Suction feeder
 Video of a red bay snook catching prey by suction feeding
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Video of a slingjaw wrasse catching prey by protruding its jaw
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Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identified by Robert McNeill Alexander. As a result, underwater predators, especially bony fish, have evolved a number of specialized feeding mechanisms, such as filter feeding, ram feeding, suction feeding, protrusion, and pivot feeding.
Most underwater predators combine more than one of these basic principles. For example, a typical generalized predator, such as the cod, combines suction with some amount of protrusion and pivot feeding.
Suction feeding is a method of ingesting a prey item in fluids by sucking the prey into the predator's mouth. This is typically accomplished by the predator expanding the volume of its oral cavity and/or throat, resulting in a pressure difference between the inside of the mouth and the outside environment. When the mouth is opened, the pressure difference causes water to flow into the predator's mouth, carrying the prey item in with the fluid flow.
Though suction feeding can be seen across fish species, those with more derived characters show an increase in the suction force as a result of more complex skull linkages that allow greater expansion of the buccal cavity and thereby creating a greater negative pressure. Most commonly this is achieved by increasing the lateral expansion of the skull. In addition, the derived character of upper jaw protrusion is acknowledged to increase the force exerted on the prey to be engulfed. Perhaps the best examples of these derived characters that are explained belong to fishes in the teleostei clade. However, a common misconception of these fishes is that suction feeding is the only or primary method employed. In Micropterus salmoides ram feeding is the primary method for prey capture; however, they can modulate between the two methods or use both as with many teleosts. Also, it is commonly thought that fishes belonging to more primitive clades exhibit suction feeding. Although suction may be created upon the mouth opening in such fishes, the criteria for suction feeding includes little or no bodily movement towards their prey.
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