Side-flattened crustaceans in weed, sediment and detritus, and most of the small crustacean life on a muck slope.
Small crustaceans, mostly between two and fifteen millimetres, compressed from side to side so that the animal lies and swims on its flank rather than on its belly. The body is arched and segmented with no carapace over the thorax, there are two pairs of antennae at the front, seven pairs of walking legs of which the first two are modified into grasping gnathopods, and the abdomen carries appendages used for swimming and for driving water over the gills. Colour runs translucent, fawn, brown, green and occasionally red or banded. Females brood the eggs in a pouch under the thorax.
Gammarid is an informal name for the shrimp-shaped amphipods, used against the two other kinds on this site: the caprellids, of which Caprella scaura is the example here, have the body reduced almost to a thread and stand upright on hydroids and rope, and the hyperiids are large-eyed animals of open water that live on and in gelatinous plankton. Within the gammarids identification needs the mouthparts and the appendages under a microscope, so a photograph is honestly logged at the order.
They are everywhere on a muck slope and are noticed mostly as movement. They live among algae, hydroids, sponge and detritus, in the surface centimetre of sediment, and on jetty structure and discarded rope, and many build tubes of silt bound with mucus. Numbers rise after dark, when they leave cover and swim, and a lamp held still over the bottom collects them. Their importance is as food: juvenile fish, gobies, cardinalfish, pipefish and seahorses take them steadily, and much of what a small fish on a muck site is eating is this.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
