Coelacanths might be monogamous, to the surprise of researchers
by Cynthia McKelvey
They evaded humans for millions of years and live very private lives. The hulking, fleshy-finned fish known as the coelacanth has beguiled scientists for generations. But the coelacanth mystique that enchants researchers also makes it difficult to study. Researchers recently revealed in Nature Communications one startling aspect of the coelacanth lifestyle: they might be monogamous.
Presumed extinct for over 60 million years, the coelacanth (SEE-lah-kanth) was known only in fossil form until a fisherman caught a live one in 1938 near South Africa. Recently, scientists have found populations of dozens of coelacanths nestled in caves hundreds of meters deep in the Indian Ocean near Kenya, Tanzania and the Comoros Islands.
Monogamy poses a risk to coelacanths in part because of the onerous three-year-long pregnancies in females. The babies are fully developed when they’re born, but the mother sacrifices a lot of energy and is more vulnerable to predators while she carries her young. If one male with a bad set of genes sires the brood, all the offspring can suffer—and those three years might be wasted…
Rebellatrix is a genus of large prehistoric coelacanth from the Lower Triassic Sulphur Mountain formation and Wapiti Lake Provincial Park of British Columbia. It is known from a single species, R. divaricerca (“forked tail”), which is the only known species of the family Rebellatricidae. Its most distinguishing feature was its tuna-like forked tail (unusual for an actinistian fish), which suggested a fast-swimming and active lifestyle, unlike coelacanths related to the living species…
The coelacanth isn’t called a “living fossil” for nothing. The 2-meter-long, 90 kg fish was thought to have gone extinct 70 million years ago—until a fisherman caught one in 1938—and the animal looks a lot like its fossil ancestors dating back 300 million years. Now, the first analysis of the coelacanth’s genome reveals why the fish may have changed so little over the ages. It also may help explain how fish like it moved onto land long ago.
"I’m very excited about this paper because coelacanths are animals that we really want to know more about," says Per Ahlberg, a paleontologist at Uppsala University in Sweden who was not involved with the study.
In order to sequence a coelacanth’s (Latimeria chalumnae) genome, scientists required fresh tissue and blood. That’s no easy task: These fish dwell in deep-sea caves and are exceedingly rare. Only 309 have been spotted in the past 75 years, off the east coast of sub-Saharan Africa and Indonesia. Moreover, caught coelacanths die immediately because of the change in pressure and temperature, and under the hot tropical sun, their DNA quickly degrades…
The coelacanth (pronounced SEE-la-kanth) is a primitive, slow-moving fish that’s sometimes called a living fossil, because it apparently existed largely unchanged for 320 million years. There are 40 known coelacanth species, 2 of which are alive today.
All other known coelacanths have broad, rounded tails designed for slow bursts of motion. But Rebellatrix had a huge, forked tail and streamlined body that likely allowed the ancient fish to cruise long distances and hunt prey at high speeds, said study leader Andrew Wendruff, a biologist at the University of Alberta in Canada. According to Wendruff, the team named the discovery Rebellatrix because, like a true rebel, “it does everything a coelacanth should not do…”
Paleontologists have long thought of the coelacanth as a stodgy old slowpoke: Two modern-day species of the fish—considered living fossils because of their remarkable similarity to ancient coelacanths—typically swim in a slow, almost dawdling manner.
As a group, coelacanths had apparently kept the same basic body plan for hundreds of millions of years. But now, researchers have found fossils of a sleeker coelacanth—one that likely was a speedy, shark-like predator in the ancient seas west of the supercontinent Pangaea about 240 million years ago. Unlike all previously known specimens, which have fleshy, three-lobed tails fringed with flexible fins, the new species (artist’s reconstruction shown) had a stiff, crescent-shaped tail like modern-day tuna and barracuda, both of which are famed for their speed.
The team has dubbed the new species Rebellatrix divaricerca, “the rebel coelacanth with the forked tail.” Although more streamlined than previously known coelacanths, Rebellatrix, which the researchers estimate grew to reach at least 1.3 meters in length, was more thick-bodied than today’s tuna and, therefore, probably slower. The new specimens, described in the May Journal of Vertebrate Paleontology,may mark the first species in a yet-to-be-discovered trove of coelacanth diversity, the researchers say. Or, the Corvette of coelacanths may have evolved to fill an ecological niche left vacant when more than 95% of the ocean’s species disappeared in a mass extinction about 252 million years ago.
The primitive-looking coelacanth (pronounced SEEL-uh-kanth) was thought to have gone extinct with the dinosaurs 65 million years ago. But its discovery in 1938 by a South African museum curator on a local fishing trawler fascinated the world and ignited a debate about how this bizarre lobe-finned fish fits into the evolution of land animals.
There are only two known species of coelacanths: one that lives near the Comoros Islands off the east coast of Africa, and one found in the waters off Sulawesi, Indonesia. Many scientists believe that the unique characteristics of the coelacanth represent an early step in the evolution of fish to terrestrial four-legged animals like amphibians.
The most striking feature of this “living fossil” is its paired lobe fins that extend away from its body like legs and move in an alternating pattern, like a trotting horse. Other unique characteristics include a hinged joint in the skull which allows the fish to widen its mouth for large prey; an oil-filled tube, called a notochord, which serves as a backbone; thick scales common only to extinct fish, and an electrosensory rostral organ in its snout likely used to detect prey…
Coelacanth is a lobe-finned fish that dates all the way back to the Devonian, and were believed to have gone extinct around the end of the Cretaceous. Before its rediscovery in 1938 it was thought to be the “missing link” between fish and tetrapods, though it is apparently no longer the case that it is the link, coelacanth is still closer related to tetrapods than to ray-finned fish. It has remained roughly unchanged for ~400 million years. There are two living species, Latimeria chalumnae (West Indian Ocean Coelacanth (pictured above,)) and L. menadoensis (the Indonesian Coelacanth.) Its rediscovery in 1938 after virtually falling off the fossil record qualifies it as probably the best example of the Lazarus Taxon.
Photo is from the Wikimedia Commons, its information can be found at this link.