Anatomy

Two very different body shapes of amphipods are found in shallow marine environments, one long and skinny (skeleton shrimps or Caprellidea), the other compact (Gammaridea). In general gammarideans are flattened from side to side, but more accurately they are defined by the presence of three pairs of uropods (tail-limbs) and usually by having the first two pairs of legs modified to help with grasping food. There is no carapace; seven thoracic and six abdominal segments are visible. The head carries two pairs of antennae, the eyes which are not on stalks, and the mouthparts. Amphipods have seven pairs of walking legs of which the first four reach forward, and the fifth to seventh reach backwards. The abdomen is divided into two parts, three segments with brush-like limbs and three with short immobile rod-like uropods.

Reproduction

In some species of Amphipoda male sexual maturity is expressed by the development of bulging eyes and chemical receptors on the antennae. At this stage the amphipod is called a swimming male and he actively searches for a mate. During reproduction, the male amphipod carries the smaller female grasped between its legs, a condition known as amplexus. These mating amphipods can be seen swimming together in rockpools. The sperm are transferred from the male to the female genital duct. The female releases the eggs into a ventral brood chamber where they are incubated during development. The eggs are not attached (as in a crab) but are free in a space enclosed by extra branches of the walking legs. Unlike crabs and shrimps amphipods are not released as zoea that develop into adults after stages of metamorphosis. Instead when released they look very much like their parents. Some species show parental care of their young after they leave the brood chamber.

Lifestyle

Amphipods live in all sorts of habitats. Species living on seaweeds may be herbivores, those in mud and sand feed on bacteria on the surface of particles, others are scavengers on dead animals or plants. One group of families lives in tubes spun from silk glands in their legs. Sometimes forming colonies, these amphipods when alarmed can completely conceal themselves inside the tubes that may be 2-3 times their body length. The only part of the body that protudes from the tubes is the antennae that have long hairs which are thought to capture food particles. Other amphipods, less commonly seen than the caprellideans and gammarideans of shallow marine environments are, the Hyperidea. These are planktonic and translucent, often with enormous eyes as big as the rest of the body.



Some groups live their lives attached to marine mammals. These whale-lice are ectoparasites which cling firmly to, and feed on, the skin of whales. Unlike other amphipods, whale-lice cannot swim so once the juveniles leave the brood chamber of the female they attach themselves close by.

Distribution

As well as being one of the most diverse group of crustaceans amphipods are also one of the most widespread. The vast majority of forms live on the seafloor in all latitudes from both poles to the equator. But there are also planktonic forms in the ocean. Terrestrial amphipods live only in moist places, on beaches or under leaf litter in forests. The shiny brown animals that leap about when pot plants are moved in the garden are amphipods.

Feedback and queries should be sent to the Discovery Centre at Museum Victoria.
Please note, the Discovery Centre can help you only with questions about crustaceans from southeastern Australia. It can not advise on the care of pet hermit crabs or on crustaceans from other regions.