Reproductive strategy of an isopod Onisocryptus ovalis, parasitizing a bioluminescent myodocope ostracod Vargula hilgendorfii

The functional morphology and the reproductive strategy of a parasitic isop od Onisocryptus ovalis in a bioluminescent ostracod Vargula hilgendorfii as its final host were studied based on video and SEM observations. During it s lifetime, Onisocryptus ovalis dramatically metamorphoses several times, c hanging sex from male to female in the final host's carapace. At nearly the last ontogenetical stage, the parasite anchors its body with a pair of tho racopods to the posterodorsal region of the host ostracod's trunk and loses all the other appendages and thus its mobility as well. Thereafter, the pa rasite reverses bodily orientation during the final moulting so as to locat e its mouth in the midst of the host eggs, and finally consumes them, leavi ng only the egg membrane. Such a mode of feeding of the parasite following the fixation of the body is interpreted in terms of the adaptation to escap e elimination from the ostracod carapace by the host's cleaning appendages (the seventh limbs) and to obtain as much space as possible for the parasit e's own eggs/embryos at the sacrifice of the mother's mobility. The synchro nization between the timing of metamorphosis of the parasite and the reprod uctive cycle of the host animal can be expected to guarantee the parasite t he opportunity to exploit sufficient nutrition from the eggs of the host.