Channichthyidae

The loss of hemoglobin was initially believed to be an adaptation to the extreme cold, as the lack of hemoglobin and red blood cells decreases blood viscosity, which is an adaptation that has been seen in species adapted to cold climates. In refuting this original hypothesis, previous analysis has proposed that the lack of hemoglobin, while not lethal, is not adaptive.[2] Any adaptive advantages incurred by reduced blood viscosity are outweighed by the fact that icefish must pump much more blood per unit of time to make up for the reduced oxygen carrying capacity of their blood.[2] The high blood volume of icefish is itself evidence that the loss of hemoglobin and myoglobin was not advantageous for the ancestor of the icefish. Their unusual cardiovascular physiology, including large heart, high blood volume, increased mitochondrial density, and extensive microvasculature, suggests that icefish have had to evolve ways of coping with the impairment of their oxygen binding and transport systems.

Recent research by Corliss et al. (2019) claims that the loss of hemoglobin has adaptive value.[24] Iron is a limiting nutrient in the environments inhabited by the icefish.[25] By no longer synthesizing hemoglobin, they claim that icefish are minimizing endogenous iron use. To demonstrate this, they obtained retinal samples of Champsocephalus gunnari and stained them to detect hemoglobin alpha 3'f. They found expression of hemoglobin alpha 3'f within the retinal vasculature of Champsocephalus gunnari, demonstrating for the first time that there is limited transcription and translation of a hemoglobin gene fragment within an icefish. Because this fragment of hemoglobin does not contain any iron binding sites, the finding suggests that hemoglobin was selected against to conserve iron.

Phylogenetic relationships indicate that the nonexpression of myoglobin in cardiac tissue has evolved at least four discrete times.[17] This repeated loss suggests that cardiac myoglobin may be vestigial or even detrimental to icefish. Sidell and O'Brien (2006) investigated this possibility. First, they performed a test using stopped flow spectrometry. They found that across all temperatures, oxygen binds and dissociates faster from icefish than it does from mammalian myoglobin. However, when they repeated the test with each organism at a temperature that accurately reflected its native environment, the myoglobin performance was roughly equivalent between icefish and mammals. So, they concluded that icefish myoglobin is neither more nor less functional than the myoglobin in other clades.[2] This means that myoglobin is unlikely to have been selected against. The same researchers then performed a test in which they selectively inhibited cardiac myoglobin in icefish with natural myoglobin expression. They found that icefish species that naturally lack cardiac myoglobin performed better without myoglobin than did fish that naturally express cardiac myoglobin.[2] This finding suggests that fish without cardiac myoglobin have undergone compensatory adaptation.

The Southern Ocean is an atypical environment. To begin with, the Southern Ocean has been characterized by extremely cold but stable temperatures for the past 10-14 million years.[26] These cold temperatures, which allow for higher water oxygen content, combined with a high degree of vertical mixing in these waters, means oxygen availability in Antarctic waters is unusually high. The loss of hemoglobin and myoglobin would have negative consequences in warmer environments.[12] The stability in temperature is also "lucky", as strong fluctuations in temperature would create a more stressful environment that would likely weed out individuals with deleterious mutations. Although most research suggests that the loss of hemoglobin in icefish was a neutral or maladaptive trait that arose due to a random evolutionary event,[27] some researchers have also suggested that the loss of hemoglobin might be tied to a necessary adaptation for the icefish.[27] Most animals require iron for hemoglobin production, and iron is often limited in ocean environments.[28] Through hemoglobin loss, icefish may minimize their iron requirements. This minimization could have aided the icefish survival 8.5 million years ago when Arctic diversity plummeted dramatically.[27]

Pagetopsis macropterus

The key to solving this conundrum is to consider the other functions that both hemoglobin and myoglobin perform. While emphasis is often placed and understandably so on the importance of hemoglobin and myoglobin in oxygen delivery and use, recent studies have found that both proteins are actually also involved in the process of breaking down nitric oxide.[29] This means that when icefish lost hemoglobin and myoglobin, it did not just mean a decreased ability to transport oxygen, but it also meant that total nitric oxide levels were elevated.[2] Nitric oxide plays a role in regulating various cardiovascular processes in icefish, such as the dilation of branchial vasculature, cardiac stroke volume, and power output.[30] The presence of nitric oxide also can increase angiogenesis, mitochondrial biogenesis, and cause muscle hypertrophy; all of these traits are characteristics of icefish. The similarity between nitric oxide-mediated trait expression and the unusual cardiovascular traits of icefish suggests that while these abnormal traits have evolved over time, much of these traits were simply an immediate physiological response to heightened levels of nitric oxide, which may in turn have led to a process of homeostatic evolution.[2] In addition, the heightened levels of nitric oxide that followed as an inevitable consequence of the loss of hemoglobin and myoglobin may have actually provided an automatic compensation, allowing for the fish to make up for the hit to their oxygen transport system and thereby providing a grace period of the fixation of these less than desirable traits.