Size does matter

21 June 2009


Giant sperm have been found inside ancient fossil crustaceans, revealing just how long ago these enormous male sex cells evolved. The oceans 100 million years ago were full of males hotly competing with each other over who got the best mates.

OstracodPublishing in the journal Science, Renate Matzke-Karasz of the Ludwig Maximilians University in Germany lead a team of researchers who studied the internal organs of some 100 million year old ostracods - a relative of crabs and lobsters that look a little like mussels (a type of mollusc of course). They are sometimes called seed shrimp, for obvious reasons if you see one.

There are around 65,0000 species of ostracod which live in marine and freshwater.

Some of the modern-day male ostracods have extraordinarily big sperm that can be up to ten times longer than they are. Now for the first time the sperm of the ancient ostracods have been discovered using a complex imaging technique called synchrotron X-ray holotomography. This non-invasive technique, developed at the European Synchrotron Radiation Facility, allows researchers to see inside these tiny creatures that are only around 1mm in size, and compile images together on a computer into a 3D model.

The team found evidence that the ancient male ostracods has huge sperm ducts (Zenker organs) inside them, and the females - like modern ostracods - have huge internal cavities that receive the sperm after mating.

Ostracods aren't the only animals that have such enormous sperm. A more familiar creature, the fruit fly, also produce 6mm sperm - an impressive feat for a 1-2mm fly. It seems bizarre to spend so much energy making such huge sperm, but it all comes down to competition for mates.

For a male that has to compete with lots of other males to get a mate, and when females mate with many different males, one way of ensuring he passes on his genes to the next generation is by producing huge sperm. In animals like fruit flies and ostracods bigger sperm are more likely to fertilize eggs inside the female than smaller ones.

It's best not to think of an analogy of this in human beings, but this study does shed light onto the evolution is this peculiar male trait - which in ostracods probably evolved just once - and showing that is been the case that size really does matter for a very long time indeed.


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