RHYTHMS IN LARVAL RELEASE BY AN ESTUARINE CRAB (RHITHROPANOPEUS HARRISH)

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Abstract
Ovigerous females of the crab R. harrisii were collected subtidally and their rhythms in larval release monitored under constant conditions in the laboratory. Larvae from a single crab are generally released as a burst lasting < 15 min. Larval release by crabs from an estuary lacking regular tides mainly occurs in the 2-h interval after sunset and is not related to coastal tides, which suggests a circadian rhythm. This rhythm can be entrained on an altered light-dark cycle. Larval release by crabs from an estuary with semi-diurnal tides begins at high tides and continues for 2 h, suggesting a circatidal rhythm. Significantly more releases occur during the night. Crabs from the estuary without regular tides change from a circadian to a circatidal rhythm after being in the estuary with semi-diurnal tides. Alternatively, crabs from the estuary with semi-diurnal tides change to a circadian rhythm when exposed to a light-dark cycle and nontidal conditions in the laboratory. R. harrisii has both circadian and circatidal rhythms in larval release with the expressed rhythm dependent upon prior environmental conditions. Nighttime release may reduce predation, while release at high tide may minimize larval exposure to stressful, low salinity water.

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