Some Thermal Requirements of Fiddler Crabs of the Temperate and Tropical Zones and Their Influence on Geographic Distribution

Abstract

Synopsis. Thermal stress is one environmental parameter that has greatly influenced the migration of crustaceans from the sea to land. Since a greater number of species of terrestrial crabs are found in the tropics than in the temperate zone, comparative studies of the influence of temperature on latitudinally separated populations were undertaken. Two tropical species, U. rapax and U. thayeri, may occur as far north as St. Augustine, Florida, or, following a severe winter, may be rare north of Cape Kennedy. The lethal effect of the low temperatures recorded during one severe winter (1957-58) is supported by laboratory studies in which LDso deaths occurred in 4.5 days at 10?C for U. rapax acclimated to 18?C. The experiment demonstrates that U. rapax cannot acclimate to and survive low temperatures. This contrasts markedly with the situation in semi-terrestrial crabs of the temperate zone, which are able to acclimate to cold. The distribution of Uca around Cape Cod Bay correlates well with the coastal hydrographic thermal gradient and supports Passano's suggestion that temperatures below 20? may be limiting as they inhibit proecdysis in U. pugnax. Such an inhibition is found experimentally in U. pugilator and in the tropical species, U. rapax. It is hypothesized that a shift in the thermodynamics of the processes underlying molting has not occurred in Uca of the temperate zone. The paucity of semi-terrestrial Brachyura in the temperate zone may be due to the failure of many species to evolve capacity-adaptations to carry out all requisite life processes at temperatures below 20?, or the resistance-adaptations necessary to survive the low temperatures of winter.