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 in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

s, especially

bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...

, have evolved a number of specialized feeding mechanisms, such as

filter feeding Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a spe ...

, 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 (: cod) is the common name for the demersal fish genus ''Gadus'', belonging to the family (biology), 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 ''Gad ...

, 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 (usu ...

, 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 jaws. 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, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

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 in which 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 ki ...

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

Teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

ei 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 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 t ...

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 (watercraft), 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 (physics), wake. A large bow wave slows the ship down, is a risk t ...

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 cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s and

herring Herring are various species of forage fish, belonging to the Order (biology), order Clupeiformes. Herring often move in large Shoaling and schooling, schools around fishing banks and near the coast, found particularly in shallow, temperate wate ...

. A number of species have evolved narrow snouts, as in gar fish and water snakes.

Herring Herring are various species of forage fish, belonging to the Order (biology), order Clupeiformes. Herring often move in large Shoaling and schooling, schools around fishing banks and near the coast, found particularly in shallow, temperate wate ...

s 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 th ...

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 The reef manta ray (''Mobula alfredi'') is a species of ray in the family Mobulidae, one of the largest rays in the world. Among generally recognized species, it is the second-largest species of ray, only surpassed by the giant oceanic manta ...

'' ram-feeding, swimming against the tidal current with its mouth open and sieving

zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

from the water File:Herringramkils.jpg,

ram-feeding on a school of

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

s File:Group of fish near the beach of Sharm El Naga.jpg,

School A school is the educational institution (and, in the case of in-person learning, the Educational architecture, building) designed to provide learning environments for the teaching of students, usually under the direction of teachers. Most co ...

of adult

Indian mackerel The Indian mackerel (''Rastrelliger kanagurta'') or bigmouth mackerel is a species of mackerel in the family Scombridae. It is commonly found in the Indian Ocean, Indian and Pacific Ocean, West Pacific oceans, and their surrounding seas. It i ...

ram feeding on

macroplankton Plankton are the diverse collection of organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they pr ...

Lunge feeding

Rorqual Rorquals () are the largest clade, group of baleen whales, comprising the family (biology), family Balaenopteridae, which contains nine extant taxon, extant species in two genus, genera. They include the largest known animal that has ever lived, ...

s feed on

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

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 Baleen is a filter feeder, filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by th ...

. Biomechanically this is a unique and extreme feeding method, for which the animal at first must accelerate to gain enough

momentum In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...

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 MUSCULAR (DS-200B), located in the United Kingdom, is the name of a surveillance program jointly operated by Britain's Government Communications Headquarters (GCHQ) and the U.S. National Security Agency (NSA) that was revealed by documents release ...

buccal rills are a specialized adaptation to this feeding mode.

Jaw protrusion

Jaw protrusion is the outward movement of the

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 mammals h ...

or mouthparts towards the prey, which is achieved via more mobile mechanical linkages in the articulations of the mouth. Vertebrate jaw protrusion is known only among modern

bony fishes Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes (cartila ...

, which possess many forms of coupled linkages in their head. Remarkable examples are the slingjaw wrasse and the sand eel, which can protrude their mouth by several centimeters. This is usually done to extend the striking range of suction feeding, and the retraction of the jaw after protrusion can also help retrieval once the prey has been engulfed. Another example of mouthpart protrusion is seen in

dragonfly A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of dragonflies are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threat ...

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e (

nymph A nymph (; ; sometimes spelled nymphe) is a minor female nature deity in ancient Greek folklore. Distinct from other Greek goddesses, nymphs are generally regarded as personifications of nature; they are typically tied to a specific place, land ...

s), which have

hydraulic Hydraulics () 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 counterpart of pneumatics, which concer ...

lower

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

s that can extend rapidly, protruding forward to seize prey and bring it to the top jaw.

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 such as sea horses and sea dragons are specialized on this feeding mechanism. 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 The pharyngeal arches, also known as visceral arches'','' are transient structures seen in the embryonic development of humans and other vertebrates, that are recognisable precursors for many structures. In fish, the arches support the gills a ...

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 o ...

. A

four-bar linkage In the study of Mechanism (engineering), mechanisms, a four-bar linkage, also called a four-bar, is the simplest closed-Kinematic chain, chain movable linkage (mechanical), linkage. It consists of four Rigid body, bodies, called ''bars'' or ''link ...

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 Osteichthyes, bony fish in the genus ''Hippocampus''. The genus name comes from the Ancient Greek (), itself from () meaning "horse" and () meanin ...

s rely on stealth to ambush small prey such as

s. They use pivot feeding to catch the copepod, which involves rotating their snout at high speed and then sucking in the copepod.

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 animal, aquatic placental mammal, placental marine mammals. As an informal and Colloquialism, colloquial grouping, they correspond to large members of the infraorder Cetacea ...

, or the ostia of

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

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 Crinoids are marine invertebrates that make up the Class (biology), class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or ...

References

External links

FishBase FishBase is a global species database of fish species (specifically finfish). It is the largest and most extensively accessed online database on adult finfish on the web.

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