Diving Under Antarctic Ice

EXPLORER'S COVE, NEW HARBOR

Explorer's Cove is at the northwest head of New Harbor, on the western side of McMurdo Sound (Ross Island with McMurdo Station is on the eastern side of McMurdo Sound). Explorer's Cove was named by the US Advisory Committee on Antarctic Names in recognition of the large number of explorers that have worked in area. New Harbor was discovered by Scott's British National Antarctic Expedition (1901-1904) and was named because this new harbor was found while the Discovery was seeking a southernmost anchorage along the coast of Victoria Land.

Here at Explorer's Cove, the crinoid Promachocrinus kerguelensis is perched on a large volcano sponge Anoxycalyx (Scolymastra) joubini, surrounded by a muddy sandy bottom. The criniod is perched up high to better position itself for filter feeding where it can take advantage of near-bottom currents. The crinoid's arms are edged with feathery pinnules containing sensory tube feet and reproductive organs; the arms are used to trap drifting plankton and they have grooves down which food particles are carried by hair-like cilia to the upward-facing mouth. On the left of the sponge is the Antarctic scallop Adamussium colbecki and on the right is the brittle star Ophionotus victoriae.

Here is a typical bottom scene at Explorer's Cove: Antarctic scallops Adamussium colbecki (on the left and upper right); the brittle star Ophionotus victoriae (in the middle); the seastar Notasterias armata (on the right). The western side of McMurdo Sound which includes New Harbor differs ecologically from the eastern McMurdo Sound along Ross Island [1]. The large shallow-water filter feeders seen along Ross Island are relatively rare at New Harbor and along western McMurdo Sound; there are fewer benthic species and benthic animal densities are an order of magnitude less compared to the eastern McMurdo Sound [1]. Bathed by a northerly current from under the permanent Ross Ice Shelf and more completely covered throughout the year by sea ice, New Harbor is a food-scarce environment (oligotrophic) and doesn't experience the plankton blooms as does eastern McMurdo Sound [1].

Here at Explorer's Cove is a pencil urchin Ctenocidaris perrieri surrounded by Antarctic scallops Adamussium colbecki. New Harbor has been described as similar to a deep sea environment, with a soft bottom with occasional biological structures like sponges and foraminiferans, fecal pellets, and depressions left by animals [1]. The Antarctic scallop Adamussium colbecki is the dominant species in shallow water and frequently found in shallow depressions it makes in the seafloor; this digging resuspends bottom detritus for filter feeding by A. colbecki [2,3]. As seen on the left here, juvenile A. colbecki are usually attached to larger individuals [2]. This survival enhancement for the juvenile A. colbecki gives them a better position for filter feeding in the water column where they can take advantage of water flow generated by adults as well as near- bottom currents [3]. The bush sponge Homaxinella balfourensis is seen attached to the spines of the pencil urchin. There is little hard substrate in Explorer's Cove so pencil urchin spines and scallop shells are what is available; the sponge's resulting elevated position as well as the urchin's movement undoubtedly facilitates its filter feeding.

Other similarities between New Harbor at 30-45 meters depth and the deep seafloor from 180 to 1800 meters depth (bathyal) are water temperature, constancy of physical environment, low terrestrial sedimentations, and the brittle star Ophionotus victoriae, which is similar to a particular deep sea brittle star in population density and distribution, size, and morphology [1]. Ophionotus victoriae uses its two leading arms in a typical brittle star rowing fashion as it moves on the seafloor searching for food [6].

The predatory brittle star Ophiosparte gigas moves quickly and efficiently on the muddy sand bottom of Explorer's Cove using its paddle-like arm spines and stilt-like tube feet [5]. Ophiosparte gigas preys on the brittle star Ophionotus victoriae above, which responds to O. gigas contact by quickly fleeing [6,7,8]. If successful in capture, Ophiosparte gigas holds the disc of Ophionotus victoriae under its own and clips off arms to ingest [7].

The nephtheid soft coral Gersemia antarctica is occasionally seen by the diver at Explorer's Cove.

Soft corals have internal skeletons consisting of calcium-containing needle-like spicules and are colonial, composed of numerous cylindrical polyps with mouths surrounded by eight tentacles. The tentacles are armed with harpoon-like stinging structures, called nematocysts, that paralyze prey.

G. antarctica colonies can inflate to over two meters in height [4]. .

In addition to an upright feeding posture, Gersemia antarctica can bend its entire colony down so that the polyps reach the bottom to feed there [4].

The food that G. antarctica seeks on the bottom includes benthic diatoms, foraminiferans, and particulate organic matter [4].

This grazing behavior has likely evolved to supplement plankton capture from the water and is useful in Antarctica where plankton in the water column is seasonal [4].

Here is a partially buried brittle star Ophionotus victoriae in front of Gersemia antarctica stretched out on the bottom.

A G. antarctica colony can move along the bottom like an inch worm, to reach undisturbed sediments for grazing [4].

G. antarctica colonies have been observed moving over fourteen meters in one year's time [4].

When a moving G. antarctica colony encounters sediment previously grazed by G. antarctica, it contracts from it [4].

1: Science 197:55-58, 1977 ; 2: Marine Biology 78(2):171-178, 1984; 3: Ecology of the Circumpolar Antarctic Scallop, Adamussium colbecki (Smith, 1902). Paul Arthur Berkman. Ph. D. Dissertation, University of Rhode Island, 1988; 4: Marine Ecology Progress Series 149(1-3):299-304, 1997; 5: The Fauna of the Ross Sea, Part 1, Ophiuroidea. HB Fell. New Zealand Department of Scientific and Industrial Research Bulletin 142, New Zealand Oceanographic Institute Memoir 18, 1961; 6: Adaptations within Antarctic Ecosystems : Proceedings of the Third SCAR Symposium on Antarctic Biology. George A. Llano, ed. Washington : Smithsonian Institution ; Houston, Tex. : distributed by Gulf Pub. Co., 1977. pp.293-326; 7: Polar Biology 16(5):309-320, 1996; 8: Norbert Wu, personal communication, 1999

Text ©Peter Brueggeman. Photographs ©Norbert Wu.Photographs may not be used in any form without the express written permission of Norbert Wu. Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?