Published: Fri, September 18, 2020

Oldest animal sperm discovered in 100-million-year-old amber

Oldest animal sperm discovered in 100-million-year-old amber

Scientists have discovered the world's oldest preserved sperm sample, which is thought to be 100 million years old.

Once in the female, the sperm are pushed up the two long sperm canals, each ending in a sac-like seminal receptacle for sperm storage; there, they finally become motile, arrange themselves into a more organized assemblage and fertilize eggs during the process of oviposition.

"The complexity of the reproductive system in these specimens raises the question of whether the investment in giant sperm cells might represent an evolutionarily stable strategy", Renate Matzke-Karasz, a geobiologist at Ludwig-Maximilians-Universitaet in Munich who was involved in the morphological analysis of the specimens, said in a statement. Their fossil shells are common and abundant but finding specimens preserved in ancient amber with their appendages and internal organs intact provides a rare and exciting opportunity to learn more about their evolution.

Using a micro-CT scanner, researchers have now revealed 39 of their ancient crustacean relatives, all in the same slice of amber.

"We really don't know why, but it (the giant sperm) must be quite functional", the paper's first author, Wang He of the Nanjing Institute of Geology and Palaeontology under the Chinese Academy of Sciences, told Sixth Tone."Otherwise, natural selection would have eliminated the trait".

The research was published Wednesday in the British journal Proceedings of the Royal Society B.

In fact, this is the oldest fossil in which sperm cells have been conclusively identified. Previous fossilized remains found trapped in amber in the area include frogs, snakes, and part of a supposed dinosaur.

They predict that the animals copulated just before their entrapment in the piece of amber (tree resin), which formed in the Cretaceous period, about 100 million years ago.

The team, led by researchers from Chinese Academy of Science, found the sperm in a new species of crustacean they named Myanmarcypris hui.

Screen Shot 2020 09 15
Ancient ostracod sperms in a female

Using 3D X-ray reconstructive technology, scientists analyzed several ostracod specimens, studying their limbs and reproductive organs.

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.

"The results were fantastic - not only did we find their tiny appendages to be preserved inside their shells, we could also see their reproductive organs", said He Wang of the Chinese Academy of Science.

The female ostracod had been fossilized alongside others in amber that was found at a site in the province of Kachin in Myanmar.

Fossilised sperms are exceptionally rare; previously the oldest known examples were only 17 million years old.

Sperm counts in men from developed world have dropped by more than 50% in less than 40 years, researchers said.

"For example, experiments have shown that in one group, a high degree of competition between males can lead to a longer sperm life, while in another group, a low degree of competition also led to a longer sperm life", she added.

"Sexual reproduction with giant sperm must be very advantageous" says Matzke-Karasz. Sexual reproduction involving giant sperm, shared by three superfamilies of living ostracod crustaceans, is among the most fascinating behaviours.

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