Size did matter

The mystery of giant sperm present in some living animal groups today has taken on a new dimension. In one group of micro-crustaceans new evidence shows the feature is at least 100 million years old. Renate Matzke-Karasz, from Ludwig-Maximilians-Universitaet Munich, has led an international team of scientists, 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.

Matzke-Karasz said, 'In these microfossils, we detected organs that are required for transferring giant spermatozoa. Since modern ostracods still produce giant sperm and manoeuvre them with the same organs as 100 million years ago, it's safe to say this distinctive feature evolved only once in this group. It seems to be 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 international team analysed Harbinia micropapillosa specimens from the Cretaceous Period that had remains of the soft body intact. These fossils had been collected, investigated and then donated to the Natural History Museum in 2000 by Robin Smith. Now at the Lake Biwa Museum, Japan, Smith is a member of the research team. Eight years later, the same specimens were analysed using synchrotron X-ray holotomography at the European Synchrotron Radiation Facility (ESRF) in Grenoble, through collaboration with Paul Tafforeau, a palaeontologist at the ESRF. This method is currently the most powerful and sensitive way to investigate in three dimensions and at a microscopic scale, the internal anatomy of exceptional fossils 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 thousand times higher,' explains Paul Tafforeau of ESRF. 'It is since very recently that palaeontologists use 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.

The X-ray examination of the fossilised ostracods revealed direct parallels with the complex reproductive apparatus of modern relatives of these Cretaceous fossils. The team also came across something of a surprise: two of the female specimens had inflated cavities that only occur in modern ostracods that have recently mated, meaning fossil evidence for an insemination had been uncovered.

The team was completed by Radka Symonova, scientist at the Charles University in Prague and Giles Miller, Micropalaeontology Curator at the Natural History Museum.

A human sperm would have to be over 17 metres long in order to measure up against one group of modern ostracods, whose sperm are up to ten times as big as the animals themselves. Roughly 34,000 of the 50 micron-long human sperm would have to line up to match the body length of a man (of 1,70 m).

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.