Damselfishes are one of the largest fish families in the world, with at least 422 different species, including one of the most instantly recognizable fish of all, the clownfish of Finding Nemo fame. These small, colorful fish mostly live around coral reefs in tropical and temperate waters and make popular pets for aquariums.
They’re also a very diverse group of fish, with different body sizes, feeding habits, and unique behaviors. Some species, like the three-spot damselfish in Florida and the Caribbean, are territorial and aggressively defend their habitats. Others, like the longfin damselfish in Belize, tend to patches of algae like farmers, herding and protecting shrimp that fertilize the “crops” with their excrement.
Because of this fascinating diversity, damselfish are of keen interest to evolutionary biologists. Mark Westneat, Professor of Organismal Biology and Anatomy at the University of Chicago, has been studying them for more than 25 years, SCUBA diving and collecting samples around the world from the Florida Keys to French Polynesia.
“I'm interested in the patterns of evolution, historical ecology and feeding mechanisms in marine fishes that are really diverse,” Westneat said. While he grew up in landlocked Ohio, he had the chance to visit the Caribbean in high school and later spend much of three years in Australia as a graduate student. Ever since then he’s been fascinated by marine biodiversity.
“The damselfishes are a case study in how we can tackle an important group that's ecologically important and evolutionarily interesting, building a tree of life and studying traits to yield some insight into how their history has been shaped over the years," he said.
In a study published this October in PLoS One, Westneat continued his study of this remarkable group of fishes, building a new phylogeny, or tree of life that shows how different species are evolutionarily related to each other, for 345 of the 422 known species of damselfishes. The phylogeny was created by analyzing DNA sequences taken from samples collected by Westneat and his colleagues over the years, and from collections at the Field Museum, where Westneat was a Curator of Fishes from 1991-2013 and continues as a Research Associate, involving students in collection-based research.
The study is an academic multigenerational effort, with a team consisting of all former or current UChicago graduate students. Lead author Charlene McCord, now an Assistant Professor at California State University, Dominguez Hills, earned a PhD at UChicago in 2014, and co-author W. James Cooper, now Assistant Professor of Biology at Western Washington University, earned his PhD in 2006. The final co-author, Chloe Nash, is currently a PhD candidate in the Committee on Evolutionary Biology.
Using the newly completed tree of life, the team used computational and statistical methods to look for clues that could be driving the diversification of different damselfish species over time, since the group first emerged about 55 million years ago. In particular, they wanted to study the associations between body size and feeding habits with patterns of species diversification.
Algae farmers and the ‘wall of mouths’
Damselfishes generally have three different feeding behaviors. Some are herbivores that hang around the bottom of the ocean, tending to their algae farms or feeding on other plant detritus. Others spend their time higher up in the water column, hovering around the edges of coral reefs and forming a “wall of mouths” to feed on plankton. A third group does a bit of both, mostly spending time at the bottom but occasionally popping up to feed on plankton.
The team’s phylogenic analysis showed that medium-sized, planktivorous damselfishes tended to diversify into different species at the highest rates. Large and small body sizes have evolved independently at least 40 times, and species transition between different feeding styles as they diversify as well. Some groups of damselfishes like the small, brightly colored, planktivorous Chromis genus, find an evolutionary niche that works and remain relatively stable with few new species. But species from the opposite end of the spectrum, like the herbivorous, bottom-feeding giant damselfishes, find the same success as well.
“We see a rich mosaic of ways of life, with free and frequent changes in some groups, and kind of stasis in others,” Westneat said.
Solving the tree of life
Damselfishes, like all the creatures in the sea, are affected by climate change, especially since they live around coral reefs that are undergoing rapid bleaching and die-offs. Developing such a complete and detailed phylogeny will be important for understanding how new conditions might impact their ability to thrive.
“We're teasing apart the history of evolution and the biology of one group of coral reef fishes that are being impacted by climate change and threatened in various parts of their range,” Westneat said. He and his colleagues plan to keep searching for more rare species of damselfishes and learning what they can.
“The more species we have in our tree of life, the more we can be certain that the patterns we’re seeing are real,” he said. “Solving the tree of life is one of the grand challenges of science. Once you have a well resolved family tree, you can do a lot of cool evolutionary biology to understand where these species came from, and where they might be headed.”
