Filter feeder

Filter feeders are aquatic animals that acquire nutrients by feeding on organic matter, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a specialized filtering organ that sieves out and/or traps solids. Filter feeders can play an important role in condensing biomass and removing excess nutrients (such as nitrogen and phosphate) from the local waterbody, and are therefore considered water-cleaning ecosystem engineers. They are also important in bioaccumulation and, as a result, as indicator organisms.

Filter feeders can be sessile, planktonic, nektonic or even neustonic (in the case of the buoy barnacle) depending on the species and the niches they have evolved to occupy. Extant species that rely on such method of feeding encompass numerous phyla, including poriferans (sponges), cnidarians (jellyfish, sea pens and corals), arthropods (krill, mysids and barnacles), molluscs (bivalves, such as clams, scallops and oysters), echinoderms (sea lilies) and chordates (lancelets, sea squirts and salps, as well as many marine vertebrates such as most species of forage fish, American paddlefish, silver and bighead carps, baleen whales, manta ray and three species of sharks—the whale shark, basking shark and megamouth shark). Some water birds such as flamingos and certain duck species, though predominantly terrestrial, are also filter feeders when foraging.

Most forage fish are filter feeders. For example, the Atlantic menhaden, a type of herring, lives on plankton 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 deadly red tide.^[1]

In addition to these bony fish, four types of cartilaginous fishes are also filter feeders. The whale shark sucks in a mouthful of water, closes its mouth and expels the water through its gills. During the slight delay between closing the mouth and opening the gill flaps, plankton is trapped against the dermal denticles which line its gill plates and pharynx. 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] The megamouth shark likely convergently evolved a similar filter feeding mechanism.^[5] The basking shark is a passive filter feeder, filtering zooplankton, small fish, and invertebrates 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 large olfactory 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]

The baleen whales (parvorder Mysticeti), one of the two extant groups of the marine mammalian infraorder Cetacea (whales, dolphins and porpoises), are characterized by having baleen plates for filtering food such as krill from water. This distinguishes them from the other parvorder of cetaceans, the toothed whales (Odontoceti). Baleen whales contains four families and fourteen species. They 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 of keratin 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.^[8] Right whales are slow swimmers with large heads and mouths. Their baleen plates are narrow and very long — up to 4 m (13 ft) in bowheads — 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.^[8] Rorquals such as the blue 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.^[8] Baleen whales typically eat krill in polar or subpolar waters during summers, but can also take schooling fish, especially in the Northern Hemisphere. All baleen whales except the gray 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 as amphipods.^[8]

Flamingos filter-feed on brine 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 called lamellae which line the mandibles, and the large rough-surfaced tongue.^[9]

Prions are specialised petrels with filter-feeding habits. Their name comes from their saw-like jaw edges, used to scope out small planktionic animals.^[10]

The extinct swan Annakacygna is speculated to be a filter-feeder due to its bill proportions being similar to those of shoveler 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, only Pterodaustro 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 been spoonbill-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 of Pterodaustro.^[11]

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 of Pterodaustro. In essence, their foraging mechanism was similar to that of modern young Platanista "dolphins".^[11][12]

Filter feeding habits are conspicuously rare among Mesozoic marine reptiles, the main filter feeding niche being seemingly instead occupied by pachycormid 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.^[13][14] In particular, it was probably a herbivore, filtering out algae and other small-sized flora from the substrates.^[15] Stomatosuchidae is a family of freshwater crocodylomorphs with rorqual-like jaws and minuscule teeth, and the unrelated Cenozoic Mourasuchus shares similar adaptations. Hupehsuchia is a lineage of bizarre Triassic reptiles adapted for suspension feeding.^[16] Some plesiosaurs might have had filter-feeding habits.^[17]

Lancelets (subphylum Cephalochordata) are fish-like chordates that form a sister group to vertebrates. They are benthic animals that typically inhabit the seafloor, burrowing into well-ventilated substrates characterized by a soft texture and minimal organic content, with a distinct preference for coarse sand with low levels of fine particles. Lancelets are passive filter feeders,^{[citation needed]} spending most of the time half-buried in sand with only their frontal part protruding,^[18] and their diet include a wide variety of small planktonic organisms such as bacteria, fungi, diatoms, and zooplankton, as well as detritus.^[19]

Lancelets have buccal cirri, thin tentacle-like strands that hang in front of the mouth that act both as sensory devices and as a filter-feeding organ. Water passes from the mouth into the large pharynx, which is lined by numerous pharyngeal slits. The ventral surface of the pharynx contains a groove called the endostyle, which, connected to a structure known as Hatschek's pit, produces a film of mucus. Ciliary action pushes the mucus in a film over the surface of the pharyngeal slits, trapping suspended food particles. The mucus is collected in a second, dorsal groove known as the epipharyngeal groove, and passed back to the rest of the digestive tract. Having passed through the pharyngeal slits, the water enters an atrium surrounding the pharynx, then exits the body via the atriopore.^[20] Both adult and larva lancelets exhibit a "cough" reflex to clear the mouth or throat of debris or items too large to swallow. In larvae the action is mediated by the pharyngeal muscles while in the adult animal it is accomplished by atrial contraction.^[21][22]

Tunicates (subphylum Tunicata or Urochordata) such as ascidians, salps and larvaceans are chordates which form a sister group to vertebrates and lancelets. Nearly all tunicates are suspension feeders, capturing planktonic particles by filtering sea water through their bodies. Water is drawn into the body through the inhalant buccal siphon by the action of cilia lining the pharyngeal 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.^[23]

Aquatic arthropods such as crustaceans are ecdysozoans, a clade without cilia, which play an important role for other filter feeding animals. Crustaceans instead use modified extremities for filter feeding.^[24] Mysidaceans live close to shore and hover above the sea floor, constantly collecting particles with their filter basket. They are an important food source for larger animals such as herring, cod, flounder, and striped bass. Mysids have a high resistance to toxins in polluted areas, and may contribute to high toxin levels in their predators due to biomagnification.^{[citation needed]} Antarctic krill manages to directly utilize the minute phytoplankton 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:^[25] the six thoracopods 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.^[26] Most species of barnacles are filter feeders, using their highly modified legs to sift plankton from the water.^[27]

Also some insects with aquatic larvae or nymphs are filter feeders during their aquatic stage. Such as some species of mayfly nymphs,^[28] mosquito larvae,^[29] and black fly larvae.^[30] Instead of using modified limbs or mouthparts, some caddisfly larvae produce nets of silk used for filter feeding.^[31]

External image
Movie clip of siphon feeding

Bivalves are aquatic molluscs which have two-part shells. Typically both shells (or valves) are symmetrical along the hinge line. The class has 30,000 species, including scallops, clams, oysters and mussels. 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 of kidneys, 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 of cilia. 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 or pseudofeces. Each oyster filters up to five litres of water per hour. Scientists believe that the Chesapeake 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,^[32] and sediment, nutrients, and algae can cause problems in local waters. Oysters filter these pollutants,^[33] 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".^[34] 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 of paralytic shellfish poisoning (PSP). For example, the average harvested mussel contains: 0.8–1.2% nitrogen and 0.06–0.08% phosphorus^[35] 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, where mussel bioextraction efforts have been evaluated and shown to be a highly effective source of fertilizer and animal feed^[36] 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.^[37]

Bivalves are also largely used as bioindicators to monitor the health of an aquatic environment, either fresh- or seawater. Their population status or structure, physiology, behaviour,^[38] 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 the Mussel Watch Programme in America.

Lophophorates are a sister group to molluscs that include brachiopods, bryozoans and phoronids, all characterized by the lophophore, a filter-feeding organ made of a ring of ciliated tentacles. Water flow into the lophophore from the sides and exits at the front. In lingulids, the entrance and exit channels are formed by groups of chaetae that function as funnels.^[39] In other brachiopods the entry and exit channels are organized by the shape of the lophophore.^[40] The lophophore captures food particles, especially phytoplankton (tiny photosynthetic planktons), and deliver them to the mouth via the brachial grooves along the bases of the tentacles.^[39] The mouth is a tiny slit at the base of the lophophore.^[41] Food passes through the mouth, muscular pharynx ("throat") and oesophagus ("gullet"),^[39] all of which are lined with cilia and cells that secrete mucus and digestive enzymes.^[40] The stomach wall has branched ceca ("pouches") where food is digested, mainly within the cells.^[39] Nutrients are transported throughout the coelom, including the mantle lobes, by cilia.^[40] The wastes produced by metabolism are broken into ammonia, which is eliminated by diffusion through the mantle and lophophore.^[39]

Sponges have no true circulatory system; instead, they create a water current which is used for circulation. Dissolved gases are brought to cells and enter the cells via simple diffusion. 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, because Leuconia 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.^[42] Such a flow rate allows easy food capture by the collar cells. Water is expelled through a single osculum at a velocity of about 8.5 cm/second: a jet force capable of carrying waste products some distance away from the sponge.

The moon jellyfish has a grid of fibres which are slowly pulled through the water. The motion is so slow that copepods cannot sense it and do not react with an escape response.^{[citation needed]}

Other filter-feeding cnidarians include corals, sea pens, sea fans, plumose anemones, and Xenia.^{[citation needed]}

• 
  An undulating live Aurelia in the Baltic Sea showing the grid in action
• 
  Higher magnification showing a prey item, probably a copepod
• 
  The prey is then drawn to the body by contracting the fibres in a corkscrew fashion (image taken with an ecoSCOPE).

Crinoids, also known as feather stars or sea lilies are echinoderms that have a stalk attaching to the substrate via a holdfast, but many live attached only as juveniles and become free-swimming as adults. Crinoids are passive suspension feeders of plankton and small particles of detritus, which they catch from the sea water flowing past with feather-like arms (pinnules) that are held perpendicular to the current like a fan. Mobile crinoids move to perch on rocks, coral heads or other eminences to maximise their feeding opportunities.

The food particles are caught by the primary (longest) tube feet fully extended and held erect from the pinnules, forming a food-trapping mesh, while the secondary and tertiary tube feet are involved in manipulating anything encountered.^[43] The tube feet are covered with sticky mucus that traps any particles which come in contact. Once they have caught a particle of food, the tube feet flick it into the ambulacral groove, where the cilia propel the mucus and food particles towards the mouth. Lappets at the side of the groove help keep the mucus stream in place. The total length of the food-trapping surface may be very large; the 56 arms of a Japanese sea lily with 24 cm (9 in) arms, have a total length of 80 m (260 ft) including the pinnules. Generally speaking, crinoids living in environments with relatively little plankton have longer and more highly branched arms than those living in food-rich environments.^[43]

The mouth descends into a short oesophagus. There is no true stomach, so the oesophagus connects directly to the intestine, which runs in a single loop right around the inside of the calyx. The intestine often includes numerous diverticulae, some of which may be long or branched. The end of the intestine opens into a short muscular rectum. This ascends towards the anus, which projects from a small conical protuberance at the edge of the tegmen. Faecal matter is formed into large, mucous-cemented pellets which fall onto the tegmen and thence the substrate.^[43]

• Particle (ecology)
• Planktivore
• Predation
• Spider webs, which function as aerial biofilters to catch and retain airborne food such as flying insects,^[44] with analogies to aquatic filter feeding

• Bullivant, JS (July 1968). "A Revised Classification of Suspension Feeders". Tuatara. 16 (2): 151–160.
• Ostroumov, S. A. (July 2005). "Some Aspects of Water Filtering Activity of Filter-Feeders". Hydrobiologia. 542 (1): 275–286. Bibcode:2005HyBio.542..275O. doi:10.1007/s10750-004-1875-1.

• Filter feeder of krill Archived 1 May 2002 at the Wayback Machine
• Mussel Watch Programme