Chinese scientists have unveiled the oldest animal sperm ever discovered, found in a crustacean the size of a poppy seed that lived over 100 million years ago. Despite the organism’s diminutive stature, its seed is four times longer than human sperm.
Ostracods are a type of tiny aquatic arthropod that have remained largely unchanged for millennia. Sometimes called mussel shrimp for resembling shrimp living inside protective shells, they’ve been around for at least 500 million years, predating dinosaurs and mammals, and surviving multiple extinction events.
The sperm of modern ostracods can measure over 100 times longer than that of humans, earning the designation “giant sperm” from scientists.
New fossil evidence suggests that ancient ostracods had developed giant sperm as far back as the Cretaceous Period. The research was published Wednesday in the British journal Proceedings of the Royal Society B.
The 3D reconstruction of the fossilized ostracods. From left to right: a reproductive organ of a male ostracod; the lump of sperm found in the female ostracod; and a detailed look at the fossilized sperm. Wang He for Sixth Tone
“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.”
Wang and his colleagues obtained a piece of amber from Myanmar a few years ago. At first, the team simply wanted to check whether the black dots inside it were ostracods, which, being aquatic animals, don’t usually get trapped in tree sap, Wang said.
The findings were more interesting than the team had expected: The fossil was nature’s take on a set of matryoshka dolls, with new surprises nestled in each layer.
CT scans revealed that not only were the animals trapped inside ostracods, but their soft tissues were well-preserved. Wang found a lump in a female ostracod’s reproductive organs, and a 3D reconstruction of the animal revealed the mass to be a blob of squiggly, stringy sperm.
A visual reconstruction of the ostracod fossil discovered in amber. Wang He for Sixth Tone
“This new discovery is indisputable evidence that giant sperm is at least 100 million years old, and probably much older,” Robin Smith, an ostracod expert at Lake Biwa Museum in Japan, told Sixth Tone.
Smith, who is not affiliated with the study, said that before this visual evidence, scientists only had indirect proof of giant sperm in ancient ostracods from the size of their reproductive organs in fossils.
Based on where the sperm was found in the female’s body, Wang believes copulation must have occurred just before the specimen became trapped in the viscous resin. While a male ostracod was present in the amber in close proximity to the female, the team could not confirm whether the little fellow had contributed the spilled seed.
Though it was difficult to see fine detail in the sperm ball, the researchers measured the fossilized sperm to be at least 200 micrometers long, or about four times longer than human sperm.
The anatomy of ostracods, found in almost all bodies of water on Earth, has remained largely unchanged for millions of years, and it’s unclear why.
“Ostracods have been using these traits for so long, so the systems are likely to be very efficient and stable,” Wang said. “I mean, if it’s not broken, why fix it?”
Editor: David Paulk.
(Header image: An artist’s rendering of ancient ostracods mating. Yang Dinghua for Sixth Tone)