The Antarctic is one of the most inhospitable environments on Earth, yet despite this, its waters are rich in oxygen and nutrients. The southern oceans support diverse marine ecosystems, in which many species have developed unusual and fascinating adaptations to deal with the harsh Antarctic conditions. The icefish are no exception, and this unique family of fish has developed several novel adaptations to this extreme environment, such as the evolution of antifreeze proteins (AFPs) in the blood (11). Like other icefish, the blackfin icefish relies on these AFPs to lower the point at which the body freezes, allowing it to survive in the sub-zero temperatures of the Antarctic (11).
Icefish are the only vertebrates that do not have haemoglobin in their blood (3) (4) (5) (6) (7). Without haemoglobin, icefish blood is only able to carry around 10 percent of the amount of oxygen that normal blood can transport. However, icefish are known to use less energy in comparison to other fish, and have been shown to have a number of other remarkable adaptations to help them cope with their severe environment. These adaptations include the ability to absorb oxygen through the skin, a bigger heart which pumps the blood around the body at a faster rate than most fish, and thinner blood to increase the amount of blood flow around the body (3).
The blackfin icefish feeds primarily on fish, particularly mackerel icefish (Champsocephalus gunnari), as well as krill (Euphausia superba) and other crustaceans (3) (4) (6). The blackfin icefish is thought to change its diet as it grows, with juveniles and smaller blackfin icefish feeding mainly on krill, and older, larger individuals (usually over 30 centimetres in length) feeding almost exclusively on fish (2) (4) (6). The blackfin icefish is a benthic predator, and an adult fish will typically sit motionless on the substrate for long periods of time while it waits for suitable prey species to pass. Juvenile icefish are more mobile, moving up and down the water column in search of krill (12).
Most icefish species spawn during the autumn and winter (3). The blackfin icefish is known to move inshore to spawn from February and March onwards, although this seasonal migration occurs later in the more southerly parts of its range (2) (4) (9). Spawning typically occurs between March and May around South Georgia, and April to May around the South Orkney Islands (9).
The blackfin icefish lays large, yolky, yellow-orange eggs (3) (7) (9), which are usually deposited as a flattened, rounded mass in a shallow, circular depression in the substrate (3) (9). It is thought that these nesting depressions are excavated and cleared of other benthic organisms by the adult blackfin icefish to protect the mass of eggs (9), and the male is known to tenaciously guard the eggs against predators and other potential threats (3) (9) (10). The incubation period of the eggs lasts for around two to three months in the north of the Southern Ocean, to more than six months in the more southern portions of the blackfin icefish’s range (3). At hatching, the larvae of the blackfin icefish are relatively large. The larvae grow around 6 to 10 centimetres each year during their juvenile pelagic phase, and will remain at sea until they reach maturity at around 5 to 8 years old (or 50 centimetres in length) (2) (3) (10).