Hatching rhythms and dispersion of decapod crustacean larvae in a brackish coastal lagoon in Argentina

Abstract

Mar Chiquita, a brackish coastal lagoon in central Argentina, is inhabited by dense populations of two intertidal grapsid crab species, Cyrtograpsus angulatus and Chasrnagnathus granulata. During a prehminary 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 oligohahne 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 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 larval abundance was recorded in C. angulatus, with almost 8000. m -3. Significantly more C. angulatus and C. 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 of C. granulata in low salinities (--< 12%o) and at a particular sampling site may be related to local distribution patterns of the reproducing adult population. Unhke 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 hke 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.

Distribution and habitat preferences of two grapsid crab species in Mar Chiquita Lagoon (Province of Buenos Aires, Argentina)

“…11). A similar linear relationship between salinity and pH had been found also in water samples taken from different parts of Mar Chiquita Lagoon (Anger et al, 1994); however, the present regression line differs significantly in both the slope and intercept values (P<0.01; Fig. 11).…”

Salinity tolerance of northern Brazilian mangrove crab larvae, Ucides cordatus (Ocypodidae): Necessity for larval export?

“…Rhitropanopeus harrisii: Cronin, 1982;Armases miersii: Anger, 1996). Larval export is common in many estuarine brachyurans and is generally interpreted as an avoidance of stressful salinities and or high densities of estuarine predators (Christy and Stancyk, 1982;Morgan, 1990;Anger et al,1994;Morgan, 1995;Charmantier et al, 2002). In U. cordatus, the cumulative duration of the euryhaline ZI and ZII stages was 7.74 ± 0.31 days, a time span that seems long enough for ensuring that currents transport the larvae sufficiently off-shore.…”

“…Scheltema, 1986;Botsford et al, 1989;Underwood and Fairweather, 1989;Anger et al, 1994, Shanks et al, 2003. In commercially exploited species the question whether local crab assemblages, e.g.…”

Interpopulational differences in the female reproductive cycle of the southwestern Atlantic estuarine crab <i>Chasmagnathus granulatus Dana</i>, 1851 (Brachyura: Grapsoidea: Varunidae)

“…A continuous supply of freshwater comes from several creeks and frequent rains (800 mm/year). The water temperature and salinity show strong seasonal, daily and local variation, especially in the southern part of the lagoon which is more influenced by the sea (Anger et al, 1994). Intertidal environments in the lagoon comprise mud flats and large surrounding areas dominated by the "dense-flower cordgrass" Spartina densiflora Brongniart, 1829 (Isacch et al, 2006).…”