Watch the Wild, Never-Before-Seen Orgies of the Humble Shipworm

The mollusks give and receive sperm simultaneously, and pack tiny males into the pouches of much larger females.
December 9, 2020, 7:00pm
​Shipworm siphon wrestling. Image: Dr. Reuben Shipway, University of Portsmouth
Shipworm siphon wrestling. Image: Dr. Reuben Shipway, Uni

Shipworms, a family of marine mollusks famous for their wood-eating skills, don’t look like much upon first glance. But these weird wriggly creatures have shaped landscapes, inspired novel drug compounds, and altered the course of human history.

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Most salaciously, however, shipworms are also into all kinds of freaky sex stuff that nobody has ever seen before, according to a new study. Giving and receiving sperm simultaneously? No biggie for shipworms. Wrestling rivals with their reproductive organs? They do that, too. Housing harems of tiny males in cute pouches inside the far larger bodies of females? Some shipworms go for that.  

Taken together, this “stunning diversity of reproductive strategies” many of which are “exceptionally rare,” can shed light on the evolution and management of these bizarre animals, reports a paper published on Wednesday in Biology Letters.

“Describing the behavior, and saying we've seen this for the first time, is the first step,” said lead author Reuben Shipway, a marine biologist at the University of Portsmouth, in a call. “I'm hoping this paper is like a springboard for renewing interest in shipworms.” 

Shipworms engage in pseudocopulation, a form of direct fertilization. Individuals inseminate each other with the help of two tube-like “siphons” that roam outside the wooden burrows. The siphons grope around for each other until a donor finds a recipient, and then intertwine to trade sperm. Fertilized eggs are later released into the sea, and those lucky enough to land on wooden habitats can begin the cycle again.

Shipworm reproduction has not been extensively studied, which prompted Shipway—and co-authors Nancy Treneman of the Oregon Institute of Marine Biology and Daniel Distel of Northeastern University—to deploy pine panels off the coast of Charleston, Oregon, in November 2016. 

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By the time the researchers removed the structures in May 2017, dozens of giant feathery shipworms, or Bankia setacea, had bored into them (though they have “giant” in their name, the individuals are small, with millimeter-scale siphons). The inhabited wood was brought back to an aquarium to be monitored.

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Aquarium holding a shipworm-infested pine panel. Image: Dr. Reuben Shipway, University of Portsmouth

Shipway and his colleagues expected to observe pseudocopulation, but they did not anticipate the frenzied orgies and novel competitive behaviors performed by the bacchanalian bivalves. Footage of the animals reveals wrestling between siphons, the unexplained rejection of certain sperm donors, and in one memorable case, a siphon apparently removing a rival’s spent shot off of a potential mate.

“You had this one individual come over and it tried to perform copulation, and it basically missed,” Shipway said of the latter event. “It put its spermatozoa around the outside of the siphon. Almost immediately, another one comes over and instead of pulling or batting the other one away, it just wiped.” 

“Whether or not it meant to do that, or whether it was being clumsy, I don’t know,” he continued. “But it’s intriguing.”

Shipworms display “simultaneous, consecutive, and rhythmical-consecutive hermaphroditism,” according to the study, which means it can be hard to tell whether an individual is male, female, or perhaps both, at any given time during pseudocopulation

Individuals that rapidly grew to large sizes were generally more sexually successful because of the upper hand afforded by longer siphons. However, there are still plenty of open questions about the complicated sexual dynamics of the giant feathery shipworms, to say nothing of the many other diverse species of shipworm on Earth. 

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Given that these animals hide inside their wooden homes, they are tricky to observe and identify in the wild, which is why the new study revealed so many previously unseen marvels. “Because they're cryptic, you can't tell what species they are just from looking at a piece of wood and seeing siphons,” Shipway explained.

Watching shipworms have weird sex makes for great entertainment, but it could also have big implications for protecting coastal infrastructure, understanding the carbon cycle, and developing novel artificial compounds.

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A siphon depositing sperm. Image: Dr. Reuben Shipway, University of Portsmouth

As the only animals in the sea that have perfected the art of eating wood, shipworms cause billions of dollars of damage to piers, boats, and other marine structures each year. While this is a headache for beach dwellers and seafarers, especially in developing nations where wooden infrastructure is more common, shipworms also enrich their ecosystems with nutrients, such as carbon, extracted from their timber meals.  

“On an ecological level, these animals are simply amazing,” Shipway said. “They're processing all this carbon through their guts. In terms of the ecosystem, it is massively beneficial.”

“If we don't understand how they reproduce, we don't understand that process,” he added.

Moreover, the unique enzymes and bacteria that allow shipworms to digest cellulose are being used to develop new drugs and technologies, including antibiotics and biofuels. 

“There's this boom of research that's going on into them,” Shipway said. “We culturally forgot about them for a number of decades and now they are going through a little bit of a renaissance at the moment, which is really cool.”