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Antarctic Cod (also known as the Antarctic toothfish)
Dissostichus mawsoni, the Antarctic toothfish, is a species of cod icefish native to the Southern Ocean. It is often mistakenly referred to as an Antarctic cod, consistent with the misnaming of other notothenioid Antarctic fish as rock cods. However, notothenioid fishes are unrelated to cods, which are in another taxonomic order, the Gadiformes. The generic name Dissostichus is from the Greek dissos (twofold) and stichus (line) and refers to the presence of two long lateral lines, which are very important to the species' ecology. The common name "toothfish" refers to the presence of biserial dentition in the upper jaw, thought to give it a shark-like appearance. The habitat of the Antarctic toothfish is in subzero degree water below latitude 60ºS.
Fully grown, these fish (and their warmer-water relative, the Patagonian toothfish, D. eleginoides) can grow in excess of 1.7 m in length and 80 kg in weight, twice as large as the next largest Antarctic fish. Being large, and consistent with the unstructured food webs of the ocean (i.e., big fish eat little fish regardless of identity, even eating their own offspring), the Antarctic toothfish has been characterized as a voracious predator. Furthermore, by being by far the largest midwater fish in the Southern Ocean, it is thought to fill the ecological role that sharks play in other oceans. Aiding in that role, the Antarctic toothfish is one of only five notothenioid species that, as adults, are neutrally buoyant. This buoyancy is attained at 100-120 cm in length and enables them to spend time above the bottom without expending extra energy. Both bottom-dwelling and mid-water prey are therefore available to them. Most other notothenioid fish and the majority of all Antarctic fish, including smaller toothfish, are confined to the bottom. Coloring is black to olive brown, sometimes lighter on the undersides, with a mottled pattern on body and fins. Small fish blend in very well among the benthic sponges and corals. They have a broad head, an elongated body, long dorsal and anal fins, large pectoral fins and a rudder-like caudal fin. They typically move slowly, but are capable of speed bursts that can elude predatory seals. Therefore, competition for prey among toothfish and these other mesopredators (middle trophic level predators) could be very important. The large Antarctic toothfish are eaten by sperm whales Physeter macrocephalus, killer whales Orcinus orca, Weddell seals and colossal squid Mesonychoteuthis hamiltoni. Toothfish that are dwelling on the bottom, especially in ice free waters of the continental slope, eat grenadiers (Macrouridae) and skates (Raja spp.). Antarctic toothfish have been caught to depths of 2200m, though based on commercial fishing effort, few occur that deep.
Aging data indicate Antarctic toothfish are relatively fast-growing when young, but then growth slows later in life. They reach about one-third of maximum size after five years, and half maximum by 10 years, after which growth slows considerably. To grow fast when small is an adaptation of most predatory fish, e.g., sharks, so as not to be small for very long. The maximum age recorded so far has been 48 years. Antarctic toothfish take a long time to mature (13 years for males, 17 years for females) and once mature may not spawn every year, though the actual spawning interval is unknown. Only a few Antarctic toothfish with mature eggs have ever been caught, meaning knowledge is sparse about fecundity. They spawn sometime during winter. Large, mature, older fish have been caught among the seamounts of the Pacific-Antarctic Ridge, a location thus thought to be important for spawning. Smaller, subadult Antarctic toothfish tend to concentrate on the bottom hidden among sponges and corals of the continental shelf, and a large portion of the larger ones remain in the particularly food-rich waters of the continental slope. This sequestering by size and age could be another adaptation for small fish to avoid being eaten by large ones. The recruitment potential of Antarctic toothfish, a measure of both fecundity and survival to spawning age, is not known.
The Antarctic toothfish has a lightweight, partially cartilaginous skeleton, lacks a swim bladder, and has fatty deposits which act as a stored energy source, particularly during spawning. This fat also makes large toothfish neutrally buoyant. Many toothfish caught over the seamounts are very depleted of fat, and this is thought perhaps to be related to spawning and spawning migration, which are energy-demanding activities. It is not known what happens to these fat-depleted fish, including whether they reach, or how long it takes them to reach, breeding condition again; this ostensibly occurs upon returning to continental slope waters. Antarctic toothfish have vision and lateral line systems well adapted to find prey in low light levels. Since ice covers the surface of the ocean where Antarctic toothfish occur even in summer, these sensory specializations likely evolved to enable survival in the reduced light levels found under ice and in the Antarctic winter, as well as at deep depths. Antarctic toothfish also have a very well developed sense of smell, which is why they are easily caught by baited hooks and also scavenge the remains of penguins killed by other predators.
The Antarctic toothfish is noteworthy, like most other Antarctic notothenioids, for producing antifreeze glycoproteins, a feature not seen in its closest relative, the Patagonian toothfish, which typically inhabits slightly warmer waters. The presence of antifreeze glycroproteins allows the Antarctic toothfish (and other notothenioids) to thrive in sub-zero waters of the Southern Ocean surrounding Antarctica. The Antarctic toothfish's voracious appetite also is important in coping with cold water. It is mainly caught in the Ross Sea in the austral summer, but has also been recorded from Antarctic coastal waters south of the Indian Ocean sector, in the vicinity of the Antarctic Peninsula, and near the South Sandwich Islands.