There’s something very strange living in Loch Creran, Scotland. Between around six and 10 metres deep is a reef unlike any other on the planet.
In an area of around 108 hectares – that’s more than twice the size of Vatican City – bursting with colour exploding from tiny, wispy fireworks.
But this isn’t one of the vibrant coral reefs you might be familiar with. It’s made by another animal: a marine tubeworm called the serpulid worm (Serpula vermicularis), also sometimes known as the organ-pipe worm, plume worm or fan worm.
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“The organ-pipe or serpulid worm (Serpula vermicularis) is a beautiful marine tubeworm with a showy crown of feathery tentacles,” writes NatureScot on its website. “Bright red, pink and orange, these contrast with its hard white tube.”
The worm creates these calcium carbonate tubes to protect itself from predators. These are attached to something solid, such as rocks, shells or the seafloor.
When the worm detects movement in the water that could signal potential danger, it dashes back into the safety of its cylindrical home and plugs the entrance shut with its operculum, which is a bit like a trap door.
When the worms are sealed tight inside, the tubes look like drinking straws poking out. When it’s safe to do so, they extend their feather-duster-like tentacles into the water column to try to catch a morsel to eat. To feed, they filter tiny particles, such as plankton and algae from the water.
The most famous type of serpulid worm – the festively named Christmas tree worm – adds a pop of colour to many reefs around the world. It’s commonly seen by scuba divers and snorkellers but not in the same numbers that plume worms are found in Loch Creran.
“Individual organ-pipe worms occur almost worldwide, but only rarely grow together as ‘reefs’,” adds NatureScot.
The largest of all these rare reefs is in Loch Creran where hundreds of tubeworms have built their homes. Here, the bushy mounds of worm reefs can each grow up to around 75cm high – about as tall as a Labrador – and one metre across.
When all these worms are feeding together, the reef is ablaze with the colour of their plumes.
In the same way that corals support other animals, the reefs at Loch Creran give many other creatures somewhere to stay.
“By cementing these tubes together, they can create fantastic underwater castles,” says Argyll Hope Spot on its website, adding that these habitats “provide shelter and habitat for other animals, such as sponges, sea squirts, seaweeds, spider crabs, squat lobsters, starfish, sea urchins, brittle stars and terebellid worms,” sometimes called spaghetti worms.
“In one survey, a reef of 0.1m2 supported 163 different species,” they add.
These spectacular but fragile reefs are easily damaged by fishing gear, boat anchors and divers’ fins. Loch Creran became a Marine Protected Area in 2015 to try to protect these special habitats.
Reefs like this one have been found elsewhere but some have been wiped out – and experts aren’t entirely sure why. “Living reefs were once found in Linne Mhuirich, an inlet by Loch Sween, but these died out in the 1990s for reasons still not understood,” says NatureScot.
There seems to be a boom-and-bust pattern. “We now believe that serpulid reefs go through natural cycles of growth and collapse. Individual structures eventually get so large that they can no longer support themselves,” they add. “Fallen worm tubes can continue to grow up from the seabed. But if they die, the dead tubes form new habitat for the settlement of serpulid larvae and the reef continues to grow and spread.”
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