Amphipod (order Amphipoda)

CrustaceanInvertebrateFood web

Overview

Amphipods (order Amphipoda) are small crustaceans — typically just a few millimetres long — with bodies that are flattened from side to side, giving them a curved, shrimp-like shape and a habit of lying or hopping on their sides. They are enormously diverse and abundant: the sandhoppers (beach fleas) that jump in seaweed on the shore, the “scuds” that swarm in streams and ponds, the clinging skeleton shrimps of the reef, and strange deep-sea and cave species are all amphipods.

Though individually tiny, amphipods are everywhere in water and damp places, and in sheer numbers they are one of the most important groups in aquatic food webs — recycling dead material and feeding a huge range of fish, birds, and other animals.

Note: “amphipod” covers a vast order; details here describe the group broadly (the photos show a freshwater example). Treat general statements as approximate and verify against authoritative sources.

Habitat & Range

Amphipods are found in water and moist habitats around the world — in the sea from the shoreline to the deepest ocean trenches, in fresh waters such as streams, lakes, and groundwater, and even in damp places on land (the terrestrial sandhoppers). They are especially abundant among seaweed, sediment, leaf litter, and on the seabed, where many burrow, cling, or scavenge.

Diet

Most amphipods are detritivores and scavengers, feeding on decaying plant and animal matter, algae, and organic debris, while some graze, filter particles, or prey on smaller animals. By breaking down dead material — including, in the deep sea, falling carcasses, which deep-sea amphipods swarm in huge numbers — they are key recyclers that return nutrients to aquatic ecosystems.

Behavior

The sideways-flattened body is the amphipod hallmark, and it shapes how they move: shore sandhoppers flick their bodies to leap, freshwater scuds scoot and swim on their sides among plants, and skeleton shrimps cling to seaweed and hydroids with grasping limbs, swaying like tiny mantises. Many amphipods are important to people indirectly, as a major food for fish (including farmed and game fish), shorebirds, whales, and countless other animals. Deep-sea species include surprising giants — “supergiant” amphipods that grow far larger than their shallow-water relatives. Females typically carry their eggs and young in a brood pouch beneath the body until the young are ready to fend for themselves.

Human Interaction & Conservation

Amphipods are harmless to people and are ecologically vital, forming a huge part of the diet of fish and other wildlife and helping recycle organic matter in seas and fresh waters; some are even used as indicators of water quality. A few species can become invasive when carried to new regions, and some shore amphipods are familiar as the “sand fleas” that hop in beach wrack. As a group they are extremely common and not of conservation concern, though specialised cave and spring species can be vulnerable. Consult authoritative sources for details.

More photos of the amphipod