Amphipods (Amphipoda)

The species group usually requires a (stereo) loupe to be identified
  • Amphipod (Gammarus sp.)
  • Skeleton shrimp (Caprella sp.)

Amphipods belong to phylum Arthropoda in the same class as crabs and shrimp, the Malacostraca. Animals belonging to the order Amphipoda do not possess a carapace like crabs and their laterally compressed bodies are similar to those of shrimps. That is why in some areas they are wrongly given the name of “shrimps”. The term Amphipoda is derived from 2 root words: “amphi” from modern Latin meaning “of both kinds” and the Greek word “pous” meaning “foot”. This refers to the two types of thoracic legs that amphipods possess. 
Amphipods have segmented bodies which are divided into 3 groups the: the cephalothorax, the thorax and the abdomen. Along the body are paired appendages, which perform different functions, from feeding to swimming or for respiration and sexual reproduction. Amphipods have small bodies and except for some deep-sea species, do not grow to more than 2 centimeters in length.  They have a pair of unstalked eyes. 

Amphipods are poor swimmers but are very efficient surface crawlers. They are mostly scavengers and detritus feeders and many pelagic forms are predators to small zooplankton. 

Amphipods are dioecious meaning the sexes are separate. During reproduction, the male amphipod holds on to a female with specialized appendages and deposits the sperm cells in a special pouch in the female. Fertilisation is external and there are no larval stages. Young amphipods hatch directly from the egg and can be carried in a brood pouch by the female from 2 – 35 days before they are released. 

The most common amphipods you may find on the discs are the scuds and the skeleton shrimps.