Thomas G. Wolcott

Ecological rôle of ghost crabs, Ocypode quadrata (Fabricius) on an ocean beach: Scavengers or predators?

Abstract Food habits and ecological role of the ghost crab. Ocypode quadrata ( Fabricius) on a North Carolina barrier beach have been investigated in field and laboratory studies. Despite previous reports that they are scavengers, the crabs spent little time in ureas where drifted material accumulated. Dead material accounted for less than 10% of the food in the field. The crabs did. however, give evidence of being facultative scavengers, readily consuming virtually any form of organic matter. Live prey, consisting almost exclusively of mole crabs, Emerita talpoida (Say), and coquina clams, Donax variabilis Say, made up more than 90% of the diet. Handling times indicate that about equal weights of E. talpoida and D. variabilis are consumed. Because of its higher caloric content E. talpoida provides ≈60% of the energy and D. variabilis ≈25%. The effect of O. quadrata on the prey species was assessed by comparing the estimated rates of its feeding (based on resting metabolism) with estimated production of E. talpoida and D. variabilis . Calculations indicate that ghost crabs consume most of the production of both species. Ghost crabs have essentially no terrestrial competitors or predators on the beaches concerned and the stability of this simple food web in such a physically unstable environment may be attributed to the flexible feeding behavior of the predators and their ability to endure long periods of starvation, and to the prey having high biotic potential and dispersal rates. Ghost crabs are the top carnivores in a simple, filter-feeding based food chain.

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.

The influence of male mating history on male–male competition and female choice in mating associations in the blue crab, Callinectes sapidus (Rathbun)

Male Callinectes sapidus allowed complete recovery of sperm resources and then mated a single time had significantly lower vas deferens weight than males allowed complete recovery of sperm resources but prevented from mating. In laboratory experiments, when a recently mated male (having low sperm volume) competed with a male that had not recently mated (having high sperm volume) for a single pubertal female, the female was just as likely to initiate pairing with the recently mated male as with a male that had not recently mated, despite possible reduction in her fertilization potential. At the end of trials in which stable pair formation occurred, recently mated males were paired significantly more often than males that had not recently mated. The combined effects of the lack of mate choice by females and high mating frequency of some males may result in many females in the population receiving low quantities of sperm.

A new acoustic tailbeat transmitter for fishes

A new acoustic transmitter for monitoring the tailbeat frequency of free-swimming fishes and sharks is described. The transmitter is externally mounted on the fishs caudal peduncle and produces an acoustic pulse with every lateral movement of the tail. Field tests on obligate-swimming juvenile scalloped hammerhead sharks (Sphyrna lewini) show these transmitters accurately telemeter tailbeat deflections from free-swimming sharks. Tests in a flume show sharks with transmitters work harder and swim slower than sharks without transmitters, but these laboratory data can be used to correct for instrument artifacts on free-swimming sharks. A shark tracked with a tailbeat-sensing transmitter exhibited similar movement and activity patterns to shark pups tracked with smaller, orally inserted internal transmitters. The similarity of tracks indicate that the shark equipped with the tailbeat transmitter behaved normally. Thus, this new transmitter design offers a cost-effective, minimally intrusive method for accurately measuring activity rate and energy consumption of active fishes and sharks in the field.

NEW OPTIONS IN PHYSIOLOGICAL AND BEHAVIOURAL ECOLOGY THROUGH MULTICHANNEL TELEMETRY

Abstract This paper is intended to acquaint physiological and behavioural ecologists with the options offered by modern telemetry technology, and encourage collaboration between biologists and engineers. When direct observation is impossible, telemetry can be used to acquire a wide spectrum of environmental, physiological and behavioural data. This spectrum is expanding rapidly due to dramatic increases in the availability of miniaturized, micropower transducers and electronic components. The “pros and cons” of telemetry are presented, particularly with reference to crustaceans in marine systems. The kinds of phenomena that now can be telemetered are listed, alongside representative devices that transduce them into electrical signals. Transduced data may be stored for subsequent uploading, or transmitted over radio or ultrasound carrier waves. Radio is most useful in air, ultrasound in saline (conductive) water. Data can be encoded on pulsed or continuous carriers, depending on the data rate needed; common schemes are summarized. A sequence of questions about crab ecology is used to illustrate how both simple and complex telemetry systems can provide unique data sets and provide significant insights into the biological system. Simple tracking transmitters provided time budgets for ghost crabs [ Ocypode quadrata (Fabr.)], revealing that they are predators rather than scavengers. Single-channel behavioural transmitters have provided detailed spatial and temporal data on how blue crabs ( Callinectes sapidus Rathbun) move about while seeking moulting sites or food, and where they feed or moult. A multichannel system incorporating microcontroller-based “smart” transmitters and multiple receivers integrated with a data-reduction unit is providing automated logging of feeding and threat behaviours, as well as location within a field enclosure, for multiple blue crabs. A rationale is presented for incorporating telemetry technology into a research programme. An incremental approach that involves experienced collaborators is most likely to yield results quickly and minimise expenditure of resources.

Adaptive significance of habitat selection by molting adult blue crabs Callinectes sapidus (Rathbun) within a subestuary of central Chesapeake Bay

Abstract In the Rhode River subestuary, higher proportions of molting male blue crabs occupy a tidal creek, while molting and mating pubertal females are more common in the adjacent river basin. Predation pressure, mate availability, and physiological costs or benefits (competency to molt, survive ecdysis, and postmolt size increment) were estimated for these two habitats using a combination of caging and tethering techniques. Predation on molting crabs was less in the tidal creek, however, cannibalism may not explain these site-specific predation differences. There were no site-specific differences in survival, postmolt size increment, or competency for premolt crabs to molt when protected from predators. The results suggest that by molting in the creek males avoid higher predation in the river. Habitat selection by pubertal females may depend more on mate availability. Females may actively select the river or simply molt in the habitat in which they encounter a mate. Further experiments are needed to resolve the selection pressures eliciting the observed distribution of pubertal females.