Diane J. Brousseau

Laboratory investigations of the effects of predator sex and size on prey selection by the Asian crab, Hemigrapsus sanguineus

Laboratory studies have shown that the nonindigenous Asian shore crab, Hemigrapsus sanguineus, readily consumes three species of commercial bivalves: blue mussels, Mytilus edulis, soft-shell clams, Mya arenaria, and oysters, Crassostrea virginica. Although crabs can eat bivalves of a wide size range, they preferred the smaller prey (</=10 mm SL). Prey critical size limits exist for M. edulis and C. virginica, but not M. arenaria, possibly because of differences in shell characteristics among the three species. Crabs preferred M. arenaria over both M. edulis and C. virginica, and M. edulis was strongly preferred over C. virginica in pairwise comparison tests. Experiments to determine feeding rates on mussels showed that H. sanguineus can consume large numbers of mussels daily (12.7+/-11.6 mussels day(-1); sexes pooled; N=59). Mussel consumption rates increased with size of the predator and male crabs consumed more mussels than did similarly sized female crabs. The high densities of Hemigrapsus that occur in the wild, their effectiveness as predators of juvenile bivalves and their large appetites suggest an important role for these predators in restructuring the prey communities in habitats into which they have been introduced.

LABORATORY INVESTIGATIONS OF FOOD SELECTION BY THE ASIAN SHORE CRAB, HEMIGRAPSUS SANGUINEUS: ALGAL VERSUS ANIMAL PREFERENCE

Abstract The Asian shore crab, Hemigrapsus sanguineus (De Haan, 1853), is an opportunistic omnivore with well-developed predatory tendencies and a strong preference for animal food items over algae. In laboratory experiments, 71 percent (319/448) of the crabs given a choice between macroalgae (Enteromorpha spp. and/or Chondrus crispus) and benthic invertebrates (Mytilus edulis and/or Semibalanus balanoides) consumed animals only. There were no significant differences in food preference between sexes or between juvenile and adult crabs. Relative abundance of food type, either algal or animal, in small food patches did not affect crab food preference. Crabs with prolonged starvation periods (5-d), however, consumed both food types more often than those that had been starved for 1-d only. Experiments to determine the effect of conspecifics on food selectivity showed that increased crab density leads to increased diet breadth, suggesting that competition for food can alter food selection patterns of H. sanguineus. Results reported here and in previous studies provide strong evidence that predation pressure exerted by H. sanguineus could play an important role in structuring the post-settlement population dynamics of its invertebrate prey, possibly leading to population declines of commercial shellfish, especially blue mussels.

Seasonal aspects of sarcomatous neoplasia in Mya arenaria (soft-shell clam) from Long Island Sound

Abstract Mya arenaria were collected monthly for 2.5 years from three populations in Long Island Sound. Histopathological examination revealed that 6.1% of the clams from Stonington, Connecticut, 12.9% of the clams from the Saugatuck River, Westport, Connecticut, and 12.7% of those from Old Mill Beach, Westport, also in Connecticut, had sarcomatous neoplasms. This is the first documented account of the occurrence of clam neoplasm in populations from this geographic area. Peak prevalences of 45, 59, and 60%, respectively, were found in clams from the three study sites. The prevalence of neoplasms in clams collected from three epizootic areas showed a pronounced seasonal pattern, with the highest incidences occurring in the late fall-winter of each year studied. The regular, seasonal occurrence of neoplasia in field populations does not support the hypothesis that pollution alone is the cause of the disorder.

Age and growth rate determinations for the Atlantic ribbed mussel, Geukensia demissa Dillwyn (Bivalvia: Mytilidae)

Field studies were conducted in Westport, Connecticut, to determine linear shell growth rates forGeukensia demissa. 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 clearly recognized in all size mussels and the rings were a reliable method for determining age in mussels from Westport. No significant sexual dimorphism in mean annual growth rates was detected. Age-size relationships based on internal shell lines appear to be less reliable, probably because of the interference by shell erosion at the umbo in this species. No well-defined latitudinal patterns in growth could be established forGeukensia demissa.

AN EXPERIMENTAL FIELD STUDY OF SITE FIDELITY AND MOBILITY IN THE ASIAN SHORE CRAB, HEMIGRAPSUS SANGUINEUS

Abstract Experimental field studies at two sites in Long Island Sound have demonstrated that the nonindigenous Asian shore crab, Hemigrapsus sanguineus deHaan 1853, is a highly mobile grapsid crab that shows limited fidelity to a particular shelter or feeding site. Recovery rates of tagged crabs differed significantly at the two sites, but no differences in site fidelity were measurable between males and females at either site. Between-site differences in percent crabs recovered may be due to differences in food and shelter availability at the two sites. There is some suggestion that familiarity with a shelter site may influence site fidelity by leading to reduced mobility. A conservative estimate (based on recovered crabs only) of the mean distance traveled in 24 hours (n = 38) was 7.43 ± 1.54 m; among those crabs recovered a distance > 5 m from the release point (n = 15) the mean distance traveled was 16.87 ± 2.23 m. The rapid, widespread dispersal characteristic of the Asian crab invasion along the east coast of the United States may be due in part to the high adult mobility and low site fidelity exhibited by H. sanguineus.

FIDDLER CRAB BURROW USAGE BY THE ASIAN CRAB, HEMIGRAPSUS SANGUINEUS, IN A LONG ISLAND SOUND SALT MARSH

Abstract The Asian shore crab, Hemigrapsus sanguineus, was observed occupying fiddler crab burrows (Uca pugnax) at low tide in a Spartina salt marsh at Sherwood Island, Westport, Connecticut. Forty-seven percent (48/103) of the fiddler crab burrows censused were occupied by crabs. Of those, 81% held fiddler crabs while the remainder held Asian crabs. Fiddler crabs and Asian crabs were never found in the same burrow. Unlike the Asian crab, fiddler crabs preferred areas of the “marsh edge” where rocks and small stones were not present. Hemigrapsus sanguineus, which can be found under the shelter of rocks, shells, and other debris on tidal flats along the fringes of the marsh, probably searches the marsh edge as the tide recedes for unused burrows to occupy. Field caging experiments used to investigate possible competitive interactions between these two species indicated that the presence of the Asian crab had no effect on burrow utilization by the fiddler crab. It is unlikely that patterns of habitat use by the east coast salt marsh fiddler crab, Uca pugnax, will be significantly affected by the recent introduction of the Asian crab, H. sanguineus, to this area.

Field and laboratory comparisons of mortality in normal and neoplastic Mya arenaria

The results of a 6-month mark and recapture experiment involving approximately 900 adult Mya arenaria demonstrated that under natural conditions, significantly higher (P much less than .001, chi 2 test) mortality occurred among animals with neoplasia than those diagnosed as normal. 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): Nonneoplastic (NN), 0%; low severity neoplastic (LSN), less than 50%; and high severity neoplastic (HSN), greater than 50%. Fifty-one percent of those clams initially diagnosed as HSN died by the end of the test period as compared to 8% of the LSN clams and only 3% of the normals. Both progression and remission of the disease were also evident. Approximately 10% of the clams in the NN and LSN groups progressed to a LSN or HSN condition, whereas 16% of those clams initially identified as LSN, and that were recovered alive, underwent complete remission during the test period. Comparison of the field results with those of an 18-week laboratory study suggests that studies of mortality done under laboratory conditions may not provide useful data for the interpretation of the quantitative effects of a disease process, such as molluscan neoplasia, on the natural population of the animal studied.

A comparison of reproductive patterns and adult dispersal in sympatric introduced and native marine crabs: implications for species characteristics of invaders.

The introduction and translocation of nonindigenous marine species is widespread and can pose severe threats to biodiversity and ecosystem functioning. Predicting which species are potential invaders is of particular interest to ecologists. One approach is to identify characteristics that predispose a species to becoming a successful invader. Since its introduction in the 1980’s, the invasive Asian shore crab, Hemigrapsus sanguineus, has shown a remarkable ability to colonize rocky intertidal habitats along the east coast of the United States. In Long Island Sound H. sanguineus occurs sympatrically with the functionally equivalent, but non-invasive, native Atlantic mud crab, Panopeus herbstii. The presence of both species at the same site allowed us to make a detailed, simultaneous assessment of life history traits and adult dispersibility of co-occurring invading and native crab species. We investigated fecundity and maturation rates, length of breeding season and brood production for both species, and conducted field experiments using mark–recapture techniques to determine mobility patterns. Our results show that the nonindigenous Asian crab has a greater reproductive potential than the native mud crab as evidenced by a longer breeding season, multiple brood production and higher fecundity rates. Field experiments confirmed previous studies indicating H. sanguineus is a highly mobile crab, and further demonstrated that adult Asian crabs are more likely than mud crabs to disperse from their shelter/refuge sites. Recovery rates for native mud crabs were significantly higher than those for Asian crabs in three experimental trials, across sites and years. This work provides new information about life history characteristics of both species and supports the hypothesis that high reproductive potential combined with high adult dispersal ability may be important factors associated with the invasion/establishment success of the Asian shore crab. More study is needed, however, to determine the applicability of these findings to other highly successful marine invaders.

Impact of Predation by the Invasive Crab Hemigrapsus sanguineus on Survival of Juvenile Blue Mussels in Western Long Island Sound

Abstract Hemigrapsus sanguineus (Asian Shore Crab) has shown a remarkable ability to colonize rocky intertidal communities along the east coast of the United States since its introduction in the late 1980s and is an important predator of juvenile Mytilus edulis (Blue Mussel) in invaded habitats. In this study, we used two field-caging experiments and the Kaplan-Meier model to assess the impact of predation by Asian Shore Crab on the survival of juvenile Blue Mussels in an intertidal habitat of western Long Island Sound along the Connecticut coastline. Five treatment levels (high-density enclosure, low-density enclosure, exclosure, partial cage, and open plot) were used in the 2007 experiment. The high-density enclosure treatment was omitted in the 2010 experiment since there was no statistically significant difference in the proportion of mussels surviving between low- and high-density crab treatments in 2007. In 2007, we measured a statistically significant difference in mussel mortality between exclosure and crab-enclosure cages, with crabs lowering the median survival time for mussels from 15.4 to 7.6 days. In 2010, we again measured a statistically significant difference in mussel mortality between exclosure and crab-enclosure cages, suggesting a crab effect on mussel survival. In the 2010 experiment, approximately 25% of the mussel mortality was attributable to crab predation, which reduced median survival time for mussels from 12.8 to 5.6 days. The median survival time for mussels exposed to the full complement of factors affecting survival (open plots and partial cages) was only 2–3 days. Our study shows that predation by Asian crabs may account for up to 25% of the Blue Mussel mortality in the intertidal zone at Black Rock Harbor. Further studies focusing on the importance of other biotic and abiotic factors are needed to understand the apparent declines in Blue Mussel populations and the interannual variability in recruitment success in this area.

Antimicrobial Activity in the Pallial Cavity Fluids of the Oyster Crassostrea virginica (Gmelin) from a Highly Impacted Harbor in Western Long Island Sound

ABSTRACT Fluid and its associatedmucus from the pallial (mantle) cavity of eastern oysters Crassostrea virginica (Gmelin) from Black RockHarbor, Bridgeport, Connecticut, inhibited growth of both Gram-positive (Bacillus subtilis, Staphylococcus aureus) and Gram-negative (Escherichia coli, Serratia marcescens, and Vibrio parahaemolyticus) bacteria in antimicrobial assays. In the presence of oyster fluid, E. coli resulted in significant reduction in growth after 26 h. Soluble lysozyme activity in pallial cavity fluid of oysters collected in the fall was 3 times greater than that measured in combined winter—spring—summer samples (P = 0.0008). During the course of the study, copper concentrations in pallial cavity fluid ranged from 0.60–2.49 ppm and zinc concentrations ranged from 9.7–61.0 ppm. Copper concentrations remained relatively constant throughout the study; the highest zinc concentrations were recorded in the fall. Fall antimicrobial assays showed heightened antimicrobial activity compared with the spring, which may be the result of increased lysozyme activity and higher zinc concentrations present in the pallial cavity fluid at that time of year. Results of this study suggest that pallial cavity fluid and its associated mucus likely serve an important role in defense-related functions as the first line of defense against infections from environmental pathogens in Crassostrea virginica.