Heterotrophic organisms can be classified by how they acquire their nutrition. Saprobes (also called saprotrophs) are organisms that absorb nutrients from dead organic matter. Most saprobes are either protists or fungi. Detritivores, or decomposers, such as earthworms and crabs, actively feed on dead organic material. Animals that feed on living organisms are predators: herbivores prey on plants, carnivores prey on animals, and omnivores prey on both. Filter feeders, such as clams and blue whales, prey on small organisms by filtering them from the aquatic environment. Fluid feeders include mosquitoes, aphids, leeches, and hummingbirds. The anatomical adaptations that enable a species to exploit a particular source of nutrition are usually obvious, but physiological and biochemical adaptations are also important.