How host size may constrain the evolution of parasite body size and clutchsize. The parasitic isopod Ichthyoxenus fushanensis and its host fish, Varicorhinus bacbatulus, as an example

The flesh-burrowing parasitic isopod Ichthyoxenus fushanensis was found inf ecting the body cavity of a freshwater fish, Varicorhinus bacbutulus, in he terosexual pairs. Herein we investigate the question of how the host body s ize may constrain the parasite size and clutch size by analyzing the intera ctions among the body size and clutch size of the parasite, and host size. Due to the low transmission rate of I. fushanensis to its host and the posi tive relationship between clutch size and female size, selection may favor larger females with larger clutch sizes to compensate for massive losses of manca (the free-living juveniles). The path model reveals that clutch size depends not only directly on female size, but also on the sizes of her hos t and male. Female size also depends on the sizes of the host and her mate. A negative correlation exists between the body sizes of the paired males a nd females. This negative correlation may be regarded as a consequence of c ompetition for limited available space or other resources provided by the h ost. The effects of host size on parasite size, however, act on the total v olume of both sexes as a whole, not specifically on either the female or th e male. In this case, the available space/resources may not allow both indi viduals of different sexes to evolve toward a larger size simultaneously. U nder the constraint of host size, a strategy of reducing the body size of t he paired male may provide a way to increase the body size of the paired fe male and achieve a larger clutch size.