Filter feeders are aquatic animals that obtain nourishment by straining suspended matter and tiny food particles from the water column. This feeding strategy is widespread and ancient, used in both marine and freshwater environments. Organisms employ specialized structures to capture everything from microscopic bacteria and phytoplankton to small crustaceans. The process involves moving a large volume of water past a filter and selectively retaining the suspended organic material. This mechanism allows organisms of varying complexity, from stationary invertebrates to the largest marine mammals, to thrive.
The Mechanism of Filtration
Passive Filtration
Filter feeders use distinct physical methods defined by how water moves across the filtering apparatus. Some organisms rely on passive filtration, holding their structure steady and waiting for natural water currents to deliver food. Crinoids, for example, are echinoderms that use feathery arms to ensnare drifting particles as water moves past.
Active Filtration
Many species employ active filtration, expending energy to create a current that draws water into or through their bodies. This is accomplished either by pumping water using muscular contractions or by swimming continuously with the mouth open, known as ram filtration. The whale shark uses both ram filtration while swimming and active suction feeding while stationary to gulp water and strain prey through specialized gill pads.
Specialized Invertebrate Mechanisms
Invertebrates often utilize microscopic, hair-like structures called cilia to generate the necessary water flow. Bivalves like clams and oysters have gills lined with cilia that draw water in through an incurrent siphon and pass it over the gills. Suspended food particles become trapped in a layer of mucus coating the gills, which is then transported to the mouth. Other filter feeders, such as giant larvaceans, construct elaborate, external filtering webs of mucus. This sticky net allows them to capture the smallest plankton before the entire apparatus is discarded and replaced.
Diverse Examples in the Animal Kingdom
Filter feeding is adopted by a wide range of life forms, demonstrating how different evolutionary paths converged on this solution.
Invertebrates
Sponges, among the simplest multicellular animals, are sessile filter feeders that pump water through thousands of tiny pores. Specialized collar cells called choanocytes line internal chambers, creating the current with flagella and engulfing the trapped particles. Tunicates, including sea squirts, are barrel-shaped organisms that draw water in through one siphon and expel it through another, filtering it through an internal mucous sheet.
Tiny crustaceans like krill use their bristled, basket-like thoracic legs to actively filter phytoplankton from the water. Krill form a large biomass in the Southern Ocean and are a foundational part of the marine food web, supporting numerous larger species.
Vertebrates
The largest examples of this strategy are the baleen whales. Instead of teeth, whales like the humpback have hundreds of plates made of keratin hanging from their upper jaw. These plates form a dense sieve; the whales engulf large volumes of water and prey, then use their tongue to force the water out through the baleen, trapping krill and small fish inside. Filter-feeding is also seen in fish like the basking shark, which swims with its mouth open, passively sieving plankton through modified gill rakers.
Their Vital Role in Aquatic Ecosystems
The feeding activities of filter feeders have widespread effects on the health of aquatic environments, making them ecosystem engineers. By continuously removing suspended particles, including excess algae, sediment, and organic detritus, filter feeders increase water clarity. This improved transparency allows more sunlight to penetrate the water, which supports the growth of submerged aquatic plants and coastal habitats.
Nutrient Cycling
Filter feeders are linked to nutrient cycling because they remove nutrients like nitrogen and phosphorus from the water column and incorporate them into their tissues. Bivalves, such as mussels and oysters, sequester these nutrients and package undigested material into dense feces or pseudofeces, which drop to the seabed. This process, known as benthic coupling, moves nutrients from the water column to the bottom sediment, influencing local composition and fertilizing benthic communities.
Carbon Sequestration
Filter feeders also play a role in the global carbon cycle. Krill feed on carbon-capturing phytoplankton near the surface and produce dense, carbon-rich fecal pellets that sink rapidly to the deep ocean, effectively sequestering carbon. Baleen whales enhance this process through the “whale pump,” where their iron-rich feces fertilize surface waters after consuming krill at depth. This iron stimulates further phytoplankton growth, supporting the marine food chain and helping to remove atmospheric carbon dioxide.