Effect of salinity on osmoregulation and survival of a rhizocephalan parasite, Loxothylacus panopaei, and its crab host, Rhithropanopeus harrisii
The rhizocephalan Loxothylacus panopaei parasitizes the estuarine crab Rhithropanopeus harrisii. Parasitized crabs are abundant during summers when salinities increase to around 15‰ in the crab–s habitat and scarce when salinities are lower. The two hypotheses that were proposed to explain this pattern were (1) that the parasite interferes with crab osmoregulation causing the host to die in low salinity water and (2) that salinity tolerance of the parasite larvae controls the incidence of parasitism. The first hypothesis was shown to be incorrect because (1) osmoregulation of infected crabs was, not altered by the parasite and (2) crab mortality did not increase in low salinity water down to 1‰. Unparasitized and parasitized crabs and the parasite itself were hypersomotic at low salinities (below 27‰ for the crabs). The parasite became slightly hyperosmotic at high salinities while the crabs were slightly hypoosmotic. The second hypothesis appears correct, because L. panopaei larvae survived poorly in salinities below 10‰ but well in salinities from 10 to 15‰. Thus R. harrisii, have a reproductive refuge at salinities below 10‰, because parasite larvae cannot survive and infect the mud crab at these low salinities. © 1991, Estuarine Research Federation. All rights reserved.
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