There鈥檚 sperm and then there鈥檚 super sperm 鈥� gigantic reproductive cells many times longer than the minute crustaceans that produce them. Now, scientists have discovered that ostracods, or seed shrimp, have been cranking out these giant sperm for at least 100 million years.
The outsized cells likely increase a male鈥檚 chances of impregnating a female in the face of intense competition from sperm from other males.
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Yet researchers thought that this 鈥渂igger is better鈥� reproductive strategy couldn鈥檛 last for long, certainly not 100 million years, says , a biologist at Ludwig Maximilians University in Munich, who led the study. 鈥淲e would expect the development of these strange things to stop at a certain point and it would not be an advantageous anymore.鈥�
Big investment
Males invest vast stores of energy in making the super sperm and females must maintain similarly large ducts to store the cells for later fertilization.
鈥淚t seems to be a successful strategy and this is strange,鈥� says Matzke-Karasz, whose team constructed three-dimensional images of millimetre-long male and female ostracod fossils from the Cretaceous-era Santana formation in north-eastern Brazil. It鈥檚 rare for internal organs to exist in a fossil, but the ostracods鈥� calcified carapaces may have enabled their preservation.
The images, made by firing X-ray beams through intact fossils, clearly show two large sperm pumps called Zenker organs. The pumps resemble those found in contemporary species of giant sperm-producing species of ostracods, which devote a third of their body volume to reproduction.
The female fossils, for their part, boast a pair of large seminal receptacles, which 鈥渕ust have been filled with sperm in order to be preserved,鈥� Matzke-Karasz and her colleagues say.
Sexual competition
Matzke-Karasz says it鈥檚 impossible to know just how long their sperm cells were, but closely related species that are still around produce sperm at least the length of their body. Such a long history for giant sperm points to intense sexual competition between males, she says. Females gather sperm from multiple males before fertilizing their eggs, but researchers don鈥檛 have a clue what determines success.
鈥淚t would be very interesting to find out which sperm the female actually uses, if she takes the first one or those of the last mating partner,鈥� says Matzke-Karasz, who plans to perform DNA paternity tests on offspring.
, an ostracod expert at Cornell University in Ithaca, New York, is less surprised to discover that giant sperm has been around so long.
鈥淚f it works, there鈥檚 no reason to get rid of it,鈥� he says, noting that a 425 million-year-old ostracod fossil contains the world鈥檚 oldest penis. 鈥淵ou could make the same argument for copulatory limbs.鈥�
Journal reference: (DOI: 10.1126/science.1173898)
