James M. Welch

HERDING LIMITS WATER LOSS IN THE SAND FIDDLER CRAB, UCA PUGILATOR

Abstract Adult sand fiddler crabs, Uca pugilator, forage in large herds on exposed sediment during low tide. These herds form as a response to optimal foraging conditions but may have an additional benefit of water conservation. We determined water loss rates for crabs in groups of different sizes. Crabs in groups of 10 and 20, but not in a group of 5 or isolated individuals, showed a water-conserving group effect, yielding nearly a two-fold decrease in water loss rates. Other characteristics of U. pugilator (water content, dehydration tolerance, activation energies for water loss, critical transition temperature, and critical equilibrium humidity) are consistent with a hydrophilic water balance profile. Because crabs did not replenish water supplies from water vapor like some terrestrial arthropods, water conservation is vital to crab survival outside burrows. Enhanced water retention generated by herding likely permits more time for feeding outside the burrow, while also increasing feeding efficiency by minimizing frequency of water uptake. The fact that U. pugilator is semiterrestrial suggests that a reduction in water loss due to group effects may provide a mechanism for terrestrialization.

Light induced larval release of a colonial ascidian

Larval release and photobehavior were studied in the colonial ascidian Polyandrocarpa zorritensis. The test hypothesis was that if larval release is induced by light, then larvae should be attracted to settlement areas where light is sufficient for larval release. Light induced larval release but the time course varied with light intensity. As the intensity of either sunlight or blue-green light decreased (1) the time until the beginning of larval release (latency) became longer, (2) the mean time of larval release increased, and (3) the time interval over which larvae were released increased. The threshold light intensity to induce larval release in blue-green light (8.75x10(12) photons cm(-2) s(-1)) was lower than that in sunlight (3.6x10(13) photons cm(-2) s(-1)). Light induced larval release was not affected by currents up to 15 cm s(-1). Larvae aggregate in light when given a choice between light and dark. This response did not vary with larval age. The lowest light intensity, at which larvae could distinguish between light and dark was 5.0x10(12) photons cm(-2) s(-1) in blue-green light and 2.9x10(14) photons cm(-2) s(-1) in sunlight. Thus, the hypothesis is supported because larvae are attracted to areas where light intensity is sufficient for larval release.

Effect of arm loss on escape speed of two epibenthic brittle stars

Abstract. Despite its obvious costs, autotomy of a body part is a common and effective behavioral strategy to avoid predation. For several species, this limb loss may impact the ability to escape future predation events. Lizards that lose tails are slower to climb or run, and crickets that autotomize legs can jump less far. Epibenthic brittle stars frequently autotomize arms but the effect of arm loss on their escape speed has not been examined. Using two species, spiny (Ophiocoma echinata) and banded-arm (Ophioderma appressum) brittle stars, we predicted that brittle stars missing an arm would move more slowly than intact individuals. Individual brittle stars, either intact or with one arm cut off, were placed in the center of a sandy, well-lit, subtidal arena and allowed to move under the rocks surrounding the arena. We found a positive relationship between the size of the brittle star and its speed. However, there was no difference between escape speeds of intact brittle stars and ones missing an arm for either species. Our data suggest that autotomy has little impact on speed. Thus, unlike many other species, autotomy of an arm does not appear to negatively impact the ability of brittle stars to escape future predation events.