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Clams engage in wacky, competitive group sex, study finds

"I’d periodically leave and come back a few hours later and they were still at it."

It's a tough life for a shipworm. Stuck to the same piece of wood since they were teeny larvae, these rapidly-growing clams need to get creative to feed themselves, and also reproduce with nearby shipworms.

To manage this task, the wood-eating, giant feathery shipworm has evolved a frenzied, no-holds-barred approach to sex, a new study published in the journal Biology Letters reveals.

The researchers captured this shockingly competitive sexual behavior, known as pseudocopulation, for the first time on video.

Capture on video for the first time, shipworms engage in pseuodocopulation - a unique sexual behavior.Dr Reuben Shipway, University of Portsmouth

What they discovered — To survive, the giant feathery shipworm (N. fusticular) feeds on wood which scarcely leaves the water.

These clams grow fast — a blessing and a curse. To increase in size, the shipworm eats more wood, reducing the size of their home.

As a result, shipworms have to be cutthroat. Many shipworms might live on a single piece of wood, fighting pitched battles for food — and love. For the first time ever, researchers have managed to film one of the clams' frenzied battles for sexual partners in action.

“Most of the time, they’re not very interesting to look at. I just happened to be in the aquarium one day, and I noticed there was this thick plume of creamy liquid in the tank, which I knew had to be eggs and sperm,” Reuben Shipway, lead author on the study and a fellow at the University of Portsmouth, explains in a press release accompanying the research.

“When I got a little closer, I realized the siphons of these animals were just going crazy and I was witnessing a sexual frenzy, and I decided to film it," Shipway says.

Shipworms in pseudocopulation.Dr Reuben Shipway, University of Portsmouth

These marine bivalves grow rapidly to sexual maturity. Some become shockingly large — up to 1.8 meters long — a height advantage that ups their chances of reaching to inseminate other shipworms which are far away. In the case of these clams, bigger really is better.

How they did it — Researchers studied 79 shipworms in an aquarium setting, recording hours of footage of this spunky creature having some of the wildest sex the animal kingdom has witnessed.

“I’d periodically leave and come back a few hours later and they were still at it," Shipway says.

What's new — If you're going to successfully transfer your sperm to a nearby shipworm, you're often going to have to fight for the privilege, the study reveals.

The scientists observed for the first time the shipworms engaging in a unique competitive behavior known as pseudocopulation, which involves wrestling with your rivals to win the right to inseminate a nearby shipworm.

"When we first noticed these animals reproducing in the aquarium, we couldn’t believe what we were seeing. They were using their siphons to wrestle and inseminate one another and trade sperm,” Shipway says.

Shipworms display two fairly long siphons — excurrent and incurrent — which provide their only contact with the outside world beyond their wooden burrow.

Shipworms in pseudocopulation.Dr Reuben Shipway, University of Portsmouth

In pseudocopulation, the excurrent siphon teases the surface of the wood until it finds its prize: the incurrent siphon of another shipworm, which is wide open and ready for sex.

The shipworm then roughly winds its siphon around the other's, penetrating and transferring its sperm. Finally, the receiving shipworm expels the fertilized eggs into the water.

But the shipworm can be picky when it comes to their sexual partners. Scientists noticed some recipient shipworms brutally rejecting and expelling the sperm out of its incurrent siphon.

Digging into the details — The researchers observed up to six individuals engaging in a sexual orgy.

In these instances, many shipworms would fight with rival mates by pulling their desired recipient's siphon out of reach while pushing a competitor's siphon away. In one case, a shipworm even removed its rival's sperm from the surface of an incurrent siphon.

But the ferocious clam can multitask, too. The scientists recorded shipworms both received and donating sperm — at the same time.

While the clam was giving sperm via the excurrent siphon, it would be receiving it from another shipworm through their incurrent siphon.

A shipworm.Dr Reuben Shipway, University of Portsmouth

We're familiar with male animals engaging in heavy competition for sex. But these clams can switch sexes throughout the breeding season, and also occasionally display hermaphroditism.

As a result, the researchers observed both male and female shipworms initiating sexual behavior by introducing their excurrent siphons to nearby recipients — a relatively distinct trait.

There's also a chance that shipworms may favor those who demonstrate reciprocity in sex, since many were both recipients and donors of sperm. In other words: if you give, you shall receive.

What's next — But further research on this clam's intense sex life is needed to confirm these scientific curiosities.

"Shipworms have evolved a stunning diversity of reproductive strategies, some simply spawn their eggs and sperm into the water, some recruit a harem of dwarf males to mate with, and now we know they compete to directly inseminate each other using their siphons,” Shipway says.

Abstract: Shipworms are predominantly wood-eating bivalves that play fundamental roles in biodegradation, niche creation and nutrient cycling across a range of marine ecosystems. Shipworms remain confined to the wood they colonize as larvae; however, continual feeding and rapid growth to large sites degrade both food source and habitat. This unique lifestyle has led to the evolution of a stunning diversity of reproductive strategies, from broadcast spawning to spermcasting, larval brooding and extreme sexual size dimorphism with male dwarfism. Some species also engage in pseudocopulation, a form of direct fertilization where groups of neighboring individuals simultaneously inseminate one another via their siphons—the only part of the animal extending beyond the burrow. Among the Bivalvia, this exceptionally rare behaviour is unique to shipworms and remains infrequently observed and poorly understood. Herein, we document pseudocopulation with video footage in the giant feathery shipworm (Bankiasetacea) and novel competitive behaviours, including siphon wrestling, mate guarding and the removal of a rival’s spermatozoa from the siphons of a recipient. As successful sperm transfer is likely greater for larger individuals with longer siphons, we suggest that these competitive behaviours are a factor selecting for rapid growth and large size in species that engage in pseudocopulation.
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