The “living fossils” are also Methusalems of the seas, reveals a study: Coelacids can live almost a century and thus live five times longer than previously thought. The life-clocks of these animals tick very slowly, according to the studies: Coelacids only reach sexual maturity at the age of over 40 and the gestation period of viviparous fish lasts five years, the researchers report.
Due to its astonishing history of discovery, the coelacanth became an icon of evolutionary biology. At first, these bony fish with their strangely leg-like fins were only known from fossils up to 400 million years old. It was assumed that relatives of these creatures belonged to the ancestors of the terrestrial vertebrates. In 1938 the sensation followed: a fish that looked like the fossil coelacanth was caught off the African coast. In fact, the species called Latimeria chalumnae turned out to be a surviving representative of this ancient family. Apparently this fish has hardly changed its archaic appearance over the course of millions of years. Presumably that was not necessary in his habitat: expeditions showed that the coelacanth, up to two meters long and over 100 kilograms, live in caves at a depth of 150 to 400 meters.
Far underestimated age
Since then, the Comoros coelacanth has been studied intensively. Researchers have already investigated the question of how old these fish get. The assessments were based on the examination of apparent growth rings (macrocirculi) on the scales of twelve specimens. The results led to the assumption that the fish did not live to be more than 20 years old. According to this, however, they should reach their considerable size comparatively quickly. This seemed surprising, because the other biological and ecological characteristics of the coelacanth seemed to be more typical of fish with a slow development. For this reason, the researchers around Kélig Mahé from the fisheries laboratory of the marine research institute IFREMER in Boulogne Sur Mer have now carried out a new study of the life expectancy and growth processes of the prominent fish.
They were able to access sample material from the French National Museum of Natural History in Paris. It houses the largest coelacanth collection in the world, ranging from fish embryos to individuals almost two meters in length. In total, the scientists examined material from 27 specimens. In contrast to earlier studies, Mahé and his colleagues used modern methods to analyze the tiny structures of the scales – so-called circuli.
“We were able to show that these circuli are actually annual growth markers, while the macrocirculi observed previously were not,” says Mahé. Specifically, the investigations have now shown that the oldest specimen in the sample has reached the age of 84 years. “This means that the maximum life expectancy of coelacanth is five times longer than previously assumed – it is probably around a century”. This results in a new assessment of the body growth of the coelacanth, says the scientist: “It is therefore one of the slowest growing marine fish of this size,” says Mahé.
Five years of gestation
The other results also fit into this picture: It was shown that the coelaceans do not become sexually mature until they are between 40 and 69 years old. In addition, the researchers uncovered details of an interesting characteristic of the coelacanth: They are viviparous. The study now shows that the young animals are carried to term in the womb for a surprisingly long time: the examination of two coelacanth embryos showed that both were around five years old. Apparently they were about to give birth, as is evident from their size. “Coelacids seem to have one of the, if not the slowest life stories among marine fish,” says Mahé.
As the scientists finally emphasize, the results also have an impact on the protection of the rare cave dwellers. “Long-lived species, which have a slow life history and relatively low fertility, are known to be extremely susceptible to natural or anthropogenic disturbances,” says Mahé. “Our results therefore suggest that the Comoros coelacanth, due to their special way of life, could be threatened even more than previously assumed. Therefore, this new information about their biology is of great importance for the conservation and management of this species, ”said the scientist.
Source: Cell Press, technical article: Current Biology, doi: 10.1016 / j.cub.2021.05.054