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Dozens of perfectly preserved sperm coiled up inside the reproductive tract of a 100-million-year-old female microcrustacean have been identified as the oldest animal sperm ever found.

This tiny crustacean (Myanmarcypris hui)聽had a sexual encounter just before getting trapped in resin that had flowed from a nearby tree which later turned to amber, says Renate Matzke-Karasz at Ludwig Maximilian University in Munich, Germany.

The roughly 50 intact sperm cells nestled inside the organs of this newly discovered species of ostracod 鈥� small crustaceans known as 鈥渟eed shrimp鈥� that are usually about half a millimetre long 鈥� are considered giant, as they are at least as long as the adult animals鈥� body themselves, and probably much more, Matzke-Karasz says.

More than a decade ago, Matzke-Karasz and her team scanned petrified ostracods from the early Cretaceous period and found organs that would support giant sperm. But recently excavated amber from northern Myanmar has now allowed scientists to take direct images of well-preserved soft tissue in ostracods from this same period, including their sperm and eggs.

The research team scanned the preserved animals using computed tomography and other imaging techniques. The eggs aren鈥檛 giant, Matzke-Karasz says, but the sperm are exceptionally long.

鈥淭he difficulty of following individual sperm in the tangled mass makes precise length measurements impossible,鈥� she says. They found that the minimum length of the sperm could be 200 nanometres, but estimate that, based on the arrangement, the sperm are at least as long as the ostracod that produced them.

Many species of ostracod, which live in most bodies of freshwater and saltwater throughout the world, still have giant sperm, says He Wang at the Chinese Academy of Sciences, who led the study.

Some species of ostracods have shorter, quick-moving sperm. The fact that some ostracods had already evolved to produce slow, giant sperm rather than fast, tiny sperm by the age of the dinosaurs, and that they continue to do so today, suggests an important evolutionary advantage that merits further investigation, says Wang.