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

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.