Glass sponges in reefs off the coast of British Columbia “cough” to keep sediment out of their pores, according to new research that is shedding light on these mysterious but ecologically important invertebrates.
A team from the University of Alberta has discovered that glass sponges exhibit a range of responses to sediment in the water. Small particles of sand and dirt can clog the pores the sponges use to filter silica out of the water to build up their intricate skeletons, so the sponges appear to have evolved defensive behaviours to protect themselves from suffocation.
“Sponges do the equivalent of holding their breath,” says Nathan Grant, a Masters student in the department of biological sciences who presented the team’s findings at the 2018 World Conference on Marine Biodiversity (WCMB) in Montreal last weekend.
Glass sponges don’t have nerves, but they do have a sensory system. When sediment is detected, the organism sends an electrical signal through its tissues, suspending the pumping mechanism that takes in water. They are essentially turning themselves off and on again, Grant explains, much like a person would do with a malfunctioning computer.
When sediment hangs about in the water for longer periods of time, the sponges may repeat this process several times. “When you look at the data, it almost looks like the sponge is coughing, trying to test the water to see whether it's clear enough to be able to breathe and feed again,” Grant says.
The team also identified a new behaviour which Grant calls a “prolonged arrest.” This behaviour was observed in a species of sponge that seems to be highly sensitive to sedimentation — the fingered goblet sponge.
While bouts of “coughing” can last anywhere from two to 10 minutes each, prolonged arrests can be up to six hours, says Grant. That’s a long time for an organism that needs to constantly pump water throughout its body.
Muddying the waters
Sponges require a certain amount of sediment over time to help them build up their reefs, says Sally Leys, who leads the sponge research team at the U of A and has studied the organisms for almost 30 years. “But we humans...have a terrible tendency to kick up more [than they need].”
In 2017, the federal government designated the glass sponge reefs of Hecate Strait, B.C., a Marine Protected Area (MPA) and restricted fishing activity within the MPA’s borders.
But these protections don’t shelter the sponges from the large amounts of sediment thrown up by bottom trawling next to the reefs.
And trawling isn’t the only problem: last month, a video surfaced showing what looked like a lifeless glass sponge reef, apparently buried in waste from an ocean pen salmon farm at the surface. “The amount of fallout from something like a salmon farm is just ridiculous, so it wouldn't take very long for sponges to completely be smothered,” says Leys.
Glass sponge reefs were thought to have gone extinct some 40 million years ago, but in the late 1980s, living reefs believed to be 9,000 years old were discovered off the B.C. coast in Hecate Strait and Queen Charlotte Sound. Rising out of the blackness of the deep ocean in shape-shifting structures of ethereal white or yellow, these “living fossils” provide vital ecosystem services.
Their physical structures provide refuge to creatures ranging from shrimp to juvenile fish, including some commercially important species, and the sponges help clean the water by filtering out bacteria and particulate.
“The sponges rely on the animals that they harbour and the animals they harbour rely on the sponges,” says Leys.
Without them, she adds, these aquatic neighbourhoods would become quiet places, with more than tenfold fewer animals.
Leys and Grant hope that by shedding light on how excess sedimentation affects glass sponges, their research will lead to stronger protections for these unique animals.