Antarctic scallop Adamussium colbecki

Adamussium colbecki is found throughout Antarctica and the Antarctic Peninsula, South Shetland Islands, South Orkney Islands, and South Sandwich Islands at depths from 0 to 1,500 meters [3,7,8]. A. colbecki is a free-living scallop and has a thin, fragile, sometimes flexible shell with 15 to 22 radial ribs, fine concentric striations, and small ears [4,8]. The shell of A. colbecki is plum to reddish-purple to brown [8,9]. A. colbecki has a maximum recorded length of twelve centimeters [3].

The whitish calcareous foraminiferan Cibicides refulgens is often found living on the shell of A. colbecki and has three feeding modes useful for survival in a food-scarce (oligotrophic) and seasonal environment: grazing algae and bacteria living on the surface of the scallop shell; suspension feeding through a pseudopodial net deployed from a superstructure of agglutinated tubes extending from the foram's calcareous test; and, parasitism by eroding through the scallop shell, and using free amino acids from the scallop's extrapallial cavity [2].

Here Adamussium colbecki is under attack by the seastar Notasterias armata.

A. colbecki predators include the seastars Notasterias armata, Lophaster gaini, and Odontaster validus, the brittle star Ophiosparte gigas, the proboscis worm Parborlasia corrugatus, the Antarctic whelk Neobuccinum eatoni, and the fish Trematomus bernacchii (T. bernacchii preys on the scallop size range 25-64 millimeters) [1,6,10,13,14]. A. colbecki has a swimming escape response to predators and disturbances, and has been observed swimming twenty meters above the bottom [2,5,14]. Swimming duration time can be over ten seconds long, covering up to 45 centimeters, with level swimming speeds between 12 and 23.5 centimeters per second [14].

Here Adamussium colbecki is seen with one of its predators, the brittle star Ophiosparte gigas. A. colbecki is a filter-feeder living on the seafloor; its food includes benthic diatoms, foraminifera, and detritus [6]. A. colbecki may be found in shallow depressions it makes in the seafloor; this digging resuspends bottom detritus for filter feeding by A. colbecki [4,6]. A. colbecki of all sizes may attach with byssal threads to hard rocky substrates [4,6,14]. Though A. colbecki can be seen in abundance in some locations (60-80 per square meter in Terra Nova Bay), it grows much more slowly than scallops in temperate water (an order of magnitude more slowly) [3,4,17]. Small A. colbecki specimens (less then fifty millimeters in shell length) grow at a rate of ten millimeters per year, while larger specimens grow at a rate of 0.8 millimeters per year [19]. Based on growth curves, A. colbecki that are eight centimeters in shell length are estimated to be 14-18 years old, so A. colbecki is very long lived [19]. Other studies using less direct methods estimate that A. colbecki that are eight centimeters in shell length are about twelve years old and that it may live up to twenty years of age [3,4]. A relatively low level of fishing pressure could cause the collapse of A. colbecki scallop populations [3].

Adamussium colbecki spawns and fertilizes in September, producing unprotected larvae in the water where they feed on plankton, living as larvae possibly for more than 100 days [2,20]. Juvenile A. colbecki, up to five centimeters in size, are oftentimes attached by byssus threads to the upper shell of the larger scallops [14,15]. 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 [6]. Attachment of juvenile A. colbecki scallops to adults helps the juveniles escape predation, both by avoiding predators that target only small prey and by taking advantage of the much faster and stronger swimming escape response of the adults [13]. The most vulnerable life stage may be after the growing juvenile detachs from an adult [13,14,15].

Adamussium colbecki may have the bush sponge Homaxinella balfourensis attached [6,11]. H. balfourensis is found on scallops larger than seven centimeters, with the sponge up to fourteen centimeters in height [6]. The usual position of H. balfourensis on the scallops is near the shell's peripheral margin, suggesting that the sponge is seeking the water flow over the scallop shell in order to facilitate its own filter feeding [6].

A. colbecki may be colonized on either shell by small (two millimeters high) hydroids Hydractinia angusta [16]. H. angusta hydroids eat tube feet and pedicellariae of sea urchins including Sterechinus neumayeri, which grazes on the algal film growing on the surface of the scallop's shell, but is not a predator of the scallop [16]. A. colbecki shells are very thin and such urchin grazing may damage the shell; thus the hydroids act in defense of the scallop [16]. H. angusta hydroids eat the film (includes agglutinated diatoms) it can remove with its tentacles from the scallop shell, as well as bottom sediment exposed to it due to clapping activity of the scallop [16]. H. angusta hydroids also reduce the settling of young A. colbecki scallop larvae onto the shells of adult scallops, competing successfully for shell space with the young scallops [18].

1: Antarctic Science 6(1):61-65, 1994; 2: Antarctic Science 3(2):151-157, 1991; 3: Antarctic Ecosystems: Ecological Change and Conservation. KR Kerry & G Hempel, eds. Berlin : Springer- Verlag, 1990. pp.281-288; 4: Marine Biology 78(2):171-178, 1984; 5: Marine Biology 94:479-487, 1987; 6: Ecology of the Circumpolar Antarctic Scallop, Adamussium colbecki (Smith, 1902). Paul Arthur Berkman. Ph. D. Dissertation, University of Rhode Island, 1988; 7: Antarctic Mollusca : with Special Reference to the Fauna of the Ross Sea. RK Dell. Wellington, NZ : Royal Society of New Zealand, 1990. Bulletin 27, Royal Society of New Zealand; 8: 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; 9: EA Smith, Report on the Collections of Mollusca Made in Antarctica during the Voyage of the "Southern Cross." IN: Report on the Collections of Natural History Made in the Antarctic Regions During the Voyage of the "Southern Cross" Part 7, London : Printed by Order of the Trustees, 1902. page 212; 10: Polar Biology 16(5):309-320, 1996; 11: Tethys Supplement 4:9-24, 1972; 12: Biological Bulletin 173(1), 136-159, 1987; 13: Antarctic Science 12(1):64-68, 2000; 14: Antarctic Science 10(4):369-375, 1998; 15: Scientia Marina 61 (Supplement 2):15-24, 1997; 16: Polar Biology 23(7):488-494, 2000; 17: Scientia Marina 63(Supplement 1):113- 121, 1999; 18: Polar Biology 24(8):577-581, 2001; 19: Polar Biology 26(6):416-419, 2003; 20: Polar Biology 26(11):727-733, 2003

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