Stimulation of metamorphosis in an estuarine crab, Chasmagnathus granulata (Dana, 1851): temporal window of cue receptivity
Introduction
Planktonic larvae are the principal agents of dispersal in the life cycles of most benthic marine invertebrates (Thorson, 1950). During the larval period, there is initially a precompetent phase in which the larvae are morphologically and/or physiologically incapable of settling and passing through metamorphosis. This is followed by a phase of metamorphic competence during which, in some species, the larvae respond to stimulating physical and/or chemical cues by initiating the developmental and behavioural changes that comprise the process of metamorphosis Doyle, 1975, Burke, 1983, Burke, 1986, Pawlik, 1992. Effective cues are generally associated with characteristics of the benthic adult habitat, including humic acids from rivers and estuaries, reduced salinities, microbial films, odors from conspecific adults, sediments, algae, or specific ions or organic chemical compounds (for recent review, see Anger, 2001, Forward et al., 2001, Gebauer et al., 2003). When effective external cues are absent, competent and receptive larvae may prolong their planktonic phase for several days or months. If such cues remain absent for longer periods, the larval phase may end with a spontaneous metamorphosis or larval death Pechenik, 1990, Zimmerman and Pechenik, 1991, Zaslow and Benayahu, 1996, Gebauer et al., 1998.
In the present study, the period of maximum receptivity of metamorphosis-stimulating cues was experimentally investigated in the final larval stage, the megalopa, of an estuarine grapsoid crab species, Chasmagnathus granulata Dana, exposed for differential periods during the moulting cycle. This species occurs in coastal salt marshes, brackish lagoons, and other estuarine habitats in Argentina, Uruguay and southern Brazil Boschi, 1964, Boschi et al., 1992. Its life history is characterised by an export strategy, i.e. the first larval stage leaves the brackish parental environment soon after hatching, zoeal development takes place in lower estuarine or coastal marine regions, and the megalopa returns later to recruit to the adult populations Anger et al., 1994, Luppi et al., 2002, Giménez, 2003. The megalopa of C. granulata responds to cues associated with the parental habitat, in particular to muddy sediments, the presence of conspecific adults, and combinations thereof (Gebauer et al., 1998). If such cues are absent, the megalopa can postpone metamorphosis by about 2–3 days (ca. 26–30%), although this developmental delay also implies energetic costs that may reduce juvenile growth (Gebauer et al., 1999).
In laboratory experiments with the megalopa stage of C. granulata, we studied the temporal window of receptivity for previously identified external cues, addressing the question during which parts of the moulting cycle the larvae must be exposed to those cues to induce a measurable response in development time. This should indicate the optimal period for returning from coastal waters to estuaries, where settlement and metamorphosis are known to occur (Luppi et al., 2002).
Section snippets
Origin of materials, larval rearing
Larvae of C. granulata were obtained from crabs originating from Mar Chiquita lagoon (Province of Buenos Aires, Argentina; see Spivak et al., 1994) and cultured since 1991 in the laboratory of the Helgoland Marine Biological Station (Germany). Our standard cultivation conditions for this species are 18 °C, 32‰ salinity and a 12:12h light/dark cycle. Crabs are fed with frozen isopods (Idotea spp.), mussels (Mytilus edulis), and shrimp (Crangon crangon) from the North Sea. From hatching to the
Results
The survival of megalopae through metamorphosis varied in the two experimental series from 44% to 100%. In larvae obtained from F1 but not in those from F2, an R×C test of independence indicated in experiment A a statistically significant variation of mortality rates (G=34.1; P<0.05). This was caused by an exceptionally high mortality occurring exclusively in treatment A10 (56% vs. 4–20% in all other treatments and controls). However, no relationship of the mortality pattern with the timing or
Discussion
Metamorphosis from the megalopa to the first benthic juvenile stage of a highly gregarious salt marsh crab, C. granulata, is triggered by chemical cues that are associated with the adult habitat, in particular by presence of mud and/or odors from conspecific adults (Gebauer et al., 1998). Adult odors may also stimulate metamorphosis in related species, although the effectiveness appears to decrease with increasing phylogenetic distance (Gebauer et al., 2003). Lack of exogenous cues causes a
Acknowledgements
The first author acknowledges a grant from the Biologische Anstalt Helgoland and the Dirección de Investigación y Desarrollo (project DID D2001-12-Universidad Austral de Chile). The second author was funded by Deutscher Akademischer Austauschdienst (DAAD; Bonn, Germany), the Universidad Austral de Chile (MECESUP AUS-9908), and the Dirección de Investigación y Desarrollo (project DID S200159-Universidad Austral de Chile). [SS]
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