Specialist Feeders: Some Freshwater Fish Have Found Food Niches

Whereas most fishes tend to choose their food from within the same broad range of invertebrates, small fish, and vegetable matter, a significant minority have rather more unusual tastes.This kind of diet specialization is most commonly seen in habitats where a large number of species compete for the same kinds of food. By diversifying their diet into new and unexploited areas, they gain what is known as competitive release.

Are there any truly parasitic fish?

Parasites are animals that obtain their food from another, usually larger, animal species, but “ideally” without killing them. Nearly all parasite species are invertebrates — animals without backbones, such as ticks and tapeworms. But in the Amazon there are fish that have adopted this lifestyle with considerable success. The infamous candhiru is a tiny catfish, seldom more than 2 inches (6 cm) long that seeks out its victims by following the traces of urea that they emit from their gills as they breathe and excrete.

Once it picks up an odor trail, it follows it to the source and slips inside the gill cover. It then locks itself in place with specialized crampon-like spines on its gill covers, bites into the rich blood vessels of its host's gills and settles down to a blood meal. Once it has drunk its fill, it leaves its host and retires to the safety of the leaf litter at the bottom of the river, where it can contentedly digest. The way the candhiru finds its food — using urea trails — means that it can sometimes mistakenly follow trails produced by bathing mammals, including people, if they should urinate into the water. If this happens, and the candhiru swims somewhere it should not, the consequences for both the fish and the bather are serious.

Do fish have other unusual diets?

A number of fish species, including representatives of the catfish and cichlid families, have evolved the habit of dining out on the scales of other species. Where there are plenty of fish about this is a clever ploy — scales are rich in protein and can be nipped from their owners fairly simply with the right dental tool kit. Some scale-eaters have evolved color patterns that mimic those of their hosts. This allows them to mix with them almost like “wolves in sheep's clothing,” only revealing their gory intent at the last minute. Others feed obligately (meaning that this is their sole food source) on the mucus of various fish species. Mucus protects fish from infections and contains protein compounds, so once again, this is a pretty good diet if there are enough fish around.

Does the infamous eye-biter cichlid really live up to its name?

Some species, including Dimidiochromis compressiceps, are famed for another gruesome habit, that of eating the eyes from their living victims. Although this is quite conceivable, there is a lack of hard evidence to support the idea. Even if eye-biting is just a rare dietary excursion, it is clear that in fish-rich communities only a tiny number of fish species may feed by taking the body tissues of others, whether it be fins, scales, mucus, or eyes. Hands up anyone who still thinks it's easy being a fish!