Chaenocephalus aceratus

Chaenocephalus aceratus
Conservation status
Not evaluated (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Perciformes
Family:	Channichthyidae
Genus:	Chaenocephalus Regan, 1913
Species:	C. aceratus
Binomial name
Chaenocephalus aceratus (Lönnberg, 1906)

Chaenocephalus aceratus, commonly known as the blackfin or Scotia Sea icefish, are a species of crocodile icefish belonging to the family Chaenichthyidae. Blackfin icefish are notothenioids, a suborder of fish mainly found in the Antarctic which have evolved antifreeze glycoproteins to survive the cold temperatures of the Southern Ocean.^[1] Icefish, also called white-blooded fish, 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.^[2] The absence of this respiratory pigment means chaenichthyids must have behavioural and physiological adaptations to obtain sufficient oxygen. Additionally, without the iron molecules found in haemoglobin, icefish have translucent blood and creamy white gills.^[3]

Morphology

Blackfin icefish have elongated, tapered bodies with relatively weak skeletons. They also lack scales and have thin, highly vascularised skin. Their body structure makes them vulnerable to injury from trawling, with very high mortality rates when they are caught as by-catch.^[4] Their heads and snouts are depressed with a single nostril, and a large mouth with small teeth. They range from grey to brown in colour, and have dark vertical stripes along their sides. They have two dorsal fins and thickened skin on their pelvic fins, which is thought to be an adaptation to allow them to rest on the ocean substrate since they lack swim bladders to help keep them buoyant.^[5] Blackfin icefish are sexually dimorphic, with the males having taller 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.^[6]

Habitat

Chaenocephalus aceratus are found in the Southern Ocean around Antarctica and the tip of South America. Most research on blackfin icefish has been carried out around the Scotia Arc, including South Georgia, the South Shetland Islands, Bouvet Island, the South Orkney Islands, and the Antarctic Peninsula.^[7] The ocean temperature in these regions usually remains within a few degrees of the freezing point, allowing blackfin icefish to be stenothermal ectotherms, only being able to survive in a narrow range of temperatures (-1.9 to +2.0 °C).^[8] The low temperatures mean the water has a high oxygen content, requiring less water to be pumped over the gills in order to obtain oxygen. This helps to understand how the fish can survive without haemoglobin. The cold waters around Antarctica also mean there is a lot of productivity, so the fish don’t likely have to travel far or expend much energy in order to feed. Blackfin icefish are typically found at depths of 0-770m, 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 behaviour.^[9]

Evolution

There are 15 known species of icefish, and they are thought to make up a monophyletic group.^[10] It is still unknown when the icefish evolved, but there are two main competing hypotheses. The first is that they are only about 6 million years old, appearing after the Southern Ocean cooled significantly. The second predicts that they are much older, evolving 15-20 million years ago, after the formation of the Antarctic Circumpolar Current.3 Although the evolution of icefish is still disputed, it is widely accepted that the formation of the Antarctic Polar Frontal Zone (APFZ) marks the beginning of the evolution of Antarctic fishes. The APFZ is an oceanic current that moves in a clockwise northeast direction, and can be up to 10,000 km wide. This current formed 25-22 million years ago, and thermally isolated the Southern Ocean by separating it from the warm subtropical gyres to the north. The various lifestyles of icefish are thought to have caused speciation within the family. For example, C. aceratus adults have a very sedentary lifestyle which could have geographically isolated them from other members of the family. Other species, on the other hand, are more pelagic and seem to have avoided speciation.^[11]

Diet

Blackfin icefish primarily eat fish and krill, but have occasionally been found with crustaceans in their stomachs.^[12] Younger icefish tend to eat krill, and then switch to mackerel icefish when they get bigger (about 30 cm). From data collected in different locations, researchers have determined that Chaenocephalus aceratus probably feed sporadically, consuming large quantities of fish and krill at a time, but at irregular intervals.^[13] Their lack of haemoglobin supports this behaviour because burst activity would allow them to obtain energy anaerobically, reducing the need for oxygen. While larvae and juveniles are more active in the water column searching for krill, adult blackfin icefish usually work as ambush predators, sitting on the substrate until prey swim by.^[14]

Reproduction

C. aceratus take between 5 and 7 years to reach reproductive age, and expend a lot of energy when it comes to reproducing and parenting. Females typically spawn large, slow-developing embryos that are up to 5mm in diameter.^[15] However, the large yolky eggs mean that females rarely produce over 10,000 eggs per spawning.^[16] Males dig shallow, circular depressions on the seafloor and clear the surrounding area of debris for a female to spawn her eggs. The embryos are at high risk of predation because they are demersal, so males spend the months between spawning and hatching ferociously guarding the eggs. However, this also makes males particularly vulnerable to being caught as trawling by-catch. Spawning time seems to follow a latitudinal trend, with spawning primarily taking place between autumn and winter, but progressively later the closer to Antarctica the fish are.^[17] During spawning season, blackfin icefish tend to migrate closer to shore. Female blackfin icefish are total spawners with determinate fecundity, and typically spawn every year. However, because reproduction requires a lot of energy and icefish are limited by their lack of haemoglobin, sexually mature females may skip a season of spawning if food has been scarce or of poor quality. The incubation period can be between 2 and 6 months depending on the latitude (more southerly regions have longer incubation periods), and the larvae remain pelagic for 5–7 years until maturity, growing relatively quickly at about 6–10 cm each year.^[18]

Adaptations

The Southern Ocean is one of the most extreme habitats on the planet, so to survive there Chaenocephalus aceratus have had to adapt behavioural and physiological methods to survive. While the cold temperatures allow icefish to survive without haemoglobin, the fish also have to be able to not freeze. The presence of antifreeze proteins in the blood lowers its freezing temperature, allowing the fish to survive in water that’s below 0 °C. Other adaptations to deal with their harsh environment include an enlarged ventricle, vascularised skin to absorb oxygen, and thinner blood for easier flow around the body. Even though icefish lack haemoglobin, they have retained some erythrocytes, as these contain essential enzymes. However, having a reduced number of erythrocytes also allows the blood to be less viscous.^[19] This means blood can flow more easily around the body at low temperatures, reducing the amount of energy expended, as well as transporting oxygen more efficiently. Icefish contain a hypertrophied heart which allows it to pump very large volumes of blood, but at low pressure and speed. The stroke volume of the icefish heart is 6-15 times that of other teleosts, and the blood volume in general is 2-4 times as large as other teleosts.^[20] C. aceratus also have very high concentrations of mitochondria in their cardiac muscle cells. All of these adaptations allow icefish to survive in low temperatures and without haemoglobin.

Threats

Although blackfin icefish are not caught commercially, they are often caught as by-catch when fishermen trawl for mackerel icefish. Even if Chaenocephalus aceratus are thrown back after being caught, they have a high mortality rate because of their sensitivity to environmental change. Additionally, their benthic behaviour makes them more vulnerable to being caught in trawling nets. Researchers are currently working to understand how marine protected areas can be created for this species, and the best ways to handle them to reduce stress and injury.^[21] Another major threat facing Chaenocephalus aceratus is climate change. Blackfin icefish can only survive within a very narrow temperature range due to dissolved oxygen levels. Increasing sea temperatures pose a serious threat to this species, and scientists are studying their physiology to see how severely climate change will affect them, especially at embryonic and larval stages.^[22]

References