LEGERE, J. K. D. *; WITTEN, P.; HALL, B.; Dalhousie University: Examination of Skeletal Remodelling and Resorption in Echinoderm Larvae

Skeletal remodelling and resorption are observed in both vertebrates and echinoderms. Vertebrate bone is resorbed by osteoclasts that secrete the acidic lysosomal enzyme acid phosphatase (AcP). Resorption and remodelling have also been observed in the calcium carbonate adult skeleton of some echinoderms. Many echinoderm larvae have a skeleton of CaCO₃ rods, called spicules, which support the larval body and arms, and are believed to be remodelled and resorbed throughout larval life, especially during metamorphosis. Previous studies indicate that spicule resorption may be a cell-mediated process as it is in vertebrates. This study examines skeleton remodelling and resorption in larvae of the common sand dollar Echinarachnius parma and the green sea urchin Strongylocentrotus droebachiensis. Larvae of both species have been successfully reared through metamorphosis. During this process the spicules break off near the proximal end, and the remaining spicule material may be resorbed or incorporated into the juvenile skeleton. Enzyme histochemistry was used to investigate AcP and alkaline phosphatase (AP) activity in embryos and larvae of each species at various stages of development. Zebrafish larvae (Danio rerio) with known patterns of enzyme activity served as controls. Results to date show that sand dollar larvae seven weeks old through metamorphosis do not express AcP, but that AP is present in the gut, rudiment and ciliated band of larvae of the same age. Studies using histology, to examine cells that may be involved in resorption, acridine orange labelling to examine acidic compartments in the larval body, and calcein labelling to investigate the fate of spicules, are in progress. This research is supported by NSERC (Canada) and Dalhousie University.