M. Zachary Darnell

Thermoregulation as an alternate function of the sexually dimorphic fiddler crab claw.

Fiddler crabs are highly sexually dimorphic. Males possess one small (minor) feeding claw and one greatly enlarged (major) claw; females possess two small claws. The major claw is used to attract mates and for burrow defense, but it is costly for the male to possess. We tested the hypothesis that the major claw also functions as a thermoregulatory structure, a function that would allow males to spend a greater amount of time at the surface, foraging and attracting potential mates. Fiddler crabs Uca panacea were exposed to a source of radiant heat and body temperatures were monitored. Four groups of crabs were tested: intact males, males with the minor claw removed, males with the major claw removed, and females. The body temperatures of males without the major claw increased more rapidly and reached higher values than did those of males with the major claw intact, but the results from these animals were similar to those of females. These results support the hypothesized thermoregulatory function of the major claw. The major claw may function as a heat sink, transferring heat away from the body and dissipating it into the air. Enhanced thermoregulatory ability provided by the major claw may partially ameliorate the energetic costs of possessing such a large claw.

Habitat use and population dynamics of blue crabs, Callinectes sapidus, in a high-salinity embayment.

ABSTRACT A high-salinity embayment located near Beaufort, North Carolina was surveyed to create a bathymetric profile of the main basin. To examine population dynamics and habitat use within the embayment, over 3,000 blue crab locations, sex, size, and egg stage were recorded during nocturnal low tides. Males and females partitioned habitat, with males concentrated in the shallower upper portion of the embayment, and mature females concentrated near the deeper mouth of the embayment. Juvenile females were predominantly found in shallower areas within mature male habitat. Location of ovigerous crabs varied with egg developmental stage. Crabs with late-stage eggs were found closest to the mouth of the embayment, and 92.9% (156 of 168) of ovigerous crabs left the embayment before larval release and may not have returned. Because the embayment is homogenous at 35 psu, a salinity gradient is not the primary mechanism underlying this segregation. We hypothesize that tidal activity rhythms and microhabitat selection contribute to the observed spatial patterns.

Entrainment of the circadian rhythm of the supratidal amphipod Talorchestia longicornis by light and temperature: mechanisms of detection and hierarchical organization

The amphipod Talorchestia longicornis is active on the substrate surface during the night and inactive in its burrows during the day. The underlying circadian rhythm in activity can be entrained by the light : dark cycle. This study considered other aspects of entrainment, such as: whether the compound eyes or extraocular photoreceptors are used to detect entrainment cues, which visual pigment mediates the entrainment process, whether entrainment can also occur in response to diel temperature cycles and whether when presented with conflicting entrainment cues, the light : dark or temperature cycle dominates for entrainment. Entrainment was determined by monitoring temporal changes in surface activity using a video system, and assessed as the number of animals active on a sand substrate at 0.5 h intervals. The compound eyes and not extraocular photoreceptors are used for entrainment to the light : dark cycle. Although T. longicornis has two visual pigments with absorption maxima near 420 and 520 nm, entrainment occurred only by light that simulated the 520 nm absorbing pigment, perhaps because its absorption is matched to the spectrum of ambient light at twilight. The rhythm is also entrained by a diel temperature cycle. If amphipods are simultaneously exposed to a temperature and light : dark cycles phased differently, they entrain to the temperature cycle. These results suggest that for a burrowing amphipod, diel temperature cycles may be a more reliable indicator of environmental conditions than light cycles.

Postcapture Survival and Future Reproductive Potential of Ovigerous Blue Crabs Callinectes sapidus Caught in the Central North Carolina Pot Fishery

Abstract Harvest restrictions by sex or reproductive status are used to protect many spawning stocks. In most U.S. states, fishery regulations for blue crabs Callinectes sapidus require release of ovigerous crabs. Ovigerous crabs caught in pots become stressed by capture and physically damage the egg mass and remove eggs. We conducted a survey to assess the extent of egg mass damage in pot-caught crabs as well as crabs caught by hand and not subjected to pot stress. Egg mass damage was more prevalent in pot-caught crabs (>45% during all months) than in crabs caught by hand (<6% during all months). We investigated the postcapture survival, reproductive output, and larval viability of crabs with varying amounts of egg mass damage by collecting ovigerous crabs from the pot fishery and confining them in the field for the duration of their lifetimes. Over 80% of the crabs survived to release the clutch present at capture, and crabs produced up to six clutches of eggs. Of 156 clutches produced in confinement, o...

Temperature-Dependent Growth and Molting in Early Juvenile Blue Crabs Callinectes sapidus

ABSTRACT Understanding how growth rates are influenced by environmental conditions is important for proper management of commercially harvested species, especially under current and predicted future conditions of global climate change. Blue crabs are of high commercial value in the United States, but are declining in number throughout their range. Previous laboratory and field studies suggest that temperature and salinity play a role in blue crab growth rates, although the results of these studies have been inconclusive and, in some cases, contradictory. Furthermore, these studies have focused primarily on later juvenile stages [>20 mm carapace width (CW)]. This study examined the effects of temperature and salinity on early juvenile (∼2.5–16 mm CW) blue crab Callinectes sapidus growth rates. Crabs were reared in the laboratory under six different temperature and salinity treatments from the megalopal stage until the sixth juvenile instar. Compared with crabs reared at lower temperature, crabs reared at higher temperature exhibited a decreased intermolt period (IMP) and also decreased growth per molt. Salinity had no significant effects on IMP or growth per molt at 20°C and only slight effects on IMP at 30°C. These results suggest that temperature plays a major role in determining blue crab growth rates during the early juvenile instars, but the effects of salinity are relatively minor. Given forecasted increases in temperature throughout the geographic range of the species, these results suggest that blue crabs will reach maturity faster in the future, but will be smaller in size. Future studies should investigate potential causes of the correlation between salinity and blue crab size, and consider additional environmental influences.

A novel running wheel apparatus to monitor locomotor rhythms in land crabs

This article was downloaded by: [Duke University]On: 23 August 2009Access details: Access Details: [subscription number 906453949]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK