Aquatic feeding mechanisms

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. Cranial kinesis Suction feeding is a method of ingesting a prey item in fluids by sucking the prey into the predator's mouth. It is a highly coordinated behavior achieved by the dorsal rotation of the dermatocranium, lateral expansion of the suspensorium, and the depression of the lower jaw and hyoid. Suction feeding leads to successful prey capture through rapid movements creating a drop in pressure in the buccal cavity causing the water in front of the mouth to rush into the oral cavity, entrapping the prey in this flow. This mode of feeding has two main phases: expansion and compression. The expansion phase involves the initial opening of the jaws to capture prey. These movements during the expansion phase are similar across all suction feeders with the kinesis of the skull leading to slight variations. During the compression phase the jaws close and water is compressed out of the gills. Though suction feeding can be seen across fish species, those with more cranial kinesis show an increase in suction potential as a result of more complex skull linkages that allow greater expansion of the buccal cavity and thereby create a greater negative pressure. Most commonly, this is achieved by increasing the lateral expansion of the skull. In addition, the derived trait of anterior protrusion via the premaxillary bone in the upper jaw is acknowledged to increase the force exerted on the prey to be engulfed.