Blackfin icefish  (Chaenocephalus aceratus)

The blackfin icefish has not yet been assessed by the IUCN.

An intriguing and almost prehistoric-looking fish, the blackfin icefish (Chaenocephalus aceratus) belongs to the unique Channichthyidae family. Also known as ‘icefish’, the species within this family are remarkable among vertebrates in that their blood lacks the oxygen-binding pigment haemoglobin (3) (4) (5) (6) (7). In most vertebrate species, haemoglobin in the red blood cells carries oxygen from the respiratory organs (such as the lungs or gills) to the rest of the body. Haemoglobin also usually contains iron molecules, giving vertebrate blood its characteristic red colour. The lack of haemoglobin in icefish means that these unusual species have translucent blood, and that their gills are creamy-white (2) (3).

The blackfin icefish is one of the most dominant fish species in Antarctica (4), inhabiting a unique marine environment that became isolated from the rest of the world’s oceans approximately 25 million years ago (5). It has a robust and somewhat elongated body, with a large, slightly flattened head and a fairly long, broad snout which is wider and more rounded at the end (2) (3). The blackfin icefish’s mouth is relatively large and is equipped with small teeth for grasping its prey (3).

This peculiar species’ body is entirely devoid of scales and is generally greyish or pale brown, becoming whitish underneath, with a series of crossbars along its sides. It has two blackish dorsal fins, the first of which has a number of long, flexible spines. The paler pectoral fins are long and fan-like (2). Species within the icefish family do not have the swim bladders that most fish use to regulate buoyancy, and therefore most species are primarily bottom-dwelling (7). Perhaps as a result, the blackfin icefish has developed thickened skin on its pelvic fins, which are used when the fish rests on the substrate (3).  

The male blackfin icefish grows slower and is generally much smaller than the female (3).

Found only in the Antarctic, the blackfin icefish occurs around Bouvet Island and in the Scotia Sea around South Georgia and the South Sandwich Islands, the South Orkney Islands and the South Shetland Islands. It also occurs in the northern part of the Antarctic Peninsula (2) (3) (8) (9) (10). 

The blackfin icefish is found predominantly in Antarctic shelf waters to depths of 450 metres, although some individuals occur down to depths of 770 metres (1) (2) (4) (9).

Adult blackfin icefish are mostly bottom-dwelling, but mature adults are known to move inwards to nearshore waters and fjords to spawn (2) (3) (4). The larvae and juveniles of this species are pelagic, typically inhabiting surface waters close to the shore (2).

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).

The blackfin icefish is caught in relatively low numbers as bycatch in the mackerel icefish (Champsocephalus gunnari) and Antarctic krill (Euphausia superba) commercial fisheries (3) (4) (12). Despite limitations on the levels of bycatch, the blackfin icefish remains vulnerable to exploitation at different stages throughout its life cycle (4).

This species may be adversely affected by changes in climatic and oceanographic conditions. It is thought that icefish in general are particularly sensitive to habitat changes due to their highly specialised biology, which allows them to cope with the harsh Antarctic environment (3). This is particularly true of icefish species living at higher latitudes close to their upper limits of thermal tolerance (3) (12), and is something which may affect the blackfin icefish if climate change continues to lead to rising sea temperatures in the Antarctic region.

The Commission of the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) has prohibited direct fishing of the blackfin icefish in Antarctica, and has limited levels of annual bycatch of this species to 2,200 tonnes (13). CCAMLR’s ecosystem approach to management aims to ensure that all Antarctic fish stocks are managed sustainably, taking into account the impact on non-target and dependent species in setting catch levels for commercial fisheries (4).

Studies on the nesting behaviour of the blackfin icefish have resulted in recommendations to map the spawning territories of this species and to protect its spawning areas as marine sanctuaries (9).

Find out more about the blackfin icefish and other Antarctic fish:

• Hureau, J-C. (1985) Channichthyidae. In: Fischer, W. and Hureau, J-C. (Eds.) FAO species identification sheets for fishery purposes. Southern Ocean (Fishing areas 48, 58 and 88). Food and Agriculture Organisation of the United Nations, Rome. Available at:
  http://www.fao.org/docrep/010/ah842e/ah842e00.HTM
• Kock, K-H. (2005) Antarctic icefishes (Channichthyidae): a unique family of fishes. A review, Part I. Polar Biology, 28: 862-895.

Find out more about the Antarctic:

• British Antarctic Survey:
  http://www.antarctica.ac.uk/
• Discovering Antarctica:
  http://www.discoveringantarctica.org.uk/

Visit ARKive’s Antarctic ecoregion page:

• ARKive - Antarctic:
  http://www.arkive.org/eco-regions/antarctic/