Evidence of an atypical scale development during the settlement phase of a coral reef fish

As the majority of coral reef fishes, the Convict Surgeonfish Acanthurus triostegus (Acanthuridae) has a complex life cycle that involves an ontogenetic change in morphology, physiology and behaviour as its pelagic larval stage colonizes the benthic habitat. Abrupt and spectacular changes in skeletal structures occurring when a fish takes on its juvenile form were highlighted in flatfish, bonefish, tarpon, eels, pearlfish and lampreys. However, few studies are devoted to the changes in skeleton during the settlement period of demersal coral reef fishes. In the present study conducted at Rangiroa Atoll (French Polynesia), we highlight an unexpected scales development in A. trisostegus during a fifteen days period just after the reef settlement. Fish was collected during the settlement and reared in aquaria. The osseous skeleton was displayed by a standard Alizarin red S staining technique. At settlement (t0) (SL = 22-25 mm), A. triostegus showed calcified and very long plates, lying in the dermis on the whole body. After three days, some small scales developed on the caudal peduncle. The plates seemed unchanged from the head to the pectoral girdle but were thinner on the trunk. The thin plates are pricked with whitish spots, which seem to indicate a poorer fixation of the alizarin corresponding to a decalcification process. Six days after the settlement, the squamation extended anteriorly to the pectoral girdle by the addition of new scales. Thin plates were always present on the head. Then the density of scales rapidly increased along the trunk during the following three days. The scales appeared on the head nine days after the settlement. Clearly, the plates do not transform into scales. The plates disappearance and the scales appearance appear as two parallel phenomena in the development.