Parasites are part of a healthy aquatic ecosystem

Parasites are so common in aquatic ecosystems that parasitism must be a successful way of life. Parasites’ life cycles can be highly complex, however. Many things must go just right before the next parasite generation is produced.

A freshwater bream seems to be in good health. When you clean the fish, however, you come across a surprise: the stomach cavity is full of light-coloured ribbon resembling cooked tagliatelle. And there are slim red worms wriggling inside it!

You need not worry about parasites you see in the stomach cavity or on the skin of a fish. The fish is perfectly edible despite them. In fact, parasites are a sign of a well-functioning ecosystem. They make sure that it is not too easy for their hosts to take over as the dominant species. To researchers, parasites provide information about such things as the nutrition of the fish and the status of the water body.

Most of the numerous aquatic parasites are so small that they are difficult to see with the naked eye. Many parasites also have highly complex life cycles. The microscopic Diplostomum spathaceum worm reproduces in the guts of fish-eating birds, mainly seagulls. With bird droppings, their eggs end up at the bottom of a lake. If all goes well, a gastropod eats one of the eggs. In this case, the parasite grows inside the gastropod and starts producing free-swimming larvae. If a larva finds a fish, it infects the lens of the host’s eye. It then waits for a bird to catch the fish, enabling the parasite to start its life cycle all over again.

The parasite also has ways of improving its chances. Once the larva has entered a fish, it starts causing changes in the host: the fish starts swimming closer to the surface without being able to adjust its protective colouring to the new living environment. This makes it easier for birds to spot it.

Aquatic parasites are mainly harmless to humans – with the exception of the tapeworm, which is extremely rare. Swimmer’s itch can also be a minor nuisance sometimes.