Indonesian and French scientists have identified what they believe to be a new species of coelacanth, shedding light on the biogeographical mechanisms at the origin of the divergence between the populations known to date. The history of the evolution of this group of fish, little changed in comparison to its ancestors, at the origin of all terrestrial vertebrates, has excited interest far beyond the community of specialists.
In an extraordinary temporal collision point, one of the oldest animals still alive has just been revealed to the world on social media! A snapshot of a coelacanth caught off the coast of western New Guinea, released by a proud sports fisherman, aroused a great deal of excitement among experts. “We had long suspected that the discovery of a specimen in eastern Indonesia, in western New Guinea in particular, would signal the existence of a new species, distinct from that already known on the coasts of Sulawesi, explains Régis Hocdé, oceanographer of the Franco-Indonesian team that led this work. This is indeed the case, and a genetic study of the specimen caught has clarified issues surrounding the evolution of this group of fish, which comes down to us from the depths of time, survivors of a line of aquatic vertebrates at the origin of all terrestrial vertebrates.”
Significant genetic divergences
Only two contemporary species of coelacanths - a group long believed to have been extinct - have been discovered and characterised to date, one in the Comoros islands, off East Africa, the other in Sulawesi, west of the Indonesia. It was a close run thing that we might have left the question there: the angler in western New Guinea, unaware of what was on his plate, had already eaten a good part of his fish before scientists were alerted to the find by pictures circulating on the internet! Fortunately, molecular analysis does not require large quantities of biological material, and the remains of his dinner were enough for scientists to conduct a genetic study of this coelacanth, caught 750 km east of Sulawesi. “Sequencing of the mitochondrial DNA of this specimen, and comparing it to those of its neighbours revealed significant genetic divergences, of the order of 1.2%, sufficient to suggest that it is a new species,” says evolutionary biologist Emmanuel Paradis. Specialists have managed to date the divergence with the neighbouring Sulawesian type to 13 million years
The study of one or more whole specimens will be necessary to complement the genetic characterisation with a morphological description, rendered impossible this time by the appetite of the fisherman and his friends. This is a prerequisite for the formalisation of the new species from New Guinea, and to give it a name.
Chronicle of a discovery
“Studies of the region’s sea currents suggest that an oceanographic border exists between the islands of Sulawesi and New Guinea, which a fish with a slow metabolism would find impossible to crossthe coelacanth is not a fast swimmer, and its reproductive cycle is rather long, and is largely sufficient to act as a reproductive isolatory mechanism between two marine populations of the same species,” says evolutionary geneticist Laurent Pouyaudco-discoverer of Latimeria menadoensis, the species of Sulawesi to explain their conjectures regarding the existence of a distinct species east of Indonesia. A strong current from the eastern Pacific, mobilising the first 300 to 400 metres below the surface of the ocean runs along the northern coast of the island of New Guinea. A reverse current from the north runs through the archipelago to the south along the island of Sulawesi. These shearing currents in the neighbourhood of the Moluccas reinforce this natural barrier. It has already been shown that other marine organisms, including shrimp, have divergent forms on each side of this oceanographic obstacle.
Local tectonic history is also consistent with the results obtained at the genetic level: “The region is subject to major episodes of tectonic plate reorganisation, one of which occurred 20 million years ago, causing the migration of the Moluccas archipelago between Sulawesi and western New Guinea. This event blocked connectivity between the coelacanth populations of New Guinea and Sulawesi, by modifying the bathymetry and the currentology, and led to the genetic divergence observed between these two species,” explains Régis Hocdé.
The New Guinea epicentre
A biogeographical and evolutionary scenario is taking shape, explaining the trajectory of coelacanths through the tertiary era. The vast group of Coelacanthiformes, abundant and highly varied, once populated all the seas and many rivers of the globe. Their descendants were able to survive mass extinction in pockets along the coast of New Guinea, north of the Australian plate - the epicentre of a profound reconfiguration of islands and continents over a period of more than 100 million years, following the fracture of Gondwanaland and the migration of Australia from Antarctica to its present position. This population may have spread initially, 40 million years ago, towards the Comoros and Madagascar - Australia being at that time half as far from the African continent as it is today. Continuing its slow drift to the north, the Australian plate struck the Asian plate some 20 million years ago, giving birth to the Indonesian archipelago and causing the fragmentation of the original coelacanth population into two distinct populations, that of Sulawesi and that of western New Guinea. “Only an exhaustive study of the genetic variability and the geographic distribution of the New Guinea population along the coasts of this island will allow this scenario to be validated, notes evolutionist geneticist Kadarusman, who sequenced the DNA of the recently discovered specimen. Our goal is also to better understand the ecological and ethological characteristics of Indonesian coelacanths and to highlight any differences between their populations - all to best ensure their preservation,” concludes the Indonesian specialist.
A fish like no other
Coelacanths are no ordinary fish. They were first known as fossils, as much as 400 million years old, dating from long before the appearance of the dinosaurs. Like the dinosaurs, they seemed to have disappeared from the Earth completely, until one was caught by a fisherman in the middle of the 20th century on the eastern coast of Africa. The contemporary species, morphologically very similar to its fossil ancestors, was described and identified. Other catches in Indonesia, along the coasts of Sulawesi about twenty years ago and in western New Guinea, recently (2018), have made it possible to characterise two more living species.
In addition to their ongoing game of hide and seek with scientists, coelacanths represent a key element in the transition from aquatic life to terrestrial life: they have the beginnings of a lung and, unlike all other fish, fleshy lobed fins, the precursors of legs. They belong to a family some members of which emerged from the water to give rise to the terrestrial vertebrates 350 million years ago… there’s a little coelacanth in every one of us!