Field Observations of Moulting and Moult Increment in the Red Land Crab, Gecarcoidea Natalis (Brachyura, Gecarcinidae), on Christmas Island (Indian Ocean)

Abstract

Red land crabs, Gecarcoidea natalis (Pocock, 1888) are the most abundant land crabs on Christmas Island (Indian Ocean; 10?3(ys 105?4(yE), occurring at mean densities and biomasses from 0.4 to 1.8 crabs m-2 and 1.0 to 1.5 t ha-1 (Green, 1997). With the exception of some breeding-related activities (Hicks, 1985; Adamczewska & Morris, 2001) these crabs are completely terrestrial, and inhabit dry, shallow burrows throughout rain forest on the island. The largest males grow to > 120 mm carapace width and >500 g mass, but virtually nothing is known of growth rates and longevity in this species. Hicks et al. (1990) estimated that one, two, and three year old crabs were 9-20, 17-34, and 22-47 mm carapace width, respectively, and Hicks et al. (1984) guessed that the largest crabs were around 12 years old. Adult crabs moult in sealed burrows without access to free-standing water, but detailed observations of the moulting process, or moult increments, have not been reported for G. natalis. In this note I describe observations of moulting red crabs held in semi-natural conditions in primary rain forest near Murray Hill in the western area of the island. Red crabs were held in artificial burrows, which consisted of an inclined tunnel (a longitudinally cut section of PVC pipe with a timber cover) leading to an expanded chamber (a vertical section of larger diameter pipe). This chamber approximated the size of natural burrows for crabs in the 60-80 mm size range. The burrows were buried in the ground so that just the top of the chamber and tunnel entrance were exposed. The chamber was fitted with a rubber-lined cover, such that it formed a tight seal to trap moisture within the artificial burrow.