First Finding of the Pelagic Crab Planes Marinus (Decapoda: Grapsidae) in the Southwestern Atlantic

Most grapsid crabs are benthic species that live in intertidal or subtidal marine or brackish water. Columbus crabs, Planes spp., which live associated with living or inanimate floating objects in the open ocean are an exception. A group of crabs was found on flotsam stranded on a beach near Mar Chiquita (37°45'S, 57°26'W), Argentina, and identified as Planes marinus. This is the first report of this oceanic species in the southwestern Atlantic Ocean. The crab was previously collected in the Pacific Ocean, from British Columbia to New Zealand, and at St. Helena Island (southeastern Atlantic Ocean). Surface oceanic currents are postulated as a possible transport mechanism between the highly isolated collection sites. This paper includes morphometrical data that allowed the identification of the species and several biological characteristics (sex ratio, relative growth and sexual dimorphism, incidence of limb autotomy and regeneration, epibiosis, and fecundity). Both P. marinus and most littoral marine grapsids have strong sexual dimorphism (male chelae are larger), a high incidence of autotomy, and a similar egg size and number (the latter related to female size). This information suggests that the life history and behavior of P. marinus, a typical "pelagic" crab, is similar to its benthic littoral relatives.