Abstract
Orchitophrya stellarum Cépède, 1907 is a facultative ciliate that parasitizes male asteriid sea stars in the North Atlantic and Pacific oceans. The ciliate also exists in seawater where it feeds on bacteria associated with tissue or yeast. This study is designed to determine the tolerance, growth rate, contractile vacuole activity, and infectivity to Leptasterias Verrill, 1866 spp. as a function of the sea star’s reproductive cycle stage. The tolerance range of O. stellarum in seawater containing bacteria is 2 practical salinity units (PSU) at 8 °C and 12.5 PSU at 25 °C; the ciliate tolerated a temperature range of 3–27 °C at 30 PSU. Ciliate population growth is most rapid at 24 °C and is minimal at 3 and 27 °C at 30 PSU. The ciliate’s contractile vacuole cycle frequency increases at low salinity, indicating increased water volume regulation as a function of decreased salinity. Unparasitized male Leptasterias spp. can be infected experimentally by injection of a culture of O. stellarum into the rays or from a culture in ambient seawater only when the testes are at full size, but the ciliates do not invade ovarian tissue. The normal path of entry of O. stellarum into the testes of reproductively mature male sea stars probably takes place through gonopores.
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