Biology of the Polyclad Prosthiostomum (Prosthiostomum) sp., a New Coral Parasite from Hawaii

Abstract

Prosthiostomum (Prosthiostomum) sp., a species of polyclad flatworm yet to be described, is an obligate ectoparasitic symbiont of the hermatypic coral Montipora. Field and laboratory studies have demonstrated an intimate parasite/host association involving the utilization of host corals as food and substrate by the parasite. Development of larvae is within the immediate host environment; consequently, infections are produced through direct infection. Various aspects of the biology, such as the developmental history, feeding habits, and parasite/host response to thermal environment, are reported. It is concluded that all aspects of the life history of this species show adaptations toward host specificity. This represents a rare example of true coral parasitism since most animals known to feed on coral tissues are considered to be facultative predators. The optimal thermal environment for the parasite appears to coincide with that of the coral host, a phenomenon which may tend to produce a seasonally stable parasite/host interaction. The parasite appears to become a serious coral pest only in disrupted systems such as artificial laboratory situations or in the polluted sections of Kaneohe Bay, Oahu.