in the July–August period, suggesting that energy investment was directed towards reproductive activities. Adults also showed negative growth rates (shrinkage of individuals) in winter, suggesting possible dissolution of shell margins. Age estimation based on two 1‐year periods suggests that large numbers of P. japanensis individuals were >10 years old, and the oldest individuals were >20 years old for both study populations. Our findings suggest that anthropogenic activities conducted in spring may have strong influences on juveniles and population dynamics of P. japanensis and underscore the need for accurately determining age and longevity of remaining populations of unionid mussels.

Can bivalve veligers escape feeding currents of adult bivalves?

While the stock of introduced Pacific oysters ( Crassostrea gigas) increased in the Oosterschelde estuary (SW Netherlands), so did the filtration pressure of all bivalve species together. In the same period, stocks of native bivalves declined slightly. The expansion of Pacific oysters in Dutch estuaries might be partially due to better abilities of their larvae to avoid or escape filtration, compared to larvae of native bivalves. In this context, escape and swimming abilities of Pacific oyster larvae and the larvae of the native blue mussel ( Mytilus edulis) were compared. Swimming behaviour of C. gigas larvae and larvae of M. edulis was recorded in still water and in a suction current mimicking a bivalve feeding current, in a horizontal and in a vertical plane. Larval swimming behaviour in a suction flow field was reconstructed by subtracting local water movement vectors from the total movement of larvae, yielding movement paths due to larval swimming alone. Swimming speeds and the rate of displacement in vertical direction of C. gigas and M. edulis larvae were related to larval shell length, and to the pitch of up- or downward swimming. Larvae of both species did not show escape reactions in a suction flow field. With increasing shell length, larval swimming speeds of both species increased significantly. Swimming speeds of C. gigas larvae were significantly higher than swimming speeds of M. edulis larvae, resulting in a faster vertical displacement. The ability to migrate to more favourable water layers faster may offer C. gigas an advantage over native bivalves with slower swimming larvae.

Burrowing behaviour of the softshell clam ( Mya arenaria) following erosion and transport

Erosion and transport of juvenile individuals may alter the distribution pattern of intertidal bivalves. The burrowing success of recently transported juvenile softshell clams ( Mya arenaria) was studied in a laboratory flume under a wide range of hydrosedimentary environments. Juvenile individuals (5–20 mm) were observed under a simulated 30 min slack tide before initiating the flow for a period of 60 min. Five different free-stream velocities (0, 3, 5, 10 and 24 cm s − 1 ) and four sediment types (mud, sandy-mud, sand and gravel) were used. The mean proportion of juvenile clams that initiated (MPI) or completed (MPC) a burial decreased with increasing shell length. Erosion from the sediment was more important in large juveniles suggesting that large juveniles may have more difficulty successfully relocating once transported. The MPI increased with increasing flow speed in experimental runs held at speed < 24 cm s − 1 . This was observed in all sediment types. Most individuals were unable to burrow at 24 cm s − 1 because they got eroded. The MPC also increased with increasing flow speed in mud, sandy-mud and sand. The MPC's response to flow was more complex in gravel because of a shell length × flow speed interaction effect. Our observations suggest that water movement may induce the burrowing behaviour of recently eroded juvenile clams. Results are discussed in an ecological and aquacultural context.

Genetic variation among life-history stages of mussels in a Mytilus edulis?M . trossulus hybrid zone

The mussel species Mytilus edulis L. and M. trossulus Gould coexist and hybridize throughout a large area that includes the north coast of Maine and Atlantic Canada. Previous studies provided genetic evidence for limited hybridization between the two species for mussels >15 mm. The present study used two genetic markers (ITS, Glu-5′) to examine the genetic composition of early life-history stages by sampling veliger and pediveliger larvae, juveniles ( 15 mm shell length) in Trinity Bay, Newfoundland, during three consecutive years (1995–1997) to determine if differential mortality limits the relative abundance of hybrids. The relative frequency of the two species and the different hybrid genotypes was similar among the larvae, juveniles and small adult mussels. The double hybrid genotype (F1-like) was the rarest genotype observed. There was no evidence for differential mortality during the early life-history stages, and factors limiting production of hybrids appear to operate before the late larval stage. The observed frequency of hybrids is probably due to a combination of pre- and postzygotic reproductive isolating mechanisms operating early in the mussel’s life history. M. trossulus dominated the early life-history stages, possibly due to a higher population density and a greater reproductive output than M. edulis. Differential mortality may explain the observed decrease in frequency of M. trossulus and increase in frequency of M. edulis with increasing shell length. A similar frequency of hybrid mussels from larvae to the size class of 55 mm shell length may indicate a rate of mortality intermediate between the two parental species. The M. edulis–M. trossulus hybrid zone appears to be maintained by reproductive isolating mechanisms limiting the production of hybrids and life-history differences that allow the two species to coexist.

Distribution of Mytilus edulis , M . galloprovincialis , and their hybrids in open-coast populations of mussels in southwestern England

The distribution of Mytilus edulis Linnaeus, M. galloprovincialis Lamarck, and their hybrids was examined in mussel populations in southwest England in 1996 and 1998. This is a region where both parental taxa and populations containing large numbers of hybrids co-occur yet a fine-scale mapping of the hybrid populations has not been conducted. In this study the geographic distribution of hybrid populations was determined in southwest England over 360 km of coast from Tintagel Castle in north Cornwall to Beer in south Devon. Sample localities were spaced from 5 to 20 km apart. Genotypes for individual mussels were determined using PCR to amplify a size polymorphism at the Glu-5′ locus that is completely differentiated between M. edulis and M. galloprovincialis. Hybrid populations, characterized by high frequencies of individuals with heterozygous genotypes and a pattern of decreasing frequency of M. edulis alleles with increasing shell length, were continuously distributed along 180 km of open coast in southwest England. This "hybrid patch" was bordered at one end by geographically extensive populations of pure M. edulis and at the other end by nearly pure populations of M. galloprovincialis. Strong natural selection in hybrid populations results in a decline in the frequency of M. edulis alleles with increasing size. Wave-exposure has previously been implicated as the agent producing this pattern of selection, but in the present study the relationship between allele frequency and body size was not correlated with variation in wave shock intensity among localities within the hybrid zone. The transition between hybrid populations and those containing pure populations of M. edulis or M. galloprovincialis is abrupt which suggests that coastal circulation patterns may provide strong barriers to larval dispersal which accounts for the position and maintenance of the hybrid zone.

Maximizing lifetime egg production in a Wadden Sea population of the tellinid bivalve Macoma balthica: a trade-off between immediate and future reproductive outputs

The number of eggs produced in the annual spawning season by an adult female Macoma balthica (L.) of Wadden Sea origin increases in a non-linear manner with increasing shell length and body weight. It depends primarily on the weight-at-length immediately before spawning. Lean individuals with a low body-mass index (BMI) of 5.6 mg cm -3 or less spawn no eggs. Gamete output increases to about one-third of soft-body weight (in the order of 100 000 eggs) at the maximal BMIs observed (~15 mg cm -3 ). Long-term monitoring (>20 yr) of densities of a Wadden Sea M. balth- ica population revealed that survival to the spawning season of the next year declined from about 65% at higher-than-average values of BMI (>8 mg cm -3 ) to about 30% at the lowest values of BMI observed (5 to 6 mg cm -3 ). Annual growth was also significantly lower in years starting with a low BMI value. Because spawning lowers body weight and thus the BMI, spawning is bound to reduce subsequent survival rates and body-weight increments. Thus, a trade-off situation exists between immediate and future spawning outputs. The consequences for total lifetime egg production of vari- ous levels of gamete output at first spawning (at an age of 2 yr) were estimated by model calculations, starting from a wide range of body weights at a standard length. The model results realistically pre- dict that maximal lifetime egg production can be achieved only if M. balthica refrain from spawning at BMIs below a certain threshold value (viz. ~6 mg cm -3 , which is close to their actual threshold of 5.6 mg cm -3 ) and if their spawning output increases with weight-at-length (again in a manner close to what actually happens). It is concluded that M. balthica populations in the Wadden Sea apply a spawning strategy adapted to the local conditions.

Resource Allocation in the Intertidal Salt-Marsh Mussel Geukensia demissa in Relation to Shore Level

Ribbed mussel populations at two levels of the intertidal shore were studied for 3 yr at a salt marsh site in Jamaica Bay (New York). Length-specific dry body weight was determined monthly and shell growth was monitored between April and November of each year. Production (Pi) was measured as monthly changes in body dry weight. Annual reproductive output (PR) was estimated indirectly for mussels between 10 mm and 80 mm by determining the gain in body weight prior to spawning (Weight Gained Method). Using a second method (Basal Weight Method), monthly rates of somatic production (Pg) were determined by assuming that, as mussels grow in length, a fraction of monthly Pt is allocated to increasing body weight. The monthly production allocated to growth (Pg) was estimated based on the length-specific body weight (basal weight) in March, when body weights in these populations begin to increase due to gametogenesis. Monthly rates of reproductive output (Pr) were then estimated be substraction as the difference between monthly Pt and Pg. Total PR determined by both methods were very similar. Shell and body growth were concurrent in sexually immature mussels, but body weight preceeded shell growth in older, larger animals. Annual shell increments were correlated with annual PG. In sexually mature mussels, Pr preceeds Pg in larger mussels, that is, allocations of net production to reproduction and somatic growth diverge in time with increasing shell length. Between April and November, shell growth rates were correlated with water temperature. At the higher shore level, mussels allocated a much smaller proportion of total production to growth, exceeding the reproductive effort of the lower-shore population. Variation in mussel production between years probably is related to variability in nannoplankton biomass. Although mussels higher on the shore can reduce somatic growth in order to increase fecundity and reproductive effort, total reproductive output is much reduced as compared to the marsh edge.

COMPARISONS OF THE BIOLOGY OF THE INTERTIDAL SUBANTARCTIC LIMPETS NACELLA CONCINNA AND KERGUELENELLA LATERALIS

Two limpet species occur intertidally on subantarctic South Georgia, the patellid Nacella concinna and the siphonanid Kerguelenella lateralis. N. concinna is confined to the lower shore close to LWS; K. lateralis occurs in middle shore pools, so their distributions do not overlap. N. concinna has a much narrower thermal niche (-12.9°C to +15.6°C) than K. lateralis (-17.8°C to +31.8°C). Environmental data are presented to show that the upper lethal temperature of N. concinna is low enough to prevent the limpet living higher on the shore. Both limpet species are slow-moving, but K. lateralis shows increasing speed with rising temperature, peaking at 15-20°C. In contrast, N. concinna moves actively down to — 1.9°C (when sea water freezes), but there is a steady decrease in speed of locomotion above +2°C. Locomotion ceases at 14°C in N. concinna (c.f. 30°C in K. lateralis). Both species exhibit very low tenacities, but in N. concinna tenacity decreases with increasing shell length. In K. lateralis there is no effect of temperature on tenacity. Both species show a positive allometric relationship between foot area and shell length. N. concinna feeds upon microbial films and microepiflora, but K. lateralis eats colonial diatoms and Enteromorpha bulbosa. Observations on shell middens of the kelp gull Larus dominicanus showed that the gulls did not eat K. lateralis, though they ate great quantities of the less accessible N. concinna. Gulls ate N. concinna as small as 11 mm shell length (within the size range of K. lateralis). Experiments on gulls demonstrated an unwillingness to eat K. lateralis, probably because the siphonariid extrudes a viscid white mucus when the foot is touched.

LABORATORY EXPERIMENTS ON BEHAVIOUR AND MOVEMENT OF A FRESHWATER MUSSEL, LIMNOPERNA FORTUNEI (DUNKER)

Experiments on the movement and behaviour of a freshwater mussel Limnoperna fortunet (Dunker) were conducted m the laboratory. Small mussels of 5-12 mm shell lengths moved over 100 cm during a day within plastic containers, with a maximum distance of 300 cm. However, their movement was concentrated during the first five hours of the experiments. The distance moved decreased with increasing shell length, and mussels larger than 27 mm moved less than 20 cm. Both small (< 15 mm) and large (»15 mm) mussels showed negative phototaxis and positive geotaxis under the light, but positive geotaxis of small mussels tended to be weak in darkness or under the shade Small mussels had strong thigmotaxis with a preference for settling in the angled crevices between the vertical walls and the flat bottoms of the containers Small mussels tended to aggregate, after movement, and the presence of large mussels facilitated secretion of byssal threads by the small mussels. The possible adaptive significance of these behaviours and movement are discussed in relation to response to predation and dislodgement by waves or water current

Ontogeny of shell morphology and shell strength of the marine snails Littorina obtusata and Littorina mariae: different defence strategies in a pair of sympatric, sibling species

The sibling marine snails Littorina obtusata (L.) and Littorina mariae Sacchi &amp; Rastelli are sympatrically distributed and the shells of both species are subject to similar breaking forces by predatory crabs. Nevertheless, the two species exhibit rather different growth and defence strategies. To determine growth patterns, we measured changes in five morphological variables with increasing shell length: body whorl thickness at the point of crushing force application, shell height (related to globosity), shell mass, body mass, and apertural lip thickness. We also measured ontogenetic changes in the ability to withstand shell crushing. For most morphological variables, L. mariae showed uniformly allometric growth of juveniles into adults. In contrast, L. obtusata usually exhibited a distinct change in growth pattern upon reaching maturity. As adults, L. mariae showed a more sustained increase in overall shell mass and in body whorl thickness (defence against crushing attacks) and also had proportionally thicker apertural lips (defence against peeling attacks). Littorina obtusata, however, grew to a larger size and their shells could accommodate larger bodies at all sizes. Furthermore, the strength of L. obtusata shells increased faster than could be accounted for by either overall shell mass or thickness at the point of force application, suggesting strengthening by other means such as changes in shell microstructure or shape (other than globosity). These results illustrate the viability of two contrasting antipredator strategies, despite a highly similar phylogenetic history and selective regime.

Allometry of shell and body weight in relation to shore level in the intertidal bivalve Geukensia demissa (Bivalvia: Mytilidae)

Absolute and allometric growth rates were determined for two shore-level populations of Geukensia demissa at a site within the Jamaica Bay National Wildlife Refuge (NY). Growth rates of both shell and biomass decreased with increasing shore level, but annual growth increments as a proportion of size increased. Shell metrics (length, height, width, weight) did not differ between shore levels. However, shell weight was supported by a smaller biomass at the higher shore level than at the marsh edge. Dry body weight increases allometrically with shell length in Geukensia ( slope = 2.77). Gill height was greater at the higher site for mussels of equal length, and total gill area per gram of body weight was significantly greater at the higher site. Since gill area can be increased only by increasing shell length, mussels respond to shorter feeding periods at higher shore levels by maximizing shell growth at the expense of body growth (although both decrease).