Sexually reproducing ancient crustaceans had sperm that were literally larger than life, according to a new study to be published in next week's issue of Science. The finding suggests that despite the extreme energetic costs of producing such sperm, the trait might be quite evolutionarily stable, the researchers say.

Electron micrograph image of ostracode sperm Image: Renate Matzke-Karasz

"This study fills a real gap by, for the first time, actually going back into the fossil record and examining the structure of sperm and of the female reproductive tract," said evolutionary biologist Scott Pitnick of Syracuse University in New York, who was not involved in the research. "They've gone back 100 million years and found the signature of giant sperm and of the interacting female sperm storage organs."

Giant sperm are found in a variety of extant animal taxa, but until now, there has been no evidence of the enormous gametes in ancient species. Thanks to the rare protection of the fine-grained sediment at a geological site called the Santana Formation in Brazil, an extinct species of aquatic bivalve fossilized completely, soft innards and all. Geobiologist Renate Matzke-Karasz of Ludwig-Maximilians-University in Germany and her colleagues examined the fortuitously preserved reproductive organs of this ancient ostracode, Harbinia micropapillosa, using a new technology known as synchrotron tomography, which combines the penetrating ability of x-rays with the extreme resolution of electron microscopy.

The researchers generated three-dimensional displays of the reproductive tracts of three male and two female H. micropapillosa. In the male fossils, the researchers found evidence of sperm pumps known as Zenker organs, which are necessary for inseminating females with giant sperm in living ostracodes. In the female fossils, they found the paired cavities characteristic of giant sperm storage. "We now have a signal that these 100 million-year-old fossils already used giant sperm for reproduction," Matzke-Karasz said.

The average size of extant freshwater ostracodes is one millimeter, Matzke-Karasz said, and several are much smaller. The giant sperm of these crustaceans, on the other hand, can reach up to ten times that length.

"It was always held that a sperm was a sperm was a sperm," Pitnick said, when in fact, "sperm are the most diverse cell type there is." The discovery of a 10 mm sperm in a 1.4 mm ostracode in the middle of the 20th century contradicted the accepted dogma that sperm were cheap and that the male sexual strategy was to maximize the number of sperm produced. Since then, researchers have reported male gametes reaching 58 mm in length.

"The only way giant sperm are going to evolve are if the benefits exceed the costs," Pitnick said. "And the costs are great."

The extreme variation in sperm size and shape is believed to be a direct result of sperm competition. With the advancement of DNA technology, paternity testing revealed that, across a variety of species, "promiscuity was more the rule than the exception," Pitnick explained. When the sperm of different males overlap inside the female reproductive tract, sexual selection continues well after mating, and any variation that made one male's sperm more likely to fertilize the female's eggs than another would be under intense selection.

So far, the functional significance of most sperm morphologies are unknown, but that's exactly what Matzke-Karasz hopes to learn about the giant sperm of ostracodes. How many sperm are transferred during one copulation event? How long are they stored in the receptacles before they fertilize an egg? Which sperm are the most successful - the ones of the first copulatory partner or the ones of the last? "All these things are unknown," she said. "There's a lot to do."


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