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].
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