Metamorphosis in a semiterrestrial crab, Sesarma curacaoense: intra- and interspecific settlement cues from adult odors

The larvae of many marine invertebrate species are able to delay their settlement and metamorphosis in the absence of characteristic cues from the adult habitat. This phenomenon was experimentally studied in the megalopa stage of Sesarma curacaoense de Man, 1892, a semiterrestrial grapsid crab that lives in the shallow coastal mangrove habitats in the Caribbean region. Duration of the development and survival to metamorphosis to the first juvenile crab stage were compared between experimental treatments, where the water was conditioned with adult crabs (“adult-conditioned water,” ACW) and control groups reared in filtered seawater. In the experiments with larvae from two different females, development duration was significantly shorter and mortality lower in water conditioned with conspecific adults. In the two control groups, the effects of supply with an artificial substrate (nylon gauze) were tested. This comparison showed that the presence of substrate did not significantly influence the time to metamorphosis, but did reduce the mortality rate. In all later experiments, the megalopae were thus routinely provided with nylon gauze as a substrate. In each of the three subsequent replicate experiments conducted with larvae from different females, survival rate and development time to metamorphosis were compared between one control group and four treatments with ACW. The effectiveness of conspecific ( S. curacaoense) adult odors as metamorphosis-stimulating cue was, in these experiments, compared with that of ACW from one congener ( S. rectum) and two species belonging to different genera within the Grapsidae ( Armases miersii, Chasmagnathus granulata). While the rate of survival showed inconsistent patterns among repeated experiments, the development was consistently fastest with conspecific ACW, followed by ACW from S. rectum, A. miersii and C. granulata. Only the conspecific and congeneric cues had statistically significant effects (i.e. shorter development than in the controls). These response patterns suggest that chemically similar factors (presumably pheromones) are produced by closely related species and, thus, their chemical structure may reflect phylogenetical relationships within a clade.