Alla V. Silina

The Effect of Muddy Bottom Sediment on the Abundance and Life Span of the Barnacle, Hesperibalanus hesperius, Epizoic on Scallop Shells

Barnacles are an important fouling component of molluscs of commercial value, and they may adversely affect their hosts. The abundance of living and dead individuals, survival of newly-settled young and the maximum life span of the barnacle, Hesperibalanus hesperius, encrusting the shells of the scallop, Patinopecten (Mizuhopecten) yessoensis, were compared for two sites differing in bottom sediment composition. It was found that muddy sediment (mud content > 25.5%) had an adverse effect on the survival of this barnacle. The valves of scallops collected from a sandy substratum had significantly more barnacles than scallops sampled from muddy sand. At the sandy site, the mean number of barnacles on the upper valves of 5-8-year-old scallops was 378.4 ± 9.6 individuals per valve, and on the lower valves, 92.3 ± 1.8 individuals per valve; of these, 38.5-48.3% were alive. At the muddy sand site, the mean number of barnacles on the upper valves of 5-8-year-old scallops was only 154.5 ± 4.5 individuals per valve, and on the lower valves, 31.7 ± 1.4 individuals per valve. However, living barnacles made up only 20.1-22.6% of the total number counted. On muddy sand, the number of barnacles per cm2 of scallop valve increased with scallop age up to 3-5 years, after which it declined. However, on sand, the number of barnacles per cm2 of scallop valve did not change with increase in host age from 5 to 8 years. The young barnacles settled in significantly greater numbers on the shells of scallops living on sand than on muddy sand. The mortality of barnacles during the first month after settlement on the upper scallop valves was about 45 and 80% on sand and muddy sand respectively. Old, large barnacles (with a shell base >10 mm), both living and dead, occurred more often on scallops living on the sandy substratum. The largest barnacles had a base length of 15-20 and 13-15 mm, at the sandy and muddy sand sites respectively. The possible reasons for these differences are discussed.

Trophic relationships in the community of a bivalve mollusk and a boring polychaete

The community of the Japanese scallop Mizuhopecten yessoensis with its dominant endobiont polychaete Polydora brevipalpa, which bores into the scallop’s shell, was studied. It was established that the bulk of the scallop’s food ration consists of diatom algae and, to a lesser extent, flagellates and small invertebrates. The polychaete species considered mainly assimilates diatom algae and, to a significant degree, bacteria. Thus, the elements of the community enter competitive food relations. In the process of feeding, the scallop excretes detritus with its pseudofeces and this way increases the concentration of food available for polychaetes. In the course of its motion and filtration, the scallop enhances the water flow around the polychaete, which also favors the feeding of the latter. Meanwhile, negative relations between the degree of the population of the scallop’s shell by polychaetes and the volume of its internal cavity were recognized; this indicates a decrease in the potential for water filtering and, correspondingly, for food procuring by the host mollusk. With the increase in the polychaete-related bioerosion of the shell, the linear sizes and weight parameters of the scallop fall together with the rate of its growth.

Feeding and growth of Japanese scallop inhabiting different bottom sediment types

Two parts of the population of Japanese scallop Mizuhopecten yessoensis inhabiting sites with different bottom sediments in an open part of Peter the Great Bay of the Sea of Japan were compared. The scallops grew slower on muddy site compared to sandy site despite better food availability at muddy site. The food sources were determined using fatty acids as biomarkers. Analysis of the fatty acid composition of the scallops has identified diatom plankton, flagellates, and invertebrate larvae as the main scallop food sources. Benthic bacteria insignificantly contributed to the scallop diet. The food composition slightly varied in scallops on different sediment types. Most likely, low oxygen content in water, high resuspension of fine sediment particles rich in dead organic matter, and high content of contaminants accumulated in muddy sediments are the main factors of decelerated growth of scallops on muddy sites.

Population Dynamics of the Japanese Scallop Mizuhopecten yessoensis (Bivalvia) Under Conditions of Enhanced Hydrodynamics

A population of the Japanese scallop, Mizuhopecten yessoensis, found in the open area of Peter the Great Bay (Sea of Japan), which was not protected against southeastern summer monsoons, has been studied. Specimens of different ages, from 1 year old to 11 years old, were found in the population. According to a provisional estimation, the total reserve of the Japanese scallop in the population reached 30 000 specimens. After strong typhoons, up to 6 000 specimens were stranded along the shoreline. Even after repeatedly occurring typhoons observed during one particular summer season, the population of M. yessoensis remained rather stable, although its age structure demonstrated both uneven annual recruitment in the population and different survival rates in different generations. Specimens of the highly productive generation of 1999 constituted the bulk of the bottom population and coastal wreckage. At older ages, the probability for the Japanese scallop to be stranded during a storm decreases significantly.

Spatial heterogeneity and long-term changes in bivalveAnadara broughtoni population: influence of river run-off and fishery

A comparison was made of population of the economically important cockleAnadara (=Scapharca)broughtoni (Bivalvia, Arcidae) inhabiting different areas of the Razdolnaya River estuary at the head of Amurskii Bay (Peter the Great Gulf, East Sea). Also, changes in cockle population density and structure, as well as in cockle growth rates during the last 20 years were studied. In all years of investigation, the morphometrical parameters and growth rates of cockles were smaller at the sites located close to the River mouth than farther down-estuary. The differences can be attributed to higher concentration of suspended particulate matter, decreased salinity and water temperature, as well as a longer exposure to these unfavorable environmental factors at sites located close to the River mouth, compared to farther sites. For two decades, cockle population density had decreased by almost 30 times at some sites in the River estuary. The main reason for this population decline is commercial over-fishing of the cockle. Besides, for the last 20 years, linear parameters of the cockles in the population decreased approximately by 30% and weight parameters, almost two times. Cockle growth rates also decreased for this period. Evidently, these facts are due to the damaging effect of dredging.

The Benthic Association between a Bivalve and a Shell Boring Polychaete and Their Potential Food Sources

The trophic relationships in the association of the Yeso scallop Mizuhopecten yessoensis and its dominant endobiontic polychaete Polydora brevipalpa, which burrows into the scallop’s shell, and their potential food sources were studied using the method of fatty acid biochemical markers. It is shown that the differences in the diet of the scallop and the polychaete allow them to coexist in a close association. The trophic role of the association in the benthic community was revealed. The association selectively utilizes the food sources of the environment. It almost does not consume organic matter of bottom sediments, which allows it to coexist with other species in the community of higher order. There is minimal food competition between the association and detritivorous species; however, association can limit the development of species mostly foraging on diatom algae.

A comparative study of samples of the bivalve Ruditapes philippinarum (Adams et Reeve, 1850) from populations and shell assemblages

According to the results of a comparative analysis of the size and age structures of samples from a population of the bivalve mollusk Ruditapes philippinarum (Manila clam) and from shell assemblages of this species, the possibility of data acquisition on populations using samples from shell assemblages was assessed. A considerable coincidence of these parameters after taking the losses of small individuals during formation of shell assemblage into account, which depended on the location and conditions of assemblage formation, was recorded. Due to the good level of preservation of the shells of older individuals, the sample from the assemblage gave a more comprehensive appreciation of the maximum size and life span of the species than the sample of live mollusks from the natural habitat. Samples from shell assemblages may be used for growth rate estimation of clams without catching live individuals, as the results from studying the linear growth of mollusks based on samples from live populations and from shell assemblages did not differ significantly. Samples from many-year shell accumulations can also be useful for revealing the periods of the highest death rates, the most vulnerable periods in the ontogenesis of the mollusks, and, in some cases, the causes of their death.

Seasonal Variability in the Levels of Fe, Zn, Cu, Mn, and Cd in the Hepatopancreas of the Japanese Scallop Mizuhopecten yessoensis

The relationships between concentrations (μg/g of tissue) and content (μg/organ) of the metals Fe, Zn, Cu, Mn, and Cd with seasonal changes in the weight of the hepatopancreas in the Japanese scallop Mizuhopecten yessoensis are studied. It is shown that, during a year, the weight of the hepatopancreas in the mollusk shows significant (more than twofold) changes. A positive correlation is revealed between the seasonal changes in the concentrations of the physiologically important metals Fe, Zn, Cu, and Mn and the weight of this organ, although no general consistent pattern of seasonal variability in the metal concentrations is found. For toxic Cd, a negative correlation is found between its concentration and the weight of the hepatopancreas.

Age, size distribution and growth of native and cultured Japanese scallops in Possjet Bay, Sea of Japan, Russia

Stock size, distribution, size and age composition, and growth of Japanese scallops,Mizuhopecten yessoensis, were studied at eight sites in Possjet Bay, Sea of Japan, Russia. At seven sites, most of the scallops were cultured animals (seeded as one-year-olds in 1986–1989). At the eighth site, only native (i.e. naturally settled) scallops were present. Cultured scallops had an irregular strip-like distribution at all locations. Maximum growth of scallops occurred in the northwestern part of Reid Pallada Bight. Only at Temp Bight did both native and the majority of cultured scallops attain the harvest size of 100 mm shell height at age 3 years. At all other sites in Possjet Bay, cultured scallops reached harvest size at an age of 4 years.

Sex change in scallop Patinopecten yessoensis: response to population composition?

Sex structure is very labile between populations and specific for each population because it is a result of genetic, ontogenetic and biocenotic influences on the mollusks. In this study, the age frequency distribution, age-sex structure, and sex ratio were assessed in the wild populations of the Yeso scallop Patinopecten yessoensis (Jay) observed at fifteen sites in the northwestern Sea of Japan (=East Sea). The sex ratio varied between the populations from 0.83:1 to 1.52:1 (males/females), with the mean sex ratio being 1.03 ± 0.05:1. Within a population, the proportions of males and females in term of number differed between age classes. Males were more numerous than females in the younger age classes, and females prevailed over males in the older age classes. It was found that in different scallop populations the sex change occurred at different ages. In the populations that predominantly consisted of young (two- to four-year-old) individuals, males prevailed over females in the age class 2 yr.; the equal male/female proportion was found in the age class 3 yr.; and in older age classes, females prevailed over males. Another pattern was observed in the populations that consisted mainly of middle-aged (five- to six-year-old) individuals. Here, the age-sex ratio became equal at an age of 4–6 years. In the old populations (mainly 6–12-year-olds) the equal male/female proportion was observed at an age of 8–10 years. Thus, the age of sex change was not uniform for the scallop populations. It depended on the age structure of the population and, thus, was socially controlled. The greater number of females in the older age classes suggests a protandric sex change.