Amphipoda
Geographic and vertical distribution

—Gammarids

The geographic distribution of epipelagic gammarids is similar to that of the warm-water hyperiids (Synopia is more closely associated with the tropics s.s.). Deep-water gammarids of the South Atlantic are mainly pan-oceanic or circum-oceanic. On the other hand, records of deep-water pelagic gammarids are very scarce in the Indian and the Pacific oceans, and even in the North Atlantic. Some of these species have restricted ranges; for example, abyssal and hadal animals strictly associated with a few deep-sea trenches, (like Hirondellea gigas from the Indian-Pacific region). But others do not seem thus limited. The inventory of South Atlantic pelagic gammarids probably is still far from complete. Coastal plankton samples may contain many bottom-dwelling gammarids, practically all of which venture high up in the water column in their diel migrations, but these species are not included in the present review.

Pelagic gammarids (including those which have not lost connection with the bottom completely) inhabit all depths at all latitudes. Most of them are meso- and bathypelagic, but the mesopelagic animals often move to subsurface layers (especially Cyphocaris challengeri), or sink to abyssal and even hadal depths. Strictly epipelagic gammarids are also known: in the South Atlantic Stenopleura atlantica, Eusiropsis riisei, and all species of Synopia (the latter practically hypo-neustonic animals). And a few pelagic gammarids have been recorded only from abyssal and hadal samples (e.g., Scopelocheirus shellenbergi, several locations, including the Orkney Trench). Bentho-pelagic scavenging gammarids are a major component of abyssal epibenthic communities. Some never venture far above the bottom (many lysianassoids), and are therefore not included in this review. Others, however, are often recorded in plankton samples (Eurythenes, Paracallisoma, Orchomenella).