Genetic analysis shows that the sessile crustaceans can broadcast sperm in water

By Susan Milius

Web edition: January 15, 2013
Print edition: February 23, 2013; Vol.183 #4 (p. 15)

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Among a dense crowd of Pollicipes polymerus gooseneck barnacles, scientists found one leaking sperm (center). The barnacles reproduce in part by releasing sperm into seawater that will hopefully reach a receptive partner, a new study finds.

Credit: M. Barazandeh

Confounding more than a century of received wisdom about crustacean sex, genetic tests show that at least one kind of barnacle can transfer sperm without making direct contact via their famously extendable parts.

The Pollicipes polymerus gooseneck barnacles along the coast of the northeast Pacific have sperm-delivery organs that stretch out about half a body length, which is modest by barnacle standards. Many biologists presumed that barnacles deliver sperm to a mate only by reaching with these impressive organs into the space within a neighbor’s shell.   

Yet genetic markers show that the barnacles also reproduce using sperm transported by water, Richard Palmer of the University of Alberta in Edmonton, Canada, and his colleagues report January 15 in the Proceedings of the Royal Society B.

“A landmark paper,” says Diana S. Jones of the Western Australian Museum in Welshpool, who reeled off plenty of new questions about basic biology and theoretical implications after seeing the new paper’s results. Biologists have thought that barnacles weren’t able to reproduce just by casting sperm into the water, though creatures in nine other phyla, such as sponges and mollusks, manage quite well.

”The surprise is more that it hasn’t been investigated before,” says invertebrate zoologist John Zardus of The Citadel in Charleston, S.C. The discovery “is a good example of how we can get stuck thinking inside the box.”

Biologists have certainly recognized that barnacles are in a fix:  The crustaceans essentially glue themselves head-end down in one spot for their entire adult lives, which can last decades. Plenty of kinds have evolved sperm-delivery organs two to three times as long as their bodies, and one species can reach eight body lengths. Many barnacles are hermaphrodites, and individuals beyond the reach of potential partners have been presumed to fertilize themselves.

Testing that idea genetically proved challenging. Study coauthor Marjan Barazandeh, also at the University of Alberta, spent more than a year struggling to find distinctive stretches of repetitive DNA the researchers could use as reliable markers to distinguish parents. Finally she abandoned that strategy and developed a set of oddities in a single “letter” in stretches of DNA. This approach allowed her to test egg masses from isolated barnacles or barnacles living in pairs.

The loner barnacles’ egg masses included markers that couldn’t have come from the single parent, and thus indicated fertilization by an out-of-reach individual. Even among barnacles with a potential mate within reach, nearly a quarter of them were fertilized by some out-of-reach donor.

As far as he knows, Palmer says, these results mark the first documented spermcasting among any of the crustaceans. Now researchers need to see whether the capacity evolved in just one kind of barnacle or has been overlooked elsewhere.

The new genetic evidence “confirms what a handful of barnacle geeks have suspected for a long time,” says marine ecologist Steve Hawkins of the University of Southampton in England.

The most famous of barnacle geeks, one Charles Darwin, didn’t show any signs of anticipating such a result, Palmer says. Even though the keen-eyed naturalist may have missed the phenomenon during the eight years he devoted to barnacle research, “Darwin would be thrilled,” Palmer says. “He was that kind of guy.”