Amphipoda
Geographic and vertical distribution

—Hyperiids

Although records of hyperiids in nearshore areas are not uncommon, they are essentially pelagic animals (only Lestrigonus bengalensis tends to inhabit coastal waters; Bowman, 1973), though they do not particularly avoid them. Based on their thermal relationships, hyperiids can be divided into cold-water and warm-water species.

In the southern hemisphere, cold-water hyperiids are associated with circum-Antarctic, bipolar (antitropical) or pseudo-antitropical areas. Bipolar and circum-Antarctic species have similar distributions here, but the former are present also in boreal and Arctic regions. Sometimes it is not clear if a given species occurs in only one hemisphere or both (see Themisto gaudichaudi, Primno macropa), and distinction between circum-Antarctic and bipolar species is partly formal. There are, however, some clearly circum-Antarctic species with no representatives in the northern hemisphere (e. g., Cyllopus). The northernmost distributional boundary of these species is usually the Antarctic Convergence (= Polar Front, 45-60°S), but they range occasionally to the Subtropical Convergence (35-40°S, e.g., Hyperiella antarctica). Aided by northerly cold currents, some of these species may at times reach even lower latitudes (e. g., Themisto gaudichaudi). The third distributional pattern includes the pseudo-antitropical species, characterized by many species of the genus Hyperia. These animals are abundant in cold-water regions and very rare in subtropical and tropical waters. As a rule, they inhabit very broad depth-ranges, at the surface in cold-water areas, deeper in warm-water regions, avoiding the warmer surface layers. In the South Atlantic this type of distribution is well exemplified by Hyperia spinigera and Hyperia medusarum. In cold-water regions, Themisto is by far the most widely distributed genus.

Warm-water hyperiid species may also have fairly broad ranges, covering not only the tropical, but also subtropical areas (down to the Subtropical Convergence). Some species, however, are restricted fairly well to the tropics. Most warm-water hyperiids are circumtropical. These include representatives of all hyperiid families and practically all genera (except Cyllopus, Hyperiella, and probably some abyssal Physosomata). Some families (mainly Oxycephalidae, Platyscelidae, Parascelidae, Lycaeopsidae, Anapronoidae, but also Pronoidae, Brachyscelidae, Lycaeidae) are entirely warm-water. Some hyperiids (especially several deep-water Physosomata) have pan-oceanic or circum-oceanic distributions (see Table Amphipoda).

Large high-speed nets came into common use in the 1970's. Thus, detailed data on the hyperiid fauna of various parts of the ocean have become available only recently, particularly from the tropical and subtropical areas of the Pacific and Indian Oceans (Shulenberger, 1977; Siegel-Causey, 1982; Young and Anderson, 1987; Barkhatov and Vinogradov, 1988; Young, 1989; Vinogradov, 1990a, 1991, 1993b). These studies show that species compositions are relatively homogeneous within the vast tropical anticyclonic gyres, and dominant species are also more or less similar in the various collections. Although the most abundant species may vary somewhat between areas, they invariably include Phronima atlantica, Phronimella elongata, Phrosina semilunata, and one species of Primno. Also, Scina crassicornis, Anchylomera blossevillei, Hemityphis tenuimanus, Vibilia armata, and a representative of Brachyscelus are often among the first ten. Unfortunately, large collections have not been taken or published from the Atlantic Ocean.

Hyperiids inhabit the entire water column, but the vertical distribution of the two infraorders differs strongly. The more primitive Physosomata inhabit mostly deeper layers, including the hadal, only a few (some Lanceola, especially Lanceola felina, as well as some Scinidae) having colonized the surface and subsurface layers. Adult females of Mimonectes with eggs are found near the surface. Most common Atlantic Scina are usually found at 500 m (Thurston, 1976b). Strictly abyssal subspecies of Scina are also known (e.g., Scina wagleri abyssalis), and many Scina species (sometimes those caught at 100-200 m) inhabit depths of 4000 m or more (Vinogradov et al., 1982). Other Physosomata are almost exclusively deep-water animals, inhabiting depths mainly below 3000 m.

The Physocephalata are restricted entirely to surface layers (with the exception of the family Cystisomatidae, which occurs to depths of 3000 m or more). The lower limit of most Physocephalata is about 200-500 m, only a few venturing deeper (for example, some Vibiliidae or Primno macropa, to depths of 1500-2000 m). However, isolated specimens of subsurface, and even epipelagic, species may also be found much deeper than usual, occasionally to 1000-2000 m. For example, during the 1990 Russian expedition to the hydrothermal vent fields of the tropical Pacific Ocean, a slurp-gun from the deep-sea manned submersible "Mir" caught a few Lycaeopsis zamboangae and Parascelus edwardsi near the bottom at depths of 1500-2300 m (Vinogradov, 1993a, 1994).

It is generally assumed that hyperiids decrease sharply in the thin near-bottom layer (e.g., Vinogradov, 1990c). However, from the "Mir" submersible I have observed swarms of Themisto abyssorum near the bottom of the Norwegian Sea, with isolated animals even contacting the sea-floor for short periods of time.

Many Physocephalata (and some Physosomata, like Scina) have diel activity cycles and limited vertical migrations. The range of these migrations is usually less than 100 m, but Scina crassicornis, Vibilia armata and sometimes Primno macropa, may rise as much as 400 m from their daytime levels (Thurston, 1976b), and Themisto has well marked diel vertical migrations (Bowman et al., 1982; Hiroki, 1988). Land (1992) reported some very interesting investigations on locomotion of hyperiids as a function of light. He showed that some species (Phronima, Platyscelus) cease swimming or swim downwards under high ambient light levels, and start swimming upwards as the light level falls below a given threshold, thus staying within a selected lighting regime. In other hyperiids (Brachyscelus, Phrosina), however, this mechanism has not been observed, suggesting that vertical movements are triggered by other factors.