Occurrence of the protozoan parasite, Perkinsus olseni in the wild and farmed Pearl Oyster, Pinctada fucata (Gould) from the Southeast coast of India

Abstract

The pearl oyster, Pinctada fucata (Gould), is a commercially important bivalve distributed in the Gulf of Mannar along the southeast coast of India and had supported a healthy, traditional pearl fishery until the 1950s. But, during the past few decades, the natural pearl oyster beds in the Gulf of Mannar have showed a sharp decline leading to the closure of the traditional pearl fishery and was presumed to be due to overexploitation and pollution. Except for a preliminary report on Perkinsus marinus infection in Crassostrea madrasensis in 1988, no incidence of perkinsosis or other Office International des Epizooties (OIE) notified protozoan infections in mollusks has been reported from the Indian subcontinent and the pathogen profile of P. fucata from the region has not been studied. Since Perkinsus spp. is known to have destroyed many oyster beds worldwide, the present study was taken up to screen the pearl oyster population along the Gulf of Mannar coast for the presence of Perkinsus spp. and examine the probable role of Perkinsus in the decline of the natural pearl oyster beds. Thirty individuals of adult P. fucata were collected from wild populations at three different locations in the Gulf of Mannar and 10 individuals from a pearl farm containing hatchery reared stock at Tuticorin. The tissue samples were subjected to Ray's fluid thioglycollate medium (RFTM) culture, histology and polymerase chain reaction (PCR). All the samples showed enlarged blue-black hypnospores in RFTM, indicating the presence of Perkinsus spp. Perkinsus-like organisms were also observed in the histological preparations. Screening of the tissues using the Perkinsus genus specific internal transcribed spacer (ITS) 85 and ITS 750 primers, amplified the product specific to the genus Perkinsus (ca. 700 base pairs) and further, the specific identity of the parasite was determined by sequencing the amplified PCR products which showed 99% identity to Perkinsus olseni. The pairwise genetic distance values and phylogenetic analysis also confirm that the present isolate from P. fucata is a member of the P. olseni clade. This preliminary investigation suggests a possibility that perkinsosis could be one of the major reasons for the decline of the P. fucata beds in the Gulf of Mannar over a period of time. This forms the first report on the existence of P. olseni, an OIE listed pathogen in the wild and cultured P. fucata populations from the Indian subcontinent.