Invertebrates: Yucky Bugs to Us, Yummy Morsels to Fish

What could be better than to snack on a spider or feast on a fly larva? You and I might disagree, but invertebrates of all kinds are the food of choice for countless fish species. They represent nutritious, bite-sized morsels for many aquarium fishes.

How do invertebrates try to stay off the menu?

Being small and tasty is not an ideal strategy in the competitive underwater world. Consequently, invertebrates have evolved a wide range of defenses against predation. Many, such as worms, hide; molluscs, including snails and mussels, put their stock in a protective shell; and crustaceans, such as Daphnia, grow spiky appendages when they detect the odor of fish in the water. But in each case the defense does not provide a total solution. Fish have evolved ways of overcoming these defenses, although sometimes only specialists in a given kind of prey are consistently successful.

Do many fish eat plankton and other "midwater" animals?

Yes! Any small animal in the water column will be investigated by fish and eaten if it is bite-sized. The so-called water fleas (Daphnia spp.) and copepods are small relatives of crabs. They swim in the water column but can be difficult to catch, evading their hunters with bursts of speed. Often, they live in dense vegetation where they have a refuge from the attentions of most fishes. However, pencilfish possess the perfect narrow body for pursuing them into this aquatic undergrowth, and their small terminal mouths allow for accuracy in picking off their prey one by one.

Why are mosquitofish so called?

It is estimated that mosquitoes have killed more humans by spreading malaria than any other animal. Man's response has been to introduce fish into the swamps and wetlands that act as nurseries for so many mosquitoes and biting flies in the tropics. Both the livebearing mosquitofish and guppies reproduce rapidly and were thought to have an insatiable appetite for these larvae. However, recent studies of the diet of guppies in the wild show that algae makes up 75% of their intake and larvae only 24%.Thus their effectiveness is unproven and they often damage the indigenous aquatic animals as well. Nonetheless, larvae make excellent food, relying mainly on concealment to escape the fishe's attention.This strategy is often undermined by species such as mormyrids, which detect their presence electrically, and catfish, which explore the sediment using sensitive barbels.

Do any fish prey on snails?

Al Aquatic habitats are rich in molluscs, and snails are especially common in fresh water.Their tough shells put them off-limits for most fishes, but a number of species specialize in overcoming this problem. Pufferfish have bony beaks with which to bite open the snail shell, exposing the succulent flesh inside. The beaks grow continuously and without a tough diet to wear them down, overgrowth of the beak can prove a real problem in the aquarium. Doradid catfish also specialize in snails, as do clown loaches (Chromobotia macracanthus) and a number of cichlids. In these cases, the fish have specialized, platelike pharyngeal teeth and powerful muscles that generate remarkable force to crush the snail shells.

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