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.[1] 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.
Foraging Mobula alfredi ram-feeding, swimming against the tidal current with its mouth open and sieving zooplankton from the water[23]
![Foraging Mobula alfredi ram-feeding, swimming against the tidal current with its mouth open and sieving zooplankton from the water[23]](http://upload.wikimedia.org/wikipedia/commons/thumb/3/33/Manta_alfredi_ram_feeding.png/193px-Manta_alfredi_ram_feeding.png)
Ram feeding is a method of feeding underwater in which the predator moves forward with its mouth open, engulfing the prey along with the water surrounding it. During ram feeding, the prey remains fixed in space, and the predator moves its jaws past the prey to capture it. The motion of the head may induce a bow wave in the fluid which pushes the prey away from the jaws, but this can be avoided by allowing water to flow through the jaw. This can be accomplished by means of a swept-back mouth, as in balaenid whales,[21] or by allowing water to flow out through the gills, as in sharks and herring. A number of species have evolved narrow snouts, as in gar fish and water snakes.[22]
Herrings often hunt copepods. If they encounter copepods schooling in high concentrations, the herrings switch to ram feeding. They swim with their mouth wide open and their opercula fully expanded. Every several feet, they close and clean their gill rakers for a few milliseconds (filter feeding). The fish all open their mouths and opercula wide at the same time (the red gills are visible in the photo below—click to enlarge). The fish swim in a grid where the distance between them is the same as the jump length of the copepods.
![Seahorses rely on stealth to ambush small prey such as copepods. They use pivot feeding to catch the copepod, which involves rotating their snout at high speed and then sucking in the copepod.[29][30]](http://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Black_Sea_fauna_Seahorse.JPG/192px-Black_Sea_fauna_Seahorse.JPG)
Pivot feeding is a method to transport the mouth towards the prey by an upward turning of the head, which is pivoting on the neck joint. Pipefish such as sea horses and sea dragons are specialized on this feeding mechanism.[28] With prey capture times of down to 5 ms (shrimpfish Centriscus scutatus) this method is used by the fastest feeders in the animal kingdom.
The secret of the speed of pivot feeding is in a locking mechanism, in which the hyoid arch is folded under the head and is aligned with the urohyal which connects to the shoulder girdle. A four-bar linkage at first locks the head in a ventrally bent position by the alignment of two bars. The release of the locking mechanism jets the head up and moves the mouth toward the prey within 5–10 ms. The trigger mechanism of unlocking is debated, but is probably in lateral adduction.