Big or small: Research describes how fish get their shapes
CLEMSON, South Carolina — With more than 4 million specimens, the fish collection at the Smithsonian National Museum of Natural History in Washington, D.C., contains the largest ichthyology assemblage in the world.
Stacked shelf upon shelf in the museum are jars of preserved fishes, some collected before the United States officially became a country. After several years of study, Clemson University assistant professor of biological sciences Samantha Price knows many of the specimens all too well.
For the past three summers, Price has been recording the shapes and sizes of the fishes at the Smithsonian as part of her research aimed at understanding the factors that control body-shape diversity.
“A typical fish, like a trout – that’s what we think of as fish-shaped. But a seahorse, that’s decidedly quite different from a little fish. Then you’ve got things like puffer fish, boxfish or ribbon fish that literally look like a ribbon,” Price said. “So you have all of this incredible diversity, but what drives it? Are there particular patterns within those shapes determined maybe by their ecology or their habitat or feeding?”
Plenty of hypotheses exist to explain why the 36,000 species of fishes belonging to the Teleostei infraclass are so varied. Many ideas focus on the importance of habitat or dietary differences for driving changes in body shape. For example, it is known that fishes that eat tiny microscopic prey, called zooplankton, have larger eyes, smaller mouths and streamlined body shapes.
The caveat, however, is that many of these hypotheses are based on scientists’ observations, with little data to back them up. This is where Price’s research earns its significance.
By wielding a caliper and taking photographs, Price and her students – a team comprised of Clemson University undergraduates, graduates and postdoctoral scholars – collected wide-ranging measurements of the fishes, including their standard length, body depth and jaw width. By doing so, the team has begun to quantitatively test the long-standing hypotheses of body-shape diversity. While measuring with a caliper renders an exact measurement for each category, taking a photo of a fish allows Price to make an overall estimate of the fish’s shape. Ultimately, the two datasets will be compared – with one dataset complementing the other, Price hopes.
This summer marks the last round of data collection for Price and her co-principal investigator, professor Peter Wainwright of the University of California-Davis. The pair and their students are now moving toward analyzing their data. With so many hypotheses to test, they are starting with two basic questions: Does size constrain shape? Have fishes living in freshwater evolved different shapes compared to those in saltwater? A third project is on the horizon as a Creative Inquiry group of 11 undergraduates are developing their own hypothesis that they will test using the data.
“We have these three-dimensional shapes that we’ve delineated for teleost fishes, so now we can look at a particular family and see how it is taking up that space. Do all families occupy similar amounts of space? Do some occupy unique regions of shape space? Do fishes with extreme ecologies, such as deep-sea fishes, also have extreme shapes?” Price said. “To analyze these data, we need to use a phylogeny (evolutionary history) with all of the species, which requires thousands of DNA sequences. The work involves an abundance of data.”
Over the course of the project, the team has gathered records on more than 5,000 species of fishes; although, being that there are often multiple specimens of each species, the actual number of fishes studied tops 7,000.
Seven-thousand and one specimens later, Price says she still is amazed by the variability found in each fish.
“We did this project because fishes are so diverse, but then when you actually start measuring, it’s just incredible how diverse they actually are. For instance, I thought catfish were very speciose – meaning they have lots of species – but their morphology is incredibly diverse as well,” Price said.
In the final step of the study, Price and Wainwright will work to digitize the Smithsonian’s Fish Collection as well as the collections of several other natural history museums into a database that is freely available to the public.
The team’s research is supported by a National Science Foundation grant (award number 1556953) titled “Disentangling the ecological drivers of body form diversity in teleost fish species.” The researchers are wholly responsible for the content of this study, of which the funder had no input.