Giant Sperm Is Ancient Evolutionary Tool, Study Finds

Kate Ravilious
for National Geographic News
June 18, 2009

Size does matter, at least for the seed shrimp.

The tiny creatures' giant sperm are an evolutionary strategy that stretches back at least a hundred million years, scientists discovered in a new study.

The giant sperm can be up to ten times the animals' body lengths. By comparison an average sperm from a man is around 0.002 inch (0.05 millimeter) long, less than a thirty-thousandth of his height.

To find out whether giant sperm is an ancient adaptation, researchers x-rayed the innards of five well-preserved seed shrimp, or ostracods, from hundred-million-year-old sediment from Brazil. Although the giant sperm had rotted away, the scientists could still see the remains of perhaps the ultimate male organ: a sperm pump, used to push the giant sperm out of the body.

"Only [shrimp] that produce giant sperm have this organ," study co-author Robin Smith, of Lake Biwa Museum in Japan, said by email.

Furthermore, two female specimens also found in the sediment had huge reproductive cavities.

"These sperm receptacles only inflate when they carry sperm, meaning the [two] females must have mated only shortly before they died," said lead study author Renate Matzke-Karasz, of Ludwig Maximilians University in Munich, Germany.

(Related: how a jungle bird doles out sperm with precision.)

Why Giant Sperm?

The giant sperm could be the equivalent of a peacock's tail—a way of attracting females, researchers say.

Also, the large reproductive cells may "cork up" females, ensuring no other males have the opportunity to mate. Or perhaps the giant sperm are sources of nutrients for developing embryos, the researchers say.

If giant sperm is an effective adaptation, there ought to be other animals benefiting from it—and there are, the researchers note. Certain frogs, gastropods, and insects join seed shrimp in producing larger-than-life reproductive cells.

(Related: dragonfly mating pictures in National Geographic magazine.)

Findings appear tomorrow in the journal Science.

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