Castrating trematodes!

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.


  1. Volvox says

    In his book “A River Ran Out Of Eden” Richard Dawkins said

    “I cannot persuade myself,” Darwin wrote, “that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars.”

    Most likely he would feel the same way about the trematode/whelk interaction.


  2. Zarquon says

    Zorba: Anyway the majority of the molluscs are included in three large groups, the gastropods, the lamellibranchs and the cephalopods…

    Mrs Jalin: We knew that (she gets up and goes to the set)

    Zorba: However, what is more interesting, er … is the molluscs’s er … sex life.

    Mrs Jalin: (stopping dead) Oh!

    Zorba: Yes, the mollusc is a randy little fellow whose primitive brain scarcely strays from the subject of the you know what.

    Mrs Jalin: (going back to sofa) Disgusting!

    Mr Jalin: Ought not to be allowed.

    Zorba: The randiest of the gastropods is the limpet. This hot-blooded little beast with its tent-like shell is always on the job. Its extra-marital activities are something startling. Frankly I don’t know how the female limpet finds the time to adhere to the rock-face. How am I doing?

    Mrs Jalin: Disgusting.

    Mr Jalin: But more interesting.

    Mrs Jalin: Oh yes, tch, tch, tch.

    Zorba: Another loose-living gastropod is the periwinkle. This shameless little libertine with its characteristic ventral locomotion … is not the marrying kind: ‘Anywhere anytime’ is its motto. Up with the shell and they’re at it.

    Mrs Jalin: How about the lamellibranchs?

    Zorba: I’m coming to them … the great scallop (holds one up) … this tatty, scrofulous old rapist, is second in depravity only to the common clam. (holds up a clam) This latter is a right whore, a harlot, a trollop, a cynical bed-hopping firm-breasted Rabelaisian bit of sea food that makes Fanny Hill look like a dead Pope… and finally among the lamellibranch bivalves, that most depraved of the whole sub-species – the whelk. The whelk is nothing but a homosexual of the worst kind. This gay boy of the gastropods, this queer crustacean, this mincing mollusc, this screaming, prancing, limp-wristed queen of the deep makes me sick.

  3. SEF says

    healthy whelks … with their shells removed

    Somehow I don’t think the whelks themselves would view that as healthy … and certainly not conducive to continued health and whelk-being.

  4. craig says

    Whelks are my favorite. About the only fun I’ve had here in FL is finding a 13 inch whelk shell.

  5. plunge says

    The way parasites affect their hosts and even change their behavior is mindbogglingly fascinating. Did everyone read the recent Loom story on the common cat parasite that actually changes rat brains so that they are more likely to get killed by cats, continuing the cycle? Amazing stuff!

  6. says

    Yes, they didn’t see it in this particular species, but the authors mention that gigantism is one common consequence of castration.

  7. says

    I have now officially added trematodes to my “unlikeable critters” list. You are going to get some messed up google searches from this post, I believe.

  8. kmeson says

    Must recommend “Parasite Rex” by Carl Zimmer. Absolutely fascinating. Don’t read before bed time…

  9. says

    Perhaps this explains the evolution of the surprisingly hardy Lawrence Whelk, which hides all traces of its reproductive readiness behind garish colors and curious “bubble” displays.

  10. Torbjorn Larsson says

    The smart (successful) parasite ought to promote the population size of its target. So the whelk trematode is obviously not an ID creation…

    Fascinating that not only does the parasite adversely affect the host (one assume) but also its procreation and still gets away with it.

  11. malcolm says

    I did some TEM work on a trematode back in the 70’s in Australia, when I was, alas all too briefly, a biologist. They have spectacularly complex lifecycles with dramatically different forms inhabiting quite different environments. The one I studied spent one phase in the cloaca of a gull, as well as two forms that live in snails. One form is a cyst, and in the stage before this you could cells full of what looked just like rolls of carpet, ready to unfurl into one component of the cyst wall.

  12. says

    Forgive my complete ignorance of mollusc sex organs- is the penis itself the source of molluscan testosterone? That is, are there testes (and sperm) separate from the glans, or it it a single package. Or does fluke infection mow it all down, lock, stock and barrel?