Elongated, scaleless, pikelike body (length/depth ratio, about 7:2) with an elongated snout. The jaws do not protract. In coloring it is silvery gray, darker on the back, and silvery on the belly, with dark vertical stipes on the sides, reminiscent of those seen in mackerel. The gills are a pale yellowish; the blood is colorless, completely lacking hemoglobin. The absence of hemoglobin is a remarkable feature of all 15 icefish species, apparently caused by a single massive mutation that deleted the j8-globin gene. Far from being an adaptation to Antarctic conditions, this has been termed a "disaptation." Several factors have enabled channichthyids to survive this evolutionary catastrophe: frigid seawater and blood plasma can carry more dissolved oxygen, and low body temperatures limit metabolic requirements. Subsequent evolution has compensated for the loss of an oxygen-binding pigment: blood volume, heart, vessel, and gill size and the perfusion rate of gills and blood vessels all have increased. As a consequence, channichthyids are surprisingly active, competing successfully with other notothe-nioids.
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