Bone Worm Mating Revealed: Males Have “Penis” on Head

By Bradley Balukjian

Thousands of feet under the sea, bone worms give new meaning to the song “It’s Raining Men.”

These frilly pink creatures live on whale carcasses in harems that make free lovers of the 1960s look tame—up to 607 males cohabitating with a single female.

A female Osedax mucofloris. Photograph by the Natural History Museum London, Alamy

That’s only possible space-wise because the males are little more than bags of sperm—orders of magnitude smaller than the females—and they spend their lives hanging out inside the tubes of the females.

Greg Rouse, an anatomist at the Scripps Institution of Oceanography at the University of California, San Diego, was determined to figure out how these worms reproduce, given their bizarre anatomy and living situation.

For instance, do the females spray eggs into the open water, where males fertilize them, or does the love happen on the inside?

Rouse’s latest study, published in the current issue of Invertebrate Biology, suggests the latter—males, in fact, appear to have an “intromittent organ” (read: penis) at the tip of their heads that penetrates the females’ lady parts, in this case, the oviduct (which leads to the uterus).

“The males are putting their penis or their head up against the oviduct and somehow pushing their sperm through,” Rouse said.

How exactly this happens is still a mystery—it could be gentle, or it may be rough, as in “traumatic insemination,” in which the penis tears through the thin wall of the oviduct. (Also see “‘Torture’ Phalluses Give Beetles Breeding Boost.”)

Roll Out the Red Carpet

Internal fertilization is just the latest in a series of surprising discoveries about these marine worms, which were discovered in 2002 during an expedition to Monterey Canyon off the coast of California. (See “New Species of Naked Bone-Eating Worms in Antarctica.”)

Biologist Robert Vrijenhoek was looking for deep-sea clams when he stumbled on a whalefall (whale carcass) almost 10,000 feet (3,000 meters) down.

“It was covered in this red carpet of stuff and he had no idea what it was,” Rouse said.

Vrijenhoek sent samples to Rouse, who determined that it was part of the family Siboglinidae.

The “red stuff” was so different to science that Rouse put them in a new genus called Osedax; he quickly realized that the worms were confined to undersea bones—usually whale, but sometimes others.

No Walking Dead

Rouse and his collaborators gave Osedax the nickname “bone worms” for their preferred habitat and for the females’ ability to root themselves to bone and then secrete an acid to dissolve it for nutrition. (See more bone-worm pictures.)

Completely lacking a mouth and a gut, bone worms eat through their skin, although how this happens is still a mystery. Somehow they break down the collagen inside bones and pull it through their skin, where symbiotic bacteria further break it down, Rouse said.

To top it off, Rouse found that the worms sometimes turn on their own employees, eating the bacteria themselves.

Despite this diet of bone and bacteria, Osedax have been often called “zombie worms” by the media, a trend that baffles Rouse. (Related: “‘Zombie’ Worms Mate Inside Whale Bones.”)

“I don’t understand why they’re called zombie worms,” Rouse said. “Zombies, as far as I know, eat human flesh and brains.”

While Osedax may not get a guest spot on Walking Dead anytime soon, according to their Wikipedia page, they guest-starred on another popular TV show. Which one?

Bones, naturally.

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