The oldest evidence for reproduction with giant sperm has been uncovered in the Natural History Museum’s collections, a report in the journal Science announces today.
The giant sperm feature was detected in 100-million-year-old fossils of a group of tiny aquatic crustaceans, called ostracods.
Modern ostracods also have this feature and the sperm length of some can reach 10 times that of their body.
Electron micrograph of Ostracod, Harbinia micropapillosa, shows soft body parts inside.
An international team of scientists, led by Dr Renate Matzke-Karasz from Ludwig Maximilians Universität Munich, studied Museum specimens of a Cretaceous species called Harbinia micropapillosa. The specimens were so well preserved that they had remains of the soft body still intact.
The team analysed the specimens using synchrotron X-ray holotomography at the European Synchrotron Radiation Facility (ESRF) in France using new methods developed by Dr Paul Tafforeau.
It is one of the most powerful and sensitive ways to investigate the internal anatomy of fossils at a microscopic scale, in 3D, without damaging them.
The X-ray examination revealed that the complex reproductive organs existed in both these ancient animals as well as their modern-day relatives.
Dr Giles Miller, Curator of Fossil Ostracods at the Natural History Museum said, ‘It’s exciting when new methods of analysis can be used on our specimens to give us insights into the past and begin to answer evolutionary conundrums’.
As well as ostracods, there are several frogs, moths, featherwing beetles, backswimmer species and fruit flies that reproduce using giant sperm. The sperm of the fruit fly, Drosophila bifurca, measures around 6cm while the fly is only a few millimetres in length.
A human sperm would have to be over 17m long in order to measure up against one group of modern ostracods whose sperm can be 10 times as big as the animals themselves.
It is a bit of a mystery to scientists as to why some living animal groups have giant sperm.
‘It seems to be an evolutionarily successful reproduction strategy,' says Dr Matzke-Karasz. '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, including Radka Symonova of Charles University, Prague also uncovered fossil evidence for an insemination. Two of the female specimens had inflated cavities that only occur in modern ostracods that have recently mated.
The Harbinia micropapillosa fossils used in the study were uncovered from the Santana Formation of Brazil. They were collected, investigated and then donated to the Natural History Museum in 1998 by Dr Robin Smith, who is also a member of the research team.
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.