Scientists uncover the oldest sperm sample ever found

"This is the evidence that giant sperm are no extravagant whim."

Entombed in amber during the Cretaceous period, and hidden away in a collection for decades, sperm's evolutionary grandfather was patiently waiting for scientists to discover it.

In 2020, that day finally arrived. In a new study, scientists report that they've discovered the oldest known sperm cells to date.

The cells, reported in a paper published Wednesday in Proceedings of the Royal Society B, were found in the reproductive tract of a hitherto undiscovered species of ostracod – a class of small crustaceans about 1 millimeter in size.

The entire amber sample dates back to the cretaceous period, which means for as many as 100 million years, the specimen was undisturbed.

This creature, dubbed Myanmarcypris hui, lived in the coastal and inland waters of Myanmar, which are lined with dense trees. M. hui, likely copulated with the owner of what is now the world's oldest sperm, shortly before becoming encased in a coat of amber, which preserved both its body and the soft sperm cells inside.

Renate Matzke-Karasz is the study's second author and a paleontologist at Ludwig Maximilian University of Munich. She tells Inverse that these sperm cells are approximately 50 million years older than the previously identified oldest sperm cells. The previous oldest sperm cells that can be attributed to a source were found in the cocoon of an extinct species of Antarctic worm, which were reported in 2015.

The amber piece analyzed as part of the study.Photo Courtesy of Renate Matzke-Karasz

Matzke-Karasz adds that her team came across the specimen in a private collection in China, owned by a collector named Cheng Hu.

"He realized that there were these tiny ostracods in the amber," she tells Inverse. "We named the new species after him."

The age and condition of the find are both exciting, however, there was something extra special about the sperm encased in the Burmese amber: The sperm is far larger than you might expect for a creature that weighed less than one gram.

It's an example of sperm gigantism, and proof that an alternative sperm strategy was an effective way of copulating millions of years ago.

"This is the evidence that giant sperm are no extravagant whim," she explains.

An artists' reconstruction of M. Hui mating in during the Cretaceous period. Because the sperm cells were still in the female's reproductive tract, scientists propose that the pair mated shortly before the female was entombed in amber.

What does this discovery change? – Sperm typically conjures the thought of tiny, roughly 50-micrometer swimmers found in males. Matzke-Karasz says that this type of sperm represents a concept used by vertebrates and most other animals: produce as many sperm as possible and hope that the strongest, fastest swimmers get to the female's egg.

However, that's not the only sperm strategy honed by nature. On the other end of the spectrum is sperm gigantism – the idea that a creature might produce a few, very long sperm, in comparison with their body size.

That's the case for Drosophila bifurca, a fruit fly that produces sperm that's 5.8 cm long when uncoiled (about 20 times its body length). Ostracods like M. hui are also known for having particularly long sperm, as long as 4.5 times their body length or about explains Matzke-Karasz.

For a human that's 5 feet, 5 inches tall, that would translate to a sperm length of about 23 feet, she explains.

Sperm from a living ostracod.Courtesy of Renate Matzke-Karasz

The sperm found in M .Hui is difficult to measure because of the "tangled mass" of the sample (not uncommon for ostracods), the authors note. But they estimate it is at least 200 micrometers long.

As impressive as these measurements are, it has been unclear, evolutionarily speaking, how useful this strategy was. Large sperm comes at a cost, explains Matzke-Karasz.

Long sperm, like that found in M. hui, are immobile and need to be funneled into the female's reproductive tract, which means the creature's body must invest in additional organs and energy to reproduce. Large sperm also has to be produced by larger reproductive organs, which create an additional drain on energy.

"The reproductive organs are much bigger than in other species, they are taking a lot of space in the animal, and mating lasts long," she says. "This is a lot of biological energy that must be allocated to reproduction."

A reconstruction of the sperm found in M.Hui.Courtesy of Renate Matzke-Karasz

Evolutionary speaking, "expensive" features without clear payoff are thought to be counterintuitive, though there are examples of creatures or whom extravagant traits pay off. For example, the male peacock's feathers, do help them attract mates.

Long sperm coming from tiny creatures, however, doesn't present any clear advantages. Some studies in mammals have suggested greater sperm length can lead to faster, swimming more competitive sperm, but the evidence is mixed.

The fact that this is example of sperm gigantism exists at all is a signal that this strategy of copulation persisted for millions of years in ostracods. Matzke-Karasz sees it as proof that something about that strategy must have been working, even if we don't know what that something is yet.

"Previously, we were not sure if animals that 'switched' to using these giant sperm at a certain point in their evolutionary history are doomed to become extinct very quickly," she continues. "But in ostracods, it worked for more than 100 million years."

Abstract: The bivalved crustacean ostracods have the richest fossil record of any arthropod group and display complex reproductive strategies contributing to their evolutionary success. Sexual reproduction involving giant sperm, shared by three superfamilies of living ostracod crustaceans, is among the most fascinating behaviours. However, the origin and evolution of this reproductive mechanism has remained largely unexplored because fossil preservation of such features is extremely rare. Here, we report exceptionally preserved ostracods with soft parts (appendages and reproductive organs) in a single piece of mid-Cretaceous Kachin amber (approximately 100 Mya). The ostracod assemblage is composed of 39 individuals. Thirty- one individuals belong to a new species and genus, Myanmarcypris hui gen. et sp. nov., exhibiting an ontogenetic sequence from juveniles to adults (male and female). Seven individuals are assigned to Thalassocypria sp. (Cypridoidea, Candonidae, Paracypridinae) and one to Sanyuania sp. (Cytheroidea, Loxoconchidae). Our micro-CT reconstruction provides direct evidence of the male clasper, sperm pumps (Zenker organs), hemi- penes, eggs and female seminal receptacles with giant sperm. Our results reveal that the reproduction behavioural repertoire, which is associated with considerable morphological adaptations, has remained unchanged over at least 100 million years—a paramount example of evolutionary stasis. These results also double the age of the oldest unequivocal fossil animal sperm. This discovery highlights the capacity of amber to document invertebrate soft parts that are rarely recorded by other depositional environments.

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