University of Tasmania, Australia

Amphipoda

Latreille 1816

Taxonomy

Size

• Gammarids vary in size from about 5 to 20 mm.
• Hyperiids vary in size from 1 or 2 mm to 20 mm or greater.

Distinguishing characteristics

• Body is typically slender and laterally compressed, although some members of the Hyperiidea can have rounded bodies.
• Head is fused to the thorax and there is no carapace.
• Thorax and abdomen are usually quite distinct.
• Compound eyes are sessile, i.e. flat on the head with no eyestalk.
• 1st antenna is biramous and the 2nd antenna lacks an exopodite.
• All thoracic appendages are uniramous, also lacking exopodites.
• 1st pair of thoracic limbs function as maxillipeds, i.e. accessory head appendages, while the next seven pairs, the pereiopods, differ markedly in their arrangement and function. Each of the first 4 pereiopods are directed forward with the dactyl (end) segments directed back. The last 3 pairs are directed backwards with the dactyls directed forward.
• Pereiopods 1 and 2 are typically modified as subchelate prehensile gnathopods (claw-bearing and grasping limbs) for processing food.
• Abdomen comprises 2 sections of 3 segments, the pleosome and urosome. The anterior pleosome is large and bears 3 pairs of biramous pleopods for swimming.
• The name amphipod refers to having both walking and swimming feet.
• Smaller posterior urosome has 3 pairs of biramous uropods that do not form a tail fan as seen in prawns and crayfish.
• Mature females have a pouch or marsupium, constructed from plates at the bases of the thoracic 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.
• Gammarids:
  • Long 1st and second antennae.
  • Compound eyes are small.
  • Thoracic legs are well developed.
  • Distinctive large coxal plates overlapping the bases of the legs.
• Hyperiids:
  • Large compound eyes that cover almost all of the head.
  • Both pairs of antennae are positioned on either the anterior or ventral surface of the head and vary considerably in length between species.
  • Last 2 abdominal segments are fused.

Distribution

• Gammarids:
  • 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.
• Hyperiids:
  • Entirely planktonic and marine, and occur in all oceans.

Ecology

• Algal grazers, omnivores, detritivores or carnivores.

Notes

• Please note some specimens photographed for this fact sheets were not collected in plankton nets.