The Effect of Parasitism by an Entoniscid on the Secondary Sex Characters of Pagurus longicarpus

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Abstract
Paguritherium alatum, a parasite of the hermit crab Pagurus longicarpus, causes modifications of the secondary sex characters of the female host but does not externally affect the male. In infested females the endopod/exopod ratio for the first 3 pleopods is significantly decreased. In normal and infested females the rami increase in length as the crabs grow, but parasitism causes a pronounced retardation in the growth of the endo-pod and only a slight decrease in the growth of the exopod as shown by a comparison of endopod/carapace and exopod/carapace ratios. The hairs on the external surface of the endopod and protopod of infested females are either partially or completely lost. Since the endopods and ovigerous hairs are the only characters affected, and since these are structures directly concerned with the care of the developing young, it is concluded that the result of parasitism in this case is specifically a suppression of the normal development of the chief functional secondary sex characters of the female. Failure of these structures to develop properly is attributed to the loss of possible hormonal influence from the ovaries due to castration of the host by the parasite.

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