Donna L. Wolcott

Nursery role of seagrass beds: enhanced growth of juvenile blue crabs (Callinectes sapidus Rathbun)

Abstract The role of submerged aquatic vegetation in supporting enhanced growth of Callinectes sapidus was investigated through field and laboratory experiments. In predator-free enclosures (1 m 2 ) in the lower York River, Virginia, juvenile blue crabs within Zostera marina (L.) beds grew faster than crabs in enclosures deployed outside the beds. First stage crabs were introduced into vegetated or un vegetated enclosures at either 10 or 50 crabs m −2 . After the 6 wk experimental period, both survival and growth were significantly higher in vegetated treatments (growth was estimated by change in “volume,” the product of carapace width, length and depth). In fiberglass mesocosms (2.67 m × 1.33 m × 0.67 m) divided into vegetated and unvegetated halves, juvenile blue crabs grew faster in the vegetation, consistent with field findings. Where differences existed between density treatments in field enclosures, juvenile crabs grew faster in high density than in low density treatments. Aggregate crab growth (summed “volumes” of all recaptured individuals) for vegetated enclosures was greater than for unvegetated enclosures. Potential contribution of cannibalism was sufficient to explain some within-habitat density effects, but was not sufficient to account for the entire aggregate differences, suggesting that food may not be limiting within the seagrass beds even at 50 crabs m −2 . These results show that early stage blue crabs receive a substantial growth advantage, in addition to the refuge function shown in other studies, from their association with seagrass beds. This trophic advantage may be experienced by juveniles of other species that utilize vegetated nursery areas and may help explain the ontogenetic habitat shifts that characterize many life histories.

Impact of off-road vehicles on macroinvertebrates of a mid-atlantic beach

Potential and actual impacts of off-road vehicle (ORV) use on beach macroinvertebrates were examined on the Cape Lookout National Seashore (North Carolina). Mole crabs Emerita talpoida and coquinas Donax variabilis were not damaged. Ghost crabs Ocypode quadrata were completely protected by burrows as shallow as 5 cm, and therefore were not subject to injury during the day, but they could be killed in large numbers by vehicles while feeding on the foreshore at night. Ghost crab populations on the Seashore were large (10 000 km−1 of beach) and a small proportion of the population would be killed by a single vehicle pass. Nevertheless, predicted population mortalities calculated from observed kills of ghost crabs per vehicle-km ranged from 14–98% for 100 vehicle passes. Currently vehicle use on this beach is light and essentially none occurs on the foreshore after dark. Little impact on beach macroinvertebrates would be expected from this usage pattern. Actual impact on ghost crab populations, assessed by burrow censuses, was negligible. No differences were detected between heavy-use and light-use sites in total population size, average crab size or population change through the heaviest traffic season. However, increases in traffic to levels seen on other beaches, especially night driving, would probably have devastating effects on ghost crab populations. In heavily used areas, banning of ORVs from the foreshore between dusk and dawn may be required to protect this species.

Extrarenal modification of urine for ion conservation in ghost crabs, Ucypode quadrata (Fabricius)☆

Abstract When placed under ecologically relevant hypo-osmotic conditions, ghost crabs lose salts at very low rates and osmoregulate well. How can they do this when they, like other brachyuran crabs, produce urine isosmotic with hemolymph? We tested two hypotheses to account for low rates of ion loss: that crabs reduce urine flow (abandon volume regulation); and that they osmoregulate without compromising volume regulation by producing a dilute excretory product through extrarenal modification of urine. Experimental crabs were volume-loaded and hemodiluted by infusion with deionized water; controls were volume-loaded without hemodilution by infusion with isosmotic saline. Crabs were held in containers which allowed the crabs to move fairly freely and to handle wastes normally, and allowed final excretory product to be collected without disturbing the crabs. Crabs infused with deionized water did not reduce urine flow acutely. They produced a dilute excretory fluid containing as little as 10% of the total osmolytes, Na, and Cl of hemolymph and urine. The net effect on osmoregulation was as though they, like other terrestrial animals when water-loaded, produced dilute urine.

Post-mating behavior, intramolt growth, and onset of migration to Chesapeake Bay spawning grounds by adult female blue crabs, Callinectes sapidus Rathbun

Abstract After molting to maturity, female blue crabs must rebuild muscles atrophied to permit molting and grow larger ones commensurate with the larger exoskeleton. They also must acquire energy for oogenesis and for migration to high-salinity spawning habitat, a distance of >150 km for females mating in the Upper Chesapeake Bay. Using telemetry and mark–recapture techniques, post-copulatory females in the upper bay were shown to forage at high rates, alternating between meandering and directed movement in the area of mating for weeks to months, and to begin migrating in October. Consequently, females from the Upper Chesapeake Bay probably do not spawn until the season after mating. Their priority seems to be to acquire energy before migrating. After molting, energy was allocated first into somatic tissue and eventually into hepatopancreas and gonads. Telemetry of feeding and movement showed that habitat utilization, traveling velocities, foraging patterns, and movements were similar to those already determined for males. However, females appeared to invest proportionally more energy (calories per gram dry weight) into their somatic and reproductive tissues than did males. A newly designed transmitter that telemetered depth showed that females moved during both ebbs and floods and remained at or near the bottom of the water column.

Food Quality and Cannibalism in the Red Land Crab, Gecarcinus lateralis

Nitrogen (N) is a scarce and perhaps limiting nutrient for many herbivores. Herbivore populations that are able to adjust density rapidly in response to changes in ambient N levels would be able to optimize growth, survival, and reproduction during periods when N is more available and to avoid habitat depletion and mass starvation should N availability decline. We report here that the natural plant diet is growth limiting for the red land crab Gecarcinus lateralis (Freminville) on Bermuda, and that supplementation of the diet with high-nutrient food (soybeans) results in markedly reduced cannibalism of adults on conspecific juveniles. This predicts that populations in habitats where food quality is high will have greater survival of recruits than those in nutrient-deficient habitats. Nutritionally modulated cannibalism would allow population size to expand and contract with nutrient availability while minimizing mortality in reproductively active age classes.

High Ammonia and Low pH in the Urine of the Ghost Crab, Ocypode quadrata

Nitrogen excreted into the urine (<1 mM) has generally been considered a negligible component of total nitrogen output of crustaceans. But concentrations of ammonia >100 mM were found in the urine of laboratory-held Ocypode quadrata, suggesting that this notion might not be applicable to all crustaceans. To address this issue, hemolymph and urine were removed from freshly captured O. quadrata and analyzed for nitrogenous catabolites and major ions. Hemolymph composition was similar to that of other crustaceans, but the urine was acidic ({Xbar} pH = 5.50) and contained ammonia, often at >100 mM. Other nitrogenous catabolites in the urine (urea, amino acids, and uric acid) were much less concentrated: totaling <12 mM on average. The ionic composition of the urine was similar to that of other crustaceans, with the exception that Na was much less concentrated than Cl-. Total osmolality of hemolymph and urine was similar. The Na+/K+ ATPase activity was greater in the antennal glands than in the posterior gills of O. quadrata, suggesting that this enzyme is important for ammonia concentration and Na resorption. This pattern of enzyme activity was not present in two terrestrial brachyurans whose urine contains little ammonia. The evolutionary significance of high ammonia concentrations in the urine of ghost crabs is unclear.

When Limiting Factors aren’t: Lessons from Land Crabs

Marine animals invading the intertidal zone face a number of potentially stressful or lethal physical factors (e.g. extreme temperatures, desiccation). As physiological ecologists, we have set out to investigate the various sorts of mechanisms by which animals cope with intertidal and supratidal existence, and our research has a larger physiological emphasis than most other programs represented in this symposium. However, many of the adaptations for coping with the challenges of the physical environment are behavioral, instead of (or in addition to) physiological. In this paper we discuss how the concept of physical “limiting factors” has been used to elucidate the coping mechanisms of a series of intertidal and terrestrial crabs. Two types of behavioral adaptation are included in the discussion: 1) behaviors which contribute to homeostasis during stress and might be considered part of physiological mechanisms (e.g., behaviors for taking up water); and 2) behaviors which remove the animal from stressful microenvironments (e.g., selection of specific microhabitats, locomotion, behavioral range limitation).