A reproductive strategy is essentially a cost-benefit analysis, as each organism has to determine how much energy to expend in its mating efforts. Most males, including humans, arrived at a strategy of producing vast amounts of tiny sperm, in hopes that among many matings, a few lucky sperm will manage to fertilize eggs. But some unusual organisms take a different approach and produce small amounts of “giant sperm,” and a new study reveals that some organisms had adopted the strategy as early as 100 million years ago.
The giant sperm strategy is found in a handful of modern organisms, including the fruit fly, which is only a few millimeters in size but can produce 6 cm-long (2.5 inch) coiled sperm…. Now the discovery that ostracodes, an extinct ancient class of arthropods, displayed the same trait shows that making giant sperm is a long-standing and evolutionarily successful reproduction strategy [Reuters]. Using a sophisticated imaging technique to study the fossilized soft organs in a tiny ostracode, a bivalve creature only one millimeter long, researchers proved that expending a great deal of energy to produce giant sperm has paid off for some species. “Now we can show that in spite of the costs, it must be a successful way to reproduce, since it ‘survived’ for such a long time” [LiveScience], says lead researcher Renate Matzke-Karasz.
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 … examined the fortuitously preserved reproductive organs … using a new technology known as synchrotron tomography, which combines the penetrating ability of x-rays with the extreme resolution of electron microscopy [The Scientist]. The images, published in a study in the journal Science, revealed two hollow tubes in the males that were likely sperm pumps, and two large cavities in the females that appear to be sperm receptacles. The reproductive organs took up about one-third of the body in both males and females, and had the sperm been uncoiled, it would have been as long as the male’s entire body.
Evolutionary biologist Scott Pitnick, who wasn’t involved in the current research, says it’s not yet clear what advantage the giant sperm gave to the extinct ostracode, but says it must have confered some mating benefit. 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 [The Scientist].
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Image: Renate Matzke-Karasz
