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A while ago I posted this.

No one managed to correctly guess the identity of this mystery creature, so in this post I’m letting the copepod out of the bag. Apart from wanting to reveal the identity of this creature I also need to share the story of what is another poor unfortunate.

If you’re ever down on your luck or feeling a bit sorry for yourself then spare a thought for a male sarcotacid. I’ve racked my brains to think of a more unfortunate creature, but I always return to this poor little chap.

In the bizarre world of aquatic parasites, the sarcotacids are truly in a league of their own.  Taxonomically, these enigmatic animals are copepods, although adult sarcotacids give little away to suggest this is the case. The adult female looks like a chunky maggot and the male, well, the male is almost an afterthought, barely a couple of millimetres long.

When young, it is clear to see that sarcotacids are copepods, but with infection of their host comes a huge change in appearance and most the outward crustacean characteristics are lost. The unfortunate hosts of the sarcotacids are lots of species of marine fish that are infected as the young parasite burrows through the skin into the tissues beneath the lateral line. Some have a preference for the snout, while others appear to favour the anus of their host. How uncomfortable these are for the fish is anyone’s guess, but they probably make haemorrhoids feel like a birthday present.

What happens following penetration of the host’s skin is something of a mystery, but after some months the tiny female sarcotacid has grown into a 45mm segmented blob entirely encapsulated in a fluid filled cyst derived from the host’s tissues. How she feeds and what else she gets up to in this cyst is unknown, but it seems at some point in their development they engorge themselves on fish blood, the by-product of which remains as a thick, inky fluid.

Pressed against the side of this cyst by the bulk of their relatively gigantic mate are as many as 26 miniscule males, all of whom vie to fertilise the eggs of the huge, unseemly female. If this isn’t an unfortunate existence I don’t know what is.

Sarcotacids are but one type of parasitic copepods that have so embraced a parasitic way of life that as adults they are completely unrecognisable as crustaceans. In almost all cases the males of these bizarre animals are nothing more than tiny specks destined for an unspeakable existence. As grisly as the sarcotacid way of life is they appear to have very little impact on the health of their host. Perhaps the relationship between these copepods and fish is so ancient that the typical unpleasantness of parasitism has mellowed. With more eons of evolution the descendents of the sarcotacids may be integral parts of the bodies of their fish hosts.