Since I mentioned yesterday that penis size mattered, upon stumbling on this article about the horrific effects of a trematode infestation, I thought everyone might enjoy a grim and vivid picture of what trematodes can do to a poor, innocent mollusc.
This is a photo of a trematode, or fluke. Trematodes are parasitic flatworms with very complex life cycles; this particular one is a cercaria, or tailed larva. They swim about and infest various hosts at various stages, proliferating and spreading through tissues, before moving on to infect the next host in their cycle.
This particular trematode of the genus Neophasis was isolated from a whelk, Buccinum undatum. When the trematodes invade these molluscs, they specifically congregate in the gonad, destroying it and any hormones it produces. Here are some photos of healthy whelks, a female on the left and male on the right, with their shells removed to expose the internal organs.
The gonad is on the left, labeled with a “g”, and is distinct from another gland (the digestive gland, “dg”) by its lighter color. On the male, notice the penis “p” of impressive size.
Now take a look at a female (left) and male (right) suffering from a trematode infestation.
The gonads of both sexes are infected with the parasite, and are indistinguishable from the digestive gland. The most noticeable deficiency is in the male—that once noble and prominent penis is nothing but a tiny, shriveled pimple. The deletion of the source of the hormones necessary to trigger development of the organ has castrated the animal.
This kind of hormonal change is expected to cause behavioral changes, and in some species is suspected to modify the host in such a way as to promote the shift of the trematode to the next host, by making it more likely to be eaten by that host, for example. No such specific behaviors were noted in this study, although in this case the sterilization of the host can have significant impacts on the population size.
Tétreault F, Himmelman JH, Measures L (2000) Impact of a Castrating Trematode, Neophasis sp., on the Common Whelk, Buccinum undatum, in the Northern Gulf of St. Lawrence. Biol. Bull. 198:261-271.