Decapods employ a wide variety of feeding techniques, ranging from filter feeding, grazing, and deposit feeding to predation. Some are specialists that use just one of these methods, while others are generalists that make use of several different techniques depending on the circumstances. One of the most common misconceptions about crabs and other decapods is that they are primarily scavengers, since many are harvested from baited pots. Most large marine crustaceans are actually very efficient predators and only scavenge when the opportunity arises. The decapod body plan allows for a great degree of specialization in feeding structures; this specialization is especially apparent in the structure of the claws. Fast, slender claws can be used to snatch elusive prey, while massive, strong claws armed with molars can exert tremendous force on mollusks and other hard-shelled prey. Much of the elaborate ornamentation and other features seen in marine snails and other mollusks is a side effect of predation by shell-crushing crabs, as these groups are engaged in what is essentially an evolutionary arms race.
In many cases decapods have evolved asymmetrical claws, thus providing more than one type of tool for subduing and extracting prey. For example, the pistol or snapping shrimp of the family Alphaeidae have one enormously enlarged claw that produces a loud noise when snapped and can stun or kill prey. Apart from its use as a weapon, however, the large claw is virtually useless for most other functions, and the much smaller claw on the other side is used to convey food to the shrimp's mouth. On the other hand, claws are not essential to predatory decapods: palinurid lobsters and many shrimps lack large claws, but are still remarkably effective predators. Spiny lobsters have exceptionally strong mandibles (jaws) that are used to crush mollusks and other prey, and many shrimp are quite skilled in using their walking legs to envelop worms or smaller crustaceans.
In many tropical areas land crabs play the role of earthworms, as the primary recyclers of fallen leaves and other plant material. They also help to turn over the soil with their burrowing activities. Many marine and freshwater species are herbivorous as well, but most will occasionally ingest animal material when it is available.
A number of anomuran crabs and caridean shrimps are exclusively filter feeders, pulling detritus and plankton out of the water with their maxillipeds, antennae, or modified legs. Many more are deposit feeders, consuming much the same type of material after it has settled out of the water column. Deposit feeding crabs often have downturned claws that are adapted for scooping up sediment so that it can be processed by the animal's mouth parts.
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