HATCHING RHYTHMS AND DISPERSION OF DECAPOD CRUSTACEAN LARVAE IN A BRACKISH COASTAL LAGOON IN ARGENTINA

Mar Chiquita, a brackish coastal lagoon in central Argentina, is inhab ited by dense populations of two intertidal grapsid crab species, Cyrt ograpsus angulatus and Chasmagnathus granulata. During a preliminary o ne-year study and a subsequent intensive sampling programme (November- December 1992), the physical properties and the occurrence of decapod crustacean larvae in the surface water of the lagoon were investigated . The lagoon is characterized by highly variable physical conditions, with oligohaline waters frequently predominating over extended periods . The adjacent coastal waters show a complex pattern of semidiurnal ti des that often do not influence the lagoon, due to the existence of a sandbar across its entrance. Besides frequently occurring larvae (excl usively freshly hatched zoeae and a few megalopae) of the two dominati ng crab species, those of three other brachyurans (Plathyxanthus crenu latus, Uca uruguayensis, Pinnixa patagonica) and of one anomuran (the porcellanid Pachycheles haigae) were also found occasionally. Caridean shrimp (Palaemonetes argentinus) larvae occurred in a moderate number of samples, with a maximum density of 800 (.)m(-3). The highest larva l abundance was recorded in C. angulatus, with almost 8000.m(-3). Sign ificantly more C. angulatus and C. granulata zoeae occurred at night t han during daylight conditions, and more larvae (statistically signifi cant only in the former species) during ebb (outflowing) than during f lood (inflowing) tides. In consequence, most crab zoeae were observed during nocturnal ebb, the least with diurnal flood tides. Our data sug gest that crab larvae do not develop in the lagoon, where the adult po pulations Live, but exhibit an export strategy, probably based upon ex ogenously coordinated egg hatching rhythms. Zoeal development must lak e place in coastal marine waters, from where the megalopa eventually r eturns for settlement and metamorphosis in the lagoon. Significantly h igher larval frequency of C. granulata in low salinities (less than or equal to 12 parts per thousand) and at a particular sampling site may be related to local distribution patterns of the reproducing adult po pulation. Unlike crab larvae, those of shrimp (P. argentinus) are reta ined inside the lagoon, where they develop from hatching through metam orphosis. They significantly prefer low salinity and occur at the lago on surface more often at night. These patterns cannot be explained by larval release rhythms like those in brachyuran crabs, but may reflect diel vertical migrations to the bottom. It is concluded that osmotic stress as well as predation pressure exerted by visually directed pred ators (small species or life-cycle stages of estuarine fishes) may be the principal selection factors for the evolution of hatching and migr ation rhythms in decapod larvae, and that these are characteristics of export or retention mechanisms, respectively.