Bald notothen or bald rockcod Pagothenia borchgrevinki

Pagothenia borchgrevinki is found throughout Antarctica and the Antarctic Peninsula, South Orkney Islands, and South Shetland Islands from 0 to 695 meters depth [8,11,13]. P. borchgrevinki is a commonly seen fish associated with the sea ice along the Antarctic shore, and has been observed clinging to the underside of thick ice shelves [8,16]. P. borchgrevinki can grow up to 28 centimeters in length [8]. P. borchgrevinki collected under the sea ice are pale all over in coloration while those collected in association with the bottom are a dark phase with a dark olive-brown spotted pattern above and silver-white below [9]. Color has also been recorded as yellowish with dark spots or irregular crossbars and dorsal and caudal fins with a series of spots but caudal fin without transverse bands [11].

Here Pagothenia borchgrevinki is seen in a crack on a grounded iceberg just south of Cape Evans on Ross Island. The undersurface of sea ice is a feeding and refuge site for many organisms with a profusion of amphipods, euphausiids, and fish including P. borchgrevinki. P. borchgrevinki is well adapted as a hunter; its lateral line sensory system can detect prey by recognizing the low vibration frequencies emitted by swimming crustaceans like Orchomene plebs, Euphausia crystallorophias, and Euchaeta antarctica [17]. P. borchgrevinki eats the free-swimming shelled pteropod mollusc Limacina helicina, ice krill Euphausia crystallorophias, copepods (including the calanoid copepod Euchaeta antarctica), decapod crustacean larvae, chaetognaths, amphipods (including the medusa-hitchhiking hyperiid amphipod Hyperiella dilatata, Orchomene plebs and Epimiriella macronyx), and juvenile fish (including Pleuragramma antarcticum, a key species in the food web, being eaten by fish like Dissostichus mawsoni, Weddell seals, Adelie penguins and skuas) [2,3,6,10,11,12]. P. borchgrevinki is eaten by Emperor penguins and other predators [4].

Termed cryopelagic for its lifestyle preference, Pagothenia borchgrevinki is ideally suited for its close association with sea ice. Living in such close association with sea ice crystals is a physiological challenge; you couldn't pick a colder place to live and risk freezing. P. borchgrevinki, Pleuragramma antarcticum and P. brachysoma are among the most southern fish [11]. Antarctic fish like P. borchgrevinki are well-adapted to the extremely low and stable temperatures of McMurdo Sound where seawater has a nearly constant mean annual temperature of -1.86 degrees Celsius (28.65 degrees Fahrenheit) and temperature doesn't vary much with depth or season -- 0.2 degrees Celsius (0.36 degrees Fahrenheit) [5]. The flip side is that P. borchgrevinki and some other cold-adapted Antarctic fish die of heat at approximately 6 degrees Celsius (42.8 degrees Fahrenheit) which is the lowest known heat death temperature of any animal [8].

Pagothenia borchgrevinki is protected from freezing by glycopeptide antifreeze compounds in its body fluids, that bind to emerging ice crystals and prevent their growth [1,15]. These antifreeze compounds are being commercially marketed for product development [14]. P. borchgrevinki lives in the upper six meters of water swimming beneath the sea ice undersurface and entering it to feed and take refuge where it is well-camouflaged by special adaptations in its body coloration. A silvery protective layer beneath the skin masks dark-colored internal organs (i.e. liver) from appearing on its lightly colored body; the iris and choroid of the eye are similarly masked to avoid their brown or black color [3,6].

Pagothenia borchgrevinki eggs located in a hole in a grounded iceberg south of Cape Evans being protected by a parent fish.

The species name borchgrevinki honors CD Borchgrevink, the Norwegian commander of the British Southern Cross Antarctic Expedition of 1898-1900 which established the first wintering-over base on the Antarctic continent and which first collected this fish.

The Pagothenia genus differs from the Notothenia genus in being more slender and deep bodied with pelagic rather than benthic features [7].

1: Science 172:1152-1155, 1971; 2: Antarctic Fish Biology. JT Eastman. San Diego: Academic Press, 1993; 3: Polar Biology 4:155-160, 1985; 4: The Penguins, Spheniscidae. TD Williams. Oxford: Oxford University Press, 1995. pp.152- 160; 5: Antarctic Research Series 5, Biology of the Antarctic Seas II. GA Llano, ed. Washington DC: American Geophysical Union, pp1-37; 6: Polar Biology 4:45- 52, 1985; 7: History and Atlas of the Fishes of the Antarctic Ocean. RG Miller. Carson City, Nevada: Foresta Institute for Ocean and Mountain Studies, 1993. pp. 229-235; 8: Science 156:257-258, 1967; 8: Fishes of the Southern Ocean. O Gon and PC Heemstra, eds. Grahamstown, South Africa: JLB Smith Institute of Ichthyology, 1990. pp. 308-309; 9: Hydrobiologia 165:161- 167, 1988; 10: Polar Biology 8:41-48, 1987; 11: FAO Species Identification Sheets for Fishery Purposes : Southern Ocean (Fishing Areas 48, 58 and 88) (CCAMLR Convention Area). W Fischer & JC Hureau, eds. Rome : Food and Agriculture Organization of the United Nations, 1985; 12: Polar Biology 8(1):49-54, 1987; 13: Tethys 6(3):631-653, 1974; 14: www.afprotein.com; 15: Water and Life : Comparative Analysis of Water Relationships at the Organismic, Cellular, and Molecular Levels. GN Somero, CB Osmond, CL Bolis, eds. New York : Springer-Verlag, 1992. pp. 301-315; 16: Polar Biology 25(4):320-322, 2002; 17: Science 235(4785):195-196, 1987

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