Both freshwater and marine fish have evolved an astounding array of methods of obtaining food. One species or another is able to utilize as food nearly any plant or animal to be found in the world's aquatic habitats.
Many fish are able to exploit food resources that arise in terrestrial habitats. The most common example of this is when fish consume insects that fall upon the water's surface, or scavenge animals that have drowned. In certain areas of South America, numerous fish species invade the rainforest during the rainy season, thereby gaining access to a rich source of land-based foods that are unavailable during the dry season. Some fish, moreover, are specifically adapted for consuming nonaquatic foods. Mudskippers leave the water to chase crabs across mudflats, while arowanas — active, elongated fish of Central and South America — are well suited for leaping and plucking insects and other small creatures from overhanging branches. Pacu, the large, vegetarian relatives of the infamous piranha, are equipped with jaws that make short work of the thick husks of the various seeds and fruits that grow along their native South American streams and rivers.
It is impossible to even begin to describe the lengths to which some fish have gone, concerning unique specializations, to exploit unusual food sources. While most species are limited to swallowing food items that will fit within their mouths, some fish use a remarkable behavior that enables them to eat pieces of large food items that would otherwise be denied them. Certain types of piranha, for example, rush at larger fish and clip off a piece of their fin for a meal. Cookie-cutter sharks have uniquely structured mouths that allow them to grasp onto the sides of larger fish and twist off a hunk of flesh.
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Some fish, such as barracuda, actively search for their prey, while others, such as the anglerfish, remain sedentary while manipulating a wormlike lure designed to draw prey to within striking range. From fish that stalk their prey to those that shock it with electricity or are carried to food sources after attaching themselves to larger animals, the array of the feeding behaviors utilized by these remarkable creatures seems to stretch the limits of what is possible for animals.
The actual food sources utilized by fish are also interesting and, often, improbable. Strangely, many of the largest species, such as basking sharks, whale sharks, and manta rays, feed upon plankton, the tiniest of the ocean's numerous food items. At the other end of the scale, large ocean-going predators, such as great white sharks, are quite capable of consuming large marine mammals such as sea lions and, on rare occasions, people. As you will read in more detail in the various species accounts, there are, among countless other interesting feeding techniques, fish that create and protect gardens of algae, species that have evolved long, thin snouts to reach coral polyps, and others that can crush coral with their powerful jaws.