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.     

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 and growth of the naticid gastropod <Emphasis Type="Italic">Polinices pulchellus</Emphasis> (Gastropoda: Naticidae) based on length frequency analysis and statolith growth rings

In Red Wharf Bay, UK the naticid gastropod, Polinices pulchellus, was more abundant and more highly aggregated during the summer months (June–August 2001) than during the winter (December 2000). Whilst small numbers of juvenile P. pulchellus (4–6 mm shell length) were present throughout the year the population consisted mainly of individuals of 12–14 mm shell length. Juvenile snails grew rapidly in size during the winter and early spring; growth then virtually ceased between May and June, following which there was a further period of rapid growth between August and February. Densities ranged between 57 and 4,073 ha⁻¹ and the largest individual collected during this investigation measured 16.2 mm in shell length. Statoliths from adult P. pulchellus revealed the presence of a settlement ring and two prominent growth rings (rings 1 and 2). A curvilinear relationship exists between statolith diameter and shell length in snails up to 16 mm in length. Settlement rings ranged in diameter from 19.7 to 45.2 μm (mean 29.8 μm; SE=0.41) giving an estimated shell length of the settled juvenile of 1.1 mm. The diameter of ring 1 and ring 2 were significantly correlated indicating that rapid growth during the first year is maintained during year 2. Shell lengths estimated from the diameters of the prominent statolith rings and those obtained from length frequency data analysis (LFDA), were broadly congruent strongly suggesting an annual periodicity to the statolith rings. The largest snails (>15 mm) present within this population were estimated to be between 2 and 3 years old. Von Bertallanfy seasonal growth curves obtained from the LFDA predicted values of L∞, K and t ₀ of 14.32 mm, 1.54 and −0.14 years, respectively, suggesting that P. pulchellus rapidly attains its maximum asymptotic size.     

Induced triploidy in the soft-shelled clam Mya arenaria: energetic implications

Given that triploid adult bivalves reportedly grow larger and faster than their diploid siblings, such differences should be traceable to variation in energy allocation. In one proposed mechanism, retarded gametogenesis found in triploid adults may allow them more energy for somatic growth. Another hypothesis states that triploids are more heterozygous; increased heterozygosity has been positively correlated with enhanced growth. Juvenile soft-shelled clams, Mya arenaria, were treated with cytochalasin B to induce triploidy and examined with respect to components of a balanced energy equation (C=P+R+E). The variables measured were oxygen uptake (V _{o 2}), filtration rate (FR), dry tissue weight, shell length, shell height and shell inflation. Energy budgets were constructed and diploid and triploid groups compared. Few significant differences were found between diploid and triploid juvenile clams with respect to energy budget components. However, at seven loci assayed electrophoretically the triploid individuals were nearly twice as heterozygous as their diploid siblings. Moreover, tripoloid variances were less than diploid variances for every variable measured. Increased heterozygosity has been correlated with the decreased variance of morphological parameters. This study is believed to be the first to show decreased variance of physiological properties as well as morphological characters. Overall the data clearly indicate that energy allocation in juvenile M. arenaria is not related to ploidy.     

Blooms of surf-zone diatoms along the coast of the Olympic Peninsula,Washington. XI. Regeneration of ammonium in the surf environment by the Pacific razor clam Siliqua patula

A 20 month field study was conducted on ammonium excretion rates of the Pacific razor clam Siliqua patula Dixon along the beaches of Washington State, USA. Excretion rates of all those nutrients likely to be regenerated in sufficient quantity to affect surf diatom growth were measured; ammonium appeared to be the most important metabolite. Excretion of ammonium by razor clams far exceeded that by other beach fauna. Ammonium excretion rates of razor clams were positively correlated with shell length, but no correlation between ammonium excretion rate and water temperature was evident. This may be an artifact or may represent some degree of seasonal acclimation of the species to temperature. Weight-specific ammonium excretion rates were negatively related to clam size, indicating a possible large (and unknown) contribution of regenerated ammonium by smaller clams in their first year of growth; smaller clams were rarely captured during this study.Contribution No. 1048 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Potential effects of a non-indigenous predator in its expanded range: assessing green crab, <Emphasis Type="Italic">Carcinus maenas</Emphasis>, prey preference in a productive coastal area of Atlantic Canada

The non-indigenous green crab (Carcinus maenas) is an important predator on bivalve wild beds in coastal areas worldwide. This study explored size-dependent green crab prey preference on American oysters (Crassostrea virginica), blue mussels (Mytilus edulis), and soft-shell clams (Mya arenaria) in a productive coastal system of Atlantic Canada. Using two sizes of prey and three different experimental manipulations, small, medium, and large green crabs were given a choice among these three bivalves, and their daily feeding rates were monitored over the course of 3 days. For both prey sizes, green crabs showed an early feeding preference for soft-shell clams and, only as they declined in numbers, a switch toward mussels and subsequently toward oysters. We found that such changes in the timing (order) of prey preference are related to prey differences in shell thickness, a fairly reliable indicator of prey shell strength.     

Hematopoietic neoplasia inMya arenaria: Prevalence and indices of physiological condition

A field survey of hematopoietic neoplasia (Hn) in the soft shell clamMya arenaria (L.) was undertaken using an immunoperoxidase diagnostic technique. Monthly collections ofM. arenaria were made at two sites: Little Buttermilk Bay and New Bedford Harbor, both in Buzzards Bay, Massachusetts, USA, from May 1986 to October 1987. Clams were diagnosed for leukemia and analyzed for soft-tissue dry weight, condition index, and carbon and nitrogen content of the soft tissue. Prevalence of leukemia inM. arenaria exhibited a seasonal fluctuation with a maximum prevalence in fall (September to October) and a minimum prevalence in early summer (March to July). A second maximum peak in late winter (January to March) was observed at one site. Leukemia primarily affected clams that were 3 to 4 yr post-settlement. Lower prevalence levels were observed in both younger and older clams. LeukemicM. arenaria with advanced stages of the disease, were in poorer physiological condition based on dry weight of the soft tissue, condition index, and carbon content of the tissue. Nitrogen metabolism appeared to be unimpaired. Significant differences were observed between the two sites with respect to prevalence of Hn and the physiological condition of clams. Differences in disease prevalence between the two sites may be the result of unknown environmental factors that facilitate initiation of the disease or, that compromise the defense mechanisms of the clams.     

Genotype-specific growth of hard clams (genus Mercenaria) in a hybrid zone: variation among habitats

Shell growth rate is an important component of fitness in bivalve molluscs. Using the parameter computed from the von Bertalanffy growth equation, we quantitatively compared rates of annual shell grwoth among the hard clams Mercenaria mercenaria, M. campechiensis, and their hybrids sampled from a variety of habitats in the Indian River lagoon, Florida, USA, a zone of species overlap and natural hybridization. Our results indicate that the classical paradigm describing hard clam growth, in which growth rate is fastest in M. campechiensis, intermediate in hybrids, and slowest in M. mercenaria is not supported in the Indian River lagoon. Instead, M. campechiensis has a growth advantage in deep-water habitats in the northern section of our study area. In the central and southern sections of our study area, hybrids have a growth advantage over M. mercenaria in shallow-water habitats, but M. mercenaria has a growth advantage over hybrids in deep-water habitats. In all other sampled habitats, either growth rate among genotype classes is equal, or M. mercenaria has a growth advantage. This complex relationship between genotype and habitat-specific growth provides a mechanism for selection to act on hard clams in the Indian River.     

Growth of juvenile Mercenaria mercenaria and the effect of resuspended bottom sediments

The influence of silt on growth of juvenile hard clams Mercenaria mercenaria (L.) (9 mm in mean shell length) was investigated in the laboratory using mixed suspensions of algae (50x10⁶ Pseudoisochrysis paradoxa cells l^-1) and fine-grained bottom sediments (0 to 44 mg l^-1). Growth rates, expressed as percent increase in ash-free dry tissue weight, were not significantly affected by sediment concentrations up to 25 mg l^-1. Significant reduction in growth (by 16% relative to controls fed only algae), and condition of clams, occurred at 44 mg silt l^-1. The results of the 3-week growth experiment agree well with predictions made in an earlier study by integrating results of shortterm physiological measurements. Growth rates obtained with experimental algal-silt diets at 21°C (2.6 to 3.3% increase in dry tissue weight d^-1) were comparable to those determined at ambient concentrations of Great South Bay particulates at 20°C (0.9 to 4.0% d^-1). Levels of particulate inorganic matter in seawater from Great South Bay, New York, exhibited pronounced daily changes, and ranged from 6 to 126 mg dry weight l^-1. Growth enhancement by the addition of silt to an algal diet, reported in mussels, surf clams and oysters, was not found in M. mercenaria. It is suggested that these three species are better suited than hard clams for culturing efforts in inshore turbid waters above uncompacted, muddy bottoms.Contribution No. 452 from the Marine Sciences Research Center, State University of New York at Stony Brook, USA     

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.     

Gill pump characteristics of the soft clamMya arenaria

Pumping rates in the soft clamMya arenaria, collected in June 1987 in the Great Belt, Denmark, were determined as rates at which clams cleared suspensions of algae,Dunaliella marina. The frictional resistance of the siphons to water flow was estimated by studying the effect of excision of the siphons at the base on the rate of water pumping. The frictional resistance was also calculated from the Poiseuille equation. Excision of the siphons had no measurable effect on the pumping rate, and calculations indicated pressure losses ranging between 0.3 to 1.2 mm H₂O. This is consistent with the conception that the capacity of filter feeding bivalves to process large amounts of water depends upon a low resistance to water flow in the siphons.     

Growth in the bivalve Yoldia eightsi at Signy Island,Antarctica, determined from internal shell increments and calcium-45 incorporation

The growth rate of the infaunal nuculanid bivalve Yoldia eightsi at Factory Cove, Signy Island, South Orkney Islands (maritime Antarctica), was estimated from internal shell increments and ⁴⁵Ca incorporation of individuals collected monthly from December 1987 to April 1989. Acetate peels of etched shells revealed clear first-order increments, with less well defined, narrower, second-and third-order increments. The first-order increments were assumed to be annual, although there is no independent confirmation of this assumption. Unfortunately abrasion of the umbo region and the small thin shells of Y. eightsi meant that in no case could a complete sequence of increments be measured realiably on any individual shell. Measurements of 1043 first-order increments from 130 shells where a minimum of two consecutive increments could be detected were therefore pooled, and a population growth curve constructed from a Ford-Walford plot. This indicated a slow growth rate, with a maximum shell height of 22.3 mm (equivalent to a shell length of 35.6 mm) being reached at an age >60 yr. The size-frequency distribution of 1521 individuals pooled from winter (July to October) samples revealed a distinct lack of smaller (younger) individuals, possibly reflecting poor recruitment in areas of dense adult populations. The largest shell recovered in the samples was 33.5 mm in length, with an estimated age of 52 yr. Short-term ⁴⁵Ca-incorporation experiments indicated a mean daily rate of growth increment of 3.8 m for individuals of 12 mm shell height, which matches the proposed annual growth rate if growth is assumed to occur for about 150 d each year and the first-order increments are assumed to be annual.     

Diet and food preferences of the adult horseshoe crab Limulus polyphemus in Delaware Bay,New Jersey,USA

Adult horseshoe crabs Limulus polyphemus (L.) feed on a wide variety of infaunal and epifaunal invertebrates during their spring spawning migration in Delaware Bay, New Jersey, USA. Comparison of the gut contents with estimates of available prey showed that the most abundant potential prey item, the bivalve Gemma gemma, was avoided. The thinner shelled but comparatively scarce clam Mulinia lateralis was a preferred prey item. In the laboratory, crabs fed on G. gemma when it was the only available item but not when M. lateralis or soft-shell clams, Mya arenaria, were offered in conjunction. Large M. lateralis (>10mm) were preferred to small M. lateralis; there was no discrimination between M. lateralis and M. arenaria of the same size. Male and female horseshoe crabs had similar gut contents and laboratory feeding preferences, despite the fact that females are larger than males. Crabs spawning later in the summer contained more food than did crabs collected at the peak of spawning activity.     

Age and growth rate determinations for the Atlantic surf clam Spisula solidissima (Bivalvia: Mactracea), based on internal growth lines in shell cross-sections

Marked and recovered surf clams, Spisula solidissima Dillwyn, from Virginia (USA) deposited one internal growth line during a period of 11/2 years, probably in response to spawning during late summer. We have used these annual growth lines to make growth curves for two samples of New Jersey (USA) clams, one from inshore (1.8 km from shore, 15 m deep), the other from offshore (17.5 km from shore, 28 m deep) waters. The offshore and inshore clams grow at approximately the same rate until Age 3 years, after which time the growth rates differ, as does the ultimate lifespan; the offshore clams grow more rapidly and attain a greater age — up to 31 years. Comparison of our growth curves with other published curves revealed a close correspondence to a curve which was based on a study of growth over a 5-year period. Curves based on external growth lines probably underestimate growth rate in early life and overestimate it in later years.     

Sclerochronological records of temperature and growth from shells of Mercenaria mercenaria from Narragansett Bay,Rhode Island

Annual internal growth increments in shells of hard clams (Mercenaria mercenaria) provide an accurate record of both growth history and some types of environmental change. These increments were used to determine age and growth rate of hard clams collected in 1984–1985 from ten sites in Narragansett Bay, Rhode Island, USA, and to assess geographic variation in growth within the bay. The regional comparisons were facilitated by modeling the clam growth using the von Bertalanffy equation and the parameter of Gallucci and Quinn. The optimum region for hard clam growth was near the head of the bay, with areas further north (Providence River) and south (lower bay) not as productive. The relationship between size and age was similar to that reported for hard clams from northern New Jersey. Using dendrochronological techniques, variations in growth were analyzed on a temporal (year-to-year) basis. The comparative longevity of the hard clams (individuals with 40 annual bands were encountered) demonstrated that sclerochronologies of several decades were possible. A 26 yr growth record based upon 100 individuals was established, covering the years 1959–1984. Significant temporal variations occurred in bivalve growth, and a broad trend of increasing growth indices over the last two decades of the record was noted. The yearly standardized growth index values were highly and positively correlated with mean annual water temperatures in Narragansett Bay for the same time interval. The incremental shell records of these bivalves provide a quantitative indication of marine climatic (temperature) variability and growth which can be used in hindcasting the effects of natural or anthropogenic environmental perturbations.     

Growth and size structure in a baltic Mytilus edulis population

Since Mytilus edulis L. has very few predators and competitors for space, it has become a biomass dominant in the Baltic proper covering hard substrates from the water surface to more than 30 m depth. In order to investigate the factors controlling size and production in a Baltic M. edulis population, growth was studied by the analysis of annual growth rings, measurements of caged individuals and the analysis of size classes in the population, and on settlement ropes. The total number of mussels in a representative mussel bed at 4 m depth varied between 36 000 and 158 000 ind · m^-2 during the year, mainly due to variations in very small mussels (<2 mm), whereas the abundance of mussels 2mm was rather constant between about 17 000 and 28 000 ind · m^-2. Maximum numbes of mussels < 2 mm, amounting to 132 000 ind · m^-2, were found after settlement in summer, but still half a year later in spring, 65 000 ind · m^-2 < 2 mm were registered, due to very strong intraspecific competition for food and space leading to the competitive suppression of small individuals and large variations in growth rates. Due to the special size-structure of the population only the analysis of annual growth rings could be used to estimate natural shell growth. From being very low in the smallest mussels, growth was linear between about 2–10 yr of age, corresponding to about 3–20 mm length, after which it decreased with a L=32 mm. Over the linear interval, growth in the populations from 3–6 m and 10–15m depth was 3.1 and 2.2 mm · yr^-1, respectively. Meat growth showed strong annual variations mainly due to gonad production. Starving mussels could, however, while utilizing energy reserves, survive losses of up to 78% of their meat biomass. This ability of M. edulis to respire away its own biomass and its apparent tolerance of weight loss has important implication. It will drastically reduce the energy flow to destruents from mussels dying naturally, which is of special significance in the Baltic, where predators and scavengers are scarce. It enables the mussels to endure bad food conditions and buffer strong seasonal variations in food abundance, maintaining the strongly food-and space-limited Baltic M. edulis population at the carrying capacity of the area.     

Age,growth and population structure of <Emphasis Type="Italic">Modiolus barbatus</Emphasis> from the Adriatic

Age, growth and population structure of Modiolus barbatus from Mali Ston Bay, Croatia were determined using modal size (age) classes in length frequency distributions, annual pallial line scars on the inner shell surface, internal annual growth lines in shell sections of the middle nacreous layer and Calcein marked and transplanted mussels. The length frequency distributions indicated that M. barbatus attain a length of ∼40 mm in 5–6 years indicating that a large proportion of the population in Mali Ston Bay is <5 years old. Some mussels of ∼60 mm were predicted to be 14 years old using the Von Bertalanffy growth (VBG) equation. Up to the first 6 pallial line scars were visible in young (<6 years) mussels but in older shells the first scars became obscured by nacre deposition as the mussel increased in length and age. The age of the older shells (>6 years) was determined from the middle nacreous lines in shell section, which formed annually in winter between February and March; the wider dark increments forming during summer (June to September). The oldest mussel, determined from the middle nacreous lines, was >12 years, with the majority of mussels aged between 3 and 6 years of age. The ages of mussels ascertained using the growth lines were not dissimilar to the ages predicted from the length frequency distributions. Age at length curves produced using modal size class data were not different from the data obtained using the pallial scar rings and internal growth lines. Taken together these data suggest that M. barbatus attains a length of 40 and 50 mm within 5 and 8 years, respectively. Eighty one percent of individual M. barbatus injected with a Calcein seawater solution (300 mg Calcein l⁻¹), into their mantle cavity successfully deposited a fluorescent line, which was visible in suitably prepared shell sections under ultra violet light. Incorporation of Calcein into the mussel shells was seasonally variable with the lowest frequency of incorporation in mussels marked in February and recovered in May. Seasonal shell growth was observed with significantly higher growth rates in mussels marked in May and removed in August (ANCOVA, F _3,149 = 23.11, P < 0.001). Mussels (∼18 to 22 mm) marked in May and recovered in August displayed maximal growth rates of >2.5 mm month⁻¹ compared with a mean mussel growth rate of 1.2 ± 0.6 mm month⁻¹. At other times of the year mussel shell growth ranged from immeasurable to 1.48 mm month⁻¹.     

The gastropod statolith: a tool for determining the age of<Emphasis Type="Italic"> Nassarius reticulatus</Emphasis>

The microstructure, shape and appearance of the growth rings in statoliths of Nassarius reticulatus (L.) were investigated. This species possesses two statocysts, each containing a single spherical statolith of calcium carbonate of up to 0.22 mm in diameter in the largest animals. The relationship between statolith diameter (SD) and total shell height (TSH) is exponential [ln(TSH)=26.3SD–0.842], although the function is site specific. Statoliths of the largest whelks (>29 mm) contained three or four clearly defined rings, corresponding to TSH values of ~1.1, 4.6–5.3, 12.0–13.5 and 18.5 mm, respectively. The first ring likely represents the metamorphic ring that was deposited at the time of larval metamorphosis when the post-larval whelk adopted a benthic lifestyle. The estimated size of the whelks at formation of the second, third and fourth statolith rings closely matched the TSH inferred from the shell rings. It is concluded that the patterns of growth rings present in statoliths can provide information about the age and growth of N. reticulatus.Communicated by O. Kinne, Oldendorf/Luhe     

Growth,production and ecological significance of Ophiura albida and O. ophiura (Echinodermata: Ophiuroidea) in the German Bight

Growth and production of the shallow-water ophiuroids Ophiura albida and O. ophiura were investigated at two stations in the German Bight from 1988 to 1991. Growth rings visible on the vertebral ossicles of the ophiuroid arms were interpreted as annual age markers. A correction for overgrown first rings allows for more exact estimations of growth and age. In both species growth could be described by Von Bertalanffy growth functions with the asymptotic disc diameter D =10.1 mm, K=0.229 and t _o=-0.192 in O. albida and D =27.7 mm, K=0.084 and t _o=0.042 in O. ophiura. Somatic production was calculated from mass specific growth rates. Annual production:biomass (P:B) ratios were estimated at 0.32 for O. albida and 0.43 for O. ophiura.AWI Publication Number: 618     

Population dynamics of naturalised Manila clams <Emphasis Type="Italic">Ruditapes philippinarum</Emphasis> in British coastal waters

The Manila clam Ruditapes philippinarum was introduced to Poole Harbour (lat 50°N) on the south coast of England in 1988 as a novel species for aquaculture. Contrary to expectations, this species naturalised. We report on individual growth patterns, recruitment, mortality and production within this population. On the intertidal mudflats the abundance of clams (>5 mm in length) varied seasonally between 18 and 56 individuals m⁻². There appear to be two recruitment events per year and there were 6 year classes in the population. A mid-summer decline in abundance was partly due to increased mortality but probably also a result of down-shore migration in response to high water temperatures and the development of anoxic conditions. A winter fishery removes c 75% of clams of fishable size (maximum shell length ≥40 mm) and c 20% of the annual production. The fishery depresses the maximum age and size attained by the clams but appears to be sustainable. Clam mortality due to factors other than fishing is highest in late-winter to early spring. The growth of the clams is intermediate in comparison with many published studies but remarkably good given their intertidal position. As on the coasts of the Adriatic Sea, where the clam is also non-native, the Manila clam has thrived in a shallow, eutrophic, lagoon-like system on the English coast. While the Poole Harbour population is currently Europe’s most northerly reported self-sustaining, naturalised population, given forecasts of increasing air and sea temperatures it might be expected that this species will eventually spread to more sites around the coasts of Northern Europe with associated economic and ecological consequences.