Volume 2, Issue 12
A Shrimp's Eye View
In the deep blue ocean a tropical mantis shrimp puts on a fantastic show for its brethren. Patterns on its sides and back light up in fluorescent yellows and greens while its legs flash red and white. The shrimp opens its mouth appendages and reveals a brilliant blue glow. These dazzling displays are invisible to the naked human eye, but UC Berkeley marine biologist Roy Caldwell has seen them. In fact, he discovered most of these secret signals.
Roy Caldwell on a boat headed to a research site on Lizard Island, Australia. (Gloria Caldwell photo)
"The mantis shrimp has the most sophisticated eyes of any animal on the planet," Caldwell says. "But we simply didn't know why they needed such a complex eye."
After nearly two decades studying the intricacies of stomatopod eyes, Caldwell and his colleagues recently determined that the eye has evolved to see things that human beings need special filters and lights to observe. Mantis shrimp, from the order Stomatopoda, use fluorescent markings and polarized patterns to signal and perhaps warn each other. After all, Caldwell and UC Berkeley integrative biologist Sheila Patek have shown that the mantis shrimp has the swiftest kick in the animal kingdom. Its calcified club appendage is a deadly weapon, capable of killing prey with one blow.
"Once they evolved that lethal weapon, it was imperative that they be able to recognize each other," Caldwell says. "One mistake and you could lose a head."
The mantis shrimp Lysiosquillina glabriuscula in threat posture, photographed near Key Largo. The stomatopod was photographed in blue light with a yellow filter (above) to show only the fluorescence, similar to what other mantis shrimp see at depth. (Roy Caldwell photo)
In late 2003, Caldwell and his collaborators reported the first documented case of sea creatures using fluorescence to signal. The breakthrough came when Caldwell and Thomas Cronin of the University of Maryland analyzed video shot by Physical Sciences Inc.'s Charles Mazel. Using a fluorescent lighting system to capture the colorful undersea life off the Bahamas, Mazel had accidentally filmed a mantis shrimp displaying two unusual "headlights." In shallow water, the yellow-green spots enable tropical mantis shrimp to recognize members of its own species. However, the animal also lives at depths of 40 meters where there's no yellow light to reflect off the spots. The video revealed that the stomatopod uses fluorescence to keep up the signal.
"At those depths, the animal does a neat trick of taking in the blue light in the ocean and fluorescing yellow," Caldwell says. "So the fluorescent pigment enables it to maintain its species-specific signal. In fact, its eyes are tuned to pick up on that color."
As he investigated further, Caldwell observed similar fluorescence in most of the species in the superfamily Lysiosquilloidae, but the story of the stomatopod signaling gets even stranger.
"I was taking photos of a mantis shrimp in our aquarium and there was a reflection I couldn't get rid of," he says. "So I grabbed a polarizing filter to knock out the reflection and as I turned it, the animal started flashing red and white at me."
This Haptosquilla banggai from Sulawesi, Indonesia has bright blue polarized patches on its first pair of mouth appendages that send a clear, but highly directional signal identifying the species and saying "This cavity is taken!" (Roy Caldwell photo)
It turned out that some mantis shrimp in the superfamily Gonodactyloidea also display and detect strong polarized signals, light waves that are oriented in a particular direction. Think of polarized sunglasses that reduce glare by blocking horizontally-oriented light. In the case of a stomatopod, the polarized patches on parts of its body look pinkish to the unaided human eye. But seen through a polarized lens, light reflected off those patches reads as bright red or white depending on the orientation of the animal.
"We already knew stomatopods had excellent polarized vision," Caldwell says. "But they see at least three angles of polarized light. It's the equivalent of wearing three different pairs of polarized sunglasses at various angles. Any time the animal moves, the eye will pick up these signals as flashing."
In May, Caldwell received a very special care package from his former graduate student Mark Erdmann, now working in Indonesia. Inside was a living stomatopod that Erdmann picked up on a deep dive near Sumatra. Much to Caldwell's surprise, the animal, a Squilloid, exhibited more polarized signaling than any of the Gonodactyloids the researchers had studied.
"Those two superfamilies are very far apart," Caldwell says. "So that suggests that this ability to produce polarization came from their common ancestor. It's a trait that's been around for a very long time."
This pair of Odontodactylus latirostris are courting. Only the male (left) has a polarized patch of cuticle on a scale near his head. The red signal is exposed by viewing the animals with the aid of a polarizing filter. (Roy Caldwell photo)
Currently, the researchers are surveying an array of stomatopod eyes and identifying the myriad polarized and fluorescent signals the animals display. Of course, the long-term goal is to deduce the purpose of each signal, if there is one, and how the signals and eyes co-evolve. The stomatopods live in a rough environment where hiding places are few and predators are many, Caldwell explains. In some cases, a stomatopod might use a polarized signal to let another shrimp know that a cavity is occupied.
"The polarized blue signal on the first pair of mouth appendages is hidden unless the animal shows it," he says. "And even then it can only been seen from a narrow angle. It's like having a spotlight you can flash at a specific individual to say: 'Hey, I'm in here. The cavity is occupied!''"
In other cases, the combination of fluorescence and flashing could signal another stomatopod's sexual intentions. A fluorescent male shrimp spinning its polarized legs like an eggbeater is not easily missed, Caldwell says.
"As we pick up little bits and pieces about these animals, we can put together an entire narrative about their evolution and how it all can be tied back to the innovation of that dangerous weapon 300 million years ago," Caldwell says.