UCSF/JULIUS LABSharks, rays, and skates can detect minute fluctuations in electric fields—signals as subtle as a small fish breathing within the vicinity—and rely on specialized electrosensory cells to navigate, and hunt for prey hidden in the sand. But how these elasmobranch fish separate signal from noise has long baffled scientists. In an environment full of tiny electrical impulses, how does the skate home in on prey?
In a study published this week (March 6) in Nature, researchers at the University of California, San Francisco (UCSF), have analyzed the electrosensory cells of the little skate (Leucoraja erinacea). They found that voltage-gated calcium channels within these cells appear to work in concert with calcium-activated potassium channels, both specifically tuned in the little skate to pick up on weak electrical signals.
“We have elucidated a molecular basis for electrosensation, at least in the little skate, which accounts for this unusual and highly sensitive mechanism for detecting electrical fields,” said coauthor Nicholas Bellono, a postdoc at USCF. “How general it is, we don’t know. But this is really the first instance in which we’ve been able to drill down and ask what molecules could be involved in this kind of system.”
“These sensory systems are not just about molecules, they’re about the anatomical properties of the system as well,” added coauthor David Julius, chair of the department of physiology at UCSF. “In all sensory systems, the organs evolve brilliantly to do what they do. The electrical system in these skates is no exception.”
Every time a fish breathes, seawater contacts the animal’s mucus membrane and generates a minor electrical field. Skates and other elasmobranch fish evolved to pick up on these electrical blips and use them to find hidden prey, navigate treacherous waters, and avoid undesirable fish.
How skates make use of these electrical fields, and the anatomy of their sensory organs, have been fairly well-studied. “It has been known for almost 40 years that two ion currents—a calcium current and a calcium-activated potassium current—are responsible for the electrical activity of the shark and the skate’s electroreceptors,” Harold Zakon, a professor at the University of Texas at Austin, who was not involved in the study, wrote in an email. “But what was not known was how electroreceptors could detect such minute voltages in the water.”
Bellono and colleagues first isolated electrosensory cells from little skate ampullary organs. “These are tough experiments,” Julius said. “The cells in these ampullary organs are very small and hard to get out. The technical aspect of this was actually quite challenging.” The researchers then measured ionic currents within the cells in response to different electrical stimuli. “We found that there were these two major currents, a calcium current and a potassium current, coupled to one another,” Bellono said, noting that these currents amplify small electrical signals. The team then carried out gene expression experiments to confirm the presence of specialized calcium and potassium channels within the cells. “We were trying to close the gap between genetics and physiology,” Julius said.
In a final experiment, the researchers used drugs to block these channels in several skate specimens, and compared their hunting abilities to those of wild-type skates. The team then hid electrical apparatuses under the sand in a skate tank, and watched as wild-type skates homed in on the signal while the altered skates did not. “In perfect world you’d use genetics, like in mice, and you’d knock out the genes of interest and ask if you’ve perturbed this behavior,” Julius said. “In skates we can’t do this, so we used the next best thing to block the ion channels—pharmacology.”
“This study was technically excellent and examined the problem from behavioral to biophysical to molecular levels,” Zakon wrote. “It was a brilliant example of how to analyze a problem.”
Christopher Braun of Hunter College in New York City agreed. “It’s really interesting, because it shows a mechanism of tuning that adjusts the sensitivity of those electroreceptor cells to the stimuli that the animal cares about,” he said. “The ionic mechanisms the paper describes make it very clear how the cells can be specifically tuned into those stimuli that are ecologically relevant.”
Braun added that his own work focuses on how animals sort stimuli, and differentiate between their own electrical discharges and that of other animals. “That difference is often based on frequency,” Braun said. “The cellular mechanisms described in this paper provide a very low level way for the brain to only receive information that’s important to it, and not be distracted by other information.”
The next steps, according to Zakon, are to sample other animals that can sense electricity and investigate whether these relatively simple channels mediate similar functions in other species. “Other species of fishes, and even some mammals like the platypus, have independently evolved electroreceptors,” he wrote. “The next exciting step in this work would be to examine electroreceptors in other species to see if they evolved the same or different mechanisms for sensitivity.”
N.W. Bellono et al., “Molecular basis of ancestral vertebrate electroreception,” Nature, doi:10.1038/nature21401, 2017.