Filter feeders areaquatic animals that acquirenutrients byfeeding onorganic matters,foodparticles or smallerorganisms (bacteria,microalgae andzooplanktons)suspended in water, typically by having the water pass over or through a specializedfilteringorgan that sieves out and/or traps solids. Filter feeders can play an important role in condensingbiomass and removingexcess nutrients (such asnitrogen andphosphate) from the localwaterbody, and are therefore considered water-cleaningecosystem engineers. They are also important inbioaccumulation and, as a result, asindicator organisms.
Filter feeders can besessile,planktonic,nektonic or evenneustonic (in the case of thebuoy barnacle) depending on thespecies and theniches they haveevolved to occupy. Extant species that rely on suchmethod of feeding encompass numerousphyla, includingporiferans (sponges),cnidarians (jellyfish,sea pens andcorals),arthropods (krill,mysids andbarnacles),molluscs (bivalves, such asclams,scallops andoysters),echinoderms (sea lilies) andchordates (lancelets,sea squirts andsalps, as well as manymarinevertebrates such as most species offorage fish,American paddlefish,silver andbighead carps,baleen whales,manta ray and three species ofsharks—thewhale shark,basking shark andmegamouth shark). Somewater birds such asflamingos and certainduck species, though predominantly terrestrial, are also filter feeders whenforaging.
Mostforage fish are filter feeders. For example, theAtlantic menhaden, a type ofherring, lives onplankton caught in midwater. Adult menhaden can filter up to four gallons of water a minute and play an important role in clarifying ocean water. They are also a natural check to the deadlyred tide.[1]
In addition to these bony fish, four types ofcartilaginous fishes are also filter feeders. Thewhale shark sucks in a mouthful of water, closes its mouth and expels the water through itsgills. During the slight delay between closing the mouth and opening the gill flaps, plankton is trapped against thedermal denticles which line its gill plates andpharynx. This fine sieve-like apparatus, which is a unique modification of the gill rakers, prevents the passage of anything but fluid out through the gills (anything above 2 to 3 mm in diameter is trapped). Any material caught in the filter between the gill bars is swallowed. Whale sharks have been observed "coughing" and it is presumed that this is a method of clearing a build up of food particles in the gill rakers.[2][3][4] Themegamouth shark has luminous organs calledphotophores around its mouth. It is believed they may exist to lure plankton or small fish into its mouth.[5] Thebasking shark is a passive filter feeder, filteringzooplankton, small fish, andinvertebrates from up to 2,000 tons of water per hour.[6] Unlike the megamouth and whale sharks, the basking shark does not appear to actively seek its quarry; but it does possess largeolfactory bulbs that may guide it in the right direction. Unlike the other large filter feeders, it relies only on the water that is pushed through the gills by swimming; the megamouth shark and whale shark can suck or pump water through their gills.[6]Manta rays can time their arrival at the spawning of large shoals of fish and feed on the free-floating eggs and sperm. This stratagem is also employed by whale sharks.[7]
Like all arthropods, crustaceans areecdysozoans, a clade withoutcilia. Cilia play an important role for many filter feeding animals, but because crustaceans don't have them, they need to use modified extremities for filter feeding instead.[8]Mysidaceans live close to shore and hover above the sea floor, constantly collecting particles with their filter basket. They are an important food source forherring,cod,flounder, andstriped bass. Mysids have a high resistance to toxins in polluted areas, and may contribute to high toxin levels in their predators.[citation needed]Antarctic krill manages to directly utilize the minutephytoplankton cells, which no other higher animal of krill size can do. This is accomplished through filter feeding, using the krill's developed front legs, providing for a very efficient filtering apparatus:[9] the sixthoracopods form a very effective "feeding basket" used to collect phytoplankton from the open water. In the animation at the top of this page, the krill is hovering at a 55° angle on the spot. In lower food concentrations, the feeding basket is pushed through the water for over half a meter in an opened position, and then the algae are combed to the mouth opening with special setae on the inner side of the thoracopods.Porcelain crabs have feeding appendages covered with setae to filter food particles from the flowing water.[10]Most species ofbarnacles are filter feeders, using their highly modified legs to sift plankton from the water.[11]
Also some insects with aquatic larvae or nymphs are filter feeders during their aquatic stage. Such as some species ofmayfly nymphs,[12]mosquito larvae,[13] andblack fly larvae.[14] Instead of using modified limbs or mouthparts, somecaddisfly larvae produce nets of silk used for filter feeding.[15]
Thebaleen whales (Mysticeti), one of two suborders of theCetacea (whales, dolphins, and porpoises), are characterized by havingbaleen plates for filtering food from water, rather than teeth. This distinguishes them from the other suborder of cetaceans, thetoothed whales (Odontoceti). The suborder contains four families and fourteen species. Baleen whales typically seek out a concentration of zooplankton, swim through it, either open-mouthed or gulping, and filter the prey from the water using their baleens. A baleen is a row of a large number ofkeratin plates attached to the upper jaw with a composition similar to those in human hair or fingernails. These plates are triangular in section with the largest, inward-facing side bearing fine hairs forming a filtering mat.[16]Right whales are slow swimmers with large heads and mouths. Their baleen plates are narrow and very long — up to 4 m (13 ft) inbowheads — and accommodated inside the enlarged lower lip which fits onto the bowed upper jaw. As the right whale swims, a front gap between the two rows of baleen plates lets the water in together with the prey, while the baleens filter out the water.[16]Rorquals such as theblue whale, in contrast, have smaller heads, are fast swimmers with short and broad baleen plates. To catch prey, they widely open their lower jaw — almost 90° — swim through a swarm gulping, while lowering their tongue so that the head's ventral grooves expand and vastly increase the amount of water taken in.[16] Baleen whales typically eatkrill in polar or subpolar waters during summers, but can also take schooling fish, especially in the Northern Hemisphere. All baleen whales except thegray whale feed near the water surface, rarely diving deeper than 100 m (330 ft) or for extended periods. Gray whales live in shallow waters feeding primarily on bottom-living organisms such asamphipods.[16]
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Bivalves are aquaticmolluscs which havetwo-part shells. Typically both shells (or valves) aresymmetrical along the hinge line. The class has 30,000species, includingscallops,clams,oysters andmussels. Most bivalves are filter feeders (although some have taken up scavenging and predation), extracting organic matter from the sea in which they live.Nephridia, the shellfish version ofkidneys, remove the waste material. Buried bivalves feed by extending a siphon to the surface. For example,oysters draw water in over their gills through the beating ofcilia. Suspended food (phytoplankton,zooplankton,algae and other water-borne nutrients and particles) are trapped in the mucus of a gill, and from there are transported to the mouth, where they are eaten, digested and expelled as feces orpseudofeces. Each oyster filters up to five litres of water per hour. Scientists believe that theChesapeake Bay's once-flourishing oyster population historically filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process would take almost a year,[17] and sediment, nutrients, and algae can cause problems in local waters. Oysters filter these pollutants,[18] and either eat them or shape them into small packets that are deposited on the bottom where they are harmless.
Bivalve shellfish recycle nutrients that enter waterways from human and agricultural sources.Nutrient bioextraction is "an environmental management strategy by which nutrients are removed from an aquatic ecosystem through the harvest of enhanced biological production, including the aquaculture of suspension-feeding shellfish or algae".[19] Nutrient removal by shellfish, which are then harvested from the system, has the potential to help address environmental issues including excess inputs of nutrients (eutrophication), low dissolved oxygen, reduced light availability and impacts on eelgrass, harmful algal blooms, and increases in incidence ofparalytic shellfish poisoning (PSP). For example, the average harvested mussel contains: 0.8–1.2% nitrogen and 0.06–0.08% phosphorus[20] Removal of enhanced biomass can not only combat eutrophication and also support the local economy by providing product for animal feed or compost. In Sweden, environmental agencies utilize mussel farming as a management tool in improving water quality conditions, wheremussel bioextraction efforts have been evaluated and shown to be a highly effective source of fertilizer and animal feed[21] In the U.S., researchers are investigating potential to model the use of shellfish and seaweed for nutrient mitigation in certain areas of Long Island Sound.[22]
Bivalves are also largely used asbioindicators to monitor the health of an aquatic environment, either fresh- or seawater. Their population status or structure, physiology, behaviour,[23] or their content of certain elements or compounds can reveal the contamination status of any aquatic ecosystem. They are useful as they are sessile, which means they are closely representative of the environment where they are sampled or placed (caging), and they breathe water all the time, exposing their gills and internal tissues:bioaccumulation. One of the most famous projects in that field is theMussel Watch Programme in America.
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Sponges have no truecirculatory system; instead, they create a water current which is used for circulation. Dissolved gases are brought to cells and enter the cells via simplediffusion.Metabolic wastes are also transferred to the water through diffusion. Sponges pump remarkable amounts of water.Leuconia, for example, is a small leuconoid sponge about 10 cm tall and 1 cm in diameter. It is estimated that water enters through more than 80,000 incurrent canals at a speed of 6 cm per minute. However, becauseLeuconia has more than 2 million flagellated chambers whose combined diameter is much greater than that of the canals, water flow through chambers slows to 3.6 cm per hour.[24] Such a flow rate allows easy food capture by the collar cells. Water is expelled through a singleosculum at a velocity of about 8.5 cm/second: a jet force capable of carrying waste products some distance away from the sponge.
Themoon jellyfish has a grid of fibres which are slowly pulled through the water. The motion is so slow thatcopepods cannot sense it and do not react with anescape response.[citation needed]
Other filter-feeding cnidarians includesea pens,sea fans,plumose anemones, andXenia.[citation needed]
Tunicates, such asascidians,salps andsea squirts, arechordates which form a sister group to thevertebrates. Nearly all tunicates aresuspension feeders, capturingplanktonic particles by filtering sea water through their bodies. Water is drawn into the body through the inhalantbuccal siphon by the action ofcilia lining the gill slits. The filtered water is then expelled through a separate exhalant siphon. To obtain enough food, a typical tunicate needs to process about one body-volume of water per second.[25]
Flamingos filter-feed onbrine shrimp. Their oddly shaped beaks are specially adapted to separate mud and silt from the food they eat, and are uniquely used upside-down. The filtering of food items is assisted by hairy structures calledlamellae which line themandibles, and the large rough-surfaced tongue.[26]
Prions are specialised petrels with filter-feeding habits. Their name comes from their saw-like jaw edges, used to scope out small planktionic animals.[27]
The extinctswanAnnakacygna is speculated to be a filter-feeder due to its bill proportions being similar to those ofshoveler ducks. It is unique in being a large, flightless marine animal, unlike the smaller still volant flamingos and prions.
Traditionally,Ctenochasmatoidea as a group has been listed as filter-feeders, due to their long, multiple slender teeth, clearly well adapted to trap prey. However, onlyPterodaustro showcases a proper pumping mechanism, having up-turned jaws and powerful jaw and tongue musculature. Other ctenochasmatoids lack these, and are now instead thought to have beenspoonbill-like catchers, using their specialised teeth simply to offer a larger surface area. Tellingly, these teeth, while small and numerous, are comparatively unspecialised to the baleen-like teeth ofPterodaustro.[28]
Boreopterids are thought to have relied on a kind of rudimentary filter feeding, using their long, slender teeth to trap small fish, though probably lacking the pumping mechanism ofPterodaustro. In essence, their foraging mechanism was similar to that of modern youngPlatanista "dolphins".[28][29]
Filter feeding habits are conspicuously rare amongMesozoicmarine reptiles, the main filter feeding niche being seemingly instead occupied bypachycormid fish. However, some sauropsids have been suggested to have engaged in filter feeding.Henodus was a placodont with unique baleen-like denticles and features of the hyoid and jaw musculature comparable to those of flamingos. Combined with its lacustrine environment, it might have occupied a similar ecological niche.[30][31] In particular, it was probably aherbivore, filtering outalgae and other small-sizedflora from the substrates.[32]Stomatosuchidae is a family of freshwatercrocodylomorphs with rorqual-like jaws and minuscule teeth, and the unrelatedCenozoicMourasuchus shares similar adaptations.Hupehsuchia is a lineage of bizarreTriassic reptiles adapted for suspension feeding.[33]Someplesiosaurs might have had filter-feeding habits.[34]
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