Spirorbis

Image: Malcolm Storey, bioimages.org.uk

Oooooh! This is what happens when snails and worms share ideas! Who new annelids were into biomimicry?

Spirorbis is a genus of at least 20 tube-building, polychaete worms. They're utterly puny at barely 5 millimetres long!

Image: Malcolm Storey, bioimages.org.uk

As you'd expect of any tube worm who respects the tube worm way of life, they live in a tube. It's made of tough calcium carbonate which they secrete from special glands. However, unlike most tube-builders, their tubes are neatly coiled into what look like teensy snail shells.

Another part of the tube worm way of life is a strict lack of wanderlust. None of this laborious sliding around from place to place as you see in snails, Spirorbis worms spend their entire lives firmly plastered to fronds of seaweed.

Video: Chincoteague Bay Field Station

They capture tiny particles of food using a small collection of stiff, transparent tentacles. There's quite a contrast between the minute Spirorbis and some of their extravagant relatives in the Serpulidae family, like the Christmas Tree Worm and the Coco Worm. Spirorbis appears to have renounced the showy feather dusters and lurid colours of their kin and taken on a life of quite contemplation halfway up a bit of seaweed.

When predators come along or low tide threatens to expose them to the ravages of fresh air, these worms close the entrance of their tube with a special, spoon-shaped tentacle.

Image: Malcolm Storey, bioimages.org.uk
Look at the little baby!

Many species are hermaphrodite and can even fertilize their own eggs. Otherwise, they release their own sperm into the water and capture someone else's with their tentacles. They can then keep hold of the sperm until their eggs are ready. Even once the eggs are fertilized the parent will keep them safe within the tube.

Image: SADERNE
Left: Larva that'll drift around before settling
Right: Recently metamorphosed youngster ready to build its tube

The eggs hatch into larvae that drift and swim around for a while before settling to build a tube of their own. Larvae don't necessarily leave their mother's shell as soon as the eggs hatch, some stay there for so long they can only swim around for a few hours before they have to settle down.

Image: Biopix, JC Schou

I suppose this limited time for travelling means they don't always get very far. Sometimes poor seaweeds get utterly smothered in hundreds of swirly worm shells. It looks like the macaroni art we all did in school!

On the other hand, one species called Spirorbis borealis appears to have spread very far indeed. It's difficult to tell species apart, but this particular one seems to have spread from the frigid waters of Iceland all the way down and around to Florida.

Sounds like the worm/snail conference needs to reconvene. I think the worms can teach the snails a thing or two about how to spread across the world without actually moving!