The Coelacanths (Coelacanthimorpha) are lobed-fin fish (sarcopterigios) that were believed extinct. Discover today everything about this amazing species.
Table Of Content
- 1 Taxonomy
- 2 Review
- 3 The Coelacanths’ Discovery
- 4 The Coelacanths’ Main characteristics
- 5 Adaptation to Ground
- 6 Closer Living fish to the Tetrapods
- 7 Immunoglobulins
- 8 The Coelacanths’ Reproduction
- 9 Behavior
- 10 Types of Coelacanths
- 11 Are Coelacanths Threatened?
- 12 Uncertain Future
Animalia Kingdom Phylum: Chordata Class: Sarcopterygii Subclass: Coelacanthimorpha Order: Coelacanthiformes
In 1938 a live specimen was captured on the east coast of South Africa. In 1998 another specimen was located on the island of Celebes (Indonesia). Together with the lungfish, coelacanths are the closest living marine creatures of terrestrial vertebrates. They appeared in the Devonian period (400 million years ago), although the largest amount of fossilized remains belong to the Carboniferous period (350 million years ago).
Only two species of Coelacanths are known: one that lives near the Comores Islands, (east coast of Africa), and another found in the waters of Sulawesi, in Indonesia. Many experts believe that the coelacanth’s unique and particular characteristics, show one of the initial stages of the evolution of fish to terrestrial four-legged animals, such as amphibians.
The Coelacanths’ Discovery
The fossil record has numerous examples of species that once populated the planet and ended up becoming extinct, either because their environment was destroyed by, for example, a planetary catastrophe or because new, better adapted species occupied their ecological niches. This is the norm imposed by natural selection and only the so-called «living fossils», that is, species that survive from remote times, are capable of avoiding it.
The most representative example of these exceptions to the general rule are the Coelacanths. According to the fossil record, this type of fish appeared in the Devonian (400 million years ago) and reached its maximum splendor in the Carboniferous (250 million years ago). From the Upper Cretaceous (65 million years ago) no coelacanth fossil is known, so it was believed that this type of fish had become extinct.
However, on December 22, 1938, Marjorie Courtenay-Latimer, curator of the local museum, discovered among fish unloaded in the docks of the city of East London (South Africa), a fish whose most distinctive feature was the possession of lobed fins (Normal fish have fins with radii). The striking resemblance of this fish with the coelacanth fossils allowed its quick identification. The second specimen of Coelacanth appeared at the end of 1952 in the Comoros Islands, located in the Indian Ocean, between Madagascar and Mozambique.
The appearance of new specimens in successive years confirmed the existence of a stable population of the species in the archipelago. Until 1998, it was thought that this was the only population of Coelacanths existing in the world. Again, the scientific world was surprised when Mark V. Erdmann, a biologist at the University of California at Berkeley, demonstrated the existence of Coelacanths in Manado Tua, one of the Celebes Islands (Sulawesi, Indonesia).
The Celebes Islands are almost 10,000 kilometers away from the Comoros Islands. Therefore, this discovery considerably extended the geographical distribution of the coelacanth, which could well have several populations throughout the Indian Ocean or even in other seas.
The Coelacanths’ Main characteristics
The most striking feature of this «living fossil» are its pairs of lobed fins that extend from its body outwards as if they were legs and move alternately, like those of a horse trotting.
Other of its unique characteristics include an intercranial joint that allows the mouth to swallow large prey, a canal called notocord filled with an oily liquid, which functions as bone marrow, thick scales that had only been seen in extinct; and a facial electro-sensor organ located in its snout and possibly used to detect its prey.
The coelacanth is an elusive abyssal creature that lives at depths of up to 700 meters. It can reach a huge size of more than 2 meters in length and 90 kilograms in weight. Scientists estimate that they can live 60 years or more.
It isn’t known how many specimens its population rises, although the studies carried out in the Comoros indicate that only about 1000 specimens could remain in those waters. Thus, it is considered an endangered species.
Adaptation to Ground
One of the great milestones of evolution is the water-earth transition process, that is, the first organisms capable of colonizing the earth. It is estimated that this event occurred in the Devonian period, approximately 400 million years ago, and to carry out this transition there must be changes at the metabolic level, such as the excretion of urea, and morphological changes, such as the development of limbs.
In regard to the morphological changes that occurred in this geological period, two hypotheses have been proposed in this regard. The first one is that it was a new feature of the tetrapods. The second conjecture would be the development of fins from a lobed fish ancestor. Darwin in his 1859 work, The Origin of Species, already emphasized the morphological similarities between the various vertebrates:
«What can be more curious than the hand of man made to take; that of the mole, made for mining; the horse leg, the fin of the porpoise and the wing of a bat, are all built according to the same pattern and enclose similar bones in the same relative positions?” Charles Darwin, the Origin of Species (1859), Chap. XIV: Mutual affinities of organic beings-Morphology.
The coelacanths have fins with a bony structure that could be the precursor of the current tips of the tetrapods. The analysis of CNEs (conserved non-coding elements) in the coelacanth in the HOX-D cluster revealed the presence of regulatory sequences shared between tetrapods and coelacanths, but not in the rest of the fish. Noting that these sequences have been maintained evolutionarily to control the expression of the genes involved in the development of the extremities of the tetrapods.
Closer Living fish to the Tetrapods
Determining which living fish is evolutionarily closer to the tetrapods is an idea that has disturbed the minds of all the scientists dedicated to this subject. The construction of phylogenetic trees based on the alignment of gene sequences points to the lungfish as the live fish with greater kinship with the tetrapods, discarding the coelacanths as a possible candidate.
Immunoglobulins play an important role in the adaptive immune response as a receptor for vertebrate B cells. Much of the vertebrates synthesize Ig M. However, the coelacanths don’t produce Ig M, since their genome codes for an immunoglobulin called Ig W. It is thought that these immunoglobulins could play a role similar to IgM.
The Coelacanths’ Reproduction
The Coelacanths carry out an oviparous reproduction with internal fertilization, although their reproductive behavior is not totally known yet. However, it is believed that sexual maturity does not occur before age 20. The gestation period is around 13 months after which the female gives birth between 5 and 25 quite developed pups, capable of surviving, over which no type of parental care is carried out.
They are predatory fish. During the day they live in caves located in deep zones (from 150 to 300 m), going up at night to the surface, where they feed on reef fish.
Types of Coelacanths
Let’s learn about the two species of coelacanhts that exist
The coelacanth of Comores measures between 1.5 and 1.8 meters and weighs between 65 and 98 kilograms. It presents a black coloration, (a little bluish), with white spots and slightly pink.
In spite of being a sarcopterigi fish, with a muscular base of the fins, the Latimeria chalumnae does not crawl along the marine bottom, but swims slowly. Its tail is trilobed, wich is an unusual feature. Their scales are hard, big and thick and it has a second dorsal fin near the tail.
As for Its head, it is large, with a wide mouth and large black eyes. The skull has hinge, the swim bladder is not functional and is full of fat. The females are larger than the males (sexual dimorphism).
Biology and behavior
This coelacanth inhabits rocky caves and volcanic bottoms, between 70 and 700 meters deep. It is a carnivorous fish, which hunts by stalking its prey.
Although the females give birth to their young, they actually incubate the eggs inside their bodies, that is, they are ovoviviparous. They are large eggs, (the largest of all fish), with 9 centimeters in diameter. The gestation period is unknown, although it probably lasts more than a year. The females live up to 48 years.
The coelacanth (Latimeria chalumnae) is a «living fossil» that was believed to be extinct from the time of the dinosaurs until the first rediscovery observed scientifically in 1938. This surprising specimen was dubbed the «most important zoological discovery of the century», species is a member of an ancient lineage that has existed for more than 360 million years
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Indonesian coelacanth (Latimeria menadoensis)
The Indonesian coelacanth (Latimeria menadoensis) is one of the two living species of coelacanth; was described in 1999. The species is listed as vulnerable on the IUCN Red List. The other known species, the coelacanth of Comores, Latimeria chalumnae, is considered critically endangered.
On September 18, 1997, a strange-looking fish was observed at a market in Manado Tua, on the island of Celebes, Indonesia, which was thought to be a coelacanth of Comores; however, it differed from this by its brown color at the back. A second specimen, 1.2 m long and weighing 29 kg, was captured alive on July 30, 1998.
It remained alive for six hours allowing it to be photographed and its color, fin movement and general behavior documented. The specimen was preserved and donated to the Bogor Zoology Museum.
The DNA tests revealed that the specimen differed genetically from the Comores population. In appearance, the Indonesian celacanths, locally called raja laut («king of the sea»), are similar to the Comorian coelacanths, except for the brownish-grayish dorsal coloration rather than bluish hue.
The species was formally described in 1999 by Pouyaud et al. in the publication Comptes Rendus de l’Académie des sciences Paris with the binomial name Latimeria menadoensis. In 2005, a study of mitochondrial DNA estimated the separation of the two species between 30 and 40 million years ago.
Let’s keep learning
Are Coelacanths Threatened?
The coelacanths are threatened by the expansion of oil exploration areas, warn environmental experts. The manager of the group Wildlands Conservation Trust, Andrew Venter, points out that the only colony on the Comoros Islands that is 40 kilometers from the northern limit of the exploration area of an oil company.
The drilling is currently 200 kilometers from its habitat, but the oil fields are expanding quickly. “If there is an oil spill in that area in the east of South Africa, it is very likely that it will kill the coelacanths”. Says Venter
The Ichthyologist Mike Bruton shared this concern because the fish, he said, “are creatures sensitive to environmental disturbance.» Anything that interferes with the coelacanths’ ability to absorb oxygen, such as oil contamination, would threaten their survival, «he warned.
He also said that a special danger comes from the works of the oil company in block ER236, which is closest to the canyon where the celacanths are located. «The risk must be meticulously evaluated before the commercial project has gone too far and it is too late,» the expert defended.
The coelacanths’ future is uncertain. These fish, which survived the massive extinctions that have occurred in the last 400 million years, can disappear because of man. However, we are still in time to avoid their extinction. Strict protection of the coasts of the Comoros Islands could stop the decline of this population.
In addition, the discovery of the Indonesian population is a relief for the conservation of the species. However, it is necessary to take concrete measures to prevent the pillage and decline that occurred in the Comoros Islands from occurring again. Hopefully, this living fossil can continue to defy extinction for a few million years more.