The boat loaded, we push off from shore. We are headed out for a nighttime blue-water scuba dive in search of salps off the Pacific coast of Panama.
Salps are transparent, barrel-shaped organisms, ranging in size from about 0.5 to 5 inches long, that live in all oceans except the Arctic. Yet few people have ever seen one because they are adapted to live far from shore. This evening's objective is to videotape swimming salps using a harmless, bright green fluorescent dye to better see how they move and feed.
Salps swim and eat by rhythmic pulses. Each pulse draws seawater in through a siphon opening at the front end of the animal. Then the salp contracts muscle bands, and the water shoots out another siphon at its rear end, producing a jet that propels the animal forward. In the process, food particles in the water (mostly tiny plankton) are caught on a mesh of mucus strands inside the salp's mostly hollow body.
In other words, this simple pumping simultaneously controls both feeding and swimming. It also formed the basis of the research for my Ph.D. dissertation. The animals get energy from food and expend energy to move. If these essential functions are coupled, how do various species of s alps balance their energy resources?
There are about 40 species of salps with a wide diversity of body shapes; perhaps some species are good at filtering lots of seawater through their feeding mesh at the expense of being good swimmers, and vice versa. Since no one had explicitly considered these trade-offs before, I wasn't sure what I'd find out.
This seemingly esoteric question has ramifications for the marine food web and even Earth's climate. Salps filter huge amounts of phytoplankton, tiny photosynthetic marine plants that take up carbon dioxide in waters near the ocean surface and convert it into organic carbon. The salps package this carbon-rich material into dense, fast-sinking fecal pellets that provide carbon for other organisms that live deep below the surface. If those pellets sink deep enough, carbon is removed from the surface, where it might otherwise exchange with the atmosphere, and instead is sent into the depths, potentially for thousands of years. People today talk about sequestering the excess carbon dioxide that has built up in our atmosphere in the oceans; salps have been facilitating this process for millions of years.
Into the black
Several years of work and collaboration have led up to tonight's dive. Research objectives aside, though, it is just plain exciting to be out here, the warm wind blowing against our skin, the expanse of stars over our heads, and the anticipation of diving below the inky surface of the ocean.
The Liquid Jungle Lab, where we are doing this work tonight, is on a small forested island off Panama's Pacific coast. The lab was conceived and built by Italian businessman Jean Pigozzi, who has a passion for science, technology, and conservation. This spot is not only beautiful, it turns out to be ideal for studying oceanic plankton such as salps, which are transported close to shore by local currents.
Dan Martin, a marine biologist at the University of South Alabama, is our safety diver tonight. He will float near our central downline, which is attached to a surface float and to the boat. He will keep a watchful eye on me and marine biologist David Kushner. David runs the Kelp Forest Monitoring Program at Channel Islands National Park in California. I am extremely lucky to have him dive with me.
We will be secured to tethers attached to the same downline, but any time you're on a science dive, a working dive, you're not really focused on your equipment or where you are. You're focused on doing the task. You're in a dark, limitless space, and you can end up drifting up or sinking way down. So it's important to have someone keep an eye on you.
Once beneath the surface, the myriad forms of plankton stand out against the dark background, illuminated by our flashlights. …