Clam Mortality Research Articles

Effects of pollutants in the Ria Formosa Lagoon, Portugal

The Ria Formosa is a most significant area from the point of view of fisheries, particularly with respect to the culture of molluscean shellfish. Eighty percent of the bivalves (e.g. cross-cut carpet shell, Venerupis decussata) harvested in Portugal come from this area. There are in this coastal lagoon rapidly increasing levels of anthropogenic releases of contaminants transported by untreated sewage and industrial and agriculture effluents. High levels of bacteria, nutrients, metals and organochlorine compounds were detected in the water column as well as suspended solids and sediments in several areas of the lagoon, mainly in the areas surrounding the main cities. Some of these pollutants have been accumulating in the tissues of several bivalve species. The accumulation of these pollutants along with the increase in temperature over the summer resulted in deleterious effects on these species some of which became unsafe for human consumption. These anthropogenic releases together with an ineffective clam harvest practice resulted in a massive clam mortality. This mortality generally occurs after summer and bivalve production has shown a significant decrease in the past 5 years with a negative impact on the economy of the region. Along with field experiments, laboratory studies have been carried out to investigate the biological effects of sublethal concentrations of some of these pollutants in the most commercially important species harvested in the region.

Pathogenicity of a Birnavirus to Hard Clam(Meretrix lusoria) and Effect of Temperature Stress on Its Virulence.

A birnavirus, CV-TS-1 strain isolated from cultured hard clam (Meretrix lusoria) was inoculated to the hard clam by means of intrapalleal cavity injection or water borne method at 25°C. Cumulative mortalities of clams injected with the virus at the concentrations of 104.7, 103.7 and 102.7 TCID50/clam were 37.5%, 32.5% and 25%, and those of clams immersed to virus solutions of 106.0, 105.0and 104.0TCID50/mlwere 30%, 25% and 15%, respectively. Temperature stress (increase from 25°Cto 33°C or decrease from 25°C to 15°C) was given to clams before and after virus challenge by exposure to a virus solution of 105.0 TCID50/ml. Mortality increased markedly when temperature was increased after virus infection, though either the decrease in temperature after virus challenge or increase or decrease in temperature before the challenge did not affect the mortality.

Modification of animal habitat by large plants: mechanisms by which seagrasses influence clam growth.

Field experiments withMercenaria mercenaria in a relatively high-energy environment demonstrated that clams on unvegetated sand flats failed to grow during autumn while those within seagrass beds grew substantially. Clam growth rates at the seagrass margin that first receives the faster-flowing, flood-tidal currents were about 25% less than at the opposite edge. In a second experiment, pruning, which reduced average blade length by 50-75%, was shown to enhance near-bottom current velocities and to reduce shell growth ofMercenaria during summer by about 50%. As in the first experiment, clams in the unvegetated sand flats exhibited no net growth. Clam mortality, caused mostly by predatory crabs and whelks, was much higher on sand flats than in seagrass beds and intermediate in clipped seagrass. Although consistent with some previous reports, these growth results are still surprising given that they contradict the generalization that suspension feeders grow faster under more rapid current regimes.Three types of indirect interactions might explain the observed effect of seagrass on growth of buried clams: (1) altering food supply; (2) changing the intensity of biological disturbance on feeding clams; and/or (3) affecting the physical stability of the sediments. Previous research on this question has focused almost exclusively on processes that alter food supply rates. In this study, food concentrations, as indicated by suspended chla, were 30% higher inside than outside one seagrass bed, whereas chla concentrations in two other beds were not different from those on adjacent sand flats. This result is sufficient to show that more intense food depletion was not induced by the reduction in flow velocities under the seagrass canopy. Nevertheless, the possible small difference in food concentrations between vegetated and unvegetated bottom seems insufficient to explain the absence of growth of sand-flat clams, especially given the virtual lack of food limitation among suspension feeders in this system. Two data sets demonstrated that the effects of biological disturbance agents cannot be ignored. An outdoor laboratory experiment showed that even in the absence of physical contact between predator and prey the presence of a whelk reduces the amount of time spent feeding byMercenaria. This result suggests that sand flats, where predation rates are higher, may be sites of lower clam growth than seagrass beds because of greater consumer interference with clam feeding. Furthermore, clam siphons are proportionately larger inside seagrass than on sand flats, implying that siphon nipping may not be as intense inside seagrass. This process, too, would reduce net growth of sand-flat clams. Finally, no explicit test was conducted of the hypothesis that enhanced sediment transport in the absence of flow baffling and root binding by seagrass inhibits net growth of clams on high-energy sand flats. Nevertheless, this is a reasonable explanation for the pattern of enhanced growth of seagrass clams, and could serve to explain the otherwise unexplained pattern of lower clam growth at the edge of the seagrass bed that experiences the faster flood-tidal current velocities. Each broad process, changing fluid dynamics, altering consumer access, and varying sediment stability, represents a mechanism whereby habitat structure, provided by the dominant plant, has an important indirect influence on the functional value of the habitat for resident animals.

Growth, mortality and recruitment rates of giant clams, Tridacna gigas and T. derasa, at Michaelmas Reef, central Great Barrier Reef, Australia

Growth, recruitment and mortality rates of a population of giant clams (Tridacna gigas and T. derasa) were monitored between 1978 and 1985 in a 2.7 ha study area on Michaelmas Reef, Great Barrier Reef. The initial 1978 census revealed the presence of 1166 T. gigas and 44 T. derasa. For T. gigas, the Fabens method provided growth-parameter estimates of L∞ = 85.7 cm, K = 0.07 and to = 0.732 year. The generated von Bertalanffy growth curve was a relatively poor fit to an empirical growth curve. A better description of growth was obtained for younger clams by using a forced value of L∞, = 80 cm, yielding K = 0.105 and to = 0.145 year. There was marked variability in the growth rate of individual clams, which has implications for the aquaculture industry. Average annual mortality rates in two census intervals (1978 to 1980-81 and 1980-81 to 1985) were 3.4 and 10.7% respectively. A comparison of the calculated size structure (assuming constant recruitment) with the observed size structure clearly suggested that recruitment was not constant and had declined drastically from a peak in the 1950s. For the much smaller population of T. derasa, the Fabens routine yielded estimates of L∞ = 46.91 cm, K= 0.108 and to = -0.188 year. The average annual mortality rate was 4.4%. Trends in recruitment could not be determined because of the small sample size. This study has highlighted the need for follow-up studies of this and other populations of giant clams if we are to understand more fully the processes of growth, recruitment and mortality in wild stocks and the implications for stock management and aquaculture.

Effect of Temperature and pH on the Toxicity of Monochloramine to the Asiatic Clam

The Asiatic clam (Corbicula fluminea) has become a pest to many users of freshwater transmission systems. Monochloramine has been suggested as a biocide because it is effective and the formation of trihalomethanes is low (see the article on page 53 of this issue). Flow‐through aquariums, a laboratory incubator, and a pilot‐scale pipeline were used to show that monochloramine concentrations between 1 and 10 mg/L effectively killed juvenile clams over a wide range of water temperatures, with mortality increasing significantly with temperature. The reduction in clam mortality as water temperature decreases can be overcome by corresponding increases in biocide concentration. An equation is provided that predicts the kill that would be achieved at a given biocide residual and water temperature. The pH did not affect the efficacy of the biocide.

Assessment of the biological impact of point source discharges employing Asiatic clams.

Three thousandCorbicula fluminea were transplanted for six months along a stress gradient produced by the thermal discharge of a power plant to ascertain whether Asiatic clams could be employed to monitor the biological impact of point source discharges. Water temperature at the warmest site (station 1) was between 25 and 35°C, 7–13°C above that in the river. Almost complete clam mortality was recorded here within 60 days. Water temperature at the next cooler site (station 2) was elevated 0.5–2.0°C above ambient river temperature (stations 3 and 4). Adult and juvenile clam shell and tissue growth and condition index was less here (P 0.1). A one time measurement of juvenile clam scope for growth (SFG) also demonstrated a decreasing trend along the thermal gradient. SFG was positively correlated with juvenile clam shell and tissue growth (P P > 0.05).

Clam burrowing behaviour and mortality related to sediment copper

Gravel sediment freshly enriched with over 4.4 μg Cu g −1 significantly increased the burrowing and reburrowing times of Littleneck marine clams ( Protothaca staminea). Abnormal burrowing behaviour was observed with time-lapse video. Aged (1 day) copper-enriched sediment did not affect clam burrowing time. Copper in sediment porewater declined three days after sediment enrichment to background levels without change in total sediment copper. In a long-term experiment, clams with increased burrowing time into copper-enriched sediment had up to 25% mortality after 12–29 days burial in the sediment. The majority of clam mortality was found in clams burrowing within the first 48 h. Over 7 weeks there was blackening of sediment and clam shells proportional to sediment copper enrichment.

Crab Response to Prey Density Can Result in Density-Dependent Mortality of Clams

Field experiments involving recovery of marked clams showed that even adult native littleneck clams (Protothaca staminea) could suffer high mortality to cancrid crabs. The mortality rate increased with clam density, suggesting that these crabs preferentially forage in areas of higher prey density. Acoustic telemetry showed that at least one of the crabs, Cancer productus, is sufficiently mobile to search large areas while foraging. This is important for understanding the effects that these large, mobile predators have on the distribution and the abundance of their prey and may explain the high incidence of crab predation seen in mariculture operations where the density of clams is artificially raised.

The Importance of Predation and Intra‐ and Interspecific Competition in the Population Biology of Two Infaunal Suspension‐Feeding Bivalves, Protothaca staminea and Chione undatella

The Importance of Predation and Intra‐ and Interspecific Competition in the Population Biology of Two Infaunal Suspension‐Feeding Bivalves, Protothaca staminea and Chione undatella

Some effects of gas-supersaturated seawater on Spisula solidissima and Argopecten irradians

Two size classes of the surf clam, Spisula solidissima, and the bay scallop, Argopecten irradians, were exposed to different concentrations of gas-supersaturated seawater in a flowing seawater system. Both species tested experienced no mortality when held in the control treatment maintained at 96% oxygen and 109% nitrogen. Mortality, gill tissue damage, gas emboli, membranous tissue blisters, and abnormal secretion of shell material were induced experimentally at elevated levels of gas supersaturation. Results indicate significant mortalities of surf clams and scallops held at 114% O 2 and 195% N 2, and at higher levels of gas concentration. These values suggest a point of reference for the bivalve culturist in identifying potential problems which can be caused by gas-supersaturated seawater.

Size-selective mortalities of clams in an oil spill site

Mixed No. 2 fuel oil and JP5 jet fuel, following an oil spill into Long Cove, Searsport, Maine, U.S.A., in March 1971 became concentrated locally at levels up to more than 250 ppm in intertidal sediments from 15 to 25 cm below the surface and continued until 1976 to kill successive year class juvenile clams as in normal growth behaviour they burrowed down through redistributed overlying clean sediments into the oil concentration beneath.

The ecological, chemical and histopathological evaluation of an oil spill site: Part I. Ecological Studies

An oil spill into Long Cove, Searsport, Maine, began on 16 March and lasted until at least 30 June 1971. It resulted in immediate and continuing soft clam mortalities which, based on before and after biological surveys, had by August 1974 exceeded 85% of the estimated 50 million market-size clams occupying the area.

Bacteriological Studies of Wicomico River Soft-Shell Clam (Mya arenaria) Mortalities

Bacteriological studies were carried out to determine a causative agent of soft-shell clam mortalities occurring in Chesapeake Bay, i.e., Eastern Bay to the Wicomico River, in midwinter, 1970 and through 1971. Several screening tests were performed in preliminary experiments designed to detect the presence of potential pathogens. Growth of 57 cultures isolated from count plates was tested at cold temperatures over long periods of time. Sixteen strains grew under the conditions employed in this screening and of these, eight strains were found to cause death of all animals within 48 hours and to show good growth at 4 C within one week. Numerical taxonomy was employed to identify and classify the isolates. Two of the eight strains, one identified as a member of the Enterobacteriaceae and the other as a Vibrio sp., were considered to be potential pathogens for the soft-shell clam. Direct evidence of the pathogenicity of these strains is not yet conclusive, although pathogenicity is strongly indicated from the results of experiments carried out in this study.