Ocean - Marine Inhabitants

Roughly 30 million years ago, the temperature of the Southern Ocean dropped. Because few fish are capable of surviving waters only just above freezing, many either went extinct or migrated to warmer areas. However, one fish held strong. Due to the magic of natural selection, the successors developed certain characteristics that allowed them to survive such extreme an environment. Chaenocephalus aceratus, or Antarctic blackfin icefish, thrive today in water so ice cold with clear blood, thin bones, and no scales. Scientists and researchers have long since been fascinated with these creatures, often wondering how they can survive in such hostile environments.

Scientists continue to study their unusual traits. New findings provided them with new ways to look at how these fascinating fish managed to adapt to these extreme and adverse frigid conditions. (Klein, 2019)

Icefish are one of eight different families that dominate the suborder Notothenioidei, a suborder of fish surrounding Antarctica. These waters are consistently below freezing temperatures, (Sidell, 2006) resulting in the icefish no longer producing hemoglobins and red blood cells to carry oxygen throughout its body, despite this being an essential trait necessary for survival for the roughly sixty thousand other vertebrate species.

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This took millions and millions of years of gene hacking, natural selection, and adaptations. Although icy water carries more oxygen than warm water, it causes red blood to thicken, in turn making it harder to pump throughout the body and more prone to freezing. The icefish developed enlarged gills and lost their scales, allowing them to absorb oxygen through its thin skin.

Its circulatory system also grew, resulting in more vasculature and a heart that is almost four times the size of its closely related brethren. With time, lipids accumulated within the icefishes' bodies, along with bones to floppy and thin you can see its brain through its skull. This enables them to rise in water and eat small creatures, like krill, that cannot be found closer to the floor of the sea. (Klein, 2019)

Another fascinating asset among the species is their excess of unsaturated fats and mitochondria, for now known as a counter to the effects of frigid temperatures on the fishes' metabolisms. This elevated content poses susceptibility to oxidative stress, and this is evident within their genes. "Icefish expanded the gene families that prevent oxidative damage, including antioxidant proteins superoxide dismutase (icefish have three extra copies) and NAD(P)H-quinone dehydrogenase (there are 33 copies in icefish, compared with between 2 and 10 in other fish). Icefish are also the only vertebrates to double up on their 8-oxoguanine DNA glycosylase gene, which produces a protein that removes damaged pieces of DNA." (Borowiec, 2019)

Because the "hemoglobinlessness" shared among the icefish family, it is notably clear that the divergence between this trait and having red blood occurred near the point of separation between the channichthyidae from their ancestors, the aforementioned notothenioid. Although cold, the icy waters are incredibly stable in temperature and is abundant in oxygen, both characteristics essential for the survival of the icefish. (Sidell, 2006)