Lernaeocera branchialis

Lernaeocera branchialis is a crustacean - believe it or not.

It is a parasitic copepod and for those of you who knows what a free-living, non-parasitic copepod looks like would probably find its appearance a bit shocking (or even nauseating). But almost half of all known species of copepod are in fact parasitic, and many of them have evolved into all kinds of weird and fantastical forms which would put most horror movie creations to shame.

What differentiate L. branchialis from other parasitic copepods is that it is one of the few which requires an intermediate host (most just have a direct, single-host life-cycle). The immature larvae of L. branchialis spend the first stage of its parasitic life living on a benthic fish - such as a flounder or a lumpfish. These fish act as hook-up bars for immature stages of these parasites, where the female makes her rendezvous with a suitable male for a little one-night stand of sorts. Then happily fertilised, she goes off and attach herself to the gill of her definitive host - a cod or haddock or other fish of the Gadidae family.

Once attached, she begins metamorphosing into a large (~4 cm) grotesque lump (to our eyes anyway, I'm sure she's just plump and happy, full of blood and churning out babies). The coiled mass are actually strings of eggs, which she produces continuously. The plump body with the egg strings dangle happily off the cod's gill like a fleshy pendant, but her anterior end is inserted deep into the fish through its gills, held in place by protruding branch-like structures appropriately called "antlers". The head of the copepod taps into the ventral aorta of the host and she happily feeds on the fish's blood, converting them into eggs which are dispersed into the sea to hatch and begin the cycle anew.

You can check out a photo here if you want to make sure I didn't just make up all that crazy business above: [link]