For centuries scientists have used physical characteristics like size and color to separate one species from another. But new genetic studies show that these traits can be misleading, at least in certain kinds of fish.
Giacomo Bernardi, an associate professor of ecology and evolutionary biology at the University of California, Santa Cruz, and his colleagues investigated genetic differences in colorful coral reef fish called damselfish. Their findings, published in the March 22 issue of Proceedings of the Royal Society of London B: Biological Sciences, challenge long-held beliefs about damselfish species distinctions.
"What is a species is a fundamental question in evolutionary biology. Almost by chance we went straight to this issue," Bernardi said.
Bernardi's work focused on four closely related damselfish species in the genus Dascyllus. Three of the species are restricted to islands in the Pacific Ocean, such as Hawaii and Marquesas. The fourth, D. trimaculatus, ranges from French Polynesia to the Red Sea. Each species has a different coloration pattern; for example, D. trimaculatus is black with white spots, and D. auripinnis has yellow fins and belly.
The researchers looked at DNA from damselfish mitochondria, tiny cellular structures that provide energy to the cells of higher organisms. They sequenced a particular section of mitochondrial DNA from 122 different individuals representing the four damselfish species, which were established based on coloring. Bernardi and his colleagues compared the DNA sequences and used the similarities and differences to group closely related individuals.
Surprisingly, the groupings based on DNA sequences were not the same as the color-based species designations for two of the four species. The damselfish with the largest range, D. trimaculatus, genetically appears to be three different species. One of these new species is genetically indistinguishable from D. auripinnis, even though they have different coloring.
"We have found that the definition of 'species' is more complicated and a lot more interesting than we thought," Bernardi said.
The mitochondrial DNA region the researchers analyzed is short, but the large number of individuals studied strengthens their results. Bernardi plans to examine more DNA regions in these species to get a clearer picture of their relationships.
Damselfish are not endangered or even threatened, although they are popular in the aquarium trade. Nevertheless, genetic analyses like Bernardi's have important conservation applications. By determining true species distinctions, marine reserves can be placed where they will protect the most genetic diversity, for example.
Bernardi's coauthors include Sally Holbrook and Russell Schmitt of UC Santa Barbara, Nicole Crane of Monterey Peninsula College, and Edward DeMartini of the National Marine Fisheries Service. The collaboration of Bernardi, Holbrook, and Schmitt was supported by the University of California Multicampus Research Incentive Fund. Additional funding was provided by the National Science Foundation and a Faculty Development Award from UCSC.
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Editor's note: Reporters may contact Giacomo Bernardi at (831) 459-5124 or bernardi@biology.ucsc.edu.
