Natural history and social organization of the semiterrestrial grapsid crab Pachygrapsus transversus (Gibbes)

Pachygrapsus transversus (Gibbes) was studied in the intertidal zone of the Bay of Panama. Individuals are active at all low tides as soon as their habitat is exposed. Crabs on large rocks occupy holes and crevices that provide food, refuge from predators, and relief from thermal stress. Males and females move daily from the refuge rocks to flat smooth rocks where they graze on algae. P. transversus is omnivorous, feeding on algae, porcelain crabs, fish and conspecifics. Predators on the crabs include raccoons, fish, and birds. Behavioral displays of P. transversus are described. Males maintain non-exclusive core areas on the refuge rocks, with larger males defending larger areas. A typical area contains a single dominant male, zero to five subordinate males, three to eight mature females, and seven to 25 non-interacting juveniles. All observed matings occurred on these areas; dominant males copulated in 83–88% of the matings. We suggest that predation and heat stress are the selection pressures favoring use of holes as refuges. The fact that some holes are occupied by females may have led to the evolution of male dominance and defense of these areas.