Some insects and sea creatures produce sperm that is up to 10 times bigger than they are. Now scientists have used an innovative new x-ray technology to show how this bizarre situation evolved over hundreds of millions of years.
It sounds like a genetic aberration, but supergiant sperm can actually be an evolutionary advantage in several species. Fruit flies are a just a few millimeters long, but their sperm are 6 cm long. To top that, a human male would have to shoot out sperm that are up to 60 meters long. Another super giant sperm producer is a type of ostracod, a sea crustacean that looks like a tiny snail less than a centimeter long. Its sperm grows up to 10 times longer than its body. Here you can see an X-ray holotomograph of some super giant sperm.
Scientists have known for a while that one way males compete to inseminate females is by producing massive amounts of sperm. The more sperm you produce, the more likely that it will reach the egg before the other guys' sperm does. But in some species, it turns out that the winning sperm are always the biggest. If you can produce a sperm so big that it pushes all the other sperm out, your genetic material wins the race to the egg. This is exactly what happened in fruit flies and among some ostracods. (Pictured is the Ostrocada known as a mussel shrimp.)
Among super giant sperm producers, females have developed a special cavity that takes up to a third of their bodies to store the super giant sperm of their mates. Here you can see one such cavity, in an ostracod, which is only about half-filled with sperm. A paper published this afternoon in Science reveals how a group of scientists led by Renate Matzke-Karasz used a cutting-edge imaging technique called "synchrotron X-ray holotomography" to understand the evolution of this bizarre reproductive tool.
Matzke-Karasz and her team looked at ancient examples of the ostracod by selectively subjecting it to radiation in the synchrotron. This allowed them to examine a fossil that was over 100,000,000 years old, whose soft tissues had been preserved. It is extremely unusual for a fossil this old to contain any soft tissues, but the ancient ostracod was preserved in a region off the Brazilian coast where the soft tissues of many sea creatures have been preserved. Here is what the X-ray holotomography revealed. According to Giles Miller, of the Natural History Museum in London:
We obtained an excellent image of the reproductive apparatus of the fossil ostracods and were in for a real surprise. Our results show that these 100 million year old Cretaceous ostracods were already reproducing with giant sperm.
Added synchrotron researcher Paul Tafforeau:
Holotomography is a non-destructive imaging technique like computer tomography, where powerful and coherent synchrotron X-rays are used. With this method, a three-dimensional image of the inner structures even of microscopically small objects can be reproduced without doing any damage, with contrast and precision levels not reachable with any other techniques.
The researchers knew the fossilized ostracod had been mating with giant sperm because of this revealing holotomograph of a female ostracod. Because its soft tissues were preserved, scientists could see that it had a massive receptacle for super giant sperm. The image also showed that this female had recently been inseminated. The receptacle would only be this large if it had contained a giant sperm or two. Explained Matzke-Karasz:
Until now, it had been unknown whether giant sperm ostracod sperm arose multiple times over the course of evolution, like those of Drosophila, or whether they have been a persistent feature in certain groups for millions of years. This question can now be answered once and for all: giant sperm have been produced in at least some species over long periods of time, even though they come at an extremely high price for both, males and females. The next stage of our research is to try to understand why and how it has persisted for so long.