Take a dive in the Mediterranean and you might be lucky enough to spot a female worm that looks like Shrek's gastrointestinal tract, moves like the molten insides of a bubbling lava lamp, and produces a potent toxin that gives it the power of sex determination over its own kind.

World, meet the green spoon worm – Bonellia viridis – and prepare for a wild ride.

Named for the shape they take when their proboscis (the worm equivalent of a feeding straw) is contracted, spoon worms (class Echiura) are difficult to collect and hard to tell apart, so it's no surprise that many of them remain poorly understood. The details we have uncovered about some representatives of the group, however, are sublimely strange – and B. viridis takes the cake. 

For starters, only female members of the species exhibit that signature emerald colouration. And in the Bonella world, "wearing" green is an option reserved for the ecologically elite. 


Green spoon worms begin life colourless and adrift in a sexually undifferentiated larval state: whether they end up male or female depends on where they settle – and if that landing zone is located in "unclaimed" waters. 

"If the spoonworm larva lands on the seafloor it becomes female and begins to secrete a potent toxin called bonellin," writes deep-sea ecologist Dr Andrew Thaler over at Southern Fried Science. Bonellin is responsible for a female worm's green hue, but it also plays an important role in the survival of her genes.

"Should [another] larva come in contact with this toxin, it will be masculinized and sucked into the spoon worm's body through her feeding proboscis," explains Thaler.

Once inside, that freshly "made" male will receive a life sentence: sperm-bank duty, without parole. The female holds masculinised larvae in her genital sac to be used like an on-demand fertiliser factory.  

Formally known as "environmental sex determination", this mechanism has been confirmed only in this particular spoon-worm species, but it's possible that others employ it, too. "Entombed" males have been found in B. viridis's close kin, but we don't know if similar biochemical warfare is involved in their development as well. 

"It's such a unique phenomenon," says Kagoshima University researcher Dr Masaatsu Tanaka, who recently described a new spoon-worm species discovered in the Seto Inland Sea. "Whether other Bonellids [there are 80!] follow this method is a very interesting topic."

One such cousin, a similarly shaded spoon worm called Metabonellia haswelli, was recently encountered off the coast of Australia:

Diver PT Hirschfield found the lone worm under a rock near Victoria's Port Phillip Bay. The sighting was a first for the local resident, despite the fact that he's completed over 800 dives in the area. "It reminded me a lot of a balloon animal in the process of making itself," Hirschfield told Storyful.

Some commenters suggested that the oscillating "bulges" seen in the worm are signs of a recently consumed gluttonous meal. It's a reasonable guess, but spoon worms slurp down only tiny edible particles. The blobs you see are actually the animal's own flesh. 

They might look like Jello-stuffed sausage casings, but these worms have surprisingly well-developed muscles. In fact, they have three layers of muscle, which surround a fluid-filled body cavity. Together, both of these structures help spoon worms achieve motion in the ocean.

By contracting their muscles in a wave-like pattern, the invertebrates send their internal fluid towards one end of the body. As fluid bulges in the direction they want to go, the muscles relax, squeeze and repeat. (This is how your own body moves food down your throat, only the worms use the process to scoot themselves elegantly along.) 

Image: Adventures with SPAK/YouTube

You can see these contractions in action by taking a gander at the distantly related spoon worm Urechis unicinctus, a species found in coastal mudflats that's commonly referred to as the "fat innkeeper" or "penis fish".

[The common name doesn't lie – what follows is NSFW viewing, so proceed with caution.]

The choice of shallow habitat is one that's shared by most of the spoon worms that have been well studied over the years. But as we continue to explore the ocean's murky depths, these animals are turning up in some of our planet's most extreme environments.

"I think there are many undiscovered species, especially in the deep sea," says Tanaka. "Some have been documented at 10,000 metres!" Revealing the details of their lives, though – let alone their sex habits! – will be a slow process. 

"Even for already described species, there are many puzzles left behind by brief original descriptions, lost [specimens], misidentifications and more," says Tanaka. "Even worse, the scientists working on Echiura are few – maybe fewer than ten persons in the world. As a taxonomist of this group, however, I also enjoy this [challenging] situation and hope to solve these puzzles one by one."



Top header image: The spoon worm Metabonellia haswelli. PT Hirschfield/YouTube screengrab