VARIATION IN THE BRANCHIAL FORMULA OF SEMITERRESTRIAL CRABS (DECAPODA: BRACHYURA: GRAPSIDAE AND OCYPODIDAE) IN RELATION TO PHYSIOLOGICAL ADAPTATIONS TO THE ENVIRONMENT

Abstract

ABSTRACT Variation of the branchial formula of 18 species of crabs collected from littoral and supralittoral regions on Phuket Island, Thailand, was examined in relation to the semiterrestrial environment. All species inhabiting water-logged substrata (3 species of Macrophthalmus, 3 species of Metaplax, and Chiromantes dussumieri) had an identical branchial formula consisting of 8 pairs of well-developed branchiae. In contrast, crabs inhabiting well-drained substrata had respiratory organs other than branchiae (6 species of Uca with well-developed inner linings of the branchiostegites, forming lungs, and Ilyoplax delsmani, I. gangetica, Dotillopsis brevitarsis, and Dotilla myctiroides with tympana on the meral segments of the thoracic pereiopods). The branchiae were more reduced and/or absent in the species occupying higher ground. In general, the major branchiae except the podobranchiae were divided into 3 types by histological observation: the highly osmoregulatory branchiae, the weakly osmoregulatory branchiae, and the respiratory branchiae. The species inhabiting well-drained substrata, especially species of the genus Uca with fewer branchiae, had more highly osmoregulatory branchiae than the species inhabiting water-logged substrata, even when they lived at almost the same tidal level. The reduction and/or disappearance of branchiae in the species inhabiting well-drained substrata is discussed in relation to the morphology of branchiae, the development of respiratory organs (lungs and tympana), and semiterrestrial environments.