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Ancient Scottish fish first vertebrate to reproduce by fertilizing internally: Say hello to Microbrachius dicki

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Many animals including humans reproduce through sexual intercourse and researchers have found evidence that this process is pioneered by a fish that existed in Scotland nearly 400 million years ago.

In a new study published in the journal Nature on Oct. 19, researchers reported that a thumb-sized fish called Microbrachius dicki is the first animal to have stopped spawning in order to reproduce. The ancient fish belongs to the placoderm group considered as the earliest vertebrate ancestors of humans.

"Recent studies show that our own evolution is deeply rooted in placoderms, and that many of the features we have, such as jaws, teeth and paired limbs, first originated with this group of fishes," said study researcher John Long, from the School of Biological Sciences at the Flinders University in Australia. "Now, we reveal they gave us the intimate act of sexual intercourse as well."

Spawning, which is still largely used by fishes today, involves the external fertilization of the egg with the female fish releasing large quantities of unfertilized eggs, or ova, into the water and the male expelling sperm towards the egg at almost the same time to fertilize them.

Instead of spawning, the Microbrachius dicki, which thrived in ancient lakes of what is now Scotland about 385 million years ago, mated by having sex.

Long said that he made the discovery after checking out a collection of ancient fish fossils at the University of Technology in Tallinn, Estonia and noticed that an M. dicki specimens had a bizarre-looking appendage. The bony and L-shaped structure turned out to be the genitals of the male ancient fish.

Flinders and colleagues found that the males developed genital limbs known as claspers that were used to transfer the sperm to the females, which also developed small bony structures that locked their mate's organs into place during mating suggesting the first known use of internal fertilization and mating among vertebrates for reproductive purposes.

"Here we show that certain antiarchs possessed dermal claspers in the males, while females bore paired dermal plates inferred to have facilitated copulation," Long and colleagues wrote.

Interestingly, the researchers said that the ancient fish, reverted back to spawning as they evolved and it took millions of years again before copulation returned in the early relatives of sharks and rays.

"Biologists thought that there could not be a reversion back from internal fertilization to external fertilization, but we have shown it must have happened this way," Long said. "Our new paper suggests that after the first jawed vertebrates evolved internal fertilization, then it was lost at the point close to where the last common ancestor of modern jawed fishes evolved."

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