What were these parasites?
Some time ago, I collected approximately 40 sticklebacks. All had distended bodies, but they were not full of eggs. The swellings were sort of square shaped. I euthanised one of them and cut it open to reveal a worm-like parasite, which I’m now wondering might have been a tapeworm—possibly even
Diphyllobothrium latum. Is this likely?
BILL BRADBURY
PETER REPLIES: I’m assuming these were three-spined sticklebacks,
Gasterosteus aculeatus. The visible worm-like parasite that you found was almost certainly the larval stage of a tapeworm, Schistocephalus solidus.
Sticklebacks that harbour this tapeworm may develop a very large rounded or angular bulge to their belly region, as you describe.
In common with other tapeworm parasites, Schistocephalus has a complex life cycle: the adult worms are attached to the gut wall of fish-eating birds, such as kingfishers. Here, they shed their eggs into the bird’s gut and the eggs are then expelled with the bird’s faeces, so they often end up plopping into the water. There, they hatch and, if the larval tapeworms are lucky, they will be consumed by a freshwater copepod (Cyclops) where they develop further.
If the infested copepod is subsequently eaten by a stickleback, then the larval tapeworms will be released inside the fish and migrate to its abdominal cavity. Here they must wait it out, until such time that the stickleback is eaten by a bird, so completing the life cycle. Note that
Schistocephalus do not reside in the stickleback’s gut, but in its body cavity. Some unfortunate sticklebacks may harbour several tapeworm larvae, each weighing around 0.3g grams. Given that the fish itself may weigh only a couple of grams, then clearly the tapeworms can make up a large proportion— sometimes over half—of the stickleback’s total body weight.