The oldest animal sperm was discovered to date in 100-million-year-old amber

A 100 million-year-old chunk of amber has revealed the oldest specimen of animal sperm, and each individual cell is growing longer.

Even more impressive, this giant sperm – many times larger than human sperm – comes from a shrimp-like crustacean that is smaller than a poppy.

Just 0.6 millimeters shy, this ancient beleve still belongs to a living class of microcrystesians, known as ostracodes, which are famous for sperm ten times larger than themselves.

This may seem impossible, but when these microscopic cells are penetrated and become entangled in small balls, they can travel easily through the female reproductive tract, preventing other small tangles in competition.

Amber piece studied showing the position of the specimens. (Wang et al., Proceedings B, 2020)

Using micro-CT scanners, researchers have now revealed 39 of their ancient crustacean relatives, all in a single slice of amber. Even more surprising is that this frozen community still has some of the same reproductive properties that we still see in the ostracode, including the giant sperm.

Not only were their ancient relatives found with similar male ‘clasp’, these fossilized bodies also included sperm pumps, eggs and – all female receptacles filled with sperm.

“The fact that the female’s semicircles are in an extended state due to being filled with spermatozoa indicates that successful copulation occurred shortly before the animals were trapped in the mole,” the author writes.

It is impossible to measure individual cells in these complex masses, the authors admit, but they state that the minimum sperm are at least 200 μm long (0.2 mm). It is at least one third of the length of the entire body of an ancient creature.

It is also the oldest specimen of animal sperm by a long shot. While other ostracode fossils dating back one hundred million years have shown signs of giant reproductive organs, we have not laid our hands on actual specimens before this time.

In 2014, 16 million years old freshwater oestrodes discovered in a cave in Australia were found to contain sperm 1.2 million long. But the new specimen discovered in Myanmar is 83 million years old. In fact, it doubles the age of the oldest unequal fossil animal sperm.

All of the time, it seems that Ostracode reproduction has remained largely the same – “a paramount example of evolutionary stagnation,” as the author states.

During sexual reproduction, both ancient and modern ostracods probably use their fifth limbs as ‘hooks’ to hold onto women. Once they have a grip, they can insert their erectile tissue and pump an “extraordinarily long but immature sperm”, pushing it into two long sperm ducts in the female body.

Once these sperm reach the seminal duodenum, the authors think that they begin to settle in “systematic combinations”, so that they can start fertilizing the eggs.

Screen shot 2020 09 15 to 2.48.17 pmAncient ostracode sperm in a female. (Wang et al., Proceedings B, 2020)

While this may seem cliché at first, some of the smallest organisms on Earth produce the largest sperm. When women have sex with more than one partner, sperm must be countered; Scientists think that large units may be more profitable. That said, giant sperm – as in ostrodods – come at a higher price for both men and women.

First and foremost, their sexual machinery simply has to grow up, and already for such a small animal, this is a serious trade-off. Some modern species devote a third of their body volume to reproduction alone.

As such, it remains a mystery as to how and when these giant traits evolved. Direct evidence is indeed rare. While crustaceans cool the shale that supersedes the rich fossil record, the detection of soft soft tissue in the fossils is rare.

This discovery is indeed remarkable, not only because amber has preserved soft tissue from many individuals for a hundred million years, but also because of all the similarities.

“Male claspers, sperm pumps, hemipenes, and female sperm eclipse with giant sperm of fossil ostracods, suggesting that reproductive behavioral behavior, which is associated with considerable morphological changes, has been unchanged for at least 100 million years.” The author writes.

The study was published in Proceedings of the Royal Society B: Biological Sciences.