Today I had the pleasure of accompanying my daughter’s fourth grade class to the “Creatures of Light: Nature’s Bioluminescence” exhibit at the American Museum of Natural History. Beyond making sure that all students returned home safely, I was also interested in how this exhibit explained bioluminescence as an evolved adaptation.
When I teach Evolution, one of the most important skills that I want my students to gain is what I would call “adaptive thinking”. Adaptive thinking starts by asking the questions:
Why might this organism have this particular trait? What survival and/or reproductive benefits might this trait convey?
These questions get at the adaptive value of a trait, and should produce a list of possible explanations of the trait’s function. Each of these explanations must make it clear how the trait might provide an advantage in the context of a particular environment, with both abiotic factors (like temperature or amount of light) and biotic factors (like predators or competitors) considered part of the environment.
Coming up with hypothesized adaptive value of traits is only the first step. If you can satisfy yourself with the brilliance of your own adaptive explanation and require no further proof, you will be accused of being an “adaptationist” who fails to test your hypotheses (Gould and Lewontin 1979): not good! To finish the job of a scientist, you must actually come up with a way to test these hypotheses, which means you must answer these questions:
If my hypothesis is true, what predicted observations would support my hypothesis? Are there tests that I could perform that would produce data that would distinguish between my hypothesis and other ‘competing hypotheses’?
This second half of doing science is the most difficult, as it is far easier to invent explanations than it is to devise meaningful tests of these explanations.
As I entered the Creatures of Light exhibit, I was curious about how much adaptive thinking would be on display, and more importantly how that adaptive thinking would be communicated to visitors. Bioluminescence is the perfect theme through which to teach adaptive thinking because most people will arrive wondering why it is that certain organisms give off their own light. Does this exhibit capitalize on this curiosity to foster better understanding of evolution?
I am happy to report that it does. The exhibit is filled with examples of bioluminescent organisms ranging from microbes to fungi to deep-sea fish, and the hypothesized value of being bioluminescent to each organism is generally well-communicated. We see the many roles that self-produced light can play in sending signals to other organisms. Male fireflies try to entice females of the same species with a specific pattern of flashing, but should be wary of predatory species that can mimic the female’s response and lure him to be food rather than a father. Bioluminescent spots signal the toxicity of prey species to potential predators. Bioluminescent bays are created by dinoflagellates and present a mystery, as the suggested reasons for their bioluminesence are varied and unconfirmed.
Some of the most spectacular adaptations on display are employed by predators as a lure to prey. Through interesting sculptural models and a short-but-informative video we learn about the lack of light (in particular red light) in the deep sea and how coevolution has produced red-colored prey and predators that employ both blue light (as a lure) and red light (as a means of seeing prey). We also learn about siphonophore colonies, which are aggregates of potentially free-living cells that coalesce to capture fish with their bioluminescence (although the social dilemmas faced by such aggregates are glossed over). There are the spectacular angler fish and viper fish, each with its own bioluminescent lure dangling over its head to optimize the proximity of prey. Perhaps my favorite prey lure is used by glowworms, which hang from the ceilings of caves and secrete sticky tendrils designed to shimmer in their light and attract newly-metamorphosed flying insects.
Light is used as a defense by some prey as well, who can either illuminate their bottom (ventral) side to counter-illuminate and thus camouflage themselves (as many fish do) or even slough off a bioluminescent shell of themselves when attacked (as one species of sea cucumber does).
There is even a little bit on the cooperative benefits of bioluminescence, as a short video shows the spectacular wave-like synchronization of firefly signals that are thought to emerge from the group benefits bestowed upon those flies that signal together. We also learn about several mutualistic relationships that involve bioluminescence, including the iconic pony fish, who has a specific organ dedicated to fostering the growth of bioluminescent bacteria. Like fireflies, pony fish males must use light signals to attract a mate, but can only do so with the help of their domesticated bacteria. Visitors can also learn about how bioluminescence allows for quorum sensing in light emitting bacteria.
As I suggest above, it is important for the general public to understand the difference between an untested hypothesis (i.e. conjecture) and a well-tested hypothesis. Overall this exhibit does a good job of making this distinction, although there is no explicit explanation of the importance of this distinction. The language used is appropriately equivocal (“this creature might use its bioluminescence for…”) when a given hypothesis has not been tested, and what is unknown is even more clear when multiple hypotheses are listed. Perhaps most valuable are the short segments nested into interactive displays that report on experiments that have been performed to test bioluminescence hypotheses. There’s a short explanation of how clay models were used to show that beetles are advertising their defensive toxicity through their bioluminescent spots, and a great set of diagrams explaining how predatory firefly mimicry was discovered.
Another important evolutionary concept that is well-covered in the exhibit is convergent evolution. The role of two key biomolecules (luciferin & luciferase) in producing light across a wide variety of taxa was emphasized throughout the exhibit, and the section on glowworms explained how worms on two different continents independently evolved the same means of using bioluminescence to lure their insect prey.
There are parts of the exhibit that fail to provide an explanation for bioluminescence in particular species, and sometimes evolution is treated solely as phylogeny without any explanation for why species diversified. But in the defense of the exhibit designers these omissions occur for organisms whose bioluminescence is poorly understood.
For those interested in less ultimate biological concerns, the exhibit also provides proximate explanations of bioluminescence. Visitors will find great info on mechanisms of firefly bioluminescence, as an interactive display provides a clear step-by-step explanation of the biochemical and physiological mechanisms that allow fireflies to control their flashes of light. Oh, and in case you were concerned about taxonomy, there are plenty of places where you will be reminded that fireflies are actually beetles.
In terms of design, this is a pretty sophisticated exhibit, especially given the constraints involved in creating an exhibit about organisms whose most interesting characteristic is impossible to produce in the museum space. There are a lot of dioramas and models showing what glowing mushrooms on a forest floor or dangling glowworms in a cave might look like. There are plenty of videos that capture both aquatic and terrestrial bioluminescence in vivo. But by far the best features of this exhibit are its abundant and diverse interactive displays. The interactive displays of old (like many in the AMNH‘s Hall of Biodiversity) tend to be clunky to operate and frequently broken. But the invention of reliable and durable touch screen technology — on both the hardware and software sides — seems to have really allowed museum designers to improve the way information is provided to visitors. This exhibit features a series of small touchscreen devices in each of three sections: air, land, and sea. Each invites users to take a seat and dynamically explore the exhibit. These devices are the means by which motivated visitors can delve deeper into the adaptive value of bioluminescence and learn about the experiments used to test adaptive hypotheses. A section that explains the difference between bioluminescence and fluorescence allows visitors to manipulate a touchscreen and toggle between views of visible and UV-fluorescent spectra, uncovering secret parts of the reef only revealed by fluorescence. There is even a really fun interactive display that tracks the movement of visitors and replicates that movement as a swarm of projected virtual dinoflagellates. Throughout the exhibit the use of technology is tasteful and minimal: there are no unnecessary flashing gadgets where technology is not needed to teach.
Perhaps the only disappointing aspects of the exhibit were the two live displays. A small container of bioluminescent bacteria designed to show pony fish light was filled with either dead or uncooperative microbes, and the “flashlight fish” tank was just frustrating enough for visitors trying to make out what was actually in the tank to motivate many to ignore the “please don’t illuminate the tank with your cell phone” sign.