Benthic and Epibenthic Organisms

In Antarctica, ice is the main factor affecting the distribution of benthic organisms on the continental shelf. Glaciers entering the sea carry coarse material interspersed with boulders and gravel, which covers large areas of the shelf. The littoral and sublittoral zone is scoured to a depth of 10-30 m through the abrasion of shorelines by fast ice, pack ice and brash ice or by floes driven ashore and piled upon each other during storms (push ice), and through plucking by anchor ice. Icebergs calved from ice shelves, glaciers, and grounded ice walls and floating on the continental shelf plough furrows in the bottom down to depths of several hundred metres, with ruinous effects on benthic communities (Gutt 2001). However, it has been found that in spite of ice disturbance and scarce light penetration, Antarctica has very rich benthic communities. Over 3,000 species of benthic invertebrates have been recorded in the Southern Ocean (Arntz et al. 1994), and their characteristics are summarised in several reviews (e.g. Dell 1972; Arnaud 1977; White 1984; Dayton 1990; Knox 1994; Gambi and Bussotti 1999; Starmans et al. 1999). In general, the most commonly reported features are abundance, high levels of endemism, gigantism, longevity, slow growth rates, delayed maturation, absence of some invertebrate groups and pelagic larval stages.

The presence of ice, the unusual depth of the shelf (up to 500 m),the extensive area of deep water around the continent, and the lack of connection with temperate shelves made the Antarctic shelf an insular evolutionary site roughly equivalent to Lake Baikal or the Galapagos (Eastman and Clarke 1998). Moreover, the Southern Ocean oceanographic fronts constitute a major zoogeographic boundary for epi- and mesopelagic organisms (in the upper 1,000 m). In 1913 C.T. Regan identified the zone between the 6 and 12 °C surface isotherms as the boundary between Antarctic and sub-Antarctic regions;

these zones have since been found to approximate the position of the APF and Subtropical Convergence respectively. As stated by Ekman (1953), "no other large faunal region in the world can match the Antarctic in the sharpness of its boundaries". In general, zoogeographic faunal units for benthic organisms in the Antarctic Region distinguish a Continental (or East) Antarctic Region (or Province; most continental coasts except for the Antarctic Peninsula), a West Antarctic Region (or Province; the Antarctic Peninsula and adjacent islands), and the South Georgia District or Province. Covering most of the Antarctic Region, the Food and Agriculture Organization of the United Nations established fishing areas 48, 58 and 88, which coincide with the limits defined by the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR). In the Indian Ocean sector (between 30 and 80° E), the northern limit of the Convention Area extends to 45° S, to ensure the inclusion of productive shelves around the sub-Antarctic islands (Marion, Prince Edward, Crozet and Kerguelen). Although these islands lie north of the APF, their fish fauna has an Antarctic origin and character (Eastman 1993).

The Liparididae and Zoarcidae, two North Pacific fish families, are well represented in the Southern Ocean, but the suborder Notothenioidei is the dominant group in coastal Antarctic waters. After the establishment of the ACC some 25x 106 years ago, the notothenioids, originally a benthic nearshore group, probably survived or were the most successful in invading across the APF. With its six families (Harpagiferidae, Bovichtidae, Notothenioidae, Arte-didraconidae, Bathydraconidae and Channichthydae), this group spread into water-column or ice-associated habitats of the Antarctic shelf and the upper continental slope to fill ecological roles usually filled by a variety of fish in other seas. Most of these bottom or coastal dwellers (they lack swim bladders) are endemic (97 % endemism for species and 85 % for genera; Gon and Heem-stra 1990) and are excluded from the peripheral parts of the Antarctic Region. Through the reduction of skeletal material and the accumulation of fat, some notothenioid species are neutrally buoyant and have adapted to become pelagic species. Pleuragramma antarcticum (Antarctic herring) ranges in depth from 0 to 900 m and constitutes a major species (in number and biomass) in the water column of most Antarctic shelf areas. It feeds on krill, cope-pods and chaetognaths and, owing to its abundance and wide distribution, P. antarcticum plays a key ecological role as food for fish, penguins and other Antarctic marine vertebrates, especially in areas where krill is scarce. Other abundant pelagic fish in the Southern Ocean are the small lantern fish (Myc-tophidae). Dissostichus mawsoni and D. eleginoides (a sister species found largely north of the APF), the largest notothenioids (up to 160 cm and about 70 kg), are typical mesopelagic predators. They feed on pelagic fish, mysid shrimp and squid, and the main difference between the two species is the lack of antifreeze in D. eleginoides, which does not inhabit subzero waters. Through morphological and physiological adaptations some notothenioid fish, such as Pagothenia borchgrevinki and P. brachysoma, have specialised for life on the undersurface of sea ice (cryopelagic species; Andriashev 1970). As cryopelagic fish come into contact with minute ice crystals, their blood contains greater quantities of glycoprotein antifreeze compounds than that of benthic species (DeVries 1971). Although most Antarctic fish are benthic, some species such as Cryothenia peninsulae are semipelagic fish (i.e. they exhibit features of both pelagic and benthic notothenioids) and others, such as Trematomus loennbergii and T. eulopidotus, are typical epibenthic species. Only a brief description of benthic habitats and organisms will be given here. Readers interested in the biology and ecology of Antarctic fish can refer to specific books (e.g. Gon and Heemstra 1990; Di Prisco et al. 1991; Kock 1992; Eastman 1993; Nelson 1984; Di Prisco et al. 1998).

In the region of ice scour and disruption by anchor ice, sessile epibenthos is generally restricted to transitory groups of organisms, except in more sheltered habitats such as rock crevices. However, about 700 species of benthic macroalgae have been recorded in the upper infralittoral zone of Antarctica and in the sub-Antarctic islands (Fischer and Hureau 1985). Providing that there is some protection from ice abrasion, Rhodophyta Iridaea cordata is one of the most widespread species down to depths of about 15 m. In the Antarctic Peninsula and islands of the Scotia Arc, Phaeophyta Ascoseira mirabilis, with a thallus up to 4 m in length, is another common species inhabiting depths of up to 13 m. The endemic and common seaweed Himantothallus grandifolius, the largest Antarctic kelp with a thallus up to 10 m long and 1 m wide, together with other species such as Leptophyllum coulmanicum, may produce dense stands throughout the Antarctic inner continental shelf to depths well below 30 m. However, in terms of biomass, benthic microalgae may be more important than macroalgae. Shade-adapted benthic diatoms grow on the topmost few mm of sediments, rock surfaces, sponge spicules or attached to macroalgae and epibenthos. In the east McMurdo Sound, their standing stock has been estimated to range from 20 to 900 mg chl a m-2, depending on depth and season (Dayton et al. 1986).Vagile organisms such as echinoderms, molluscs, polychaetes, peracarid crustaceans, and fish (e.g. Trematomus bernacchii and T.hansoni) are ubiquitous in shallow waters.

There is an abundant benthic fauna below the depth of sea-ice scour. Adaptive radiation within families provides reasonable diversity at the species level, although benthic fauna at higher taxonomic levels is less diverse in Antarctica than elsewhere in the world. Some groups such as decapods or gastropods, for instance, are poorly represented. Many invertebrates are particle-feeding or scavengers, inedible or of little value as food (e.g. sponges, starfish, sea spiders or brittle stars), and several species are toxic (cf. chemical defence against predation; McClintock 1989). The more abundant sessile particle-feeding invertebrates include sponges, hydroids, tunicates, bryozoans, sedentary polychaetes, actinarians, scleractinian corals and holothurians. Many types of sponges can cover more than 50 % of the seafloor, and their spicules may form dense mats up to 1-2 m thick. Spicules and soft bottoms provide a suitable habitat for many infaunal organisms such as peracarid crustaceans, burrowing polychaetes, oligochaetes and bivalve molluscs. Motile organisms include echinoderms (particularly the echinoid Sterechinus neumayeri and the starfish Odontaster validus), bivalves (the Antarctic scallop Adamussium colbecki and the soft shell clam Laternula elliptica), gastropods (the Antarctic whelk Neobuccinum eatoni), nudibranchs (such as Austrodoris mcmurdensis, which feeds on sponges); the large isopod Glyptonotus antarcticus,the nemer-tine Paraborlasia corrugatus, and pycnogonids.

Antarctic benthic fish (mostly of the genus Trematomus) have heavier skeletons and are less buoyant than cryopelagic and epibenthic species. They are dorsoventrally depressed, with pelvic and anal fins supporting the body when resting on the substrate. In general, different species occupy different subhabitats: Trematomus nicolai lives in shallow waters (30-50 m deep), sometimes near masses of anchor ice; T. pennellii is usually found in waters less than 200 m deep, while T. bernacchii and T. hansoni can be found from shallow waters up to depths of 550 m.As a rule,benthic trematomids are feeding generalists, and their diet varies with location, according to the availability of prey organisms (i.e. polychaetes, amphipods, molluscs, crustaceans, small fish or fish eggs; Vacchi et al. 1994). Some species such as Notothenia rossii and N. coriiceps change habitat and diet at different life history stages. Larvae are pelagic, nearshore fingerlings and juveniles are benthic in beds of macroalgae, while adults are offshore semipelagic and feed heavily on krill (Duhamel and Hureau 1990).

The distribution of abyssal Antarctic benthos has received little attention. There is evidence that the distribution of main communities is strongly affected by sediment features, local hydrography and bottom dynamics (Gambi and Bussotti 1999). In general, the most represented groups of epifau-nal organisms on fine sediments are foraminifers, bryozoans, ophiuroids, asteroids, crinoids, molluscs and crustaceans (mainly amphipods, tanaids and isopods). Infaunal assemblages are often dominated by polychaetes, although on the Ross Sea continental slope, bivalve molluscs such as Yoldiella ecaudata and Genaxinus debilis may be abundant in muddy and hard substrates respectively (Cattaneo-Vietti et al. 2000). Different sampling procedures make it hard to compare qualitative and quantitative data on fish fauna; however, Eastman and Hubold (1999) found a similar size and taxonomic composition between pelagic and benthic fish in the Ross and Weddell seas (i.e. in the East Antarctic Province). As the four dominant benthic species in the southern Ross Sea are also found in the Southern Scotia Sea, this suggests that the latter region served as a point of entry of sub-Antarctic organisms into the Antarctic region.

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