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

predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

s, especially bony fish, have evolved a number of specialized feeding mechanisms, such as

filter feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

, 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 Cod is the common name for the demersal fish genus '' Gadus'', belonging to the family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gadus'' is commonly not call ...

, combines suction with some amount of protrusion and pivot feeding.

Suction feeding

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 The dermatocranium is the portion of the cranium that is composed of dermal bone, as opposed to the endocranium and splanchnocranium, which are composed of endochondral bone. The dermatocranium comprises the skull roof, the facial skeleton (usuall ...

, 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 Cranial kinesis is the term for significant movement of skull bones relative to each other in addition to movement at the joint between the upper and lower jaw. It is usually taken to mean relative movement between the upper jaw and the braincase. ...

show an increase in suction potential as a result of more complex

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

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 Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

to be engulfed. Protrusible jaws via a mobile premaxilla can only be seen in fishes within the

teleost Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tele ...

ei clade. However, a common misconception of these fishes is that suction feeding is the only or primary method employed. In ''

Micropterus salmoides The largemouth bass (''Micropterus salmoides'') is a carnivorous freshwater gamefish in the Centrarchidae ( sunfish) family, a species of black bass native to the eastern and central United States, southeastern Canada and northern Mexico, but ...

'', 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 with more primitive characteristics also exhibit suction feeding. Although suction may be created upon the mouth opening in such fishes, the criteria for pure suction feeding includes little or no bodily movement towards their prey.

Tradeoffs

The morphologies and behaviors during suction feeding have led to three main proposed tradeoffs that determine the success of prey capture: the rate of jaw opening and closing, the mobility of the bony elements in the skull, and the ratio of ram to suction feeding behavior. The first two qualifications center around the situation that results from a highly kinetic skull. Having a highly mobile skull introduces a tradeoff between the ability to have high speed jaw opening (high kinesis) or higher bite transmission (lower kinesis). While there is a more complex relationship between mechanical advantage and the speed of lower jaw depression, there is consensus that species using high-speed attacks have more cranial kinesis compared to species that exhibit low speed attacks. Species that have a

durophagous Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil tu ...

diet have also evolved skull morphologies to crush the hard-shelled prey that is a part of their diet. Durophagous species skulls consistently have more fused skulls and shorter jaw lengths. This morphology leads to the skulls being less kinetic than their piscivorous counterparts. Having shorter jaw lengths, with a more akinetic skull allows for an individual to have a higher bite force, compromising the ability to have a faster jaw opening when the jaw lengths are longer. The third main tradeoff within suction feeding occurs with the incorporation of ram feeding with suction feeding behaviors. Ram feeding involves movement of the predator with its mouth open to engulf the prey. Most species use ram feeding combined with suction feeding to increase the chances of capturing elusive prey by swimming towards their prey while using suction to draw prey into the mouth. This diversity in relative use is quantified using the Ram Suction Index (RSI) that calculates the ratio of use for ram and suction during prey capture. The RSI ratio can be influenced by the morphology of the predator and by the elusiveness of the prey. Ram feeding and suction feeding are on opposite sides of the feeding spectrum, where extreme ram feeding is when a predator swims over an immobile prey item with open jaws to engulf the prey. Extreme suction feeding is demonstrated by sit-and-wait predators that rely on rapid depression of the jaws to capture prey (e.g. frogfish, ''Antennariidae''). There is wide diversity on how much of each feeding strategy an individual uses, especially when body ram movements are considered. The relative use of ram and suction feeding is species dependent, but it can help determine the accuracy of prey capture. The mouth aperture represents another tradeoff between the ability to capture large elusive prey with more chances of failure—large gape—or to capture smaller elusive prey with greater success—smaller gape. A predator with a small mouth aperture can generate strong suction force compared to an individual with a wider gape. This was demonstrated by Wainwright et al. (2007) by comparing the feeding success of the bluegill sunfish, ''Lepomis macrochirus'', and the largemouth bass, ''Micropterus salmoides''. ''L. macrochirus'' has a smaller gape and was found to have higher accuracy with higher flow velocity and acceleration while ''M. salmoides'' has a larger gape with lower accuracy and lower flow velocity and acceleration. However, with the larger gape the largemouth bass were able to capture larger elusive prey. Using ram feeding in combination with suction feeding can also influence the direction of water into the mouth of the predator. With use of ram, predators are able to change the flow of water around the mouth and focus the flow of water into the mouth. But with too much ram, a bow wave is created in front of the predator which can push the prey away from the predator's body. The mouth aperture and RSI represent the overall tradeoff between having a large gape with lower accuracy but being able to capture larger prey vs. having a smaller gape with increased accuracy but the size of prey is limited. The three main tradeoffs within the fish skull have occurred because of the high kinesis in the skull and the elusiveness of some prey types. However, having kinesis in the skull can enable a predator to evolve new techniques on increasing the performance of prey capture.

Ram feeding

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 A bow wave is the wave that forms at the bow of a ship when it moves through the water. As the bow wave spreads out, it defines the outer limits of a ship's wake. A large bow wave slows the ship down, is a risk to smaller boats, and in a harbor ...

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, or by allowing water to flow out through the gills, as in

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s and herring. A number of species have evolved narrow snouts, as in gar fish and water snakes. 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 raker Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch (gill arch) and are involved with suspension feeding tiny prey. They are not to be confused with the gill filaments that compose the fleshy part of the ...

s for a few milliseconds (

). 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. File:Manta alfredi ram feeding.png, Foraging '' Mobula alfredi'' ram-feeding, swimming against the tidal current with its mouth open and sieving

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

from the water File:Herringramkils.jpg, Herring ram-feeding on a school of copepods File:Group of fish near the beach of Sharm El Naga.jpg,

School A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is sometimes comp ...

of adult Indian mackerel ram feeding on macroplankton

Lunge feeding

Rorquals feed on

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

by a technique called lunge feeding. Lunge feeding could be regarded as a kind of inverted suction feeding, during which a whale takes a huge gulp of water, which is then filtered through the baleen. Biomechanically this is a unique and extreme feeding method, for which the animal at first must accelerate to gain enough momentum to fold its elastic throat (

buccal cavity The buccal space (also termed the buccinator space) is a fascial space of the head and neck (sometimes also termed fascial tissue spaces or tissue spaces). It is a potential space in the cheek, and is paired on each side. The buccal space is super ...

) around the volume of water to be swallowed. Subsequently, the water flows back through the baleen, keeping back the food particles. The highly elastic and

muscular Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...

buccal rills are a specialized adaptation to this feeding mode.

Pivot feeding

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 Pipefishes or pipe-fishes (Syngnathinae) are a subfamily of small fishes, which, together with the seahorses and seadragons (''Phycodurus'' and '' Phyllopteryx''), form the family Syngnathidae. Description Pipefish look like straight-bodied se ...

such as

sea horses A seahorse (also written ''sea-horse'' and ''sea horse'') is any of 46 species of small marine fish in the genus ''Hippocampus''. "Hippocampus" comes from the Ancient Greek (), itself from () meaning "horse" and () meaning "sea monster" or ...

and sea dragons are specialized on this feeding mechanism. With prey capture times of down to 5 ms (

shrimpfish Shrimpfish, also called razorfish, are five small species of marine fishes in the subfamily Centriscinae of the family Centriscidae. The species in the genera ''Aeoliscus'' and ''Centriscus'' are found in relatively shallow tropical parts of the ...

''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 The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of ...

. A

four-bar linkage In the study of mechanisms, a four-bar linkage, also called a four-bar, is the simplest closed-chain movable linkage. It consists of four bodies, called ''bars'' or ''links'', connected in a loop by four joints. Generally, the joints are config ...

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 Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

. File:Black Sea fauna Seahorse.JPG,

Seahorse A seahorse (also written ''sea-horse'' and ''sea horse'') is any of 46 species of small marine fish in the genus ''Hippocampus''. "Hippocampus" comes from the Ancient Greek (), itself from () meaning "horse" and () meaning "sea monster" or " ...

s 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.

Protrusion

Protrusion Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

is the extension of the mouth or

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

towards the prey, via mechanical linkages. Protrusion is known only in modern bony fishes, which possess many forms of coupled linkages in their head. Remarkable examples are the slingjaw wrasse and the

sand eel Sand eel or sandeel is the common name used for a considerable number of species of fish. While they are not true eels, they are eel-like in their appearance and can grow up to in length. Many species are found off the western coasts of Europe ...

, which can protrude their mouth by several centimeters. Another example of protrusion is seen in dragonfly larvae (nymphs), which have

hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...

lower

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

s, protruding forward to catch prey and bring it to the top jaw.

Filter versus suspension feeding

These are contrasting methods for the removal of food particles from a water flow: for example, by the

s of fish, the baleen of

whales Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...

, or the ostia of

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throug ...

Filter feeding

, the water flow is primarily generated by the organism itself, for example by creating a pressure gradient, by active swimming, or by ciliary movements.

Suspension feeding

In suspension feeding, the water flow is primarily external and the particles themselves move with respect to the ambient water flow, such as in sea lilies.

References

External links

* FishBase is a comprehensive database of bony fishes
/fishbase.org
{{diversity of fish Predation Marine biology Biomechanics