Post-larval mortality of the endoparasitic isopod castrator Portunion conformis (Epicaridea: Entoniscidae) in the shore crab, Hemigrapsus oregonensis, with a description of the host response

Abstract

SummaryThe shore crabs, Hemigrapsus oregonensis and H. nudus, sometimes kill the female endoparasitic entoniscid isopod, Portunion conformis, a parasitic castrator. Studies of host populations from Baja California, Mexico to Vancouver Island, Canada, show that the incidence of parasitized hosts with dead parasites and the percentage of the parasite population found dead vary markedly with locality but only occasionally with season. Both higher incidences of hosts with dead P. conformis and higher proportions of the total parasite population found dead are associated with (1) high prevalence of parasitism, (2) female hosts and (3) large hosts. Within a host, the proportion of the parasites that are dead is not related to the degree of multiple infection. Typically, either all or none of the parasites in a multiple infection are dead. Supernumerary juvenile parasites do not suffer differential mortality. The developmental stage of the female parasite does not seem to influence the likelihood of death. The presence of dead parasites may not confer an acquired immunity to re-infection. These features suggest that parasite death is typically the result of activation of a successful host defensive process rather than indicative of a defect on the part of the parasites. Parasitized female hosts can regain their reproductive capabilities following death of the parasite. Post-parasitic broods are smaller than normal. Reproductive recovery is presumed to provide the selective pressure favouring evolution of a lethal host response. The host-produced sheath surrounding female parasites is a haemocytic response. Sheaths enclosing dead parasites are thicker and more electron dense than those containing healthy parasites. The sheath of a healthy P. conformis may actually protect the parasite from a more intense host response.