#100-million-year-old ‘giant sperm’ discovered preserved in amber

Scientists have unearthed the largest and oldest sperm ever recorded — dating back nearly 100 million years — from an ancient crustacean encased in amber.

The Cretaceous period sex cells are not only many times larger than human sperm, but come from a tiny bivalve — or mollusk — about the size of a poppy seed, about 0.6 millimeters, according to a study published Wednesday in Proceedings of the Royal Society B.

They belong in a category of “giant sperm” common among some “micro-crustaceans” known as ostracods, which today includes the minuscule seed shrimp. Sperm from an ostracod is known for being up to 10 times larger than the animal’s own body. As another example, the modern fruit fly also boasts an outsize sperm measuring 2.2 inches, which is about 20 times the bug’s body size.

Compared to their marine findings, an equivalent ratio for humans would be a 24-foot-long sperm, according to study co-author Renate Matzke-Karasz, a geobiologist at Ludwig-Maximilians-Universität (LMU) in Munich.

“Previously, we were not sure if animals that ‘switched’ to using these giant sperm at a certain point in their history are doomed to become extinct very quickly. But in ostracods, it seemed to work for more than 100 million years,” she told Vice.

Researchers called the results “a paramount example of evolutionary stasis” in their report.

It’s the swimmers’ lengthy tail that makes this possible, wrapping around themselves to form a sort of reproductive tumbleweed that can swiftly barrel through the female reproductive canal.

The researchers examined the specimen, which was found in Myanmar, using a microscopic computerized tomography (CT) scanner to analyze the amber — which, as it turns out, was packed with 39 different species of the minute shellfish. Their study sheds light on the characteristics of ancient ostracods still present today, including giant sperm and “clasper” appendages. They were also able to observe the ovum as well as ejaculate-filled “receptacles” among the females.

Biologists confess they aren’t able to precisely measure the male cells but estimate they are at least 0.2 millimeters long — about the width of two strands of hair — and about a third of the size of the primordial plankton.

The fact that ostracod sperm are comparatively large comes at an evolutionary cost. In some modern species, their reproductive system can take up a third of their entire body.

“This is a lot of biological energy that must be allocated to reproduction,” said Matzke-Karasz, “so you might think that this doesn’t make sense from an evolutionary standpoint.”

But for these sex machines, she said, “There must be an advantage. Otherwise, it wouldn’t exist anymore.”