Caldwell's Lab:

According to Integrative Biology professor Roy Caldwell, there are two kinds of organismal biologists. There is the biologist who focuses on a specific problem and, says Caldwell, "if they need to do animal research, they choose the appropriate animal for the question." The other kind of organismal biologist "gets to know the group of animals so well that the animals start asking the questions."

Caldwell is one of the later. His group of animals, stomatopods, has been asking him questions for over 40 years. And now, Caldwell has discovered that stomatopods, commonly known as mantis shrimp, are using a kind of communication that's unique among animals, and remains mostly hidden from view: they are sending each other signals using circularly polarized light.

Stomatopods are small crustaceans. They range in size from about 2 to 40 centimeters, and though they look like shrimp, they are not exactly close relatives to the curled pink creatures of cocktail fame. Stomatopods' taxonomic group split off from the decapods, the group that includes shrimps, lobsters and crabs, about 400 million years ago. Says Caldwell, "they don't really have many close relatives." One of the things that make stomatopods so unique is their amazingly complex pair of eyes. "The stomatopod eye is the most complicated eye in the animal kingdom, period. There's nothing that comes close to it," says Caldwell. "They see color using not three visual pigments like we do, or four or five as is known in a few other animals, but sixteen."

Two of Caldwell's collaborators, Tom Cronin at the University of Maryland Baltimore and Justin Marshall at the University of Queensland in Australia, investigated the cellular structure of the stomatopod eye, and found that the eye is capable of seeing linearly polarized light. Fishermen wear polarized sunglasses to help them see fish swimming underwater. When the light reflects off of the surface of the water, it becomes linearly polarized—the light rays are reflected up vertically from the water's surface. Polarized lenses block that light, so the glare is removed and the fish are easier to see.

Light can also be linearly polarized when it scattered by small particles drifting in the air or water. And a polarizing filter can cut down the resultant glare. Says Caldwell, "cross-polarization is really good to look at transparent things in the water column," especially if the water is full of particulate matter. Caldwell, Cronin and Marshall thought that perhaps the stomatopods' ability to see linearly polarized light could improve their ability to detect predators. At this point, Caldwell, Cronin and Marshall knew that stomatopods could probably distinguish polarized light, but they had no evidence that stomatopods were taking advantage of this ability.

A few years later, Caldwell, an accomplished photographer, was taking pictures of some stomatopods in a tank in his lab. "I couldn't get rid of a reflection," says Caldwell. "So I did what usually you do when you have a reflection--I went and got a polarized filter. And put it on [the camera lens] and I rotated it and the animal started flashing red and white at me... Pieces of the animal were flashing, and I quickly realized that it was strongly polarized." Caldwell had never seen this before. "So I went around the lab looking at every other stomatopod species we had, and many of them, it turned out, were polarized." After this discovery, Caldwell and his collaborators did some behavioral experiments and realized that stomatopods weren't using polarized light to detect predators--they were using it to communicate with each other. These stomatopods' exoskeletons look drab and dull to our eyes, and the eyes of potential predators, but to the stomatopods' own polarizing eyes, they look brightly colored. This allows stomatopods to recognize other individuals of the same species.

Linearly polarized light rays move in one direction--say vertically upwards, when light is reflecting off of the surface of the water. But light can also be circularly polarized; in this situation, the light rays are spiraling like a corkscrew. Caldwell knew that stomatopods were capable of emitting and receiving linearly polarized light, but could they see circularly polarized light too?

About 15 years earlier, animal vision specialist Horace Barlow pointed out that the structure of the stomatopod eye was such that they probably could detect circularly polarized light. So Caldwell and his colleagues, while doing field work in the Caribbean, would often talk about the possibility of a stomatopod species that could emit and receive circularly polarized light. They thought "there's got to be one species out there that does this," says Caldwell.

They had a prime suspect: Odontodactylus cultrifer. It has a big keel on its tail--"friends of mine describe it as a shark fin," says Caldwell. They knew that the keel was linearly polarized, "but it was particularly dull, for an animal investing in this structure. It wasn't a bright flag like you would have expected it to be. It was also sexually dimorphic, much bigger in males than in females. So I always felt that if there was one animal out there that used circular polarization that this would be the animal." But cultrifer was impossible to find. "We couldn't get them," said Caldwell. On his lab website, Caldwell has a few pages about the care and feeding of stomatopods, for amateur aquarists. He put up a notice on his website, which currently reads: "If you see one of these animals for sale, I would very much appreciate your contacting me since I need more to finish up some research on signaling."

"As luck would have it, a couple of years ago I got a call from an amateur aquarist who collected stomatopods," says Caldwell. "He saw one for sale in a tropical fish store in Fremont California, so I immediately called them and they'd already sold the animal. We eventually tracked town the person who bought it and I convinced them to give it up in the name of science." Caldwell struck a deal with the aquarist, and traded the cultrifer for "a better stomatopod for a home aquaria."

Once he had the cultrifer in his lab, Caldwell used a circular polarization analyzer to look at the keel. "It lit up like a Christmas tree." Not only that, but "one side was right circular polarized and one side is left circularly polarized"--one side had the threads of the electric field's screw wrapping in a clockwise direction, and the other side had the threads wrapping counterclockwise. Caldwell called Cronin and Marshall to tell them the good news. They had been studying the structure of the eye in more detail, to see if the species could theoretically see circularly polarized light. Says Caldwell, they "were convinced the structure was right." Next, they did some behavioral studies; cultrifer was capable of seeing circularly polarized light, and could even distinguish between the clockwise and counter-clockwise spirals of light.

Caldwell thinks that cultrifer use their ability to see circularly polarized light in their mating displays. The species are sexually dimorphic—the males have much larger keels than females. "The whole story just came together," says Caldwell. "It's got to be the most private communication channel known, because there is just a handful of animals that produce circularly polarized signals, and only one that has specifically evolved the ability to see it." Next, Caldwell plans to study interspecific communication, between the stomatopods and their predators, small octopuses. He wants to learn whether there has been co-evolution in their communication systems.

Like stomatopods and octopuses, sometimes it seems like scientists and non-scientists are separate species, and they have difficulty communicating. Caldwell has been the Principle Investigator of Understanding Evolution, an educational web site created by the University of California Museum of Paleontology (UCMP) to help people understand how evolution works, why it's relevant, and how scientists study it. So far, the site has been wildly successful: it has earned rave reviews from teachers, has received several million hits, and has been translated into several different languages.

Now Caldwell and his collaborators at the UCMP are creating another website, called Understanding Science. While they were putting together Understanding Evolution, Caldwell, UCMP Assistant Director Judy Scotchmoor and IB professor David Lindberg often talked about why the American public struggles to understand evolution. "It always fell back on, well, it isn't just evolution. They don't understand science," says Caldwell. The Understanding Science web project seeks to explain the process of science. The linear sequence of hypothesis-experiment-conclusion, as it is often taught in textbooks, is not an accurate portrayal of how science usually happens. Caldwell's research is no exception.

"I can't tell you how many times I thought I was done with stomatopods. Every time I'm about ready to give up, something new comes up." Science does not always proceed linearly--the process is often much more circular.


Caldwell teaches two animal behavior courses:




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