The effects of environmental and nutritional conditions on the development of Perkinsus olseni prezoosporangia.

Perkinsus olseni is a widely distributed protozoan pathogen that infects a wide range of marine mollusks. Prezoosporulation of P. olseni trophozoites is easily observed in Ray's fluid thioglycollate medium, but in nature, trophozoites within host tissue should be able to develop into prezoosporangia without any additional artificial medium after the host dies. How this process might work in field conditions remains poorly understood, however, partly because of the lack of appropriate in vitro assays. In this study, we observed that trophozoites of P. olseni successfully developed into prezoosporangia when mixed with minced tissue of the Manila clam Ruditapes philippinarum and placed in seawater. We were thus able to establish a new method to examine the development of P. olseni to prezoosporangia under artificially simulated natural environmental conditions. Using this method, we found that low temperatures (5 °C, 15 °C) significantly suppressed prezoosporangia development. In addition, we found that prezoosporangia were developed in a wide range of salinities (10-50 practical salinity unit) and that P. olseni requires some nutrition factors from host tissue for prezoosporulation to occur. Because the transmission of P. olseni among a host population highly depends on the developmental process of prezoosporangia, which leads to production of the infective zoospore stage, these results will help further our understanding of the parasite's infection dynamics in nature.