The conquest of freshwater and land by marine crabs: Adaptations in life-history patterns and larval bioenergetics

Life-history patterns in marine, limnic, and terrestrial brachyurans are reviewed to discern special adaptations associated with transitions in life style. Among marine crabs, the Grapsidae have been especially successful invading freshwater and terrestrial environments, in particular a group of closely related neotropical Sesarminae species, which has evolved through a conspicuous radiation process. Compared with marine grapsid crabs, freshwater and terrestrial species show the following tendencies: (1) increasing habitat specialization, (2) reduced geographic range (endemism), (3) low fecundity, (4) brood protection, (5) large egg size, (6) unusually high carbon contents and C:N ratios in eggs and early larvae (indicating an enhanced lipid content), (7) abbreviated larval development, (8) high larval tolerance of physico-chemical stress, (9) nutritional independence (lecithotrophy), (10) no or only little larval growth, (11) possibly energy saving mechanisms in exuvial and respiratory losses of larvae. Most of these tendencies were found also in two semiterrestrial species with intermediate life histories, Sesarma curacaoense and Armases miersii. Factors selecting for these life-history traits are discussed. Among them, there may be two major driving forces in the evolution of freshwater and land crabs: (1) food-limitation in freshwater/terrestrial breeding habitats selects for large egg size, lecithotrophy, and abbreviated larval development; (2) once adaptations to the adult habitat have evolved in the larvae (for instance, in semiterrestrial species with an abbreviated, partially lecithotrophic larval development, as in S. curacaoense and A. miersii), intense pelagic predation in the shallow coastal fringe should select against an export strategy (offshore transport of larvae), and thus for larval retention in and continued adaptation to nonmarine environments.