Video: Giant glowing worm lights up the ocean
Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: colonies of the genus Pyrosoma
Habitat: open ocean in warm tropical and temperate waters
Ever feel that you're not that coordinated? Just imagine what life would be like if you were part of a giant colony of tiny individuals that all have to do the same thing at the same time.
Huge free-floating coalitions of marine invertebrates known as pyrosomes have to move together to ensure the colony can feed and move in the right direction. They lack any common nerves to communicate, so they may have a different way to move in time – light signalling.
Pyrosomes are made up of hundreds or thousands of clones called zooids. The entire brightly lit colony sprouts from a single individual, and the zooids mesh themselves together as the colony grows outwards in concentric circles from a closed tip to an ever-widening mouth. When the colony is small it looks rather like a butterfly net. As it lengthens, it becomes more like a giant worm that can reach the length of a sperm whale.
The zooids can reproduce by cloning, so the colony can regenerate injured parts and theoretically live forever, shrinking and growing based on available food and physical disturbance.
Huge but invisible
These giant glowing worms can't help but stand out. Yet pyrosomes remain an enigma. With so few people actually having seen them, when a video was posted on a major news site earlier this month saying it was showing pyrosomes, it took several days before anyone spotted that what had really been caught on camera was a mass of squid eggs.
The portraits of pyrosomes are fleshed out with anecdotes. Some divers say that swarms are as soft and delicate as feathers, whereas others claim they are tough enough to ensnare and drown unfortunate penguins.
Why do we see so few of these amazing creatures? According to Mangesh Gauns of India's National Institute of Oceanography in Goa, it is not because pyrosomes are rare.
It is partly because they spend the day in the ocean depths and partly because we're not looking in the right places, he says. Go to the right areas and Gauns is confident that colonies would be abundant.
After analysing the water conditions off the coast of India where a swarm was found, he now thinks he knows what to look for: a combination of cyanobacteria that are small enough for the zooids to swallow along with the right mineral balance in the water. Gauns says these conditions should be commonplace far enough from coastal waters, which are dominated by larger plankton that can block the filter-feeding system of the pyrosomes.
"I am excited to see if we can use these results to get this organism studied more openly," he says.
Light signals
The movement and feeding of the colony is a joint effort and the whole pyrosome is like a giant filtration system. Each zooid sucks in water from outside the colony and blows it out again the other side. This not only feeds them but creates a rudimentary jet engine to give them some control over where they drift to.
Shutting off this propulsion system allows the colony to sink out of harm's way – they regularly dive down to 500-700 metres and have been collected from as far down as 3000 metres. But because they are made up of so many small zooids, coordinating their actions isn't easy. Unpublished research from David Bennett then at Bangor University, UK, offers tentative evidence that this is where a pyrosome's impressive light show comes in.
When a pyrosome is brushed by an external object, it lights up like a Christmas tree – in red or white depending on the species. The signal ripples through the individuals, and they respond by cutting off their engines. Just think of a second world war U-boat film. When the red lights start flashing, it's time to dive.
Journal reference: Zoological Studies, DOI: 10.1186/s40555-014-0075-6
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