Amphipods are an interesting and important group of crustaceans, with about 8000 species known world-wide. They are predominantly marine but there are some fresh water species and even a few terrestrial forms, known as beach fleas. These are the only crustaceans to successfully colonise the land, although they are still restricted to moist beaches and soils. Some marine amphipods are pelagic but most live on the bottom among algae or other aquatic vegetation. They are common in shallow waters but there is also an extensive deep sea fauna. They also occur in every marine habitat from the polar regions to the tropics. They are often mistaken for shrimp but are in their own group within the crustacea because of basic anatomical differences.
Amphipods are different from other crustaceans because they are laterally compressed and have no carapace. They have large compound eyes which are embedded in the head rather than being on stalks like other crustaceans. The first 'thoracic' segment is united with the head and the gills are on the thorax rather than the abdomen. The thoracic legs are uniramous (straight instead of Y-shaped) and have no exopodites (outer branches). Some of the abdominal segments are fused together.
The body is usually curved but powerful muscles can straighten it suddenly, giving a powerful kick forward. In aquatic species, this causes dart-like movements, and on land, it allows beach fleas to jump around madly. The thorax has seven pairs of walking legs while the abdomen has swimming pleopods so most amphipods can either walk upright or swim. This is also where the name amphipod, meaning double footed, comes from. The first pair of walking legs are modified with claws for grabbing prey and are called gnathopods which means jaw-feet.
Internally, amphipods have a dorsal heart and circulatory system. There is a large ganglion behind the eyes leading to a ventral nerve chord. They have a digestive system with stomach, midgut and hindgut and kidneys for removing wastes. Sexes are separate and male and female reproductive organs are simple. There are no specialised copulatory organs but the males do clasp the females much like frogs do (amplexus). Some males liberate their sperm in the respiratory currents of the female which carries the sperm into the eggs in a pouch called a marsupium. The eggs develop in this brood pouch and are released when the female moults. Some species continue to provide parental care after the young are released.
The main suborders of amphipods are the Gammaridea, Hyperiidea and Caprellidea. The Hyperiids have unusually large heads and eyes but reduced antennae. The pelagic hyperiids are transparent and some live parasitically in jellyfish.
Gammerids are ordinary amphipods without large heads or short abdomens. Most are laterally compressed but some are dorsoventrally flattened so one must look at the position of the gills to differentiate them from isopods, which have the gills on the abdomen instead of the thorax. Beach fleas are gammarids and they are common in many soils as well as on beaches. Turn over a piece of rotten wood and the little 'fleas'that leap about to get away are actually amphipods. In the US and Canada, they are called lawn shrimp.
The Caprellids are quite diverse and include some parasitic forms, such as the so-called whale lice. They have gills on the fourth and fifth pairs of thoracic legs only. A number of long, thin Caprellids are called skeleton shrimps. They have a small fused abdomen and an elongated thorax with large raptorial legs for grabbing prey and this makes them relatively easy to identify. They live in seaweeds and are fierce hunters, hanging on to the seaweed until prey comes along and then grabbing it with their big claws. They move like inchworms, putting the head forward and down, then drawing up their hind quarters.
Life styles are quite diverse. Some amphipods are filter feeders. They use their thoracic appendages to generate feeding currents and then use the hairs on the legs to filter out particles such as detritus and bacteria. These are often burrowers or live in tubes. One group makes these tubes with silk glands on their legs, much like spiders. Some amphipods burrow in sponges, "living in a shadowland between predaceous and parasitic life" (Meglitsch, p. 561). Still others are herbivorous, crawling around on seaweeds and algae and eating it, or scavengers on dead plants and animals, or active predators like the skeleton shrimp, stalking and capturing prey. In turn they are an important food source for many other marine invertebrates and fish.
References: http://museumvictoria.com.au/crust/amphbiol.html Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press http://creatures.ifas.ufl.edu/misc/amphipods.htm http://reefkeeping.com/issues/2004-09/rs/index.php