Hatching rhythms and dispersion of decapod crustacean larvae in a brackish coastal lagoon in Argentina |
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Authors: | K Anger E Spivak C Bas D Ismael T Luppi |
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Institution: | (1) Biologische Anstalt Helgoland, Meeresstation, D-27483 Helgoland, Germany;(2) Departamento de Biología, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, 7600 Mar del Plata, República Argentina |
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Abstract: | Mar Chiquita, a brackish coastal lagoon in central Argentina, is inhabited by dense populations of two intertidal grapsid
crab species,Cyrtograpsus angulatus andChasmagnathus granulata. During a preliminary one-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 tides that often do not influence the lagoon, due to the
existence of a sandbar across its entrance. Besides frequently occurring larvae (exclusively freshly hatched zoeae and a few
megalopae) of the two dominating crab species, those of three other brachyurans (Plathyxanthus crenulatus, Uca uruguayensis, Pinnixa patagonica) and of one anomuran (the porcellanidPachycheles 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 larval abundance was recorded inC. angulatus, with almost 8000°m−3. Significantly moreC. angulatus andC. granulata zoeae occurred at night than during daylight conditions, and more larvae (statistically significant only in the former species)
during ebb (outflowing) than during flood (inflowing) tides. In consequence, most crab zoeae were observed during nocturnal
ebb, the least with diurnal flood tides. Our data suggest that crab larvae do not develop in the lagoon, where the adult populations
live, but exhibit an export strategy, probably based upon exogenously coordinated egg hatching rhythms. Zoeal development
must take place in coastal marine waters, from where the megalopa eventually returns for settlement and metamorphosis in the
lagoon. Significantly higher larval frequency ofC. granulata in low salinities (≤12‰) and at a particular sampling site may be related to local distribution patterns of the reproducing
adult population. Unlike crab larvae, those of shrimp (P. argentinus) are retained inside the lagoon, where they develop from hatching through metamorphosis. They significantly prefer low salinity
and occur at the lagoon 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 predators (small species or life-cycle stages of estuarine fishes) may be
the principal selection factors for the evolution of hatching and migration rhythms in decapod larvae, and that these are
characteristics of export or retention mechanisms, respectively. |
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