The early life history of Sesarma fossarum, an endemic freshwater crab from Jamaica

The freshwater crab Sesarma fossarum (Decapoda, Brachyura, Sesarmidae) was only recently described as a new species. As in most other endemic Jamaican crabs that have radiated in limnic and terrestrial environments, its early life history has been largely unknown. In an experimental laboratory study this species was reared through embryonic, larval, and early juvenile development, and ontogenetic changes in biomass (contents of carbon, nitrogen, and hydrogen) were measured. The duration of egg development varied greatly within a single brood, such that larval hatching occurred through an extended period (12 d). Larval development invariably consisted of two non-feeding (fully lecithotrophic) zoeal stages and a facultatively lecithotrophic megalopa, followed by metamorphosis to the first juvenile crab stage (always feeding). The zoeal phase lasted on average 4–5 d, while the megalopa required 8–10 d. This abbreviated and largely food-independent mode of development is interpreted as a life-history adaptation to conditions of food limitation in the breeding habitat of this species (presumably maternal burrows dug in river banks). This strategy is based on an enhanced maternal energy investment in the production of large, yolk-rich eggs (ca. 1.4 mm size, 0.6–0.7 mg dry mass). The megalopa shows a highly flexible nutritional strategy where the appearance of a functional feeding system allows for exploitation of external energy sources, while its development remains, in principle, independent of scarcely available food. However, when megalopae were reared without any food, the first juvenile crab stage showed an enhanced mortality and a reduced body size. Decreasing C, H, and C:N values indicate that the fully endotrophic (embryonic and zoeal) phase is principally based on a utilization of lipid reserves; the same applies to the development of megalopae reared in a continued absence of food. In fed megalopae, increasing N and decreasing C:N values indicate that dietary energy was primarily invested in protein synthesis, required for the construction of new tissues and organs. Similarity in the reproductive, developmental, and bioenergetic traits of S. fossarum and other limnic or terrestrial crabs from Jamaica suggest that food limitation during the early larval phase has been one of the principal selection factors driving the evolution of the endemic non-marine clade of sesarmids towards increasing egg size and larval lecithotrophy.