Sex is Weird: Echiurans

It’s been a while since I last shared something from the weird world of sex with you. It’s time for that to change.

Strange creatures from the deep.

This episode takes us to sandy flats along the coast of the northeastern Atlantic Ocean. If you find yourself scuba diving there , you might see a long, thin, fleshy, blue-green... thing with a sort of fan on the end. No, that's not nitrogen narcosis; that's the proboscis of a green spoon worm (I swear I'm not making this up). The scientific name for this creature is Bonellia viridis, and it's a member of the echiuran worms.

You're seeing this too, right? Photo credit:  Christian Coudre  .

You're seeing this too, right? Photo credit: Christian Coudre.

The Green Spoon Worm, in all her resplendent glory. Photo Credit:  Sylvain Ledoyen

The Green Spoon Worm, in all her resplendent glory. Photo Credit: Sylvain Ledoyen

Scientists are not in complete agreement about how echiurans fit into the scheme of things (which is pretty common with taxonomy), but most evidence places them in phylum Annelida, the segmented worms (leeches, earthworms, polycheates). The main body of B. viridis looks sort of like a blue-green sausage. They burrow into soft sediment, or occasionally hide in rock crevices, where they use their proboscis, which is very elastic, to pick up tiny particles of food from the area surrounding their burrow.

Another interesting fact about those green fan-tongued sausages is that they are all female. Well, at least the ones that you see are. 

Where are the males? 

Actually, all of the males were eaten when they were juveniles. More precisely, they became males only after being eaten by a female. That’s right; larvae only develop into males if they’ve made contact with the proboscis of a female. Otherwise they will become females themselves. Female B. viridis produce a compound called bonellin, which gives them their color. This chemical also triggers newly hatched larvae to develop testes, and to make their way into the genital sack of the females. They will live the rest of their lives inside the female releasing sperm, and relying on her to provide nutrition. There can be up to 20 males inside a single female. Talk about a complicated relationship. Oh, and bonellin is highly toxic. It's a strong antibacterial agent and can also paralyze small animals. Sort of puts a new spin on the phrase "It'll make a man outta you" doesn't it?

So, an immature worm larva gets picked up by the paralytic toxin-laced fan tongue of a female that's searching for something to eat, and then it becomes an endoparasite that does little more than produce sperm. Sex is weird.

Check out these other Sex is Weird posts:

Sex is Weird: Mating Types
Sex is Weird: Echiurans


Murina, G. 2013. Bonellia viridis. Accessed through: World Register of Marine Species at on 2014-01-20

Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill, Higher Education.