Kelp bass (Paralabrax clathratus) and leopard sharks (Triakis semifasciata) are characterized by an acellular (anosteocytic) bony skeleton and a focally calcified cartilaginous endoskeleton, respectively. These skeletal forms are not considered to function as mineral reservoirs. Previous studies showed that implanted bone particles are resorbed in rats by large multinucleated cells with ultrastructural features (ruffled borders) characteristic of osteoclasts. We tested the ability of fish to resorb bone matrix and to adapt to reduced salinity conditions. Bone particles were implanted in sharks and bass maintained in seawater (34 ppt, 40.5 mg of calcium per dl) or in diluted seawater (26 ppt, 28.5 mg of calcium per dl). Sera and elicited tissues were harvested 4 weeks later. In sharks, bone particles were not resorbed, and multinucleated cells were not evident under either normal or hyposalinity conditions. Shark sera were isoosmolar with the seawater or diluted seawater, with serum chemistries of the hyposalinity group reflecting the 23% reduction in environmental minerals and electrolytes, compared to sharks in normal seawater. In marked contrast, bass adapted to diluted seawater resorbed bone particles and maintained normal serum chemistries. Electron microcopy showed that the bone particles were surrounded by large, foamy multinucleated cells, many with membrane specializations typical of osteoclasts from higher vertebrates, i.e., extensive clear zones apposed to intact bone matrix and active ruffled borders overlying areas of matrix undergoing dissolution. Although osteoclasts had not been described in these fish, this study shows that bass have stem cells that can be stimulated to differentiate into bone-resorbing osteoclasts.