In 2005 Dr. Annie Mercier was sent a strange video: sea cucumbers rolling around at the bottom of the Atlantic Ocean.
Sea cucumbers – sausage-shaped marine animals with soft, cylindrical bodies – can usually be found on the seafloor where they gather together in large herds. Suckers on their tube feet allow the animals to stick to the ocean floor as they crawl along in search of food.
“I thought it was weird behaviour for sea cucumbers, but figured a storm might have detached them from the seafloor, and then I forgot all about it,” said Dr. Mercier, professor, Department of Ocean Sciences, Faculty of Science.
“A few years later I was sent another video, this time of dozens sea cucumbers rolling around the seafloor off Nova Scotia.”
Odd behaviours
By that point, Dr. Mercier had been studying the local species of Cucumaria frondosa for a while and had seen other odd behaviours, including watching them bloat up and become rounder when predators were present.
Her students had also reported similar problems when transferring sea cucumbers from tank to tank.
“Suddenly they wouldn’t anchor anymore and sometimes it would take 24 hours or more for them to release the water they had taken in and settle down,” she said.
“Over the years, my grad students and I also had conversations at international conferences with people who were ranching sea cucumbers in aquaculture sites around the world and they said they had seen little sea cucumber seedlings floating around in their sea pens.
Dr. Mercier says she knew then it was no longer anecdotal. She set out to gather more evidence and conduct controlled lab experience to see if she could trigger the behaviour, see what was causing it and how it was developing.
Triggering reactions
The findings became the basis of a paper published earlier this year in the Journal of Animal Ecology. The team behind the paper named the behaviour active buoyancy adjustment, or ABA, and found it could be triggered when the animals were in very high densities in the tank and also when they encountered certain water conditions.
“We found when we decreased the salinity or increased the turbidity of the water, ABA was elicited,” said Dr. Mercier. “That allowed us to measure a number of body indices – the water-to-flesh ratio and the response time – to see if certain conditions created a stronger reaction.”
They ultimately concluded ABA behaviour helps sea cucumbers move away from unfavourable conditions.
“Their water-to-flesh ratio also increases by as much as 700 per cent.”
In mildly unpleasant conditions, such as when there were too many sea cucumbers competing for food in one area, they would become slightly buoyant for a short period of time so they could spread out from each other.
Strong and rapid responses occurred when the animals were in more threatening and stressful conditions, such as when water conditions were poor.
“They need to get away quickly, so they let go completely and when this happens, that’s when they roll,” said Dr. Mercier. “Their water-to-flesh ratio also increases by as much as 700 per cent, so instead of being 30 per cent water and 70 per cent flesh, it gets completely inversed, and they become 70 per cent water content. That makes them neutrally buoyant and they can stay that way for hours or days as long as the conditions persist.”
Critical implications
A partnership with a Madagascar aquaculture facility also provided additional information that allowed them to determine correlations with moon and tide cycles in another sea cucumber species.
Their findings could have critical implications for the worldwide management and conservation of these commercially and ecologically significant animals. Sea cucumbers are a highly prized, luxury seafood.
“We looked at a few conditions, but there are many others – especially for sea cucumbers that are being ranched,” said Dr. Mercier. “Facility operators will want to avoid those that trigger ABA or adapt their sea pens with nets going right up to the surface – something that wasn’t considered necessary for a benthic animal.”
She’s hoping to expand their data to include more sea cucumber species, as they come in a variety of body shapes. The behaviour has also been noted in sea stars, soft corals and sea anemones.
“This behaviour has implications for our whole conception of benthic communities and how their population structures get modulated by things such as climate change, pollution and turbidity events.”
