Sometimes the best way to answer a question like "How many animals on the bottom of the ocean glow?" is to just go down there and poke some sea creatures with a robot arm. That's how researchers found out that the pitch-black seafloor in the Bahamas is alive with bioluminescence. They also found glowing currents full of plankton, a crustacean with the world's slowest vision, and creatures that vomit light when provoked.
In the middle depths of the ocean, making your own light is ordinary. Around 80 percent of fish and crustaceans that live here are bioluminescent, and the skill is also common in squid and other cephalopods. These animals may light up to lure prey, confuse predators, or attract a mate.
Species that live on the bottom of the ocean are less well studied, for the obvious reason that they're on the bottom of the ocean. So researchers led by Sönke Johnsen of Duke University and Tamara Frank of Nova Southeastern University set out to begin a census of bottom-dwellers. They descended in a submersible to the floor of the Bahamas to knock on some doors.
The team made 19 dives to depths between 500 and 1000 meters. Once at the seafloor, they began scooping up samples of all the species they could find, using the submersible's robotic arm. On some of their dives, they turned off the sub's lights and sat in total darkness: they could see life glowing all around them. The researchers tested individual creatures for bioluminescence by gently poking them with the robotic arm. If something glowed in response, they grabbed it or sucked it up for further study.
Bioluminescence, at least in the limited regions the researchers were able to explore in their dives, was rarer than in the middle depths of the ocean. Less than 20 percent of the species they observed could glow. The skill was most common among sea anemones, bamboo corals, and coral relatives called sea pens (so named because some of them look like old quills).
Although the ability to light up was uncommon, the light on the seafloor was abundant, thanks to bioluminescent plankton. Drifts of these tiny animals are carried on the currents and ping with light whenever they collide with another object. "Where there are 'tree-like' animals...that stick up from the bottom," Frank says, "the tiny plankton that flows by in the currents get stuck on it."
The frequent flashing of drifting plankton could be one reason bioluminescence is a less common skill on the seafloor, the authors write. It might not be worthwhile to try to generate bioluminescent signals when the visual noise of plankton is drowning everything else out. Another reason not to bother with bioluminescence on the seafloor could be that the uneven terrain, punctuated by corals and other stationary creatures, blocks signals from traveling far. In the middle of the ocean, light can travel in every direction.
Yet the large eyes on some of the specimens they gathered told the researchers that light was still an important signal for them. When the team brought their subjects back to the surface to photograph them and measure the light they emitted, they found that the stationary animals (such as anemones and corals) gave off a greener light compared to the bluish glow of mid-ocean animals. This might be an adaptation that lets light travel farther through the cloudier water of the ocean floor.
Most of the bioluminescent creatures that Johnsen and Frank observed glowed suddenly when bothered, then faded over the next few seconds. But a few were more creative with their light production: they found a sea pen that pulses with half-second flashes; a bamboo coral made up of polyps that flash individually, creating a twinkling effect; and a shrimp that vomits light in self-defense. ("In the deep dark depth, squirting a blindingly bright fluid into the face of a predator is certainly going to distract it," Frank says, "allowing the spewer to get away.")
When the researchers collected crustaceans from the ocean floor using baited traps, they got even more surprises. One was an isopod with the slowest vision ever recorded in a crustacean. The researchers measured the "flicker rate" of the crustaceans' eyes, which is the number of images the eyes send to the brain every second. In humans it's about 60, which means movies shown a little faster than 60 frames per second look seamless to us. In the crustacean Booralana tricarinata, the flicker rate is just 4.
Gathering only four snapshots of the world each second, the isopod is probably unable to follow the motion of even slow-moving prey. It's possible, the researchers say, that the animal instead uses those slow eyes to scavenge. Its long-exposure vision might let it see the subtle glow of bioluminescent bacteria living on its food. In other words, though it can't see motion, the isopod sees a whole glowing ocean-floor world we'll never be able to.
Sönke Johnsen, Tamara M. Frank, Steven H. D. Haddock, Edith A. Widder, & Charles G. Messing (2012). Light and vision in the deep-sea benthos: I. Bioluminescence at 500–1000m depth in the Bahamian Islands The Journal of Experimental Biology DOI: 10.1242/jeb.072009
Tamara M. Frank, Sönke Johnsen, & Thomas W. Cronin (2012). Light and vision in the deep-sea benthos: II. Vision in deep-sea crustaceans The Journal of Experimental Biology DOI: 10.1242/jeb.072033
Image: Bioluminescent plankton by NOAA (via Wikimedia Commons).