[UK] They’ve taken sunbathing on sandy beaches to a whole new level. They also have a curious tendency to collect together in enormous groups, forming a kind of superorganism.
These curious green flatworms – found along much of Europe’s Atlantic coast – run on inbuilt solar power, meaning they never have to worry about eating. Instead, they laze about on beaches soaking up the sun’s rays and, according to new research, taking the time to socialise.
The bright-green complexion of Symsagittifera roscoffensis, commonly known as the plant-animal or mint-sauce worm, results from free-swimming Tetraselmis convolutae algae that they consume as juveniles. Instead of being digested, the algae take up residence in the worms’ tissues, where they pump out nutrients through photosynthesis.
The algae provide a constant food source that means their hosts never have to eat again, and in return the algae gain a worm-shaped home for life. This mutual arrangement has become honed over years of shared evolutionary history to the extent that algae consumption is now a crucial rite of passage that allows the worms to mature.
Safety in numbers
Nigel Franks of the University of Bristol, UK, first encountered these “wonderful creatures” while conducting fieldwork in Guernsey, and noticed them engaging in peculiar behaviour.
Franks calls it circular milling, referring to their tendency to arrange themselves into swirling formations when placed together in high densities. His team has shown in lab studies and computer simulations that individual worms interact to coordinate their movements so that even at low densities they begin to swim in small polarised groups, and at higher ones produce circular mills.
“It’s a characteristic of creatures that are highly social,” says Franks. “Prior to this, no one had thought of these worms as social at all.”
Circular milling is also seen in army ants, but Franks says this is an unfortunate by-product of their tendency to blindly follow each other’s pheromone trails – even though this can mean going round in circles until they die of exhaustion.
In contrast, the worms’ circling behaviour might serve a useful function. “They’re pulling more and more worms into a really high density,” says Franks. Ultimately, they form a large biofilm that shares a virtually continuous mucous sheath consisting of millions of individuals - creating a kind of superorganism.
By forming such large agglomerations, Franks and colleagues say, these worms could be behaving a bit like emperor penguins in the Antarctic by using fellow colony members as living shields against environmental extremes.
The collective behaviour also allows the worms to act as a kind of dynamic “social seaweed” that may improve their stability, colonising sandy areas that regular types of seaweed can’t because of a lack of places to anchor themselves.
Their photosynthetic partners provide the worms with everything they need, so they can simply wriggle into the sand for cover when the tide comes in and avoid being swept away.
The large size of the colonies also provides safety in numbers, and may even allow the worms to engage in chemical warfare with potential predators through the collective production of dimethylsulphoniopropionate.
For Franks, these worms exemplify the power of cooperation. “It’s a classic story,” he says. “When organisms are social, it can add a whole new set of possibilities to their lives.”
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2015.2946
View original article at: Coral bleaching study examines algae’s protective effect during marine heatwaves