Skeletal tumor formations in the massive coral Pavona clavus

Abstract

Skeletal tumor formations in the massive coral Pavona clavus were investigated for 3 yr on the northern Pacific coast of Costa Rica in a reef composed almost exclusively of this species. A field survey of tumor incidence showed that more than half of the population presented this phenomenon and that frequencies increased with depth. Up to 37 tumors per colony were recorded, and the largest tumor size was 37.5 m^2, covering the whole colony surface area. Tumors and healthy skeletons and tissues were studied by scanning and transmission electron microscopy, histology, in situ alizarin marking for growth rates, and x-ray radiographs of skeletal slabs. Macroscopic and microscopic analyses of skeletons revealed the presence of 3 types of tumors (Types I to III). No cytological changes or intracellular infection by parasitic agents were recorded. The only cellular change observed was the reduced numbers of zooxanthellae in tumorigenic tissues. Tumors begin their growth as a single polyp and develop for scores of years without any sign of cellular necroses. Tumors grew faster than healthy parts; however, their skeletal density was lower, with a higher Mg content making it less resistant to bioerosion. Two yr of in situ isogeneic and allogeneic contacts between healthy and tumor fragments revealed no infection or transfer of the tumor to the healthy tissue, even following isogeneic fusion combinations of healthy versus tumor fragments. We suggest that the terminology often used to characterize tumors in hard corals (‘tumor’, ‘neoplasm’, ‘hyperplasia’) does not reflect the pathogenesis and etiology of this phenomenon. The terms ‘skeletal tumor’ or ‘calicoblastic epithelioma’ seem more fitting for this de novo genesis.