Researchers have made important new discoveries about the evolution of the senses in animals by identifying a light-based, or “photic,” sense in a multicellular marine animal called hydra.
The discoveries are reported in a new article in the journal BMC Biology, written by David Plachetzki, who is currently a post doctoral fellow working in the UC Davis Center for Population Biology, along with two co-authors from UC Santa Barbara, Caitlin Fong and Todd Oakley. Plachetzki is also affiliated with the UC Davis department of evolution and ecology.
“By combining genomic and physiological approaches we now have powerful tools to unravel these first steps in the evolution of a nervous system,” said Thomas Holstein, a professor of molecular evolution and genomics at the Centre for Organismal Studies in Heidelberg, Germany, who was not involved in the current study.
Plachetzki and his research colleagues asked questions about the common ancestor of cnidarians and humans, which existed 600 million years ago.
“This is the origin of complex ‘animal-ness’ — that’s not the first animal, but the first one that has a nervous system and moves around,” Plachetzki said.
Cnidarian (pronounced “naye-DARE-ee-en” with a silent “c”) animals include hydra, jellyfish and sea anemones, among other animals, and are evolutionary cousins to humans.
“Dave [Plachetzki] had noticed that visual proteins were located around the stinging cells in hydra,” said co-author Fong, who is a member of the research team and is now a lab technician at UC Los Angeles. “This led us to believe that hydra used visual signals to regulate the usage of these stinging cells.”
“We devised behavioral tests to be the first to show that stinging cells depend on light level,” said Oakley, a professor in the department of ecology, evolution and marine biology at UC Santa Barbara.
One of the main findings of the study was that hydra are more likely to fire their stinging cells when a shadow is cast onto them, which increases their chances of catching the possible prey casting the shadow.
“Hydra fire more stinging cells in dim light conditions and less in bright light conditions,” Fong said . “We think that this is a method by which hydra conserve stinging cells and only fire them when they might catch prey.”
Hydra are freshwater polyps, which are small relatives of jellyfish, that have been studied for over 250 years. Although they don’t have eyes, Plachetzki’s research shows how a physiological pathway could function which was a precursor to vision in later animals.
“This is one way that those pathways could function before they got co-opted into a complex eye,” Plachetzki said .
The animals that developed vision in later evolution were “putting together pieces that have already evolved,” Plachetzki said.
There are three main ways that animals detect sensory cues from the environment: chemical, mechanical and photic. The current study focused on the photic, or light means of sensation. The chemical category corresponds to the sense of “taste” in animals, while the mechanical category roughly corresponds to touch and hearing.
Holstein is currently studying dinoflagellates, a type of plankton, in his lab in Heidelberg and says that Plachetzki, Fong and Oakley’s new research findings raise new questions as to whether a mechanical sense or a photic sense developed first in common ancestors of animals and plants.
“This is difficult to decide at the moment,” said Holstein, commenting on the lack of evidence one way or the other. “We just don’t know enough [about the] huge diversity in unicellular organisms.”
BRIAN RILEY can be reached at firstname.lastname@example.org.