SELECTIVE BIOMINERALIZATION OF CRAB CUTICLE: THE KEY TO A PELAGIC LIFESTYLE

Cuticular microstructure in brachyuran crustaceans, the crabs, has traditionally been characterized as having two biomineralized layers, the exocuticle and endocuticle, which are distinguished from one another by position, thickness, and nature of their laminations. Recent examination of the cuticular structure of fossil and extant crabs has demonstrated that considerable variation in microstructure exists. The differences in structure include development of pillars, nodes, pits, and others, that are hypothesized to reflect phylogenetic relationships. Another difference, not known to have been studied in a systematic manner, is the degree and nature of calcification of the exocuticle and endocuticle in crabs in the fully developed, intermolt condition. Layers of the cuticle have previously been described as being calcified or non-calcified. Our work has shown that the distribution of calcite is much more complex and does not always follow layers of the cuticle. Examination of thin sections of crab cuticle from a wide range of families documents a pattern of biomineralization that may have ecological rather than phylogenetic implications. Callinectes sapidus Rathbun, 1896, an active swimmer in the family Portunidae, exhibits a degree of calcification of less than 40% of cuticle volume on the carapace, far less complete than that of fully benthic crabs whose degree of calcification ranges from 60 to nearly 100%. Furthermore, the thickness of the cuticle of C. sapidus, relative to carapace size, is reduced. Ovalipes catharis (White, 1843), another portunoid, exhibits about 60% calcification but has a cuticle that is thinner than fully benthic forms. Thus, we hypothesize that cuticle thickness and degree of biomineralization is an ecological adaptation to lifestyle and that differences in degree of calcification may govern whether a crab can swim with a minimum expenditure of energy or whether it is negatively buoyant and, therefore, is best adapted for life on the seafloor. Highly mineralized cuticle is more brittle than non-mineralized, tanned cuticle which remains more flexible. Fully calcified cuticle may play a larger role in providing mass to stay in contact with the seafloor than protection from predators. This research is supported by NSF grant EF 0531670 to Feldmann and Schweitzer.