Extremely old, extremely large sperm and the sexual organ that propelled them. It's an awkward kind of scientific discovery but incredible nonetheless.
Australian researchers found "almost perfectly preserved" sperm cells curled up within the sexual organs of 17-million-year-old fossilized shrimp. Not only were the fossil sperm preserved but also the organ that sent those sperm into the female.
And just why were these shrimp fossils -- and their mega sperm -- so well preserved over the course of millions of years? "Tons of bat guano," says researcher Renate Matzke-Karasz.
University of New South Wales researchers published word of their discovery Wednesday in the journal Proceedings of the Royal Society B. According to the scientists, these delights are the "oldest fossilized sperm ever found in the geological record."
The scientists believe it was the bat poop at the fossil-rich Queensland site where the fossil shrimp were found that preserved the soft tissues of the shrimp so well for so many years. UNSW associate professor Suzanne Hand speculated ithat there may be "some magic ingredient in bat droppings."
The sperm were found in specimens of the species Heterocypris collaris and Newnhamia mckenziana.
Matzke-Karasz told the Los Angeles Times by email Thursday that the scientists had "29 specimens with inner organs preserved. Four of those were clear males (identified by the sperm pumps)."
Researchers put the shrimp under a microscope and virtually peeled it -- seeing its internal sexual organs, and within those its preserved giant sperm cells, and within those its nuclei, which eons ago held the animals' DNA.
The shrimp are known as ostracods, which are among the creatures with the largest sperm in the animal kingdom. Some freshwater ostracods make sperm that's up to 10 times the length of the ostracod's body. The sperm in the ancient shrimp found by the Aussie researchers were 1.3 millimeters long. (For the sake of comparison, the head of a human sperm has a width of 3.5 microns, or 0.0035 millimeters.)
What organism has the longest sperm cell? The Drosophila bifurca species of fruit fly is said to produce sperm that's 5.8 centimeters long.
Transferring such sperm, contained within tiny organisms, seems like a huge feat. But Mother Nature's genius is apparent with the Zenker organ -- the muscular "sperm pump" referred to earlier by Matzke-Karasz -- which is highly effective at moving giant sperm from Point A to Point B. These fossilized organs were also observed, well preserved, in the tiny shrimp.
Given the size of the sperm, we asked Matzke-Karasz: Wouldn't the egg of the female also have to be size XL?
No, she said: "The solution is very simple. After the sperm enters the egg, it coils up tightly, close to the egg's inner coat surface."
The researchers are celebrating their find at the Riversleigh site in Queensland. They said this rich fossil site has produced giant, toothed platypuses and flesh-eating kangaroos -- but minuscule shrimp with mega sperm? That was "totally unexpected."