Dispersal strategies of two species of swimming crab on the continental shelf adjacent to Delaware Bay

Brachyuran crab larvae were collected on 5 cruises from late July through early September (1981 and 1982) at 3 stations located along a transect extending 30 km SE from Cape Henlopen at the mouth of Delaware Bay, USA Samples were taken from the neuston, from l m beneath the surface, from mld-depth, and from l m above the bottom. All stations were dominated by 3 taxa: Callinectes sapidus, Ovalipes ocellatus, and Uca spp Two of these dominant taxa belong to the same famlly (Portunidae) and have spawning populatlons in the estuary or in the sublittoral zone of the adlacent inner shelf. C, sapidus maintained a distribution near the surface at each of the 3 stations. In contrast 0. ocellatus showed a deeper distribution at stations farther offshore. At the station farthest from the bay mouth (30 km), 0. ocellatus showed a distinct concentration at the thermocline during daylight. At that station, C. sapidus larvae maintained a surface distribution regardless of time of day, while 0. ocellatus showed a pronounced shift up the water column after sunset and a shift down the water column after sunrise. At the offshore station, very few larvae of either species were found below the thermocline regardless of time of day.