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The presence of a locomotory rhythm in the hatching stage of the crab Callinectes sapidus Rathbun was investigated. Larvae were subjected to 12 : 12 light: dark (LD) regimes for the first 48 h after hatching, then separated into groups held on the 12 : 12 LD regime or held in constant light (LL). Swimming speeds were measured at 6 h intervals for larvae in the two groups. In larvae maintained in LD regime, a peak swim speed was measured at successive 0200 readings with reduction in swim speed by the following reading at 0800. Larvae held in LL showed no such pattern. Continuous LD entrainment is necessary to maintain the rhythm. The rhythm may serve either as a depth regulatory device or as a mechanism for diurnal vertical migration. INTRODUCTION response of the pelagic larva of the brachyuran crab Callinectes sapidus Rathbun. The results are discussed The direction and rate of horizontal movement of in the context of vertical distribution and consequent planktonic animals is often determined by their vertidispersal of the larvae. cal position in the water column. Thus vertical distribution is an important factor in determining retenMATERIALS AND METHODS tion within or recruitment to estuarine habitats in benthic species with pelagic larvae (Camker, 1951; Ovigerous Callinectes sapidus were obtained from Bousfield, 1955; Thorson, 1964; Wood and Hargis, lower Chesapeake Bay and returned to the laboratory 1971; Sulkin, 1973, 1975a). The vertical position of a with egg mass intact. Eggs were then separated from particular larval stage is the result of a complex combithe egg mass, transferred to 25 m1 Erlenrneyer flasks nation of behavioral responses, involving both direccontaining seawater at 30 ppt salinity, and agitated tion and rate of movement, as well as such physical until hatching on a reciprocating shaker at 110 rpm. parameters as size and shape. Periodic variations in Room temperature was approximately 22 "C; ambient behavioral response could have substantial impact light approximated 12: 12 LD. Upon hatching, the larupon vertical distribution and thus influence direction vae were placed in shallow culture bowls measuring and extent of dispersal. 10 cm in diameter. Each bowl contained approximately Periodicity in locomotory response has been 200 larvae in 100 m1 of filtered seawater. Larvae were documented for many marine invertebrates, including fed the rotifer Brachionus plicatilis Muller ad l ibidum zooplankters (e.g. Naylor, 1958, 1963; Williams and (Sulkin and Epifanio, 1975; Sulkin, 1975b, 1978). LarNaylor, 1967, 1969; Enright and Harnner, 1967; Naylor vae were transferred daily to new bowls of fresh seaand Williams, 1968; Hughes, 1969a, b, 1972; Fincham, water and fed. Cultures were maintained at 21'-23°C. 1970; Jones and Naylor, 1970; Allen, 1972; Honegger, Larvae from three different crabs (designated as 104, 1973; Hindley, 1974; Cronin and Forward, 1979; for a 105 and 107) were used in the present experiments. pertinent review consult 'Marine Ecology', Volume 11: From each of the three aabs , four culture groups were Enright, 1975). established (designated as A, B, C, and D). Larvae in The present experiments were designed to deterall groups were subjected to 12: 12 LD regimes for the mine the presence of rhythmicity in locomotory first 48 h after hatching. The light period extended .m Contribution No. 996 from the Center for Environmental from 0800 to 2000; the dark from 2000 to 0800 the and Estuarine Studies, University of Maryland, USA. following day. After the 48 h entrainment period, the Present Address: Department of Biological Sciences, groups were set UP in the experimental light regimes Baltimore County Campus, University of Maryland, USA. indicated in Table 1.

On the Locomotory Rhythm of Brachyuran Crab Larvae and Its Significance in Vertical Migration