We often look at skeletons (when we look at them at all) as reconstructions: bones dug out of the ground or dissected out of a body, then carefully fitted and wired back together. But this process can lose the skeleton’s natural orientation, leaving scientists to wonder how it would really look inside the animal. A vivid technique called clearing and staining offers just that perspective.
As a side effect, cleared and stained skeletons are strikingly beautiful. But not many people outside the lab would ever know it—until now.
"Cleared" is an exhibit of stained fish skeletons currently on display at the Seattle Aquarium. The fish were prepared and photographed by Adam P. Summers, a biology professor at the University of Washington’s Friday Harbor Labs who also holds a research associate position at UC Berkeley. He uses the images for his research in biomechanics, a field that blends physics and biology.
To visualize a fish by clearing and staining, the body must first be fixed in formalin, then dehydrated in alcohol. The scientist-cum-artist then alternately stains the parts he wants to see (blue for cartilage, red for bone) and clears the parts that get in the way (bleach takes care of pigments, enzymes digest everything else). The result is a 3D view of the skeleton as it naturally fits together inside the body.
Such a view can offer scientific insight into otherwise intractable questions. Recently, Summers and his colleagues used a cleared and stained devil ray (a member of the group that includes manta rays) to discover how these curiously flat fish filter food out of the water.
Misty Paig-Tran, now a postdoctoral scholar at Arizona State University, led the study, which was published in the Journal of Morphology. The scientists used a variety of imaging techniques, but as Paig-Tran wrote in an e-mail, “the cleared and stained specimen helped us to understand the very strange gill arch morphology in the devil rays.”
Gill arches are structures shared by all fish (and present even in human embryos) that support the gills and are often modified for feeding. Devil rays are not the only fish that use their arches to filter out the sea's tiny edible particles, but the rays' flat bodies give their arches an unusual shape—and, it turns out, an unusual filtering technique.
Paig-Train, Summers, and collaborator Thomas Kleinteich uncovered evidence in the devil rays for something called cross-flow filtration—a mechanism that’s better known from the wine and soft drink industries. Instead of pumping fluid directly through a filter, which can easily become clogged, fluid is passed perpendicular to the filter. The water slips out and the particles stay in, while the perpendicular flow pushes the particles along toward the ray's esophagus. The filter cleans itself.
This finding was exciting enough, and the manta ray pretty enough, that it was chosen as the journal cover.
“I have been putting these images on the covers of scientific journals since I started my career,” Summers wrote in an e-mail, “and I often used them in holiday cards.” But he’d never planned an art show, until one day the director of the Seattle Aquarium visited Friday Harbor and saw some posters that Summers had hung “to tart up the lab.” Cleared was born.
The show will travel to northern California sometime in the spring; Summers plans to post dates and venues on his website and on twitter @fishguy_FHL.
An early version of this post incorrectly identified Mobula thurstoni as its cousin, Manta birostris.