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Flamingos are one of the few terrestrial animals to engage in filter feeding. See more pictures of birds.

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What is filter feeding?

Have you ever been so lazy that you didn't want to get up off the couch for a snack? Maybe you wished you could just open your mouth and have food enter? Nothing too big, of course, because you also don't want to put forth much effort to chew. Perhaps just a steady stream of crumbled crackers and cookies, popcorn or little pieces of cereal?

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If this sounds appealing to you, then you might relate to filter feeders. Filter feeding is a method of aquatic feeding in which the animal takes in many small pieces of prey at one time. As opposed to predators who seek out specialized food items, filter feeding is simply opening up your mouth and taking in whatever happens to be there, while filtering out the undesirable parts. Filter feeders are mostly underwater creatures, although ducks and flamingos get in on the action as well [source: Hecht].

­But you, as a human, would only want to rely on filter feeding some of the time, right? Eventually you'd recover from your bout of laziness to grill up a fat, juicy steak or at least order a cheesy pizza. But some species, including a few of the biggest fish in the sea, spend their whole lives filter feeding. And though a number of creatures rely on ocean currents to bring by their dinner, for other animals, it's not about laziness. If anything, these creatures have to travel long distances to find something to eat, and they have unique bodily adaptations to get the job done.

So how do they do it? Why do they do it? Is it the ultimate diet, or just an excuse to eat all day? On the next page, we'll filter out just what's going on with these creatures.

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Filter Feeders

Filter feeders range from small sponges to baleen whales. About 20 species of fish, including sardines and mackerel filter feed [source: Parker]. Scientists have even identified a dinosaur called Gallimimus that may have been a filter feeder because its fossilized beak featured a sieve that would filter the food [source: Hecht]. Filter feeding is one of the oldest forms of eating, with some sharks shifting to filter feeding between 30 and 60 million years ago [source: Parker, Martin].

­­All filter feeders have specialized equipment for their meals. Baleen whales get their name from that equipment. Instead of teeth, these whales have baleen, or plates made out of keratin, the same material that makes up our hair and fingernails. These plates also grow like fingernails, constantly replacing themselves as they're worn down by the whale's tongue. On one side of the plate are coarse, fibrous strands that make a net for capturing food like schooling fish; some whales can capture creatures smaller than 5 mm (0.2 inches) [source: Croll and Tershy].

Baleen whales also consume krill, which are tiny shrimp-like creatures. Krill also makes up 94 percent of the diet of the filter-feeding crabeater seal [source: Croll and Tershy]. Crabeater seals have modified teeth that make filter feeding easy. When their mouths take in a gulp of water, the water is filtered out, while the krill is trapped by special postcanine teeth that have developed on both their upper and lower jaws.

It may be surprising that some of the biggest fish in the sea, including baleen whales and some sharks, are filter feeders. Yet the large body size of creatures may help them be filter feeders. When schools of little fish are hard to find, the large fish can endure a little starvation, as they swim farther and longer to find more food. When they do find food, however, they're able to take in a lot at once.

Some creatures don't have to go anywhere at all to filter feed, though. Sponges are inanimate, but they have a water current system made of canals and chambers that allows them to pump in water, filter the food and eat quite a lot. Water enters the sponge through a pore called the ostra. It then travels through the system where collar cells capture the food. The sponge expels the water through an opening known as the oscula.

In addition to the sponges that rank on the smaller side of the filter feeding spectrum, we have creatures such as mussels, clams and worms. Mussels open their shells and draw in food, filtering food particles over their gills, while clams use mucus on their gills to catch plankton as they push water in and out of their siphons [source: Chesapeake Quarterly]. A worm called Chaetopterus has a bag of mucus that strains the food out of water; when the bag is full, the worm eats it and starts a new bag [source: Encyclopædia Britannica].

To learn more about dining under the sea, visit the links on the next page.

Lots More Information

Related HowStuffWorks ArticlesMore Great LinksSources
  • "A Few Good Filter Feeders." Chesapeake Quarterly Online. 2007. (May 8, 2008)http://www.mdsg.umd.edu/CQ/V06N2/side2/index.html
  • "Animal." Encyclopædia Britannica. 2008. (May 5, 2008)http://www.britannica.com/EBchecked/topic/25501/animal
  • Croll, Donald A. and Bernie R. Tershy. "Filter Feeding." Encyclopedia of Marine Mammals. 2001. (May 5, 2008)http://montereybay.noaa.gov/research/techreports/ucsc/crolltershytr.pdf
  • "Feeding Behavior." Encyclopædia Britannica. 2008. (May 5, 2008)http://www.britannica.com/EBchecked/topic/203727/feeding-behavior
  • "Filter feeding." Encyclopædia Britannica. 2008. (May 5, 2008)http://www.britannica.com/EBchecked/topic/207047/filter-feeding
  • Hecht, Jeff. "Filter-feeding dinosaur sieved its food." New Scientist. Aug. 29, 2001. (May 9, 2008)http://www.newscientist.com/channel/life/dinosaurs/dn1216
  • Martin, R. Aidan. "Building a Better Mouth Trap." ReefQuest Centre for Shark Research. (May 5, 2008)http://www.elasmo-research.org/education/topics/d_filter_feeding.htm
  • Parker. Steve and Jane. "The Encyclopedia of Sharks." Firefly Books. 2002."sponge." Encyclopædia Britannica. 2008. (May 9, 2008)http://www.britannica.com/EBchecked/topic/560783/sponge