Amphipoda
Introduction

The Amphipoda (Peracarida, Malacostraca, Crustacea) are a crustacean order that has successfully colonized the pelagic ocean. Their success can be explained to a great extent by their high diversity - over 6,000 species, exhibiting a large variety of life-forms and occupying diverse biotopes. Most amhipods are benthic, but the approximately 400 species in the pelagic domain inhabit all depths and latitudes of the world ocean.

The order Amphipoda includes 4 suborders: Gammaridea (encompassing the majority of species), Hyperiidea, Lemodipoda (= Caprellidea) and Ingolfiellidea. These are easily distinguished by such features as the morphology of mouth parts, peculiarities of segment fusion, number of gills, etc. But phyletic relationships within the Amphipoda are poorly known, such that, for example, some branches within the gammarid phylogenetic tree may rank with the hyperiids or caprellids, which are treated as "suborders" (see Bousfield and Shih, 1994). All recent pelagic amphipods originated from benthic ancestors, but representatives of the various suborders have occupied the pelagic realm at various times and in various ways. The most ancient immigrants are the Hyperiidea. They have lost their connection with the bottom completely, and none of their known 260 species is benthic (though in rare cases some individuals have non-obligatory secondary contacts with benthic communities). Hyperiid amphipods are significant components of epi- and mesopelagic communities. In the meso- and the macroplankton they are subsidiary in abundance only to copepods, euphausiids, chaetognaths, and gelatinous animals.

Hyperiids occupy various pelagic ecological niches. Approximately half of them are parasites on such gelatinous zooplankton as medusae, siphonophores, and rarely salps. Even species with free-living adults often deposit their young on gelatinous animals (Laval, 1980; Madin and Harbison, 1977). Symbiotic hyperiideans often act as ectoparasites or commensals, but they sometimes penetrate the bodies of jelly-fish, and may eat their mesogloea and gonads (Hyperia, Lestrigonus). The free-living predator Themisto uses jelly-fish as a substrate for moulting and copulation (Sheader, 1977). Phronimidae are famous for preparing barrel-like houses from the bodies of salps and the nectophores of calycophore siphonophores by eating parts of them away. They find shelter and rear their young in such gelatinous houses. Some hyperiids, among them Scinidae, Scypholanceola, Streetsia, Parapronoe crustulum, and probably some others (as well as the gammarid Cyphocaris challengeri) are luminescent (Bowman, 1967; Vinogradov et al., 1982).

In tropical regions, high concentrations of hyperiideans rarely occur. However, even in the tropics their biomass can reach 5 g mö-3 in isolated swarms of Anchylomera, Phronima or Primno (Vinogradov and Rudjakov, 1971). On the other hand, the diversity of tropical hyperiids is high, although precise data on species compositions and geographic variability are still scarce. Such lack of information may result from long-standing poor taxonomy in a number of families (e. g., Hyperiidae, Phronimidae and some others), and the difficulty of assessing faunal distributions based on small standard plankton nets (Am.2 Methods). In cold-water regions, hyperiid species diversity is fairly low, but their biomass may be very high, to the extent that whales and seals occasionally feed on hyperiid swarms. There are even records of ships having to stop because swarms of Themisto clogged the engine water-intake pipes.

As opposed to the hyperiids, representatives of the Gammaridea are much newer to the pelagic realm: only about 150 of their over 5,000 marine species are strictly pelagic, and the majority of these have close relatives on the sea bottom. On the other hand, only a few benthic gammarids (wood-burrowing, and some highly specialized epiphytic animals) fail to rise above the bottom. In many benthic gammarid species a considerable part of the population (sometimes males only) may rise above the sea-floor at night. Such short excursions require no special anatomical adaptation, and the morphology of the animals is fully adapted to their benthic habitat. These migrations usually occur in littoral regions, and these crustaceans are only seldom carried by currents into the open ocean, in which case they are bound to perish. Such animals are not included in this review.

On the other hand, a totally different assemblage of upward-migrating gammarids occurs in the near-bottom layer. These are strictly carnivorous animals, provided with special mouth parts, moving over the sea bottom in search of carrion or easy live prey. The habits of these bentho-pelagic scavengers, widely distributed at all depths, and including dozens of genera, are somewhat analogous to those of terrestrial scavenging insects. This group formed the basis for gammarid expansion into the pelagic zone. Some bentho-pelagic gammarids still stay close to the bottom, whereas others rise thousands of meters up into mesopelagic depths and may be found in plankton samples (Eurythenes, Paralicella, etc.).

Benthic amphipods that venture high in the water column consume not only animal remains from the bottom, but also feed in the pelagic zone, consuming dead organisms or preying on live ones (Smith and Baldwin, 1984). This leads directly to the next, strictly pelagic, gammarid life-forms (like Cyphocaris). Some of the latter (the lysianassoids) became either parasites of gelatinous animals (Danaella, Thoriella), or ectoparasites of large planktonic crustaceans (mysids and decapods) (Paracyphocaris), and fish (especially sharks; Thrischizostoma). The last group has not been reported from the South Atlantic.

Gammarid amphipods with a hypertrophic lateral shield (Stegocephalidae) also arrived at their present pelagic milieu through a near-bottom phase.

Planktonic gammarids with very long appendages, such as those in the families Pardaliscidae and Vitjazianidae (not reported from the south Atlantic Ocean), as well as some Eusiridae, seem to be basically best adaptated to deep, sluggish-water pelagic habitats. It seems that their predecessors lived on soft deep-sea mud, which requires thin, long legs for efficient locomotion. This morphological trait, which proved so successful and occurs repeatedly and independently in various groups, allowed the gammarids to leave the bottom. Other groups seem to have adapted to the plankton through different pathways (see Vinogradov, 1995).

As mentioned above, strictly pelagic gammarids are not usually abundant, but inhabit all latitudes and depths of the ocean. As a rule, planktonic gammarids are significantly less numerous in samples than hyperiids. Various shallow-water, and even intertidal, benthic gammarids sometimes may be found in the open sea sitting on drifting sea-weed. A few species are associated with the pelagic seaweed Sargassum, but they are not truly planktonic animals, and therefore are not included in this review.

Caprellids are generally benthic animals, inhabiting littoral areas with sea-weeds and hydroids. A few caprellids have limited swimming capabilities, wriggling like bloodworms. One benthopelagic species, Phtisica marina, often ventures into the water-column and can therefore be considered hemi-planktonic. Another large group of caprellid amphipods is the cyamids, or whale-lice, which dwell on the skin of whales. Whale-lice are highly-specialized parasites that have lost the ability to swim. Although they occur in the pelagic domain, they are not planktonic animals and therefore are not included in this review.

The last amphipod suborder, the Ingolfiellidea, have no pelagic representatives and are therefore also omitted here.

Among other crustaceans, Amphipoda are most closely related to the Isopoda or Mysidacea (see Bousfield and Shih, 1994, for a detailed discussion of phylogenetic relationships of the Amphipoda).