BIODEGRADATION OF SHELLS OF THE BLACK PEARL OYSTER, PINCTADA-MARGARITIFERA VAR CUMINGII, BY MICROBORERS AND SPONGES OF FRENCH-POLYNESIA

The composition, distribution and infestation sequence of organisms th at destroy the commercially valuable shells of the black oyster Pincta da margaritifera var. cumingii Jameson, 1901 were studied. Three ecolo gically different groups of boring (euendolithic) organisms were ident ified: (1) phototrophic boring microorganisms (cyanobacteria, Hyella c aespitosa, Hyella sp., Mastigocoleus testarum, Plectonema terebrans, a nd green algae, Phaeophila dendroides, Ostreobium quekettii); (2) hete rotrophic boring microorganisms (fungi, Ostracoblabe implexa); (3) fil ter-feeding boring organisms (sponges, Cliona margaritiferae, C. vasti fica). The phototrophic endoliths dominate the external prismatic regi on of the shell, whereas the valuable interior nacreous region is atta cked mainly by heterotrophs. Boring patterns reflect in part the shape and behaviour of the organisms and in part the structural properties of the shell, and inflict different types of damage. Infestation start s with microbial borers, which prepare the conditions for later invasi on by more damaging clionid sponges. The infestation begins always at the apex, the oldest part of the shells, from which the periostracum i s often removed by natural attrition or by cleaning procedure. The rat e of bioerosion in 1 yr-old hatchery shells is 36 times higher than in natural populations.