A number of the most highly desirable freshwater and marine aquarium animals are confirmed live-food specialists. Although certain individuals of some of these species may be induced to accept nonliving food in captivity, in nearly all cases they fare much better on a diet that approximates that which would be consumed in the wild.

We find many live-food specialists among the marine fish that routinely appear in pet stores. Perhaps the best-known of these are seahorses and pipefish (other less commonly kept members of the order Syngnathiformes, such as shrimpfish, require live food as well). Most of these animals are extremely reluctant to feed upon nonliving food and require a varied diet of tiny living invertebrates if they are to do well in captivity. Closely related to the seahorses are the sticklebacks, family Gasterostidae, a largely marine assemblage of tiny fish that are characterized by the fascinating habit of constructing enclosed nests of plant material. These also need a varied diet of small living shrimp and other such creatures, although certain species can be habituated to a diet of flake and frozen foods.

Anglerfish, order Lophiiformes, well known for their use of wormlike lures to attract prey, require as food living fish that are themselves welladjusted to the aquarium and, therefore, interested in the angler's lure. Many, if not most, of the marine angelfish, which are among the most colorful and highly desirable aquarium inhabitants, feed exclusively upon coral polyps and the tiny animals that live among them.

A number of interesting fish that are usually collected rather than purchased by aquarists, such as the lizardfish, family Synodontidae, also are stimulated to feed only by the movement of prey.

Filter feeding invertebrates such as corals, tubeworms, sponges, and sea cucumbers feed nearly exclusively upon living plankton, although they do likely take in some nonliving particulate matter as well. Commercial diets have now been formulated for many of these invertebrate species, but most fare much better in captivity when supplied with living planktonic organisms. As you will see in the discussion of fish breeding in Chapter 20, many species are brought into breeding condition by an abundance of live food in the diet.

Although most freshwater aquarium fish seem less discerning when it comes to dietary matters than do their marine relatives, many of the most interesting ones do prefer or even require live foods. Among these are the bizarre elephant-nosed mormyrid, Gnathonemus petersi, whose elongated lower jaw allows the fish to effectively snap tiny worms from the substrate. This fish is also unique in that it discharges electrical impulses to navigate the murky waters of its West African range and, perhaps, to communicate with others of its kind.

Hailing from the same waters as the elephant nose is the butterfly fish, Pantodon bucholzi. This oddly shaped fish is specifically adapted to float at the water's surface, where it catches insects with a uniquely upturned mouth. Expertly camouflaged as a dead leaf, the butterfly fish's large pectoral fins assist it in leaping from the water to catch low flying insects as well.

The interesting spiny eels and their brightly colored relatives the fire eels of the family Mastagembelidae are also reluctant to consume anything other then tiny worms, brine shrimp, and similar invertebrates. Indeed, their body shapes and uniquely formed mouths ideally suit them for borrowing under rocks and through the substrate where such prey is to be found.

Gar, ancient predatory freshwater fish of the family Lepisosteidae, can rarely be induced to feed upon anything other than small live fish.

In addition to live-food specialists, there are a number of fish species that, while not reliant on it, prefer live food, or that are hard to maintain without it. The aquarist who keeps sizable predatory fish will find that it is difficult to provide a complete diet to such animals without the use of live food, even if the fish themselves will readily take commercial diets. For example, a number of the extremely interesting freshwater fish of the family Centrarchidae, such as the North American sunfish, basses, and perch, do best when fed earthworms, insects, and small fish. Nearly all of these animals will accept cut pieces of fish and other meats, and some will even take commercially available pellets, but these do not supply all of the nutrients required for optimal health.