The oxygen transporter hemoglobin, which is found in red blood cells (erythrocytes), has generally been regarded as essential for adult vertebrate life. What a surprise it must have been to zoologists to read Ruud's seminal Nature article on icefishes (family Channichthyidae, suborder Notothenioidei), \"Vertebrates without erythrocytes and blood pigment\" (Ruud 1954). Indeed, Ruud himself harbored doubts about the existence of the blodlaus-fisk (bloodless fish) that Norwegian whalers reported to inhabit the shelf waters of South Georgia. He wrote:

I first heard about these \"bloodless fish\" on a visit to South Georgia in 1929; but no specimens were forthcoming, and I did not take them seriously. I was reminded about their existence, however, when Mr. D. Rustad, biologist in the Norvegia Expedition (1927-28), presented me with some photographs of a \"white crocodile fish\" caught by him at Bouvet Island, mentioning the fact that its blood was colourless.

When in 1953 Ruud captured four specimens of the \"white crocodile fish\" Chaenocephalus aceratus at South Georgia, he measured some of the blood properties of the species. The nearly transparent blood contained white blood cells at <1% of total blood volume but no red blood cells or hemoglobin. The oxygen capacity of the icefish blood was 10-12% that of the related, but red-blooded, South Georgian fishes Notothenia rossii and N. coriiceps. Thus, Ruud concluded that \"the blood of Chaenocephalus aceratus is to be considered a plasma with a moderate content of leucocytes.\" How can these large fishes survive without an oxygen transporter in their blood? Ruud speculated that the metabolism of these fishes was \"rather low,\" and he argued that the ecological context of these fish was critical:

Since these fish presumably descend from ancestors with haemoglobin in their blood, one imagines that only in the cold water of the polar regions could a fish survive which had lost its blood pigment. The Chaenocephalus specimens came from water rather less than 2 C. In the winter farther south the water temperature descends to around -1.7, and is always well aerated.

Implicit in Ruud's hypothesis is the concept that a character, the absence of hemoglobin, that is deleterious for fishes living at high temperature may be selectively neutral (or perhaps deleterious but nonlethal) at low temperature. Consistent with this possibility are observations that the red-blooded Antarctic notothenioid Trematomus bernacchii can survive under resting conditions when its hemoglobin is poisoned by carbon monoxide or when its hematocrit is progressively reduced to <1% by withdrawal of blood aliquots (di Prisco et al. 1992). It has been suggested that the near elimination of erythrocytes may be advantageous because it should reduce the energy required to circulate the blood. However, the development in icefishes of adaptations that reduce tissue oxygen demand and enhance oxygen delivery (e.g., modest suppression of metabolic rates, enhanced gas exchange by large gills and through a scaleless skin, and large increases in cardiac output and blood volume) argues that loss of hemoglobin and erythrocytes was probably maladaptive under conditions of physiological stress.

One of the goals of the ICEFISH Cruise is to obtain DNA from white- and red-blooded notothenioids for further analysis of the amazing capacity of icefishes to live without red blood cells.


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