The blackfin icefish (Chaenocephalus aceratus), also known as the Scotia Sea icefish, is a species of crocodile icefish belonging to the family Channichthyidae.[2] The blackfin icefish belongs to Notothenioidei, a suborder of fishes that accounts for 90% of the fish fauna on the Antarcticcontinental shelf.[3] 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.[4] Icefishes have translucent blood and creamy white gills.[5]
The blackfin icefish was first formally described as Chaenichthys aceratus in 1906 by the Swedish zoologistEinar Lönnberg with the type locality given as South Georgia Island.[6] In 1913 the EnglishichthyologistCharles 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.[7] 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 nameaceratus means “no horned” an allusion to the lack of a spine on the snout of this species in comparison to what was then thought to be its congenerChannichthys rhinoceratus.[8]
Morphology
The blackfin icefish has an elongated, tapered body with a relatively weakly ossified skeleton.[9] It lacks scales and has thin, highly vascularised skin. Its body structure makes it extremely vulnerable to injury.[9] Its head and snout are depressed with a single nostril on either side of the head, and a large mouth with small teeth.[9]
This species can range from grey to brown in colour and can have dark vertical stripes along its side.[9] 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.[5] This species is sexually dimorphic, with the males having longer and darker first dorsal fins than females.[10] At sexual maturation, females (48–49cm) are significantly larger than males (34–40cm), and spawning females are larger than non-spawning females.[10]
The ocean temperature in these regions usually remains within a few degrees of the freezing point of seawater, -2 °C (28 °F).[12] Consequently, the blackfin icefish is a stenothermalectotherm, meaning it has a narrow thermal tolerance range and a low upper thermal limit.[13] The Southern Ocean has a high oxygen content, which allows the blackfin icefish to survive without haemoglobin.[14]
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.[11]
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.[5] 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.
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.[5] Although the evolution of icefish is still disputed, it is widely accepted that the formation of the Antarctic Polar Frontal Zone (APFZ) and the Antarctic Circumpolar Current (ACC) marks the beginning of the evolution of Antarctic fishes.[3] The ACC is an oceanic current that moves in a clockwise northeast direction, and can be up to 10,000km 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.[3]
The various lifestyles of icefish are thought to have caused speciation within the family. For example, blackfin icefish adults have a very sedentary lifestyle, which may have geographically isolated them from other members of the family. Other species, such as Champsocephalus gunnari, are more pelagic and seem to have avoided isolation.[5]
Adaptation
The Southern Ocean is one of the most extreme habitats on the planet. Like most other Antarctic notothenioids, the blackfin icefish produces antifreeze glycoproteins in their blood and other body fluids.[15] These proteins reduce the internal freezing temperature, preventing ice crystallization and thus allowing the fish to survive in water below 0 °C.
In addition, icesfishes have developed a specialized cardiovascular system to compensate for the lack of hemoglobin and maximize oxygen delivery to tissues. Because icefish blood lacks erythrocytes, it is less viscous and can flow more easily around the body at low temperatures. [13] As a result, the amount of energy expended to transport oxygen is reduced.[16]
Icefish also possess an enlarged, hypertrophied heart, which allows the animal to pump very large volumes of blood at a 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.[5] The blackfin icefish also maintains very high concentrations of mitochondria in its cardiac musclecells[17] and thin, highly vascularized skin. All of these adaptations allow the blackfin icefish to maximize oxygen delivery and survive without haemoglobin.[13]
Diet
Blackfin icefish primarily eat smaller fish and krill, but have occasionally been found with crustaceans in their stomachs.[10] Younger icefish tend to eat krill, and then switch to mackerel icefish when they grow (about 30cm). From data collected in different locations, researchers have determined that the blackfin icefish likely feeds sporadically, consuming large quantities of fish and krill at a time, but at irregular intervals.[9] 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, the adult blackfin icefish usually works as an ambush predator, sitting on the substrate until prey swim past.[5]
Reproduction
The blackfin icefish takes between 5 and 7 years to reach reproductive age, and it expends a large amount of energy when it comes to reproducing and parenting. Females typically spawn large, slow-developing embryos that are up to 5 mm in diameter.[18] However, the large yolky eggs mean that females rarely produce over 10,000 eggs per spawning.[5] 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. This also makes males particularly vulnerable to being caught as trawling bycatch.[18]
Spawning appears to follow a latitudinal trend, with spawning primarily taking place between autumn and winter, but progressively later in the southernmost populations. 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 large amounts of energy and icefish are limited due metabolically to lack of haemoglobin, sexually mature females may skip a season of spawning if food has been scarce or of poor quality.[18] The incubation period can take 2 to 6 months to complete, depending on the latitude (more southerly regions have longer incubation periods). The larvae remain pelagic for 5 to 7 years until maturity, growing relatively quickly at about 6 to 10cm each year.[5]
Threats
Although blackfin icefish is not caught commercially in direct fisheries, it is often caught as bycatch when fishermen trawl for mackerel icefish, and the flesh is of excellent quality.[19][9] Even if the blackfin icefish is thrown back after being caught, it has a high mortality rate due to its fragile structure and vulnerability to stress. Additionally, its benthic behaviour makes it more vulnerable to being caught in trawling nets.[9] 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.[9]
Another major threat facing the blackfin icefish is climate change. The blackfin icefish can only survive within a very narrow temperature range.[13] This is due in part to the high dissolved oxygen content of ice-cold seawater, which it requires due to lack of haemoglobin. Rising sea temperatures pose a serious threat to this species, and scientists are studying blackfin icefish physiology to see how severely climate change will affect this species, particularly at embryonic and larval stages. [9][20]
Related Research Articles
Nototheniidae, the notothens or cod icefishes, is a family of ray-finned fishes, part of the suborder Notothenioidei which is traditionally placed within the order Perciformes. They are largely found in the Southern Ocean.
The crocodile icefish or white-blooded fish comprise a family (Channichthyidae) of notothenioid fish found in the Southern Ocean around Antarctica. They are the only known vertebrates to lack hemoglobin in their blood as adults. Icefish populations are known to reside in the Atlantic and Indian sectors of the Southern Ocean, as well as the continental shelf waters surrounding Antarctica. Water temperatures in these regions remain relatively stable, generally ranging from −1.8 to 2 °C. One icefish, Champsocephalus esox, is distributed north of the Antarctic Polar Frontal Zone. At least 16 species of crocodile icefish are currently recognized, although eight additional species have been proposed for the icefish genus Channichthys.
Notothenioidei is one of 19 suborders of the order Perciformes. The group is found mainly in Antarctic and Subantarctic waters, with some species ranging north to southern Australia and southern South America. Notothenioids constitute approximately 90% of the fish biomass in the continental shelf waters surrounding Antarctica.
The bald notothen, also known as the bald rockcod, is a species of marine ray-finned fish belonging to the family Nototheniidae, the notothens or cod icefishes. It is native to the Southern Ocean.
Chionodraco hamatus is a species of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. It is found in the Southern Ocean.
Champsocephalus is a genus of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. They are native to the Southern Ocean.
Channichthys is a genus of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. They are native to the Southern Ocean.
Chionodraco is a genus of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. They are found in the Southern Ocean.
Jonah's icefish is a benthopelagic species of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. It is the only member of the monotypic genus Neopagetopsis. It is found in the Southern Ocean at depths of from 20 to 900 metres. It has a circum-Antarctic distribution on the continental slope and continental shelf, with the northernmost records from the South Shetland and the South Orkney Islands.
Pseudochaenichthys is a monotypic genus of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. Its only member is Pseudochaenichthys georgianus, the South Georgia icefish, which is found in the Southern Ocean.
The ocellated icefish is a fish of the family Channichthyidae. It lives in the cold waters off Antarctica and is known for having transparent haemoglobin-free blood.
The emerald rockcod, also known as the emerald notothen is a species of marine ray-finned fish belonging to the family Nototheniidae, the notothens or cod icefishes. It is native to the Southern Ocean where it is a commercially important species.
Antarctic fish is a common name for a variety of fish that inhabit the Southern Ocean. There are relatively few families in this region, the most species-rich being the Liparidae (snailfishes), followed by Nototheniidae. The latter is one of eight different families that belong to the suborder Notothenioidei of the order Perciformes. They are also called notothenioids, but this name is also used to describe the other three, non-Antarctic families and some of the non-Antarctic genera in the mainly Antarctic families belonging to the suborder.
Notothenia coriiceps, also known as the black rockcod, Antarctic yellowbelly rockcod, or Antarctic bullhead notothen, is a species of marine ray-finned fish, belonging to the family Nototheniidae, the notothens or cod icefishes. It is widely spread around the Antarctic continent. Like other Antarctic notothenioid fishes, N. coriiceps evolved in the stable, ice-cold environment of the Southern Ocean. It is not currently targeted by commercial fisheries.
Cryodraco antarcticus, the long-fingered icefish, is a demersal species of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. It occurs only in deep waters of the Southern Ocean.
Channichthys richardsoni, the robust icefish, is a demersal species of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. It is endemic to the shelf waters of the Kerguelen Islands in the Southern Ocean. It lives at depths of 126–310 metres (413–1,017 ft).
Channichthys panticapaei, the charcoal icefish, is a species of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. It is endemic to the Kerguelen Islands area in the Southern Ocean. This demersal species is found at depths of 112–154 m (367–505 ft) and is sometimes caught in bottom trawls. However, it is not targeted and is of no interest to commercial fisheries at present. It is dark gray to uniformly blackish with long first to third dorsal fin spines. The charcoal icefish reaches lengths of up to 40.2 cm (15.8 in), with females generally being smaller. This planktivorous (plankton-eating) species likely spawns from June to July, with maturity reached at 30 cm TL / 27 cm SL.
Chionodraco myersi, the Myers's icefish, is a species of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. It is found in the Southern Ocean.
Cryodraco atkinsoni is a bathydemersal species of marine ray-finned fish belonging to the family Channichthyidae, the crocodile icefishes. It is endemic to the waters of the eastern part of the Southern Ocean.
Gobionotothen gibberifrons, the humped rockcod or the humphead notothen, is a species of marine ray-finned fish belonging to the family Nototheniidae, the notothens or cod icefishes. It is native to the islands of the Scotia Arc, the northern part of the Antarctic Peninsula, and Heard Island in the Southern Ocean. This species inhabits depths of 6-429 m, but is most abundant at depths of 100-400 m, at least around Elephant Island.
1 2 3 Eastman, Joseph (1993). Antarctic Fish Biology: Evolution in a Unique Environment. Academic Press, Inc.. San Diego, California.
↑ Holeton, George (30 September 1969). "Oxygen uptake and circulation by a hemoglobinless antarctic fish (Chaenocephalus aceratus Lonnberg) compared with three red-blooded antartic fish". Comparative Biochemistry and Physiology. 34 (2): 457–471. doi:10.1016/0010-406X(70)90185-4. PMID5426570.
1 2 3 4 5 6 7 8 9 Le Francois, Nathalie; Sheehan, Eileen; Desvignes, Thomas; Belzile, Claude; Postelthwait, John; Dietrich II, William (12 July 2017). "Characterization and husbandry of wild broodstock of the blackfin icefish Chaenocephalus aceratus (Lönnberg 1906) from the Palmer Archipelago (Southern Ocean) for breeding purposes". Polar Biology. 40 (12): 2499–2516. doi:10.1007/s00300-017-2161-9. S2CID27157380.
1 2 3 Dietrich III, William; Jones, Christopher; Kim, Stacy; North, Anthony; Thurber, Andrew; Vacchi, Marino (29 July 2005). "Nesting behavior of the icefish Chaenocephalus aceratus at Bouvetøya Island, Southern Ocean". Polar Biology. 28 (11): 828–832. doi:10.1007/s00300-005-0010-8. S2CID38630559.
↑ Urschel, Matthew; O'Brien, Kristin M. (2008). "High mitochondrial densities in the hearts of Antarctic icefishes are maintained by an increase in mitochondrial size rather than mitochondrial biogenesis". Journal of Experimental Biology. 211 (Pt 16): 2638–2646. doi:10.1242/jeb.018598. PMID18689417.
↑ Somero, GN (2009). "The physiology of climate change: how potentials for acclimatization and genetic adaptation will determine 'winners' and 'losers'". Journal of Experimental Biology. 213 (6): 912–920. doi:10.1242/jeb.037473. PMID20190116.
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