Filter feeders are aquatic animals that feed 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, are 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.
Feeding mechanism
Filter feeding is a feeding mechanism used by aquatic animals to gather particles of food from the water around them, by having the water pass over or through a specialized filter that traps solids. The first mechanism proposed was sieving, by J. Alder and A. Hancock in 1851. The sieve theory implies that only particles as big as the mesh of the sieve can be trapped. In 1905, H. Wallengren proposed an alternative theory, that sticky limbs of the filters of animals such as mussels could collide with and trap particles smaller than the mesh size.
Another essential component of a filter feeding mechanism is a pump to force a flow of water over the filter. Among benthic invertebrates, some use cilia to create the flow, while others use muscles. Filter-feeders using cilia include tunicates, sponges, bivalves, bryozoans, and some polychaetes. Other polychaetes use muscular mechanisms.
Ecological effects
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, are 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.
Taxonomic range
Vertebrates
Fishes
Most forage fishes 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. Extensive article on the role of menhaden in the ecosystem and possible results of overfishing.
In addition to these bony fishes, four types of cartilaginous fishes are 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 and traps anything above 2 to 3 mm in diameter. 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. The megamouth shark likely convergently evolved a similar filter feeding mechanism. The basking shark is a passive filter feeder, filtering zooplankton, small fish, and invertebrates from up to 2,000 tons of water per hour. 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. 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.
Baleen whales
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 prey from the water using their baleens. A baleen is a row of several 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. Right whales are slow swimmers with large heads and mouths. Their baleen plates are narrow and very long—up to 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. Rorquals such as the blue whale, in contrast, have smaller heads and are fast swimmers with short and broad baleen plates. To catch prey, they widely open their lower jaw—almost 90°—and 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. 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's surface, rarely diving deeper than or for extended periods. Gray whales live in shallow waters, feeding primarily on bottom-living organisms such as amphipods.
Birds
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.
Prions are specialised petrels with filter-feeding habits. Their name comes from their saw-like jaw edges, used to scope out small planktonic animals.Gotch, A. F. (1995) . "Albatrosses, Fulmars, Shearwaters, and Petrels". Latin Names Explained A Guide to the Scientific Classifications of Reptiles, Birds & Mammals. New York, NY: Facts on File. pp. 191–192. ISBN 0-8160-3377-3.
Pterosaurs
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 next to the baleen-like teeth of Pterodaustro.
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".
Marine reptiles
Filter feeding habits are 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.Rieppel, O. (2002). Feeding mechanisms in Triassic stem-group sauropterygians: the anatomy of a successful invasion of Mesozoic seas Zoological Journal of the Linnean Society, 135, 33–63 In particular, it was probably a herbivore, filtering out algae and other small-sized flora from the substrates. 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. Some plesiosaurs might have had filter-feeding habits.
Lancelets
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, spending most of the time half-buried in sand with only their frontal part protruding, and their diet include a wide variety of small planktonic organisms such as bacteria, fungi, diatoms, and zooplankton, as well as detritus.
Lancelets have buccal cirri, thin tentacle-like strands that hang in front of the mouth that act both as sensory devices and as filter-feeding organs. 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. 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.
Tunicates
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.
Arthropods