Clear as Blood: How Did Antarctic Icefish Survive Their “Evolution”?

by on

The Antarctic blackfin icefish, (Chaenocephalus aceratus), as well as other icefish of the Channichthyidae family, had been known by scientists to have clear blood since the 1950s. And, it was assumed that this was some type of adaptation to a cold-water environment by these fish. But a new study reveals just how radical this “adaptation” turns out to be.

Slow and Gradual Step-by-Step Overhaul?

These aren’t merely adaptations, from an evolutionary point of view, this is a complete body and circulatory system overhaul.

Icefish are the only vertebrates that lack red blood cells and hemoglobin as adults. In vertebrates, these are essential for binding oxygen and then transporting oxygen throughout the body. But rather than this being a “simple adaptation,” the icefish of the Channichthyidae family appear to have several major anatomical and chemical alterations compared to other similar fish (even other Antarctic species within the same order, from the Nototheniidae family). The icefish has extremely large gills for its body size, no scales (which may help it to absorb oxygen from the surrounding water), a flexible (and less dense) bony skeleton. It also has a heart four times the size of similar fish, larger blood vessels than other similar-sized fish, has accumulated a lipid layer in its bloodstream that makes it more buoyant, and makes more antifreeze-like proteins than other cold-water fish. Oxygen exists solely in physical solution in icefish blood, which has an oxygen-carrying capacity of less than 10% compared with that of their relatives with hemoglobin. These aren’t merely adaptations, from an evolutionary point of view, this is a complete body and circulatory system overhaul.

Genetic Loss Is Their Gain?

In a piece of science news write up for Quanta magazine, hemoglobin’s strengths in binding oxygen are mentioned, but its shortcomings in cold water are also highlighted.

But the disadvantage of collaborative bonding is that hemoglobin performs worse when oxygen is in short supply. Hemoglobin’s effectiveness also drops with temperature. . . . When biologists discovered that icefish had clear blood in the 1950s, they at first assumed it was an adaptation to the cold. Subsequent work, however, pointed to the icefish’s loss of hemoglobin genes as more of a lucky accident. In most environments, that mutation would have been fatal. But because the frigid Antarctic waters hold more dissolved oxygen than warmer water does, and because the ancestors of icefish probably already had some adaptations to help them prosper in the cold, the fish survived. It may be true, as Louis Pasteur said, that chance favors the prepared mind, but having a well-prepared genome doesn’t hurt.1

Wholesale Draconian Modification and Gene Deletion

The “subsequent work” statement in the above quote is from a 2006 paper in the Journal of Experimental Biology. Their characterization of the changes in Antarctic icefish is eye-opening:

Although absences of the proteins [hemoglobin (Hb) and myoglobin (Mb)] are fixed traits in icefishes, the losses do not appear to be of adaptive value. Contrary to some suggestions, loss of Hb has led to higher energetic costs for circulating blood, and losses of Mb have reduced cardiac performance. Moreover, losses of Hb and Mb have resulted in extensive modifications to the cardiovascular system to ensure adequate oxygen delivery to working muscles [emphasis added].2

. . . 

Several fairly draconian modifications of the cardiovascular system of icefishes compensate for their lack of a circulating oxygen-carrier. Icefishes possess very large hearts compared to red-blooded fishes of equivalent body size, resulting in a weight-specific cardiac output that is four- to fivefold greater than that of red-blooded species. The blood volumes of icefishes are up to fourfold those of red-blooded teleosts [ray-finned fishes], and the diameter of their capillaries is unusually large.3

. . . 

The loss of Hb expression in icefishes is the result of a wholesale gene deletion. In contrast, the decrease in Hb content that occurs in response to a decrease in temperature in red-blooded fishes, is brought about by the downregulation of an existing gene. Clearly, these are fundamentally different processes [emphasis added].4

. . . 

All of the evidence cited above strongly indicates that the losses of the ability to express Hb and Mb were not advantageous. Indeed, available information clearly suggests that each of these losses must have resulted in a decrement in physiological performance of the fishes. . .  Such a conclusion appears at odds with modern evolutionary theory, which suggests that selective pressure should lead to the retention of Hb and Mb expression, and that mutations causing their loss should be subject to negative selection and eliminated from the population. Regardless of the specific nomenclature employed to describe them, persistence of these traits appears to be a conundrum [emphasis added].5

Disadvantageous Adaptations?

The most recent study in Nature reports on the genome sequencing of the Antarctic blackfin icefish (Chaenocephalus aceratus). One of the standout results of the genome sequencing was to confirm that there are several disadvantageous traits carried by these fish, but also some adaptations in Antarctic icefish species, compared to other notothenioid fish.

Some icefishes, including C. aceratus, are more sensitive to oxidative stress than red-blooded notothenioids are. The volume of polyunsaturated-fatty-acid-rich mitochondria per volume of skeletal or cardiac muscle cell in icefishes is approximately twice as large as in red-blooded Antarctic fishes, which may make icefishes more susceptible to reactive oxygen species (ROS) formation and lipid peroxidation at current environmental temperatures. Furthermore, the lower thermal tolerance of icefishes relative to red-blooded notothenioids may be due to increased protein and lipid damage in icefish cardiac muscle. . . . The expansion of nqo1 genes [genes that encode enzymes that help to control cellular redox which could lead to things like hypoxia and sepsis] in the icefish genome was striking: we found a total of 33 genes, in contrast with the 2–10 nqo1 genes annotated in most fish genomes. . . . Also, we provide evidence for the expansion of gene families involved in the cellular redox state, including sod3 and nqo1, which might represent evolutionary adaptations to ROS production associated with mitochondrial expansion in this white-blooded clade [emphasis added].6
What stands out when reading about the Antarctic icefish is how well suited they are to their environment and how perplexing their very existence is to evolutionary thought.

What stands out when reading about the Antarctic icefish is how well suited they are to their environment and how perplexing their very existence is to evolutionary thought. Looking over the popular science write-ups and even the science journals, phrases like “disadvantageous traits,” “sublethal traits,” “unusual evolutionary history,” “non-advantageous,” “extraordinary biology,” “evolutionary mutant models,” “lucky accident,” and “extreme evolutionary adaptations” appear frequently. Clearly, these fish pose some puzzling problems for the “natural selection builds toward macroevolution” crowd.

Never Fear, a Reification and Recuing Device Is Near

But one of the Nature paper’s authors, John Postlethwait, a developmental biologist at the University of Oregon, was able to look on the bright side (?). In an interview with the New York Times, he claimed that the icefish “evolved a therapy for anemia.”7 The science writer of that article then was able to spin all the (from an evolutionary view) negatives into positives.

It [the Antarctic blackfin icefish] developed supersize gills and lost its scales, which enabled it to absorb the water’s plentiful oxygen through its skin. It also expanded its circulatory system with extra vasculature and a heart four times the size of closely related, red-blooded species.

Over evolutionary time, the icefish accumulated lipids, or fats, which, like oil, float in water. It also developed floppy bones that were less mineralized than those of their ancestors. This allowed the icefish to rise in the water column, like spaghetti in boiling water, and eat krill and other creatures that couldn’t be found near the sea floor.8

These Antarctic fish, keeping in mind the trial and error methodology of evolution, according to their viewpoint, are indeed the beneficiaries of major lucky accidents that have occurred over and over. Just think how brave these icefish were to ditch hemoglobin and red blood cells, which no other vertebrate could do—and somehow persevere through their own sheer will, in spite of several deleterious mutations and “sub-lethal” morphological changes—and that in a harsh environment! In fact, these Antarctic icefish, in cooperation with evolution, of course, decided it would be a good time to rearrange their own circulatory system, modify their chemistry and body structure and tweak a few organs.

Creation Perspective

In a biblical worldview, we do see animals that have adapted to a post-fall (and post-flood) world to fill virtually every ecological/climatological niche on this planet. But that adaptation comes at a genetic cost. Diversity is lost, and, in the case of some of the icefish, several of the mutations and/or adaptations are or would be lethal in warmer waters. In a sense, they are now prisoners of their own biology and genetics. Some can only live in a narrow range of water temperature and oxygen content. However, some hemoglobin- and myoglobin-lacking icefish, most notably Chaenocephalus aceratus, are able to tolerate habitat water temperatures as high as 13.9°C (57°F).9 There is also an icefish species which lives almost exclusively outside of Antarctic waters (Champsocephalus esox) and which has been captured in water temperatures of 10–11°C (50–52°F).10

Since we lack a complete baraminological classification of Channichthyidae icefish, (as well as red-blooded notothenioids), any creation model must be tentative. But it appears most likely that the family Channichthyidae is its own baramin, separate from the Nototheniidae family. As such, we can postulate that some of the physiological features unique to icefish were in the original created kind, perhaps higher lipid content, enlarged hearts and capillaries, and less-mineralized bones. However, since some icefish do retain myoglobin in cardiac tissue11 and one species, Neopagetopsis ionah. still retains a complete, but non-functional hemoglobin complex,12 it seems most likely that the loss of hemoglobin and myoglobin are “loss of function” adaptations to their environments in post-flood icefish. Additionally, these adaptations are probably due in large part to genetic drift. Genetic drift is the result of the mathematical probability that some individuals leave behind more descendants than others in each generation. Those individuals’ genes, therefore, are more highly represented in the later populations without any regard to fitness for survival. This effect is even more pronounced in smaller populations. This would more feasibly explain why most icefish lost myoglobin and hemoglobin through mutation, then passed this along via genetic drift, while a few species still retained cardiac myoglobin or even non-functional hemoglobin.

Contrary to the rosy picture painted in the above NY Times article, the icefish did nothing to “evolve itself,” nor did time and chance. And the concept of evolution as a thinking, planning, and directional “force” is nothing more than reification. Furthermore, the blackfin icefish is still an icefish, and one which is not as “healthy” or fit as its red-blooded notothenioid “cousins.” What we do observe is entirely consistent with Scripture, that the original created kinds were endowed with a perfect genome, which had a much greater variety that was providentially given to allow rapid adaptation to the harsh environment God knew these creatures would need to endure. Speciation led to winnowing of this originally created heterozygosity, allowing animals to adapt to different ecosystems, but causing a loss of genetic diversity along the way.

The real hero of the plot is not evolution, nor is it the icefish.

The real hero of the plot is not evolution, nor is it the icefish. God designed everything as “very good” originally (Genesis 1:31) and frontloaded each organism with a vast amount of genetic material. This allowed (and still allows) organisms to be able to adapt to their environment in a fallen world. In the last sentence of the Quanta article, the author made a very astute observation: “having a well-prepared genome doesn’t hurt.” While we might differ on the identity of the preparer, we can certainly agree on that point!


  1. John Rennie, “Icefish Study Adds Another Color to the Story of Blood,” Quanta Magazine, April 22, 2019.
  2. Bruce D. Sidell and Kristin M. O'Brien, “When bad things happen to good fish: the loss of hemoglobin and myoglobin expression in Antarctic icefishes,” Journal of Experimental Biology 209 (2006): 1791, doi: 10.1242/jeb.02091.
  3. Ibid., 1792.
  4. Ibid., 1794.
  5. Ibid., 1796.
  6. Bo-Mi Kim et al., “Antarctic blackfin icefish genome reveals adaptations to extreme environments,” Nature Ecology & Evolution 3 (2019): 474.
  7. JoAnna Klein, “How the Icefish Got Its Transparent Blood and See-Through Skull,” NY Times, Feb. 28, 2019,
  8. Ibid.
  9. Jody M. Beers and Nishad Jayasundara, “Antarctic notothenioid fish: what are the future consequences of ‘losses’ and ‘gains’ acquired during long-term evolution at cold and stable temperatures?” The Journal of Experimental Biology 218 (2015): 1839–1840,
  10. Ian A. Johnston, et al., “Latitudinal variation in the abundance and oxidative capacities of muscle mitochondria in perciform fishes,” The Journal of Experimental Biology 201 (1998):7–9.
  11. Bruce D. Sidell, “Intracellular oxygen diffusion: the roles of myoglobin and lipid at cold body temperature,” The Journal of Experimental Biology 201, (1998): 1124.
  12. Beers and Jayasundara, 1838.


Get the latest answers emailed to you or sign up for our free print newsletter.

I agree to the current Privacy Policy.

Answers in Genesis is an apologetics ministry, dedicated to helping Christians defend their faith and proclaim the gospel of Jesus Christ.

Learn more

  • Customer Service 800.778.3390