Thousands of meters below the ocean surface, deep-sea sponges appear to have a fairly motionless existence. However, their dynamic lives have been uncovered by researchers from the Monterey Bay Aquarium Research Institute (MBARI) using time-lapse footage taken over 30 years.
At a study site called Station M, located 4,000 meters (more than 13,000 feet) underwater and about 220 kilometers (136 miles) offshore of Central California, seemingly static seafloor animals have been under surveillance. Whilst scanning through some of this footage, Amanda Kahn, a former MBARI postdoctoral fellow, came across something unexpected.
“Everyone was watching sea cucumbers and urchins snuffling around on the seafloor, but I watched the sponge. And then the sponge changed size,” Kahn said in a statement. “That was the initial eureka moment,” she added, “we didn’t know to look at the sponges before.”
Glass (Hexactinellid) sponges, so-called because of their skeletal structures made of silica glass, were seen expanding and contracting over time. Unlike the brittle and fragile nature of glass that we’re more familiar with, the sponge’s glass structures, called spicules, overlap one another without fusing together. By changing the amount of overlap, this allows the sponges to cyclically expand and contract.
This was the first time that glass sponges had been seen exhibiting this behavior. However, previous studies had observed this action before in freshwater sponges and had likened it to a person sneezing.
“There is precedent for sponges contracting and expanding,” Kahn explained. “Basically, there’s an ‘ahh‘ when the sponge expands and the ‘-choo’ when it contracts those canals.”
In the previous studies, freshwater sponges appeared to be irritated by particles in the surrounding water that they had filtered for feeding, triggering them to expel the particles. Although the researchers are unsure of what caused the response in the case of the glass sponges at Station M, the “sneezes” lasted from hours to weeks.
Alongside the tulip-looking glass sponges, Kahn and her colleagues detailed a further eight species of sponges and anemones from the site in their paper published in Deep Sea Research Part II: Topical Studies in Oceanography. This included the “spotted biscuit sponge” (Docosaccus maculatus), which moved in and out of the seafloor camera’s field of view over several months.
“The deep sea is a dynamic place, but it operates on a different timescale and with different stimuli than our world,” Kahn remarked.
“It was necessary to have the time-lapse [imagery] for us to see that these animals respond to changes in their environment. We don’t know what they’re responding to yet, but we’re going to study that next."