The Ectoparasitic Gastropod Boonea (= Odostomia) impressa: Population Ecology and the Influence of Parasitism on Oyster Growth Rates

Abstract. Boonea (= Odostomia) impressa is a common ectoparasite of oysters. In the laboratory, small oysters (Crassostrea virginica) parasitized by natural densities of B. impressa produced 75 % less new shell than unparasitized oysters. Shell deposition rates of previously parasitized oysters increased significantly after all B. impressa were removed. Thus, the decrease in growth rate, although significant, apparently was not permanent. B. impressa preferentially parasitized small, living oysters (≤2.5cm) in the field, even though a higher percentage of large, living oysters (>2.5cm) was available. The snails maintained an aggregated distribution on the oyster reef. The number of B. impressa per oyster clump was positively correlated with the number of living oysters per clump, however some clumps with few or no living oysters had many B. impressa. Thus, food availability only partially explained the pattern of distribution. B. impressa was very mobile. About 50 % of the population moved in one week. Reproduction occurred throughout the year with a peak period in May. Recruitment was greatest in July, however new recruits were observed throughout the year. The reduction in growth rate of parasitized oysters, the snaiľs propensity towards parasitizing small oysters and the snail's tendency to be contagiously distributed suggests that B. impressa potentially exerts a significant influence on the population structure and health of oyster populations.     

Selectivity by dense populations of small invertebrates foraging among seagrass blade surfaces

Current information on feeding habits of the numerous small invertebrates concentrated among seagrass blades has been inadequate to predict basic food requirements, natural influences of these crowded animals on their shared foods, or the animals’ resulting influences on each other. Apparently detrital food webs in seagrass meadows are reported frequently, but recent data from various seagrass meadows and other environments indicate that such detritus is often refractory to digestion by microorganisms and small invertebrates. A summary of literature on natural feeding habits among common seagrass meadow invertebrates, and detailed analyses of foraging by the commonest invertebrates in NW Gulf of Mexico seagrass meadows, show various degrees of feeding selectivity for epiphytic algae. Different species of epiphytes progressing along a seagrass blade can provide a gradient of food types and abundance for detailed studies on selective feeding. For minimal disturbance to feeding behavior, remote photographic sampling coupled with microacoustic monitoring and immediate, high-resolution gut analyses enable one to compare foraging frequencies on different foods to the available areas of those foods. Such comparisons can be interpreted for evidence of selectivity when an animal forages on a food more frequently than is expected by chance movement over the available areas of food. Feeding appears to be most frequent while the various invertebrates are among epiphytic algae at night, not while they are on bottom detritus. Based on available evidence from various detrital and other food webs, an hypothesis for future research is derived in which particular ephemeral algae are generally selected over other foods, but detritus may be important insteads when particular epiphytic foods are scarce. Even when total detrital foods are common, highly selected foods may be limited among such densely populated animals as in seagrass meadows.