SMYTH: CALLINECTES LARVAE IN MIDDLE ATLANTIC BIGHT 



Callinectes larvae were most abundant outside the 

 coastal boundary layer in my collections as well as 

 in those of Nichols and Keney (1963). Callinectes 

 juveniles or adults were not collected during this 

 study in extensive benthic sampling (otter and 

 small biological trawls, dredge and grab 

 samplers), and small adults were collected only 

 once in plankton samples (C. similis, n = 5, sta- 

 tion CI, neuston, October 1975). Thus, this study 

 presents no evidence for recruitment to inshore 

 and estuarine populations, either by juveniles or 

 megalopae; the evidence, however, does not pre- 

 clude recruitment from the offshore larval popu- 

 lation. 



General longshore drift in the Middle Atlantic 

 Bight is south westward (Iselin 1955; Harrison et 

 al. 1967; Bumpus 1973; and others) and may occur 

 as sporadic events rather than in a continuous 

 sweep (Ruzecki et al. 1976). Reversals of the 

 longshore flow, usually confined to a narrow belt 

 close inshore, may occur from April to September 

 (Bumpus 1969, 1973). A general constraint, how- 

 ever, seems to be placed on larval origins, viz. 

 adult populations are more likely to be re- 

 plenished by recruitment from larval populations 

 spawned to the north. Given a mean longshore 

 drift of 5 crrLS (Bumpus 1973) and a megalopal 

 duration of 5-67 days (Costlow 1967), a megalopa 

 has a range of 22-290 km; at inshore tem- 

 perature-salinity ranges (20--25° C, 30-35%o) a 

 megalopa might be transported 26-56 km. 



Callinectes megalopae collected on the outer 

 shelf in the winter and spring, as well as some 

 megalopae found there in the summer and fall, 

 probably have southern origins. Water masses of 

 Gulf Stream origin, as meanders and warm-core 

 eddies, have frequently been observed in the 

 slope-outer shelf regions (Saunders 1971; see 

 Wright 1976 for a review). Although large-scale, 

 long-range transport is not evident, the presence 

 of Callinectes and Portunus megalopae at station 

 Jl in the winter indicates either transport from 

 the south or, less plausibly, delayed metamor- 

 phosis of megalopae from Middle Atlantic Bight 

 populations as a result of low winter tempera- 

 tures. Based on Costlow's (1967) response sur- 

 faces, a megalopa in Gulf Stream waters would 

 have a duration of 7-26 days and a range, at 2 kn, 

 of 600-2,300 km. Thus, some megalopae in the 

 Middle Atlantic Bight may originate from late 

 spawning populations to the south. Metamorpho- 

 sis would be further delayed by depressed temper- 

 atures of shelf and slope waters. The presence of 



definite southern larval forms ie.g.,Dromidia) in 

 outer shelf collections supports the hypothesis 

 that at least some of the Callinectes and other 

 megalopae were produced by southern crab popu- 

 lations. 



The developmental model of Callinectes, i.e., 

 larval development in shelf waters and sub- 

 sequent recruitment to inshore and estuarine 

 adult populations, reflects the evolutionary his- 

 tory of the group. Portunids are "reproductively 

 conservative," migrating to waters of oceanic, or 

 near oceanic, salinities to release larvae (Norse 

 1977). Williams (1974) described Callinectes as "a 

 portunid group evolving at the geographic limits 

 of the family, specializing in occupation of es- 

 tuaries, . . . ." In this light, the migratory se- 

 quence of larval stages is a response to the prob- 

 lems of an essentially marine group invading the 

 estuaries. Spawning areas (marine) may repre- 

 sent a primitive condition, and the spatial se- 

 quence of stages returns larvae to habitats in 

 which the adults are successful. It is, as Williams 

 (1974) described it, a "homeostatic developmental 

 feature in the life histories of the species" that has 

 not been carried to an evolutionary conclusion, 

 that is, retention within the estuary for the entire 

 life history. 



It can be argued that such a model of develop- 

 ment may in part account for the success and 

 widespread distribution o{ Callinectes. If, as Norse 

 (1977) and others have indicated, recruitment oc- 

 curs through metamorphosis of megalopae rather 

 than immigration of adults, then such a sequence 

 would allow dispersal into numerous estuaries yet 

 assure genetic continuity over broad areas. Such a 

 role has been suggested by Cole and Morgan 

 (1978). Furthermore, it would seem more likely 

 that this is a primitive mechanism retained, 

 rather than developed, through selection pres- 

 sures. 



The essential features of Callinectes develop- 

 ment appear to be spawning and hatching in or 

 near the primitive habitat (along the shore) dur- 

 ing most of the warmer months, maintenance of a 

 large larval population in the shelf waters (chiefly 

 in the surface layers), recruitment from the larval 

 pool, and exploitation of the estuarine habitat as 

 adults. 



There is, however, a paradox in the biogeog- 

 raphy of Callinectes and the spatial sequence of 

 developmental stages. Given the southern 

 affinities of the genus and general southerly 

 longshore movement of waters along most of the 



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