Latreille, 1803
Gammaridea are amphipods with a usually laterally compressed body, though a tendency to dorso-ventral compression is shown in burrowing and domicolous forms. There are three body tagmata: the cephalon (or better cephalothorax, because the first pereion segment is fused with the true head), the pereion with seven segments and the pleon with six segments. The anterior three pleon segments form the pleosome and the posterior three form the urosome. Both pleosome and urosome bear markedly different usually biramous appendages, pleopods and uropods respectively.
The head bears six pairs of appendages: biramous antennula (a1) with the second flagellum sometimes reduced or absent, uniramous antenna (a2), mandible (with mandibular palp), first maxillae (m1) with endopodal palp and basal endite, second maxillae (m2) though usually reduced, and maxillipeds. The mandibles, m1, m2, and maxillae, together with the upper and lower lip form the mouthparts.
The seven pereion segments each bear a pair of uniramous limbs, the pereiopods. The anterior two pairs of pereiopods often are modified as chela or subchela and called gnathopod.
To the last (sixth) pleon segment, the telson is attached.
The eyes are sessile and compound.
The coxa or proximal segment of each of the pereiopods is modified into a broad, flattened plate, the coxal plate that is attached firmly to the body, enhancing the overall lateral compression of the amphipod. The somewhat similar lateral expansions of the pleosome segments, the epimera or epimeral plates, are derived from the body segment and not from the proximal segment of the limb.
Some pereiopods have additional structures derived from the inner side of the basis, the coxal gills and, in females, the oostegites. The latter form the brood plates, enclosing the marsupium or brood pouch.
Larval development is epimeric, without a postlarval stage. The female carries the eggs in the brood pouch until they have hatched.
Ecology
Gammaridea are benthic or hyperbenthic animals, but many species swim also and may be found in the plankton. Pelagic occurrence may be incidental when the animal is taken away by (tidal) currents, or regular in case of swimming predators, or mating or dispersal phases of benthic species. This often ambiguous behaviour was the reason to leave gammarids out of the zooplankton key. If identification to species of gammarids in plankton samples is required, the references at the bottom of this page are advised (also to be found in the literature field).
Distribution in the North Sea
About 240 gammarid species are known from the North Sea. Examples of (occasional) planktonic occurrence of Gammaridea in the area are manifold (W. Vader, Tromsø; pers. comm.); a few are listed below:
¥ Corophium volutator (Family Corophiidae) leaves its bottom tube under unvafourable conditions and drift away on the tidal current.
Jassa spp. (Family Ischyroceridae) living on buoys, ships, or oil rigs can easily be found in plankton samples taken in the neighbourhood.
¥ Guernea coalita and Tritaeta gibbosa (both Family Dexaminidae) are typically species with 'pelagic terminal males', when in particular the males swim up from the bottom at night.
¥ Apherusa clevei, A. ovalipes, A. bispinosa, and A. macrocephala M. Sars (Family Calliopiidae) frequently can be found in the plankton.
¥ Eusirus spp. and Rhachotropis spp. (Family Eusuridae) are all swimming predators.
¥ Metopa alderi (synonym M. spectabilis) (Family Stenothoidae) Metopa alderi
may be found on the medusa of Tima bairdi.
¥ Sand burrowing forms of the Family Lysianassoidea and Family Oedicerotidae are known for 'pelagic terminal males'. Males become pelagic in some stage of the reproductive cycle.
¥ Halice abyssi, Nicippe tumida and Pardalisca spp. (all Family Pardaliscidae) are swimming species found in Norwegian fjords and in the Skagerrak.
¥ Males of Ampelisca and Byblis (both Family Ampeliscidae) are good and frequent swimmers.
¥ Gammarus locusta and especially Gammarus crinicornis (Family Gammaridae) swim around at night and may even reach the surface.
References for identification
De Kluijver et al., in prep.
Chevreux and Fage, 1925
Lincoln, 1979
Stephensen, 1935-42
[Description after Lincoln, 1979; McLaughlin, 1980]