An unidentified protistan parasite of the pearl oyster, Pinctada maxima, in tropical Australia

Abstract

The term “pearl oysters” has traditionally been applied to bivalves of the genera Pinctada and Pteria, included in the family Pteriidae, that is believed to have originated in the Triassic approximately 230 million years ago. Members of Pteriidae are characterized by a laterally compressed and obliquely ovate shell with a straight hinge line, deep byssal notch, and frequently elongated, posteriorly projecting extensions on the hinge. The genera of Pteriidae have traditionally been defined by shell shape. The shell is strongly to moderately obliquely ovate in outline. The hinge line is straight and extended anteriorly past the umbo into subtriangular to subquadrate anterior auricles and posteriorly into characteristically long and deeply sinuated posterior auricles. The exterior shell surface sculpture consists of commarginal periostracal and, in some species, calcitic scales that typically abrade with age. Most species of Pteriidae are tropical and subtropical, but the distributions of some species extend to higher latitudes. Most species of Pteriidae inhabit shallow littoral and sublittoral zones of continental shelf regions. Some species are, however, found on sandy bottoms of about 80 m depth. The shell of genera Pinctada is prosocline, inequilateral, typically rounded to anteriorly obliquely ovate in outline. Pearl oysters belong to the superfamily Pterioidea in the bivalve subclass Pteriomorphia. A contemporary taxonomic problem of considerable importance is the status of a species-complex, Pinctada fucata/martensii/radiata/imbricata.