Population dynamics of the soft-shell clam Mya arenaria

A life table was constructed for Mya arenaria from Gloucester, Massachusetts, USA, based on schedules of age-specific fecundity and mortality determined under natural conditions. Mortality rates decrease with size and age in this species, with the period of maximum mortality occurring during the summer months. Mortality rates during the fall and winter were considerably lower, perhaps due to the inactivity of natural predators. The survivorship curve for M. arenaria approximates the Type 3 curve of Deevey (1947). Mean life expectancy is low in recently-settled clams, peaks when the individual reaches 30.0 to 34.9 mm (1 year of age) and remains fairly high for most of the remainder of life. The intrinsic rate of natural increase (r _max) is very high: 4.74. This enormous rate of potential increase is offset by high rates of larval mortality in the plankton. Unlike the reproductive values of most animals studied, those in M. arenaria peak late in life, well after the know age of first reproduction. This is probably the result of increased fecundity with age. The implications of this work in the area of resource management are discussed.     

Age determination of the soft-shell clam Mya arenaria using shell internal growth lines

Shells of known-age Mya arenaria have revealed an internal annual growth pattern in this bivalve. The number of external growth rings on the shell exterior does not correspond to the age of the clams. This study provides an alternative method to using external annual rings for age determination. The internal growth lines are visible in thin sections when the shells are cut from the umbo to the ventral margin. The lines are formed by late spring of each year prior to spawning and are probably in response to decreased winter growth. When the 7 year olds were examined it was difficult to distinguish the most recent growth line and the inner shell layer. This could result in ages being underestimated. This annual pattern of internal growth lines could be useful to biologists and paleoecologists engaged in population studies.     

Toxicity evaluation of a complex metal mixture to the softshell clam Mya arenaria

Adults of the softshell clam Mya arenaria were continuously subjected to a flowing raw seawater solution containing a mixture of salts of manganese, zinc, lead, nickel, copper, and cadmium. Final calculated concentrations, in g l^-1, of the toxicant solution were 7200 Mn, 2500 Zn, 70 Pb, 50 Ni, 50 Cu and 1 Cd; these concentrations approximated highest measured levels within surficial interstitial sediment waters from mid-Narragansett Bay, Rhode Island. M. arenaria were also subjected to a 20% solution, i.e., 1440 Mn, 500 Zn, 14 Pb, 10 Ni, 10 Cu, and 0.2 g l^-1 Cd. One study was conducted for 112 days in winter at 0° to 10°C and another for 16 days in summer at 16° to 22°C. In the winter study, all clams exposed to a 100% solution died between the 4th and 10th week; soft parts of survivors at 6 weeks contained about 19 times more Pb, 15 x more Zn, 12 x more Cu, 10 x more Mn, 3 x more Ni and 0.1 x more Cd than controls; relatively minor changes in whole body elemental content of Ca, Cr, Fe, K, Mg, Na, Sr, and V were observed. Clams exposed to a 20% solution during winter survived the 112 day study; at that time these contained about 5 x more Cu, 4 x more Mn, 3 x more Zn and about 2 x more Pb than controls; comparatively minor changes were observed in other elements examined. In the summer study, all M. arenaria subjected to the 100% solution died between 6 and 14 days; survivors from this group at 7 days contained about 25 x more Pb, 13 x more Cu, 11 x more Zn, 7 x more Mn, and 3 x more Ni than controls; other changes in elemental content were not as pronounced. Mortality in the 20% group during summer was slightly higher than controls during the 16 day study; at 14 days survivors from this group contained about 12 x more Mn, 7 x more Pb, 7 x more Zn, 4 x more Cu, and 3 x more Ni than controls. Survival and bioaccumulation patterns were not altered through feeding a supplemental diet of algae. The significance of these findings are discussed in terms of potential environmental perturbations, especially local dredging practices.     

Analysis of growth rate in Mya arenaria using the Von Bertalanffy equation

Field studies were conducted in Gloucester, Massachusetts, USA, to determine linear shell growth rates for Mya arenaria. These rates were then compared with those reported for the same species from other locations. Most shell deposition occurred from March through November of each year. Winter interruptions in growth were not as marked in the small clams as in the larger ones (>60.0 mm). Annual variations in growth were slight during the period 1973–1974. Growth of mature clams (>35.0 mm) slowed during the spawning season. No significant sexual dimorphism in mean annual growth rates was detected. Winter rings were shown to be a reliable method for determining age in clams from Gloucester. Age-size relationships, based on two independent measures of annual growth, winter rings and tagging experiments, were computed using the Von Bertalanffy growth equation. No well-defined latitudinal patcerns in growth could be established for M. arenaria.     

Tissue distribution and excretion of 99mTc-disofenin in three marine species: Pleuronectes americanus (winter flounder), Homarus americanus (lobster), and Mya arenaria (soft-shell clam)

To determine the pharmacokinetics of a small lipophilic molecule in vivo, the distribution and accumulation of ^99mTc-radiolabelled disofenin (diisopropylacetanilide iminodiacetic acid) were traced during 1991–1992 by scintigraphy and gamma well counting in winter flounder (Pleuronectes americanus collected from Boston Harbor and Long Island Sound in 1992), lobsters (Homarus americanus collected from Massachusetts Bay in 1991), and soft-shell clams (Mya arenaria purchased in 1991). The agent was distributed throughout the bodies of lobsters within 12 s, throughout flounder within 40 s, and throughout clams within 2 min. It was concentrated most strongly by the liver of flounder, which contained 61.2±7.8% of the injected dose within 1 h of injection, and by the lobster hepatopancreas. Accumulation also occurred in the flounder kidney, lobster antennal glands, and the kidney and pericardial glands of clams. The compound was rapidly excreted from the flounder liver into the gall bladder, and from the lobster hepatopancreas into the stomach. The data suggest its excretion from the lobster antennal glands and clam kidneys. The rate of clearance of disofenin from the body varied among species: 99±2.1% of the initial dose remained in flounder sampled 16 to 24 h after injection, compared to 80.5±7% remaining in the lobster after 15 h, and 87.4±5.9% remaining in clams after 27 h. The clearance rates in flounder and lobster are considered to be minimum values because of the lack of gut activity in unfed individuals. Overall, these in vivo tracer studies establish the utility of scintigraphy for assessing the uptake and excretion of a lipid soluble compound in different taxa, and may be applicable for evaluating disease and/or environmental effects on organ function in marine animals.     

The capacity of the filter-feeding bivalve Mya arenaria L. to affect water transport in sandy beds

Benthic filter feeding macrofauna organisms may be an important factor linking sediment and water column. Transport of water and concomitantly of suspended matter is directly related to the size of the benthic filter feeding population. This paper aims to determine the potential for water transport by the bivalve Mya arenaria along a coastal stretch of roughly 100 km length in the southern Baltic Sea. Quantification of population filtration rates specific to the area is based on distribution, abundance and biomass of M. arenaria and calculated according to previously published filtration rate–biomass relations. Calculated rates range up to >8 m³ m^–2 day^–1 (at 5–20 m water depth in sandy sediment) with the potential to locally process a volume of water equivalent to the water column within <1 day. Data from 1991–2002 at one site suggest that the area-specific potential population filtration rate remains remarkably constant in time despite changes in population structure of M. arenaria. The related impact on pore water exchange within the permeable sediment associated with the leakage of water from the gape of M. arenaria valves is discussed.Communicated by L. Hagerman, Helsingør     

Reduction in carbon flux in Mya arenaria caused by a spill of No. 6 fuel oil

Rates of respiration, assimilation and filtration have been determined on a monthly basis for two populations of Mya arenaria in Casco Bay, Maine (USA), one of which was heavily oiled by a spill of No. 6 fuel oil. Monthly estimates of carbon flow have been calculated from these data. The same general trends were seen in each population; small negative carbon flow in winter, large negative carbon flow during the spring; large positive carbon flow during summer. However, the oiled population gained carbon at only 50% of the rate seen in the unoiled population. This difference results from the uptake of hydrocarbons by the oiled population.Contribution No. 75009 to the Bigelow Laboratory.     

Effects of Lunatia heros predation on the population dynamics of Mya arenaria and Macoma balthica in Maine,USA

The interaction between the naticid snail predator Lunatia heros and 2 iteroparous, infaunal, intertidal bivalves was investigated in Lubec, Maine, USA. The Mya arenaria population consists primarily of young, small individuals. M. arenaria survivorship is low when young (3.5% yr^-1 for the first 5 yr), then increases. M. arenaria can attain a length of 60 mm, but it is susceptible to L. heros attack only until it is 30 mm long. It delays reproduction until it is 20 mm long (3.8 yr) and diverts its resources instead into rapid early growth (4.9 mm yr^-1 for the first 5 yr). The Macoma balthica population has a larger proportion of older individuals than does that of M. arenaria. Survivorship is higher for M. balthica than for M. arenaria (76.3% yr^-1 for the first 5 yr). Unlike M. arenaria, M. balthica attains a final length of only 25 mm and all sizes are susceptible to L. heros attack. M. balthica grows slowly (2.7 mm yr^-1 for the first 5 yr) and diverts its resources into earlier reproduction at a length of 10 mm (2.9 yr). These contrasting life-history patterns and the possible relationship between bivalve resource allocation and refuges from predation are discussed.     

Strong population differentiation of softshell clams (Mya arenaria) sampled across seven biogeographic marine ecoregions: possible selection and isolation by distance

Twenty-two Mya arenaria samples spanning seven marine ecoregions mostly situated in the Cold Temperate Northwest Atlantic (CTNA) biogeographic province were collected between 2001 and 2010 and genotyped at seven highly polymorphic microsatellite loci to test for population differentiation. Results showed strong regional differentiation with six genetic clusters: (1) Northern Gulf of St. Lawrence (GSL), (2) Magdalen Archipelago, (3) Southern GSL, (4) Lower Atlantic Canada, (5) US Coasts and (6) Northern Europe. Population structure was supported no matter the statistical approach and generally does not reflect the geographical limits of marine ecoregions. A latitudinal cline in allelic richness provides evidence for a northward post-glacial expansion range for this species. While geographical distance explains the genetic variation detected in southern CTNA, increased heterogeneity observed in northern CTNA can be explained by isolation by distance, marine landscaping and presumable selective processes acting at the Mar5 locus. Exclusion of Mar5 from analyses resulted in the detection of three genetic clusters instead of six.     

Invest in arms: behavioural and energetic implications of multiple autotomy in starfish (Asterias rubens)

The autotomy of body parts as a means of escaping predation or renewing damaged tissues has evolved in a number of animal groups. Starfishes are unique in that they can autotomise >75% of their body mass and continue to survive. Presumably, multiple autotomy of tissue has energetic costs in terms of potential fitness and may affect the allocation of energy reserves accordingly. We investigated arm autotomy, predatory capabilities and subsequent regeneration in common starfish, Asterias rubens, that were induced to lose one, two or three arms. Initially, both regeneration of autotomised arms and the rate of growth of intact arms was slowest in animals that had lost the most arms (i.e. three arms missing vs two arms missing vs one arm missing). However, 8 months later, the growth of intact arms since the start of the experiment was not significantly different between groups of starfish that had autotomised different numbers of arms. However, the average dry weight per regnerating arm was significantly higher in starfish that had autotomised the most arms. Arm loss decreased the ability of starfish to open mussels and those that had autotomised two arms were significantly less likely to feed successfully on a mussel in a 24-h period than intact starfish. Our data suggest that proportionally more energy is allocated to arm regeneration in starfish that have suffered multiple arm loss and this may compensate a potential decrease in fitness that results from decreased feeding capability.     

Disease progression and mortality in neoplasticMya arenaria in the field

Four 6-mo mark-and-recapture experiments conducted in Long Island Sound, USA, from 1988 to 1990, involving approximately 2250 individual observations, demonstrated that under natural conditions significantly higher mortality (p0.001, chi-square test) occurred amongMya arenaria (L.) with hematopoietic neoplasia than those diagnosed as non-neoplastic. Using a blood-screening technique, the clams were diagnosed and placed in one of three diagnostic groups based on the severity of the disease (the percentage neoplastic cells per total number of blood cells): non-neoplastic (NN), 0%; low-severity neoplastic (LSN), < 50%; high-severity neoplastic (HSN), > 50%. Mortality of those clams initially diagnosed as HSN ranged from 48% to 78%, depending on the test period, as compared to 3% to 21% for the non-neoplastic. Mortality in the LSN treatment varied from 8% to 34%. Both progression and remission were evident in clams at all stages of the disease. Mortality and rates of progression and remission in individuals appeared to be linked to water temperatures. Differential mortality may be responsible for the apparent seasonal cycle of prevalence in populations.     

Host glutamine synthetase activities in the giant clam—zooxanthellae symbiosis: effects of clam size,elevated ammonia and continuous darkness

Host tissues and zooxanthellae of the giant clam Tridacna gigas contained glutamine synthetase, with the highest transferase activities present in the gill, followed by the kidney, mantle, zooxanthellae, foot, heart and adductor muscle, in that order. Synthetase activities of glutamine synthetase in host tissues and zooxanthellae were in a similar order, but the differences were not so marked. Host tissues also contained hexokinase, glucose-6-phosphate dehydrogenase and malate dehydrogenase. Highest hexokinase activities were present in the heart, followed, in order, by the gill, mantle, adductor muscle and foot. Highest glucose-6-phosphate dehydrogenase activities were present in the gill, followed by the mantle, heart, adductor muscle and foot. All tissues assayed contained high malate dehydrogenase activities. There was no detectable glutamate dehydrogenase activity. Glutamine synthetase activity in gill and mantle tissue decreased by 1.6% with every 1 cm increase in clam size. Host glutamine synthetase activity decreased by 80% in gill tissue and by 45% in mantle tissue in clams which were maintained for 8 d in continuous darkness. Similar effects were found when clams were kept in light in the presence of elevated ammonia concentrations. It is suggested that both host and symbionts are nitrogen-deficient in small clams and that host glutamine synthetase plays a role in ammonia assimilation by the intact association.     

Population differences in nerve resistance to paralytic shellfish toxins in softshell clam, <Emphasis Type="Italic">Mya arenaria</Emphasis>, associated with sodium channel mutations

The softshell clam, Mya arenaria, is a commercially important bivalve with wide latitudinal distribution in North America. Populations of clams with a history of repeated exposure to toxic Alexandrium spp. have developed a natural resistance to the paralytic shellfish toxins (PSTs) produced by these algae. An association between PST resistance in individual clams and a single mutation in the saxitoxin (STX) binding region of the α-subunit of the voltage-gated sodium (Na⁺) channel gene was previously identified. Here we establish that more than one mutation associated with nerve resistance to STX occurred at this locus. Both cDNA from mRNA and genomic DNA sequences from individual clams are identical demonstrating that both alleles are expressed simultaneously. In addition, one resistant allele per individual is sufficient to confer neural resistance to STX even though heterozygous individuals show an intermediate level of resistance to STX in in vitro nerve trunk assays.     

Endogenous circatidal rhythm in the Manila clam Ruditapes philippinarum (Bivalvia: Veneridae)

Manila clams, Ruditapes philippinarum, removed from their natural environment and maintained for 9 weeks in continuously immersed conditions exhibited a clear endogenous circatidal rhythm in oxygen consumption. The clams exhibited a semidiurnal rhythmicity in oxygen consumption after showing a diurnal pattern in the first few days (5 to 7 d) of the experiment. The results of the present study indicate that activity rhythms of clams are controlled not only by exogenous factors, but also by an endogenous circatidal periodicity. Received: 8 April 1998 / Accepted: 1 February 1999     

Acid and alkaline phosphatase activities in the clam Ruditapes philippinarum

Acid and alkaline phosphatase activities have been partially characterized in Ruditapes philippinarum (Adams and Reeve, 1850). Two activity peaks at pH=4.5 and pH 10.5 were detected in the gill, digestive gland, mantle, siphon and foot. Acid phosphatase activity was higher than that of alkaline phosphatase. The highest activity for both enzymes was observed in the digestive gland and, in decreasing order, the gill, foot, siphon and mantle. Alkaline phosphatase activity was similar in the mantle, siphon and foot. K _m values were determined for both enzymes in the gill and digestive gland. Hill coefficients were near 1, indicating no allosteric behaviour for either enzyme in the two organs. The optimum temperature was the same for acid phosphatase in both gill and digestive gland (50 °C), while for alkaline phosphatase it differed for these two organs (gill, 40 °C; digestive gland, 35 °C). The apparent activation energy was obtained from Arrhenius plots, and ranged from 8.61 kcal/mol for alkaline phosphatase in the gill, to 10.84 kcal/mol for acid phosphatase in the digestive gland. The effects of metals (1 mM conc) on both enzyme activities were assayed in vitro. Hg strongly inhibited the enzyme activities in the gill and digestive gland, probably because of its affinity for the sulphydryl group. Histochemically, acid phosphatase in the gill was located in a granular form throughout the gill cells, but was undetectable in the ciliate epithelium of the gill filaments. Alkaline phosphatase was located in the gill skeleton. Clam size and phosphatase activities were inversely related, probably reflecting a decrease in shell deposition with inereasing size. As a function of season, both enzymes were present in lowest amounts in winter, when undifferentiated sex cells were predominant in the germinative epithelium, and highest in summer, when ripe individuals of both sexes were more frequent.     

Mate guarding behavior in clam shrimp: the influence of mating system on intersexual conflict

Mate guarding is a male strategy to gain access to receptive females but often results in antagonistic interactions between the sexes because of different costs/benefits of guarding. In addition to social, morphological, and physiological parameters, the type of mating system should also affect the strength of the conflict and thus the guarding duration. Specifically, when compared to females, self-compatible hermaphrodites might have reduced benefits of outcrossing. We investigated mate guarding in dioecious (co-presence of females and males) and androdioecious (co-presence of hermaphrodites and males) branchiopod crustaceans. Both sexes in androdioecious systems should shift their guarding times to lower values relative to dioecious systems because (1) androdioecious males are present in lower percentages than dioecious males and thus encounter rates with receptive mates are relatively greater for them; and (2) hermaphrodites should have low incentive to incur high costs of mate guarding, having the alternative of self-fertilization, and thus should be highly eager to resist. While females preferred short guarding times, when allowed to control the guarding duration (males tethered), dioecious males did not increase their guarding duration when females (treated with muscular relaxant) could not resist, in contrast to what has previously been found for androdioecious males. This indicates that hermaphrodites are more willing to resist mate guarding than females. The among-species comparisons supported our hypotheses: compromised guarding times were significantly lower in androdioecious than in dioecious species. The introduction of a parameter (mating system) not previously investigated in mate guarding models resulted in a powerful test of mate guarding theory, adding a valuable contribution to our understanding of intersexual conflict.     

Metallothionein-like metalloproteins in the Baltic clam Macoma balthica : seasonal variations and induction upon metal exposure

Metallothionein-like proteins (MTLs) have been measured by differential pulse polarography every two months during a period of two and half years in the marine bivalve Macoma balthica from two locations of the Western Scheldt Estuary. The MTL concentrations (0.85 to 7.81 mg g⁻¹ dry wt) are comparable to most values found for other marine invertebrates. These concentrations vary significantly with the seasons (higher in winter, lower in summer), which is mainly due to body weight fluctuations. Strong correlations exist between MTL and metal levels. Upon short-term exposure to a mixture of Cd, Cu and Zn, there are on average significant MTL increases, but important seasonal variations have been observed: in winter, the clams are more sensitive to metals, uptake more Cd and Cu and induce more MTL than during the warmer months. Received: 16 May 1997 / Accepted: 6 June 1997     

Physiological evaluation of naphthalene intoxication in the tropical acrid clam Anadara granosa

Changes in vital physiological activities — shell movements and burrowing behaviour, uptake and depuration, filtration and respiration — on exposure to sublethal concentrations of naphthalene (Nap) were investigated in the tropical arcid blood clam Anadara granosa L. from the Bombay coast in 1986. On exposure to ambient concentrations exceeding 5g Nap ml^-1, shell valves opened widely within the first hour of exposure. The compressed muscular foot was stretched vertically upwards with copious secretion of mucus, and did not exhibit any evidence of burrowing behaviour. Those exposed to 5g Nap ml^-1 regained their normal physiological activities on transfer to stressor-free medium, whereas those exposed to higher levels became intoxicated and narcotized. Bioaccumulation of Nap was dependent upon environmental concentration, increasing exponentially with time over a 9h exposure period. Further exposure, up to 96h, however, did not increase tissue levels substantially. About 65% of the total body burden of Nap was depurated within 3h of transfer to Nap-free medium. Rates of filtration and oxygen consumption were significantly reduced (ca 70%, p<0.001) compared to control clams. Percent inhibition in these physiological activities was dependent upon tissue and ambient concentrations of Nap. On transfer to pollutant-free medium, clams exhibited remarkable recovery. Rates of both filtration and oxygen consumption were gradually increased and restored to normal levels, as observed in controls. However, clams exposed to upper limits lost their ability to burrow back into the sedimentary bed, leaving them susceptable to predators. Furthermore, induction of anaerobiosis and disruption of osmotic balance on exposure to Nap together with aerial exposure at low-water periods and salinity changes, acted synergistically and proved detrimental. The rates of growth and mortality observed in the natural population of blood clams, harvested from the coastal waters off Bombay, have been explained in terms of the impact of petroleum and allied waste products released from petrochemical industries.     

The persistence of chronically accumulated hydrocarbons in the hard shell clam Mercenaria mercenaria

Hard shell clams, Mercenaria mercenaria, from the chronically polluted environment of the Providence River, USA, were transferred to a clean laboratory system located in lower Narragansett Bay, Rhode Island. The hydrocarbon contents of these transplanted clams were monitored for 120 days after transfer. After this period only slight depuration had occurred. A 41.9 g g wet weight^-1 average initial hydrocarbon burden decreased to 29.3 g g^-1, a decrease that became statistically significant (P<0.1) only after 120 days. At most, only 30% of the clams' hydrocarbons were lost. These findings indicate that the duration of the exposure period and the chemical nature of the source hydrocarbons are both instrumental in determining the persistence of these compounds in filter-feeding bivalves. Chronically accumulated petroleum hydrocarbons are strongly retained by the organisms and are only very slowly depurated.     

Haematological evaluation of naphthalene intoxication in the tropical arcid blood clam Anadara granosa

In vertebrates haematological parameters are of considerable importance in the diagnosis of pathological disorders due to pollutants. Similar haematological studies in invertebrates are lacking. This is partly because a very few species have erythrocytes; arcid clams, however, are unique in having erythrocytes. Our experiments were designed therefore to evaluate the effect of naphthalene (N) on haematological parameters in the tropical arcid blood clam Anadara granosa L. The parameter studied were haemoglobin (Hb), erythrocyte sedimentation rate (ESR), haematocrit, erythrocyte fragility, haemolymph cations, free amino acids and osmolality. On exposure to naphthalene (N) in the sublethal range (5 to 15 g ml^-1) Hb content decreased appreciably (P<0.001) compared with controls. Although ESR increased significantly (P<0.001), haematocrit decreased by 17 to 37%. Treatment with N for 96 h increased erythrocyte fragility and haemolysis; the latter by more than 40%. Both inorganic phosphate and free fatty acids increased with ambient concentration of N. Haemolymph cations — K, Ca and Mg increased significantly in relation to control. Na decreased, but not significantly. Free amino acids decreased significantly (P<0.001) with the ambient concentration of naphthalene, but haemolymph osmolality was not effected.