Deep below the waves, on the seafloor of the Salish Sea, a small creature’s tiny movements could help scientists understand the complexity of deepwater environments. 

A small yellow sponge—researchers have nicknamed it Belinda—has been the centre of a four-year study conducted by UVic’s Ocean Networks Canada (ONC) to understand these complex animals. Once considered a plant by marine biologists because of its sedentary nature, this animal is more active—and resilient—than previously thought. 

Over the course of the study, Belinda was recorded making frequent “sneeze-like contractions,” lasting up to a day each time, to clear out debris built up from filter feeding. 

For Sally Leys, lead researcher on the study and resident sea sponge expert at ONC, these ongoing contractions were a surprise, especially given these animals don’t have muscles.

“I didn't know they'd be that active,” said Leys. “I honestly didn't know the thing would go up and down and bounce around and, you know, basically look like a little bouncy ball on the seafloor.”

Instead of moving like a typical animal, relying on muscle contractions, these animals use their cells to create their tiny movements. It was also found that these cell changes help sea sponges survive the winter months.

Not unlike bears, Belinda became dormant each winter, contracting to half its size by moving its cells tightly together. It appeared not to feed or move during this time. 

These little changes are miniscule if you’re not paying attention—and most scientists weren’t or couldn’t until this study. Sea sponges have been monitored in labs, but this was the first time a long-term study has been conducted on a single sponge in its natural environment. 

Belinda was selected out of chance—when ONC was laying down its deep-sea monitoring system, it was nearly placed on the sponge, but researchers made a point of avoiding it and placed their camera to directly face Belinda. The team looped Leys into the project to advise on the sponge, and the monitoring study began.

Sentinels of the sea

Leys says sea sponges are sentinels for the ocean’s well-being. Most animals on the seafloor move around and are difficult to monitor long-term.

“There's not many things on the seafloor that stay put and can be watched over the timeframes that we know climate changes at,” Leys said. 

She says coral has typically been the bellwether for changes in warmer ocean waters, but once they bleach, they die quickly. Sea sponges, however, seem to be more resilient—and Belinda continues to prove this point. Much like coral, sea sponges are a habitat builder for other animals, their “corner store” as Leys calls them. 

One fascinating behaviour witnessed was during the 2023-2026 marine heatwave—also known as “The Blob”—that saw a section of the Pacific Ocean warm above average. Belinda’s colour darkened from a healthy pale yellow to a deep orange and its texture became lumpy. Leys says it’s not clear if the sponge’s appearance changed due to damage or if it had created a kind of shield to protect itself. 

Leys thought Belinda was toast—”It looked awful”—and saw a sea slug, or nudibranch, feeding on the sponge. But Belinda survived, and it could be that the nudibranch was helping slough off its damaged outer layer, a sort of symbiotic dynamic. But The Blob was an unusual phenomenon and more data will need to be gathered before ONC can understand what this behaviour entails. 

Since no marine biologist is hoping for another damaging heatwave, Leys says she’ll reenact the conditions of The Blob in their lab at Banfield Marine Sciences Centre.

“We have just gotten some pretty nifty tanks that we will be able to control the temperature and the different parameters on, and that would allow us to address that question maybe sooner than filming another Blob,” said Leys.

There should be a lot of time in the future to study Belinda and other sea sponges—these animals can live for hundreds, sometimes thousands, of years. Belinda is a spring chicken at only 15 years of age.

“[There aren’t] many animals on the sea floor that we know exactly how old they are, but it would be really nice to know if it gets to 50—we can have Belinda birthdays,” Leys mused. 

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