Life history patterns of Discorsopagurus schmitti, a hermit crab inhabiting polychaete tubes.

Discorsopagurus schmitti is a hermit crab that inhabits empty polychaete tubes in the North Pacific. Here we describe some aspects of its life history (relative growth, population structure, reproductive biology, and incidence of parasitism) and discuss the relationships among them. Unlike most hermits, the two sexes of this species have similar size distributions. In both sexes, larger body size is accompanied by a higher reproductive output (larger clutch size in females and more intrasex competitive potential in males). The energy the females expend in egg production might be equaled in this species by the energy the males expend in supporting parasites. In fact, the extent of infestation by two rhizocephalans [Peltogaster boschmae and Thilacoplethus (= Thompsonia) reinhardi] is more pronounced in males, especially those in the larger size classes. However, rhizocephalans have little effect on their hosts; growth and secondary sexual characters are not influenced. The only morphological modification is the more frequent loss of the second pleopod. Infected hermits also showed a mock parental behavior, fanning the externae with the pleopods as ovigerous females fan their eggs. Larvae are released in sequential bursts, and hatching occurs exclusively at night, possibly to minimize predation by diurnal fishes. Hatching is also synchronized with neap tides, which might keep the larvae from being flushed out into open waters. In a species whose habitat (sabellarian bioherms) is rare and quite unpredictable, it is beneficial to retain larvae near the parental population.