The Antarctic marine environment presents a significant challenge for life, characterized by freezing temperatures. The Southern Ocean’s surface water can reach as low as -1.8°C near the coast, creating conditions inhospitable for most fish species found elsewhere. Despite these harsh conditions, Antarctic waters teem with unique marine life, supporting a rich biodiversity unlike any other ocean. This abundance is due to nutrient-rich waters, which are highly productive, especially during summer phytoplankton blooms.
Key Fish Species
Antarctica’s fish fauna is dominated by Notothenioids. This suborder of bony fish accounts for approximately 77% of the fish diversity and over 90% of the fish biomass on the Antarctic continental shelf. Within this group, families like Nototheniidae (Antarctic cods) and Channichthyidae (icefish) are prevalent. The Nototheniidae family alone comprises about 50 species adapted to these cold, nutrient-rich waters.
While Notothenioids are the most abundant, other less common groups also inhabit these frigid waters. These include skates and deep-sea fish, though their presence is less dominant. Notothenioids show diversity, with species like the Antarctic smelt (Pleuragramma antarctica) and the giant Antarctic toothfish (Dissostichus mawsoni) inhabiting different parts of the water column.
How They Survive the Cold
Antarctic fish have developed remarkable physiological adaptations to survive in sub-zero waters. One of the most significant is the production of antifreeze glycoproteins (AFGPs). These proteins bind to small ice crystals in the fish’s body fluids, preventing them from growing and causing lethal ice formation. Fish body fluids typically freeze at a higher temperature (-0.7 to -1°C) than seawater (-1.9°C), making this adaptation important. AFGPs are secreted into the intestinal lumen, protecting internal fluids from ice crystals ingested with seawater and food.
Some icefish (family Channichthyidae) lack hemoglobin and functional red blood cells. This makes their blood and much of their body translucent. To compensate, these white-blooded fish have evolved larger hearts and enhanced vascular systems to circulate oxygen more efficiently. Antarctic fish also exhibit slow metabolic rates, conserving energy in their cold environment. They also store specialized lipids, contributing to buoyancy and energy reserves in waters that are highly oxygenated.
Their Place in the Food Web
Antarctic fish play a significant role in the Southern Ocean’s food web, acting as both predators and prey. Many species, particularly the Notothenioids, primarily feed on Antarctic krill, small shrimp-like crustaceans that form the base of the marine food web. They also consume zooplankton and smaller invertebrates found in the water column or on the seafloor. The Antarctic silverfish (Pleuragramma antarcticum) and crystal krill (Euphausia crystallorophias) are food sources for various predators in coastal waters.
Antarctic fish are a food source for larger marine animals. Predators include seals (e.g., leopard and Weddell seals) and various penguin species. Seabirds like albatrosses and petrels also prey on fish near the surface. Larger fish, including the Antarctic toothfish, also prey on smaller fish. Killer whales, at the top of the food chain, consume seals, penguins, and fish.