Bunocotyle progenetica is another parasite (see also Parvatrema margaritense) that has been thoroughly studied in the White Sea. Being a hemiurid trematode, it possesses all the typical life cycle stages. But when it comes to hosts, we see something entirely different. Hydrobia snails serve as an “all-in-one” habitat throughout the parasite's life. That is, cercariae don't leave the rediae but instead continue their development, up to an adult stage, still inside the same mollusc. The photo shows a redia with adults inside, with visible eggs inside them. The eggs are transferred to neighboring Hydrobia molluscs after the host's death. This favours increased snail exploitation by B. progenetica, since it doesn't require the host to live long. Thus the entire life of B. progenetica passes inside its host, with no free-living stage at all. This phenomemon is not uncommon among parasites as it provides maximum protection against a potentially hostile environment. The serious drawback of such a strategy, though, is lack of dispersal opportunities. It's possible to overcome this by using mobile hosts, however, not the case here, thus B. progenetica is a good example of just how odd parasites can sometimes be.
The PhD thesis referenced is entirely dedicated to this parasite, while the second paper only has certain comments on it.
Levakin I.A. Realization of a one-host life cycle of Bunocotyle progenetica (Trematoda: Hemiuroidea: Bunocotylinae) at the White Sea intertidal zone. PhD thesis manuscript, 2007. (In Russian)
Gorbushin, AM, 1997: Field evidence of trematode-induced gigantism in Hydrobia spp. (Gastropoda: Prosobranchia). J. Mar. Biol. Ass. UK 77 , 785–800.
Contributed by Anya Gonchar, photo by Ivan Levakin.