All About Filter Feeders: The Aquatic Strainers

Filter Feeders

Photo: Matt_Potenski via Getty Images

Don't you sometimes wish that you could just open your mouth and the food will go to you, all while just lazing about the couch? Just the small foodstuff, like cookie crumbs, popcorn, and the occasional piece of chocolate but tons and tons of them — because who has time to chew, right?

If this sounds tempting, you might want to learn about filter feeders.

What is Filter Feeding?

Filter feeders use filter feeding to eat, as the name suggests. Filter feeding is a technique of aquatic feeding in which the organism consumes a large number of little bits of food at once. In contrast to predators that seek specific prey, filter feeders are just opening their mouths and sucking in anything that happens to be around while filtering away the bad bits. 

The Two Filter Feeding Types

There are two types of filter feeders: internal and external filter feeders.

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Internal Filter Feeders

Internal filter feeders involve a basket-like filtration system within their body cavity that is open from the outside via two siphons. They draw water in via one opening (the "incurrent siphon"), filter it to eliminate small food particles, and release it through the second opening, or the "excurrent siphon." The food particles are then transferred from the filters to the animal's mouth by cilia, which are specialized cell processes that look like moving mini-hairs.

Mussels are one of the most significant internal filter feeders. In dry conditions, their shells close up, but when underwater, the two shells or "valves" of the bivalve widen out to show a broad incurrent siphon, which is covered by pink tentacles and blocks the entry of large particles.  The gills sift out food particles within the shell, and a narrower, excurrent siphon releases the water. 

An incompletely understood "bivalve pump" moves water through the creature, with the pumping force provided by groups of lateral cilia running down the gill filaments' edges. A pair of tendrils called palps clean the foodstuff off the gills and transport it to the mouth deeper within the shell. Oysters and scallops have similar configurations. According to research, a single mussel or oyster can filter more than 1 gallon every hour.

Several other bivalves, particularly burrowing species such as clams, have simple siphons that are considerably longer than the organism's length. This enables the creature to remain safely below the mud while the siphons protrude above the surface. However, opportunistic fish and other predators frequently eat the siphons). Bivalves rely on plankton, benthic algae, and debris, and in turn, they provide sustenance for echinoderms, fishes, fowl, and other organisms.

External Filter Feeders

Some filter feeders make use of an external filter. All barnacles and different types of polychaete worms use this strategy. Barnacles are highly adapted crustaceans that perch on their heads and filter with their feet. However, rather than pushing water through the filter, these creatures utilize a grabbing action, regularly thrusting their feet upwards through the water and then swiftly pulling them down within the shell together with any collected foodstuff.

Some tube-dwelling polychaete worms sometimes called "feather dusters," adopt an identical external but retractable filtration. Some thrive in mucus and sand tubing, while others live in a stronger, hardened tube. When endangered by the tidal cycle or predators, they would withdraw and shut a door called an operculum.

Two spoon worm species known as the Fat Innkeeper Worm have evolved a new filter feeding technique. This creature builds and lives in a U-shaped tunnel, producing slime netting that filters food while the worm pushes water through the tube. Whenever the slime net is packed with sustenance, the worm eats it along with the netting and then starts a new net. 

Several symbiotic species, including a small fish called goby, pea crabs, clams, and some worms, live in the Fat Innkeeper Worm's burrow and dine on the Innkeeper's leftovers. The presence of these frequent visitors is what gives the animal its name.

What Animals Are Filter Feeders?


The majority of forage fish are filter feeders. For example, the Atlantic menhaden, a kind of herring, feeds on plankton trapped in midwater. Adult menhaden may filter up to four liters of water per minute and help to clear ocean water. They also act as a natural barrier to the catastrophic red tide. In addition to these bony fish, several cartilaginous fishes such as rays, skates, and sharks are filter feeders.


Mysidacea is little crustaceans that dwell along the coast and float over the sea bottom, capturing particulates with their filter basket at all times. Herring, cod, flounder, and striped bass rely heavily on them for sustenance. 

Baleen whales

Baleen whales (Mysticeti) are one of two suborders of the Cetacea, including whales, dolphins, and porpoises. They are distinguished by using baleen plates instead of teeth to filter food from seawater. This separates them from the toothed whales (Odontoceti) from the cetacean suborder—just like the bowhead whale!

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Baleen whales primarily hunt for a concentration of zooplankton, dart through it, often open-mouthed or swallowing, then use their baleens to sift the prey from the waters. Baleen is a series of several keratin plates connected to the upper jaw that have a structure similar to human hair or nails. Picture baleen plates as fine-toothed combs capturing food when these whales suction water through them.

Another example of filter-feeding whales are humpback whales! These timid creatures definitely have one of the coolest feeding techniques in the animal kingdom!


Bivalves are two-part shelled aquatic mollusks. Both shells (or valves) are usually symmetric at the hinge line. There are 30,000 species in this class, such as scallops, clams, oysters, and mussels. Most bivalves are filter feeders (though some have adapted to scavenging and predation), collecting organic materials from the sea where they reside.


Sponges do not have a true circulatory system; rather, they generate a water stream employed for circulation. 

They have flagellated chambers with a total diameter substantially larger than the canals, forcing water flow through the chambers to slow significantly, allowing simple food collection by the collar cells. Water is released at 8.5 cm/second through a single oscillator, creating a jet force capable of transporting waste items far away from the sponge.


Flamingos eat on brine shrimp via filter feeding. Their unusually formed beaks are specifically suited to extract mud and silt from their food. The hairy bristles called lamellae that border their mandibles and the huge rough-surfaced tongue aid in food filtration.


Ctenochasmatoidea has traditionally been classified as filter-feeders because of their long, thin teeth that are well equipped to trap food. However, Pterodaustro solely has a functional pumping system, as evidenced by its retracted jaws and robust jaw and tongue muscles. Other ctenochasmatoids lack them and are now assumed to be spoonbill-like hunters, with their specialized teeth serving just to generate a greater surface area.

The First Filter Feeder (That We Know Of)

Tamisiocaris borealis, a huge shrimp-like organism, was the first documented filter feeder. This species is an anomalocarid, an ancient group of marine animals from the Cambrian period (about 485-540 million years ago). The anomalocarid was likely to be apex predators—sitting at the top of the food chain and consuming lesser creatures. Not Tamisiocaris borealis, though.

Scientists believe that the Tamisiocaris borealis' feather-like appendages on their skull were utilized to skim plankton from the water, based on newfound fossils discovered in Greenland. The appendages possessed finely spaced barbs separated by smaller spines, making them excellent traps for minute plankton. 

The presence of filter feeders meant that maybe there was a lot more plankton in the waters, implying a sophisticated food web during the Cambrian period.



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