Biology

The coelacanth, a living fossil

According to the theory of evolution, all species have their origin in pre-existing organisms, from which they differ by accumulation of changes in successive generations. Today we know that the first vertebrates originated in the marine environment and are related to sea ​​squirts , which have a primitive dorsal nerve cord.

The most accepted theory about the passage from the marine environment to the terrestrial environment is that the first tetrapods (four-legged terrestrial vertebrates) were related to some species of fish with which they shared common characteristics. One of these characteristics would be the lobed fins, which later became legs.

a living fossil
a living fossil

This theory was supported by several pieces of evidence from the fossil record, where the remains of finned fish had been found that could have allowed them to move on land. The scientists’ surprise was great when, in the 20th century, one of these fish that were believed to be extinct, the coelacanth, was found alive.

The coelacanth: a true living fossil

Throughout the 19th century, the first coelacanth fossils were discovered . These were fish whose fins were located at the end of fleshy protrusions similar to legs in the process of development. These fish were estimated to have appeared 390 million years ago during the Devonian period, and were believed to be extinct 66 million years ago during the Cretaceous, in the same event that caused the disappearance of the dinosaurs. 

This changed on December 23, 1938, when a fishing vessel fishing off the coast of South Africa found a live coelacanth in its nets. Since then, more specimens have been found, all on the west coast of the Indian Ocean, which is why this species was called the western Indian Ocean coelacanth (or, if you prefer a more technical name,  Latimeria chalumnae ). In 1997 another specimen was found in a fish market in Indonesia, but it was a different species, which is known as Indonesian coelacanth ( L. menadoensis ).

Some 80 species of coelacanth have been described from the fossil record, but only two species are known to be alive today. This curious phenomenon of the appearance of a species that was believed to be extinct is called the taxon Lazarus (after the biblical reference to the resurrection of Lazarus). It is not the only example of this, but it is the most striking, because these fish were believed to be extinct 66 million years ago and the fossils had been found a century before the first living specimens.

Coelacanth characteristics

a living fossil
a living fossil

As already mentioned, the most striking feature of the coelacanth is the arrangement of its fins on fleshy extensions . These fish have a bony skeleton (similar to most fish and unlike sharks or rays, which have cartilaginous skeleton). They have jaws and teeth with enamel.

Its body is covered in scales and mucus. Their blood system, like ours, has a pulmonary and a systemic circuit, which provides oxygen to the body organs. Adult individuals can measure more than two meters and weigh up to 90Kg . Interestingly, the two currently existing species are larger than the known fossils. These fish are estimated to live 60 years or more.

A striking feature of coelacanths is that they have a notochord instead of a spinal cord. The notochord is a nerve cord located in a dorsal arrangement in the ancestors of vertebrate animals, present in some current animals such as sea squirts. In the case of the coelacanth, it is a hollow tube filled with oil. Another striking feature is that its skull is almost completely filled with adipose tissue, the nervous tissue occupying just 1.5% of the cavity.

In addition, these fish have a vestigial lung filled with fat , which has a function homologous to the swim bladder of other fish. It is believed that this may be an adaptation to life in deep waters, which would also explain the small number of specimens that have been found to date.

Other peculiar characteristics of his physiognomy are that the valves of his heart are arranged in the form of a tube and that both kidneys have been fused into one. They are ovoviviparous animals, so the females produce eggs that hatch inside their bodies, giving birth to juvenile fish after a gestation period of one year.

Coelacanths in extinction?

Despite the fact that one of the two known species of coelacanth was discovered in a fish market, the meat of these animals is not suitable for human consumption . This is because its meat contains high amounts of oil, urea and esters, which makes it taste oily and is difficult to digest. The mucus that covers its scales also makes its meat difficult to cook. For this reason, coelacanths have no commercial value and fishing for individuals occurs accidentally.

This is positive for the conservation of the species. However, the reduction in the number of specimens sighted each year suggests that both species could be in danger of extinction. It is estimated that there are about 500 individuals of  L. chalumnae  and about 10,000 of  L. menadoensis , but the actual figures are not known with certainty.

Evolutionary relationship with terrestrial vertebrates

The study of coelacanths is of great interest to discern the evolution of vertebrates at the time when they first adopted terrestrial life habits. There are two types of fish, closely related to each other, that have common characteristics with tetrapods. Some are coelacanths, endowed with lobed fins, and others are lungfish, endowed with real lungs capable of breathing out of water. 

A peculiarity of coelacanths is that they have barely evolved in millions of years , unlike lungfish, so they retain primitive characteristics that allow a better understanding of the transitory evolutionary process towards terrestrial life. This is due to a lack of evolutionary pressure, since coelacanths have hardly any natural predators and conditions in the great ocean depths are very stable over time. That is why these fish have reached our days without having undergone major changes with respect to the fossil record.

Lungfish, another key piece

Lungfish or dipnoos (a term from the Greek  dipnoi , meaning “two breaths”) have lungs with which they can breathe out of water. Of the six known species, only one (the Australian lungfish,  Neoceratodus forsteri ) has developed gills that allow it to breathe underwater. The rest of the species have lost this capacity and can only breathe outside the marine environment. This is an evolutionary feature homologous to the lungs of terrestrial vertebrates, which coelacanths lack.

Analysis of the DNA of both groups of fish has concluded that lungfish, and not coelacanths, would be the ancestors most closely related to the first tetrapods . However, it should be noted that the fins of today’s lungfish have lost their lobed appearance due to evolutionary pressures, so they do not resemble the fins of coelacanths. In fact, the fins of today’s lungfish have a tapered shape reminiscent of the feathers of some birds. 

Today’s lungfish species live in freshwater environments and can survive out of water. African lungfish can survive long periods out of water, in relatively dry environments, where they burrow and form a protective envelope around their bodies. The shape of their fins may be an evolutionary response to living in shallow water, and their elongated bodies allow them to move on land.

The evolutionary group that groups coelacanths, lungfish and the group of tetrapods (both current species and all those that have become extinct) is known as sarcopterygii (a word that comes from the Greek  sarcopterygii , which means “meat” and “fin”, referring to the lobed fins of these fish).

As a final curious note, the coelacanth appears in the Pokémon video game saga, represented by the name Relicanth. This name comes from the English words  relict  (in biology, relic organisms are those survivors of a population that has declined considerably) and  coelacanth  (English name of the coelacanth).

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