The blackfin icefish (Chaenocephalus aceratus), also known as the Scotia Sea icefish, is a species of crocodile icefish belonging to the family Channichthyidae. The blackfin icefish belongs to Notothenioidei, a suborder of fishes that accounts for 90% of the fish fauna on the Antarctic continental shelf. Icefishes, also called white-blooded fishes, are a unique family in that they are the only known vertebrates to lack haemoglobin, making their blood oxygen carrying capacity just 10% that of other teleosts. Icefishes have translucent blood and creamy white gills.
Taxonomy
The blackfin icefish was first formally described as Chaenichthys aceratus in 1906 by the Swedish zoologist Einar Lönnberg with the type locality given as South Georgia Island. In 1913 the English ichthyologist Charles Tate Regan described a new subgenus of Channichthys, Chaenocephalus with the blackfin icefish as its only species, its type species by monotypy. Chaenocephalus is now generally regarded as a valid monotypic genus. the genus name Chaenocephalus is a compound of chaeno meaning “gape”, a possible reference to the wide gape which is characteristic of the family Channichthyidae as well as to Chaenichthys, an alternative spelling of Channichthys, and cephalus which means “head”, so the name means “gaping head”. The specific name aceratus means “no horned” an allusion to the lack of a spine on the snout of this species in comparison to whar was then thought to be its congener Channichthys rhinoceratus.
Morphology
The blackfin icefish has an elongated, tapered body with a relatively weakly ossified skeleton. It lacks scales and has thin, highly vascularised skin. Its body structure makes it extremely vulnerable to injury. Its head and snout are depressed with a single nostril on either side of the head, and a large mouth with small teeth.
This species can range from grey to brown in colour and can have dark vertical stripes along its side. It has two dorsal fins and thickened skin on the pelvic fins, which is thought to be an adaptation to allow the fish to rest on the ocean substrate, as it lacks a swim bladder to maintain buoyancy. This species is sexually dimorphic, with the males having longer and darker first dorsal fins than females. At sexual maturation, females (48–49 cm) are significantly larger than males (34–40 cm), and spawning females are larger than non-spawning females.
Habitat
The blackfin icefish is known to reside in the Southern Ocean around Antarctica. Most research on the blackfin icefish has been carried out around the Scotia Arc, encompassing South Georgia, the South Shetland Islands, Bouvet Island, the South Orkney Islands, and the Antarctic Peninsula.
The ocean temperature in these regions usually remains within a few degrees of the freezing point of seawater, -2 °C (28 °F). Consequently, the blackfin icefish is a stenothermal ectotherm, meaning it has a narrow thermal tolerance range and a low upper thermal limit. The Southern Ocean has a high oxygen content, which allows the blackfin icefish to survive without haemoglobin.
Blackfin icefish are typically found at depths of 0-770 m, depending on their life stage. Larvae and juveniles tend to be more active in the water column but are typically found closer to the shore, while adults more often exhibit benthic swimming behaviour.
Evolution
There are at least 16 known species of icefish, and they are thought to make up a monophyletic group. The icefishes likely descended from a sluggish demersal ancestor. During the mid-Tertiary period, a species crash in the Southern Ocean opened up a wide range of empty niches to colonize. Despite the hemoglobin-less mutants being less fit, the lack of competition allowed even the mutants to leave descendants that colonized empty habitats and evolved compensations for their mutations. Later, the periodic openings of fjords created habitats that were colonized by a few individuals. These conditions may have also allowed for the loss of myoglobin.