Reference: Biol. Bull. 163: 287-300. (October, 1982) RHYTHMS IN LARVAL RELEASE BY AN ESTUARINE CRAB (RHITHROPANOPEUS HARRISII} R. B. FORWARD, JR., K. LOHMANN, AND T. W. CRONIN 1 Duke University Marine Laboratory, Beaufort, NC 28516, and Zoology Department, Duke University. Durham, NC 27706 ABSTRACT Ovigerous females of the crab Rhithropanopeus 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 less than 1 5 minutes. 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 hours, 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 non-tidal conditions in the laboratory. Thus 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. INTRODUCTION Rhythms in reproductive activity and larval release are common among crus-taceans. Timing may be related to lunar phase, time of day, and/or phase of the tide. Semilunar cycles are known for semi-terrestrial crabs (Clifford, 1962; Warner, 1967; Henning, 1975; Klassen, 1975; Saigusa and Hidaka, 1978; Seiple, 1979; Sai-gusa, 1981), intertidal fiddler crabs (von Hagen, 1970; Zucker, 1976, 1978; Christy, 1978; Wheeler, 1978), and subtidal stomatopods (Reaka, 1976). For the lobsters Nephrops norvegicus (Moller and Branford, 1979), Homarus gammarus (Ennis, 1973; Branford, 1978), and H. americanus (Ennis, 1975), larval release in the lab-oratory occurs shortly after dusk on a series of consecutive nights; no lunar or semilunar rhythm has been reported. H. americanus occasionally releases larvae during the day (Ennis, 1975). Detailed laboratory studies of estuarine intertidal fiddler crabs indicate that fe-males release their larvae within several hours after the time of the nocturnal high tide (DeCoursey, 1979; Bergin, 1981). As implied by DeCoursey (1979), precisely timed larval release may not be restricted to intertidal fiddler crabs but could also extend to other estuarine species. This study was undertaken to examine larval release by the estuarine crab Rhith-ropanopeus harrisii which occurs from the very low intertidal zone into subtidal Received 10 March 1982; accepted 16 July 1982. ' Present address: Department of Biology, Yale University, New Haven, CT 0651 1. 287
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