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| Coelacanth | |
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| Live coelacanth seen off Pumula on the KwaZulu-Natal South Coast, South Africa, 2019 | |
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| Specimen of Axelrodichthys araripensis from the Early Cretaceous of Brazil (Mawsoniidae) | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sarcopterygii |
| Class: | Actinistia Cope, 1871 |
| Order: | Coelacanthiformes Huxley, 1861 |
| Type species | |
| †Coelacanthus granulatus Agassiz, 1839
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| Families and genera | |
Others, see text | |
Coelacanths (/ˈsiːləkænθ/ SEE-lə-kanth) (order Coelacanthiformes) are an ancient group of lobe-finned fish (Sarcopterygii) in the class Actinistia.[2][3] As sarcopterygians, they are more closely related to lungfish and tetrapods (which includes amphibians, reptiles, birds and mammals) than to ray-finned fish.
Well-represented in both freshwater and marine fossils since the Devonian, they are now represented by only two extant marine species in the genus Latimeria: the West Indian Ocean coelacanth (Latimeria chalumnae), primarily found near the Comoro Islands off the east coast of Africa, and the Indonesian coelacanth (Latimeria menadoensis).[4] The name coelacanth originates from the Permian genus Coelacanthus, which was the first scientifically named coelacanth.[5]
The oldest known coelacanth fossils date back more than 410 million years. Coelacanths were thought to have become extinct in the Late Cretaceous, around 66 million years ago, but were discovered living off the coast of South Africa in 1938.[6][7]
The coelacanth was long considered a "living fossil" because scientists thought it was the sole remaining member of a taxon otherwise known only from fossils, with no close relations alive,[8] and that it evolved into roughly its current form approximately 400 million years ago.[1] However, several more recent studies have shown that coelacanth body shapes are much more diverse than previously thought.[9][10][11]
Etymology
The word Coelacanth is an adaptation of the Modern Latin Cœlacanthus ('hollow spine'), from the Greek κοῖλ-ος (koilos, 'hollow') and ἄκανθ-α (akantha, 'spine'),[12] referring to the hollow caudal fin rays of the first fossil specimen described and named by Louis Agassiz in 1839, belonging to the genus Coelacanthus.[8] The genus name Latimeria commemorates Marjorie Courtenay-Latimer, who discovered the first specimen.[13]
Discovery
The earliest fossils of coelacanths were discovered in the 19th century. Coelacanths, which are related to lungfishes and tetrapods, were believed to have become extinct at the end of the Cretaceous period.[14] More closely related to tetrapods than to the ray-finned fish, coelacanths were considered transitional species between fish and tetrapods.[15] On 23 December 1938, the first Latimeria specimen was found off the east coast of South Africa, off the Chalumna River (now Tyolomnqa).[6] Museum curator Marjorie Courtenay-Latimer discovered the fish among the catch of a local fisherman.[6] Courtenay-Latimer contacted a Rhodes University ichthyologist, J. L. B. Smith, sending him drawings of the fish, and he confirmed the fish's importance with a famous cable: "Most Important Preserve Skeleton and Gills = Fish Described."[6]
Its discovery 66 million years after its supposed extinction makes the coelacanth the best-known example of a Lazarus taxon, an evolutionary line that seems to have disappeared from the fossil record only to reappear much later. Since 1938, West Indian Ocean coelacanth have been found in the Comoros, Kenya, Tanzania, Mozambique, Madagascar, in iSimangaliso Wetland Park, and off the South Coast of Kwazulu-Natal in South Africa.[16][17]
The Comoro Islands specimen was discovered in December 1952.[18] Between 1938 and 1975, 84 specimens were caught and recorded.[19]
The second extant species, the Indonesian coelacanth, was described from Manado, North Sulawesi, Indonesia, in 1999 by Pouyaud et al.[20] based on a specimen discovered by Mark V. Erdmann in 1998[21] and deposited at the Indonesian Institute of Sciences (LIPI).[22] Erdmann and his wife Arnaz Mehta first encountered a specimen at a local market in September 1997, but took only a few photographs of the first specimen of this species before it was sold. After confirming that it was a unique discovery, Erdmann returned to Sulawesi in November 1997 to interview fishermen and look for further examples. A second specimen was caught by a fisherman in July 1998 and was then handed to Erdmann.[23][24]
Description
Latimeria chalumnae and L. menadoensis are the only two known living coelacanth species.[8][25] Coelacanths are large, plump, lobe-finned fish that can grow to more than 2 m (6.6 ft) and weigh around 90 kg (200 lb).[26] They are estimated to live up to 100 years, based on analysis of annual growth marks on scales, and reach maturity around the age of 55;[27] the oldest known specimen was 84 years old at the time of its capture in 1960.[28] Even though their estimated lifetime is similar to humans, gestation can last 5 years, which is 1.5 years more than the deep-sea frilled shark, the previous record holder.[29]
They are nocturnal piscivorous drift-hunters.[30]
The body is covered in ctenoid elasmoid scales that act as armor.[31] Coelacanths have eight fins – two dorsal fins, two pectoral fins, two pelvic fins, one anal fin and one caudal fin. The tail is very nearly equally proportioned and is split by a terminal tuft of fin rays that make up its caudal lobe. The eyes of the coelacanth are very large, while the mouth is very small.[citation needed] The eye is acclimatized to seeing in poor light by rods that absorb mostly short wavelengths. Coelacanth vision has evolved to a mainly blue-shifted color capacity.[32] Pseudomaxillary folds surround the mouth and replace the maxilla, a structure absent in coelacanths. Two nostrils, along with four other external openings, appear between the premaxilla and lateral rostral bones. The nasal sacs resemble those of many other fish and do not contain an internal nostril. The coelacanth's rostral organ, contained within the ethmoid region of the braincase, has three unguarded openings into the environment and is used as a part of the coelacanth's laterosensory system.[8] The coelacanth's auditory reception is mediated by its inner ear, which is very similar to that of tetrapods and is classified as being a basilar papilla.[33]
Coelacanths are a part of the clade Sarcopterygii, or the lobe-finned fishes. Externally, several characteristics distinguish coelacanths from other lobe-finned fish. They possess a three-lobed caudal fin, also called a trilobate fin or a diphycercal tail. A secondary tail extending past the primary tail separates the upper and lower halves of the coelacanth. Ctenoid elasmoid scales act as thick armor to protect the coelacanth's exterior. Several internal traits also aid in differentiating coelacanths from other lobe-finned fish. At the back of the skull, the coelacanth possesses a hinge, the intracranial joint, which allows it to open its mouth extremely wide. Coelacanths also retain an oil-filled notochord, a hollow, pressurized tube which is replaced by a vertebral column early in embryonic development in most other vertebrates.[34] The coelacanth's heart is shaped differently from that of most modern fish, with its chambers arranged in a straight tube. The coelacanth's braincase is 98.5% filled with fat; only 1.5% of the braincase contains brain tissue. The cheeks of the coelacanth are unique because the opercular bone is very small and holds a large soft-tissue opercular flap. A spiracular chamber is present, but the spiracle is closed and never opens during development.[35] [8][36] Also unique to extant coelacanths is the presence of a "fatty lung" or a fat-filled single-lobed vestigial lung, homologous to other fishes' swim bladders. The parallel development of a fatty organ for buoyancy control suggests a unique specialization for deep-water habitats. There are small and hard but flexible plates around the vestigial lung in adult specimens, though not around the fatty organ. The plates most likely had a regulation function for the volume of the lung.[37] Due to the size of the fatty organ, researchers assume that it is responsible for the kidney's unusual relocation. The two kidneys, which are fused into one,[38] are located ventrally within the abdominal cavity, posterior to the cloaca.[39]
DNA
In 2013, a research group published the genome sequence of the coelacanth in the scientific journal Nature.[40]
Due to their lobed fins and other features, it was once hypothesized that the coelacanth might be the most recent shared ancestor between terrestrial and marine vertebrates.[33][41] But after sequencing the full genome of the coelacanth, it was discovered that the lungfish is the most recent shared ancestor. Coelacanths and lungfish had already diverged from a common ancestor before the lungfish made the transition to land.[42]
Another important discovery made from the genome sequencing is that the coelacanths are still evolving today. While phenotypic similarity between extant and extinct coelacanths suggests there is limited evolutionary pressure on these organisms to undergo morphological divergence, they are undergoing measurable genetic divergence. Despite prior studies showing that protein coding regions are undergoing evolution at a substitution rate much lower than other tetrapods (consistent with phenotypic stasis observed between extant and fossil members of the taxa), the non-coding regions subject to higher transposable element activity show marked divergence even between the two extant coelacanth species.[40] This has been facilitated in part by a coelacanth-specific endogenous retrovirus of the Epsilon retrovirus family.[43]
Taxonomy
Cladogram showing the relationships of coelacanth genera after Torino, Soto and Perea, 2021.[44]