A reconstruction of the invasion of land by Jamaican crabs (Grapsidae : Sesarminae)

Several decapod groups independently colonized freshwater and terrestrial h abitats and became independent from the sea. These invasions were accompani ed by analogous reproductive and developmental traits such as large eggs an d an abbreviated, lecithotrophic development. Here, we present the first em pirical study on the evolution of reproductive and developmental traits tha t accompany the invasion of land by crabs. As crucial steps in the coloniza tion, we identify the transitions of the larval nursery, first, from the ma rine plankton into landlocked-brackish nurseries and, second, into fresh wa ter. During these invasions, the early life-cycle stages were facing new ec ological conditions and selective agents. We test hypotheses on the evoluti on of egg size and the mode of development in relation to the larval ecolog y of recent species and draw conclusions on their evolutionary past. As a m odel we focus on the genus Sesarma, that colonized Jamaica relatively recen tly and comprises species with a larval development in marine, brackish and freshwater habitats. In addition, we compare representatives of the crab g enera Armases, Sesarma and Uca that invaded brackish-nursery habitats indep endently. The analysis reveals that in each genus the transition from marin e to brackish nurseries resulted in fewer and larger eggs, an abbreviated d evelopment and higher endotrophic potential of larvae, and a wider toleranc e to physicochemical stress (salinity). Size at metamorphosis, however, did not change in brackish species, suggesting that it is constrained. Within the Sesarma-lineage, egg size increases considerably from marine to freshwa ter species. The duration of embryonic development, the size and endotrophi c potential of larvae are positively correlated, but the duration of the la rval phase is negatively correlated with egg size. Hypotheses suggesting th at large eggs evolved as a response to limited food or intense predation ar e inadequate to explain the initial egg-size increase in brackish species. We suggest that the specific abiotic environment of the brackish nurseries ultimately selected for increased egg size. These particular larval nurseri es of brackish species of Armases, Sesarma and Uca are nutrient rich but ep hemeral habitats with unfavourable physicochemical conditions, which strong ly favour a swift larval phase and possibly large body size and higher sali nity-stress resistance of larvae. The reason for the further and substantia l increase in egg size in freshwater species remains unknown. The 'food-lim itation' hypothesis derived from laboratory experiments, however, is inadeq uate to explain this increase. Our results support general life-history hyp otheses ('safe harbour' hypothesis) that predict the evolution of large egg s if postembryonic stages face high risk of mortality, but not the predicte d positive relationship between egg size and instantaneous egg stage mortal ity. On the contrary, we find a negative relationship, suggesting that larg er eggs are a 'safer harbour' than smaller eggs. We outline a scenario for the invasion of land by crabs and propose a two-step model: as a first step , an instant shift of the larval development from offshore into landlocked- brackish nurseries, and, as a second step, from there into freshwater nurse ries.