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Synchrotron X-ray holotomography at the ESRF has revealed why size really does matter to the sex lives of some creatures. The researchers provide evidence of ancient giant fossil sperm and hint at a link to organisms alive today. Some animals supersize their sperm. Microscopic freshwater ostracods, such as Eucypris virens, for instance have filamentous, spiralling sperm cells that can be up to ten times the body length of the organism itself. The longest known ostracod sperm cell is 10 mm long. Why some organisms have evolved such outsized sperm cells remains a mystery. Researchers in Germany have now used the relatively new technique of synchrotron X-ray holotomography to reveal that giant sperm present in some living animal groups is a phenomenon that may have arisen at least 100 million years ago. Renate Matzke-Karasz, from Ludwig-Maximilians-Universität Munich, Germany, has led an international team, including Radka Symonová, scientist at the Charles University in Prague and Giles Miller, Micropalaeontology Curator at the Natural History Museum, studying specimens from the London Natural History Museum's collections. Their research has revealed fossilised evidence for reproduction using giant sperm in a group of small aquatic crustaceans, called ostracods, dating back to 100 million years ago. "In these microfossils, we detected organs [sperm pumps] that are required for transferring giant spermatozoa," explains Matzke-Karasz. It seems that modern ostracods produce equally as large sperm as their ancient ancestors and manoeuvre their gametes using the organs as were used 100 million years ago. "It's safe to say this distinctive feature evolved only once in this group," adds Matzke-Karasz, "It is an evolutionarily successful reproduction strategy, even though it comes at an exceedingly high price for both genders, as a lot of energy is invested in producing and carrying such enormous sperm." The team studied several Harbinia micropapillosa specimens from the Cretaceous Period that had remains of the soft body intact. Robin Smith collected, investigated and then donated these fossil specimens to the Natural History Museum in 2000. Smith, who is now at the Lake Biwa Museum, in Japan, is a member of the team once more investigating H micropapillosa. The X-ray technique was employed in Grenoble, in collaboration with ESRF palaeontologist Paul Tafforeau. This technique is the most powerful and sensitive way of investigating the three-dimensional structure of such mechanisms at the microscopic level without damaging them. "Holotomography is a non-destructive imaging technique like computer tomography (CT), but we use powerful and coherent synchrotron X-rays leading to a sensitivity a thousand times higher," explains Tafforeau. "It is only very recently that palaeontologists have used this technique to image fossils, but the results achieved so far show that this technique will surely lead to many important discoveries on fossils," he adds. Twenty-one specimens from the Natural History Museum's micropalaeontology collection were used for the research and five of the specimens showed internal evidence of sexual organs. The technique has revealed direct parallels between fossilised ostracods and their modern relatives. Gratifyingly, the team also discovered that two of the female specimens had the tell-tale inflated cavities revealing that they had recently mated and presenting fossil evidence for insemination. The critical question that will have occurred to readers by now is just how big would a human sperm have to be to compete in size with that of the ostracods. Well, the researchers suggest that sperm about 17 metres long would measure up against modern ostracod sperm. Other species with giant sperm include featherwing beetles, various frogs, moths, and the fruit fly Drosophila bifurca whose sperm is about 60 mm long, while the fly itself is just a few millimetres in length. The next stage of the research from the international team is to understand why and how reproduction with giant sperm has persisted for so long.
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