Today's parasite hails from the San José Gulf of Argentina. Opechona sp. is a digenean trematode which uses the intertidal snail Buccinanops cochlidium as a first intermediate host. The parasite sets up shop within the snail's gonads where it starts cloning itself, eventually castrating the snail through physical destruction of the gonad tissue. These clonal stages (known as rediae) produce free-living larvae called cercariae (pictured) that are released from the snail into the surrounding water, where they infect the next host in the life-cycle. This parasite reaches its peak prevalence during summer when water temperature is at its highest. While the life-cycle of Opechona is not fully known, related species have been recorded to infect jellyfishes as the second intermediate host in their life-cycle, and the period of highest cercariae emission for Opechona during summer may possibly coincide with the high abundance of jellyfishes during that season.
References: Averbuj, A. and Cremonte, F. (2010) Parasitic castration of Buccinanops cochlidium (Gastropoda: Nassariidae) caused by a lepocreadiid digenean in San José Gulf, Argentina. Journal of Helminthology 84: 381–389.
Colobomatus sillaginis is a parasitic copepod that lives in the head of two species of fish (commonly known in Australia as "whiting") in the genus Sillago (Sillago maculata and Sillago analis). This copepod dwells in the system of cephalic canals in the head of the fish. Interestingly, while the gut tracts of males and juvenile females are bright green, the gut of mature female copepods are usually coloured red or black. Living in the cephalic canal alongside C. sillaginis are small ciliates that are bright green due to the symbiotic algae living within them. These ciliates can be so numerous that some fish have a greenish tinge around front of the head. The male and juvenile female copepods graze upon this turf of abundant food. However, once they become mature, the female takes to feeding on blood, probably due to the physiological demand of egg production, rather like a female mosquito which normally feeds on nectar, but needs to obtain a blood meal for egg development.
Reference: West, G.A. (1983) A new philichthyid copepod parasitic in whiting (Sillago spp.) from Australian waters. Journal of Crustacean Biology 3: 622-628.