The possession of erythrocytes containing hemoglobin has long been regarded as a sine qua non of the vertebrate condition. Imagine the surprise of zoologists when they read J. T. Ruud's Nature article on icefishes, “Vertebrates without erythrocytes and blood pigment” (1). Ruud himself was skeptical when he heard about the blodlaus-fisk (i.e., lacking hemoglobin-bearing erythrocytes) that whalers reported to inhabit the waters of South Georgia in the Antarctic. He explained:
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. Runstad, biologist of 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 (1).
When, years later (1953), he captured several specimens of the “white crocodile fish” Chaenocephalus aceratus (family Channichthyidae, suborder Notothenioidei; Figure 7.1) at South Georgia, Ruud measured the hematological parameters of its blood (1). He reported that fresh blood is nearly transparent, contains leukocytes at <1% by volume, is iron poor, and lacks erythrocytes and hemoglobin. Ruud also determined that the oxygen (O2)-carrying capacity of C. aceratus blood is ∼10% that of two red-blooded Antarctic fishes that belong to a closely related family (Nototheniidae, Notothenioidei). In all other respects, hematopoiesis in icefishes appears normal and yields the full complement of teleost nonerythroid blood cell lineages: heterophils, granulocytes, lymphocytes, and thrombocytes (2).