Barnacles Leak Sperm Into Ocean, Upending Mating Theory

Barnacles, already famous for having the longest penises in the animal kingdom (relative to size), have another reproductive quirk.

The Pacific gooseneck barnacle is the first species of ocean-dwelling arthropods—the group that includes crustaceans, insects, and spiders—known to spermcast, according to a new study. (Watch a video of Pacific gooseneck barnacles feeding.)

Just as it sounds, spermcasting occurs when a male barnacle sends out his sperm into the water, and females pick it up and fertilize their eggs. Other species such as sponges, jellyfish, and sea anenomes are known to spermcast.

Pacific gooseneck barnacles release sticky sperm masses. Photograph courtesy A. Richard Palmer

That may sound bizarre, but barnacles’ sex lives are already stranger than fiction.

Unlike most free-living arthropods, barnacles glue themselves to hard surfaces, such as rocks. Since they can’t move, the shallow-water creatures evolved superlong penises—which can also sometimes change shape and size—in order to fertilize their neighbors.

As the study authors describe it, “a functional male searches for partners by random penis movements and then deposits sperm into the partner’s mantle cavity” in a process called pseudo-copulation.

But if that’s literally a bit of a stretch, the animals have another option: self-fertilization. (Most barnacles are hermaphrodites, though they tend to lean toward one gender.)

Pacific Gooseneck Barnacles a “Puzzle”

Not so for the Pacific gooseneck barnacle, which is bit of a “technical puzzle”—the species has never been observed self-fertilizing, said study co-author Richard Palmer, an evolutionary biologist at the University of Alberta in Canada.

Yet somehow, isolated gooseneck barnacles can still reproduce even if they’re anchored far from another barnacle—what’s scientifically called the “penis range.”

Barnacles anchor themselves to a fixed spot, such as a rock. Photograph courtesy A. Richard Palmer

Palmer, who has observed Pacific gooseneck barnacles “leaking” sperm, suspected the isolated animals were getting fertilized from sperm in the water.

To investigate this mystery, a team led by Marjan Barazandeh, also of the University of Alberta, collected several Pacific gooseneck barnacles from Barkley Sound, British Columbia, in 2009 and 2010.

Barazandeh and her team then analyzed the DNA makeup of embryo-carrying barnacles that were collected both in and out of penis range. (See another barnacle picture.)

The results showed that every individual out of penis range had at least one genetic marker from a barnacle other than itself—suggesting they were getting the sperm from the ocean.

How Barnacles “Catch” Sperm Unknown

How the females get fertilized by spermy water is still unknown.

Barnacles cast sticky sperm masses into the water fairly regularly, so a “scenario that might happen is as the tide comes in and waves start to break over the top of [the male barnacles], clumps of sperm are picked up, and downshore individuals … could capture it,” Palmer said.

However they do it, the discovery is seminal: “It overturns a century of beliefs about what barnacles can, or cannot, do,” according to the study, published recently in the journal Proceedings of the Royal Society B.

Still, there’s a lot the team doesn’t know—for one, they can’t fully rule out self-fertilization. So the next logical step? Barazendah and her team plan to videotape barnacles in the wild to, well, catch them in the act.

More sperm news:

Sperm Tracked in 3-D—A First
Deep-Voiced Men Have Lower Sperm Counts, Study Says
How a Man Produces 1,500 Sperm a Second
Sperm Recognize “Brothers,” Team Up for Speed

Christine Dell'Amore, environment writer/editor for National Geographic News, has reported from six continents, including Antarctica. She has also written for Smithsonian magazine and the Washington Post. Christine holds a masters degree in journalism with a specialty in environmental reporting from the University of Colorado at Boulder. Her book, South Pole, was published in 2012.
  • Cathy

    While hiking in East Sooke in late June, I noticed “cloudy” water around some wave swept rocks. Could this have been barnacle spawn? The tide was high so I couldn’t really see what sea life was clinging to those rocks. If they were typical, there would have been mussels, gooseneck and acorn barnacles and various seaweeds. There was lots of bull kelp in the area too. I’ve been searching for clues on line and was lead to this site. Fascinating stuff. I’d love to know what I was seeing in the water.

About the Blog

Researchers, conservationists, and others share stories, insights and ideas about Our Changing Planet, Wildlife & Wild Spaces, and The Human Journey. More than 50,000 comments have been added to 10,000 posts. Explore the list alongside to dive deeper into some of the most popular categories of the National Geographic Society’s conversation platform Voices.

Opinions are those of the blogger and/or the blogger’s organization, and not necessarily those of the National Geographic Society. Posters of blogs and comments are required to observe National Geographic’s community rules and other terms of service.

Voices director: David Braun (dbraun@ngs.org)

Social Media