The world's oldest sperm is discovered in a female crustacean that mated and was trapped in amber 100 million years ago
The earliest known sperm sample was discovered in a female crustacean that was preserved in amber shortly after mating about 100 million years ago.
The specimen from Myanmar, described by paleontologists as a "spectacular find", is an ostracode – a type of microscopic crustacean that resembles a mussel.
They say the results provide "an extremely rare opportunity" to learn more about how the reproductive process evolves in animals.
Previously it was believed that the oldest known fossilized animal sperm was found in a 50-million-year-old worm cocoon from Antarctica.
The researchers say the ostracod specimen belongs to a new species called Myanmar cypris hui.
They believe these creatures lived both on the coast and in the inland waters of what is now Myanmar, surrounded by trees that produced huge amounts of resin.
Scientists discovered what they believed to be the oldest fossilized animal sperm around 100 million years ago
In their study, the geobiologist Renate Matzke-Karasz from the Ludwig Maximilians University in Munich and colleagues analyzed 39 specimens of ostracodes that were trapped in a tiny piece of amber – with the help of X-ray scans to reconstruct them in 3D.
The researchers identified giant mature sperm stored in two sperm containers in the female ostracode – and waited for their eggs to mature, which could also be the earliest direct evidence of full insemination.
While the majority of male members of animal species – including humans – produce large quantities of very small sperm to increase the likelihood of fertilization, there are exceptions.
Some creatures – like fruit flies and modern ostracods – produce small numbers of oversized sperm, each of which truant several times longer than the actual animal.
In these cases, the likelihood of egg fertilization may increase with the size of the sperm cell, the researchers said.
Knowing more about how giant sperm evolved could shed light on what researchers call "an ancient and advanced example of evolutionary specialization."
They say the results, published in the journal Proceedings of the Royal Society B, offer "an extremely rare opportunity" to learn more about the evolution of the reproductive process
The researchers say the ostracod specimen belongs to a new species called Myanmar cypris hui. They believe these creatures lived both on the coast and in the inland waters of what is now Myanmar, surrounded by trees that produced huge amounts of resin
"The most important part of our story is that we can now show that the use of giant sperm for reproduction can be a long time in the history of the earth," said Dr. Matzke-Karasz told the PA news agency.
"We used to be unsure if animals" switched "to using these giant sperm at some point in their evolutionary history were doomed to become extinct very quickly," she said.
"After all, these are enormous costs for the animals – large sperm have to be produced, the reproductive organs are much larger than in other species, they take up a lot of space in the animal and mating takes a long time."
"This is a lot of biological energy that has to be used for reproduction. So you might think that it doesn't make sense from an evolutionary point of view."
"But with ostracodes it seemed to work for more than 100 million years," she added.
"From an evolutionary point of view, sexual reproduction with the help of giant sperm must therefore be a thoroughly profitable strategy."
The full results of the study were published in the journal Proceedings of the Royal Society B.