Fossils: World’s oldest sperm are discovered inside a female crustacean from 100 million years ago

The earliest known sample of sperm has been discovered inside a female crustacean that was preserved in amber shortly after mating some 100 million years ago. 

Dubbed a ‘spectacular find’, the specimen from Myanmar’s Hukawng Valley is an ostracod — a type of microscopic crustacean that resembles a mussel.

In fact, the specimen belongs to a previously-unknown species, which the researchers have named Myanmarcypris hui.

M. hui would have lived along the coasts and inland waters of prehistoric Myanmar, surrounded by trees that produced huge quantities of the resin that forms amber.

The discovery provides ‘an extremely rare opportunity’ to learn more about the evolution of the reproductive process in animals, the researchers said.

Before now, the oldest-known fossilised animal sperm was thought to have been found in a 50-million-year-old worm cocoon recovered from Antarctica.

In their study, geobiologist Renate Matzke-Karasz of the Ludwig-Maximilians-Universitaet in Munich and colleagues analysed 39 specimens of ostracods that had been trapped in a tiny piece of amber — using X-ray scans to reconstruct them in 3D.

The researchers identified ripe giant sperm stored in a pair of sperm receptacles inside the female ostracod — kept waiting ready for her eggs to mature, in what could also be the earliest direct evidence of a completed insemination.

While the majority of the male members of animal species — humans included — produce large quantities of very small sperm as to increase the chance of fertilisation, there are exceptions.

Some creatures — such as fruit flies and modern-day ostracods — produce a small number of oversized sperm, each of which sport tails several times longer than the actual animal itself.

In these cases, chances of fertilising an ovum can increase with the size of the sperm cell, the researchers explained.

Knowing more about how giant sperm cells evolved could shed light on what the researchers describe as an ‘ancient and advanced instance of evolutionary specialisation’.

In their study, geobiologist Renate Matzke-Karasz of the Ludwig-Maximilians-Universitaet in Munich and colleagues analysed 39 specimens of ostracods that had been trapped in a tiny piece of amber — using X-ray scans to reconstruct them in 3D, as pictured 

Myanmarcypris hui — two of which are pictured here mating in this artist’s impression — would have lived along the coasts and inland waters of prehistoric Myanmar, surrounded by trees that produced huge quantities of the resin that forms amber

‘The most significant part of our story is that we can now show that using giant sperm for reproduction is something that can last long in Earth history,’ Dr Matzke-Karasz told the PA news agency.

‘Previously, we were not sure if animals that ‘switched’ to using these giant sperm at a certain point in their evolutionary history [were] doomed to become extinct very quickly,’ she explained.

‘After all, these are enormous costs for the animals — large sperm must be produced, the reproductive organs are much bigger than in other species, they take up a lot of space in the animal and mating lasts long.’

‘This is a lot of biological energy that must be allocated to reproduction — so you might think that this doesn’t make sense from an evolutionary standpoint.’

‘But in ostracods, it seemed to work for more than 100 million years,’ she added.

‘From an evolutionary point of view, sexual reproduction with the aid of giant sperm must, therefore, be a thoroughly profitable strategy.’

The full findings of the study were published in the journal Proceedings of the Royal Society B.