- Gammarids vary in size from about 5 to 20 mm.
- Hyperiids vary in size from 1 or 2 mm to 20 mm or greater.
- Body is typically slender and laterally , although some members of the Hyperiidea can have rounded bodies.
- Head is to the and there is no .
- and are usually quite distinct.
- s are sessile, i.e. flat on the head with no eyestalk.
- 1st is and the 2nd lacks an .
- All are , also lacking .
- 1st pair of limbs function as , i.e. accessory head , while the next seven pairs, the , differ markedly in their arrangement and function. Each of the first 4 are directed forward with the (end) directed back. The last 3 pairs are directed backwards with the directed forward.
- 1 and 2 are typically modified as (claw-bearing and grasping limbs) for processing food.
- comprises 2 sections of 3 , the and . The anterior is large and bears 3 pairs of pleopods for swimming.
- The name amphipod refers to having both walking and swimming feet.
- Smaller has 3 pairs of that do not form a tail fan as seen in prawns and crayfish.
- Mature females have a pouch or , constructed from plates at the bases of the legs, into which the eggs are shed and incubated until the young are liberated.
- The name of the superorder, Peracarida, means ‘pouch shrimp'.
- Two suborders of amphipod often found in plankton samples, Gammaridea and Hyperiidea.
- Long 1st and second antennae.
- s are small.
- legs are well developed.
- Distinctive large s overlapping the bases of the legs.
- Large s that cover almost all of the head.
- Both pairs of antennae are positioned on either the anterior or surface of the head and vary considerably in length between species.
- Last 2 abdominal are .
- Distributed widely throughout freshwater, estuarine and coastal marine systems.
- Few planktonic species are known, but most specimens collected in plankton nets are epibenthic species caught when they venture into the water, or when nets are towed close to the bottom or very close to shore.
- Entirely planktonic and marine, and occur in all oceans.
- Algal grazers, , or carnivores.
- Please note some specimens photographed for this fact sheets were not collected in plankton nets.