Malformation of the coconut crab Birgus latro in Yaeyama Islands, Ryukyu Archipelago, Japan

Abstract

This report covers two malformations among coconut crabs Birgus latro observed on islands of Okinawa, Japan: an extra pleopod recorded in Iriomote Island, and an abnormal ambulatory leg found in Hatoma Island. A total of 328 crabs was investigated in Iriomote Island, two individuals that have an abnormal pleopod were found. Normally, female coconut crabs have three pleopods on the left side of the second to the fourth abdominal somites. However, the abnormal individuals had an additional pleopod on the right side of the third abdominal somite. This malformation is the first report within the species. We also found one coconut crab with an abnormal ambulatory leg, having a trifid tip on the dactylus of the left second thoracic appendage, among 156 crabs observed in Hatoma Island. This abnormality is probably caused by the battles within the species or against natural predators.