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 feeding are clams , krill ,
sponges , baleen whales , and many fish (including some sharks ). Some
birds, such as flamingos and certain species of duck , are also filter
feeders. Filter feeders can play an important role in clarifying
water, and are therefore considered ecosystem engineers .
* 2 Crustaceans
* 4 Bivalves
* 5 Sponges
* 6 Cnidarians
* 7 Flamingos
* 8 Pterosaurs
* 9 Marine reptiles
* 10 See also
* 11 Notes
* 12 References
* 13 External links
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 .
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. The megamouth shark has luminous
organs called photophores around its mouth. It is believed they may
exist to lure plankton or small fish into its mouth. 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.
Filter basket of a mysid
Mysidacea are small crustaceans that live close to shore and hover
above the sea floor, constantly collecting particles with their filter
basket. They are an important food source for 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
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: 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 crab species have feeding
appendages covered with setae to filter food particles from the
flowing water. Most species of barnacles are filter feeders, using
their highly modified legs to sift plankton from the water.
Baleen of a right whale
The baleen whales (Mysticeti), one of two suborders of the Cetacea
(whales, dolphins, and porpoises), are characterized by having baleen
plates for filtering food from water, rather than teeth. This
distinguishes them from the other suborder of cetaceans, the toothed
whales (Odontoceti). The suborder contains four families and fourteen
Baleen whales typically seek out a concentration of
zooplakton, 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. 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. 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.
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 .
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 shell fish 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, and sediment, nutrients, and algae can cause
problems in local waters. Oysters filter these pollutants, 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.” 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 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 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
Bivalve 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 or their content of certain
elements or compounds can reveal the contamination status of any
aquatic ecosystem. They are extremelly useful as they are sessile -
which means they are closely representative of the environment where
they are sampled or placed (caging) -, and they are breathing water
all along the day, exposing their gills and internal tissues:
bioaccumulation . One of the most famous project in that field is the
Mussel Watch Programme in U.S. but today they are used worldwide for
that purpose (ecotoxicology ).
Tube sponges attracting small reef fish The arcuate bill
of this lesser flamingo is well adapted to bottom scooping The
pink coloring of
Pterodaustro is hypothetical, but is based on
ecological similarities to flamingoes
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. 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 .
An undulating live Aurelia in the
Baltic Sea showing the grid in
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 ).
Other filter-feeding cnidarians include sea pens , sea fans , plumose
anemones , and Xenia .
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.
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
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 ".
Filter feeding habits are conspicuously rare among
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. 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
Triassic reptiles adapted for suspension feeding. Some
plesiosaurs might have had filter-feeding habits.
Spider web - the only terrestrial equivalent of a filter feeder
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* ^ NOAA. "Nutrient Bioextraction Overview". Long Island Sound
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