Most of the world's habitable environments lie deep enough beneath the ocean that life forms have adapted to great pressures. Fish can suffer their own equivalent of the bends if brought to the surface too quickly. Consequently, it has been impossible to study these creatures in the lab. A pressurized chamber called SubCAS (Submersible for Ascending Specimens) is changing that, however, allowing us to capture species alive where we have never managed before.
“A rapid ascent can rupture a fish’s swim bladder,” said Dr Luiz Rocha of the California Academy of Sciences in a statement. Of course, people have treated the bends for decades using hyperbaric chambers, but something suitable for fish is new. In Frontiers in Marine Science Rocha describes how SubCAS incorporates a collecting jar inside a larger chamber. Both are made of transparent plastic so divers can keep watch on their charges as they ascend.
Divers using closed-circuit rebreathing equipment can reach depths beyond traditional SCUBA equipment. There they capture fish at depths of up to 150 meters (500 feet), known as mesophotic depths, or more colloquially the Twilight Zone because so little light penetrates. SubCAS operators stop twice on the way up – once to blow an air bubble into the lid of the chamber, and a second time to hand the contraption to a support team to bring to shore. “The air bubble is critical,” said co-author Matt Wandell of Monterey Bay Aquarium. “As it expands during ascent and helps maintain the pressure inside the chamber.”
“Using an adjustable pressure control valve, we ensure that the pressure inside the chamber is similar to the depth where the fishes were collected,” Rocha said. “Over the course of two to three days, we bring them to the surrounding surface pressure in a highly controlled manner.”
Lead author Dr Bart Shepherd told IFLScience SubCAS has been used on research expeditions since 2014 on reefs across the tropical Pacific. Of 174 fish brought to the surface, 155 survived. Most were successfully flown to the Steinhart Aquarium for inclusion in an exhibition of mesophotic species.
In recent years we have discovered the existence of biologically rich coral reefs at depths below 60 meters (200 feet). These reefs formed during the last Ice Age and survived the oceans' rise. They may stand a better chance of resisting human impacts than shallower equivalents, and could potentially serve as seedbanks to restore others if we stop our ocean degradation. However, for this to occur we will need to understand them better, which is where SubCAS comes in.
Shepherd noted hyperbaric chambers were used for fish collections on at least two occasions previously but “these designs were too large and cumbersome” for individual divers to carry.