Scientists have discovered the oldest sperm in the fossil record, which dates back nearly 100 million years, inside a crustacean that was preserved in amber from Myanmar.
The Cretaceous sperm cells “double the age of the oldest unequivocal fossil animal sperm” making them “by far the oldest sperm of any kind yet identified,” according to a study published on Wednesday in Proceedings of the Royal Society B.
What’s more, the cells belong to a special class of “giant sperm” that is still observable in modern ostracods, the family to which the extinct Cretaceous crustacean belongs. Whereas most animals produce high numbers of small sperm cells, some make small numbers of huge sperm cells. Modern ostracod sperm can measure up to 4.3 times the length of the male’s actual body.
Renate Matzke-Karasz, a geobiologist at Ludwig-Maximilians-Universitaet (LMU) in Munich who co-authored the study, pointed out that an equivalent ratio in humans would result in sperm cells that measured about 24-feet long.
“The interesting part of our story is that we can now show that using giant sperm for reproduction is something that can last [a long time] in Earth’s history,” Matzke-Karasz said in an email. “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 added.
This unprecedented glimpse of Cretaceous reproduction is encased in an amber fossil that contains 39 ostracods, all smaller than a millimeter in size. The group represents a new species, called Myanmarcypris hui, which lived in an aquatic habitat that was also close to trees that produce the type of fluid resin that can eventually harden into amber.
Matzke-Karasz and her colleagues used a technique called computer-assisted 3D X-ray reconstruction to examine the soft tissues preserved in the amber fossil. The process revealed that a female ostracod in the specimen was storing sperm cells inside her body, indicating that she had mated a short time before she was entombed by the ancient tree resin.
The female’s storage organs are “coiled and messy,” Matzke-Karasz said, so the researchers were not able to get a full measurement of the sperm cells. However, the cells are at least 200 microns long. For reference, a strand of human hair is about 100 microns in width.
“This is the longest part of a sperm we could follow up in the mess,” Matzke-Karasz said, adding that 200 microns “would already equal [a third] of the body length!”
The sperm cells are almost certainly longer, though, given that the fossilized ostracods have many of the same organs as their modern relatives, including sperm pumps and two penises in the male, as well as two paired vaginas in the female.
Scientists were already intrigued by the bizarre reproductive behavior of modern ostracods, and the new discovery only deepens the mysteries of giant sperm evolution.
Ostracods are not the only family that uses giant sperm; for instance, the next-oldest sperm cells on record belong to a worm that lived 50 million years ago and also had large reproductive cells. The modern fruit fly species Drosophila bifurca has the longest sperm ever recorded, with cells that measure about 2.2 inches, or 20 times the fly’s body size.
There are “enormous costs” to this adaptation in animals, Matzke-Karasz said, including bulkier reproductive systems and longer mating times.
“This is a lot of biological energy that must be allocated to reproduction,” she said, “so you might think that this doesn't make sense from an evolutionary standpoint.”
But while this odd mechanism is an outlier in the animal kingdom, almost all freshwater ostracods continue to mate with giant sperm to this day. The new amber relic shows that these huge sperm have roots (or tails?) that extend well into the age of the dinosaurs, demonstrating that they must be doing something special for the crustaceans.
“For ostracods, there must be an advantage,” Matzke-Karasz concluded, “otherwise it wouldn't exist anymore.”
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