In a sparkling tank in Dr. Karen Maruska’s fish tank room at the top of the Life Science building, two brightly colored fish the size of your thumb aggressively dance around one another, their mouths open wide, darting forwards and backwards in a fight for dominance. These fish have one thing on their minds – defending their territory and fighting for the right to breed with the female residents of their tank. A curtain shields the tank from the outside world, and the fish behave as if no one is watching. But a camera secretly captures the interaction between these two fish, so that Maruska and other researchers in her lab may study the behavior of these African cichlid fish down to the individual neurons firing in their brains as they perform their aggressive dance.
Fast forward a few days, and Maruska is inspecting delicate slices of stained brain tissue from those two male fish who were duking it out. Maruska is an assistant professor in the Department of Biological Sciences in the LSU College of Science, and her lab studies the behaviors of a little fish species from Lake Tanganyika in Africa, the African Cichlid. She looks through a microscope at slides prepared from fish brains cut with a cryostat, a very fancy brain “deli slicer.” The brain slices are stained so that the neurons that were “turned on” during the fight appear a bright purple.
“Using this technique, we can look at which brain regions are turned on in an aggressive context between two male fish, and compare that to brain regions that are turned on when a dominant male is mating with a female for example,” Maruska said. “Some of those brain regions are going to be the same, and are clustered in a brain region called the Social Decision Making Network. In my lab we are interested in understanding how the brain controls different social behaviors and how the brain makes the decisions of what behavior to perform in certain contexts. Because we don’t know how many social behaviors are controlled in the brain, for the most part.”
But let’s get back to those two fighting fish from the beginning of our story. “Male African cichlid fish are very aggressive and dominant,” Maruska said. “The males are either dominant or subordinate. Dominant males stake out a territory and aggressively defend it from all other males.” However, male fish can switch between being dominant and subordinate very quickly (within minutes!), with concomitant changes in their brain chemistry or the genes being expressed in certain parts of their brains. It is these changes in the fish brain that Maruska and her lab are studying.
Maruska has experience studying various different fish species. She completed her PhD in Hawaii studying a damselfish. She conducted basic research related to sound production and brain function in the damselfish, but she found herself wanting to get down to the deeper molecular mechanisms of brain function. But no other researchers had sequenced the genome of the damselfish or created other resources that would allow Maruska to look deeper into molecular mechanisms of brain activity with this species. So she made her way to Stanford as a postdoctoral student to study the African cichlid with Russell Fernald.
The African cichlid fish and its brain are a “dream” for neurobiology researchers like Maruska. These fish have many interesting behaviors, a fully sequenced genome and well-described brain regions. These characteristics help Maruska and her students be able to tie specific African cichlid fish behaviors down to the specific neurons and gene expression patterns that control these behaviors. In her lab, Maruska can put individual African cichlid fish in particular social situations and later remove their brains and study molecular changes in particular brain regions that are tied with specific social behaviors.
Maruska studies both the mating and breeding behaviors of the African cichlid and associated brain activity patterns. The female African cichlid is a mouthbrooder – she carries her fertilized eggs in her mouth for 2 weeks until they are released and even temporarily protects her baby fish, or fry, by taking them back up into her mouth if she thinks they are in danger. But at some point, the female fish’s brain makes a stark switch from protecting her young to rejecting them so that she can eat and regain the weight she lost while carrying her eggs in her mouth. If her brain doesn’t make this switch at the right time she will starve to death – or eat her own babies before they hatch.
The male African cichlid fish also has a large swatch of mating signals that it uses to attract females for breeding. Dominant males are not only brightly colored and perform courtship “dances,” but they also emit sounds in the form of grunts and smells and tastes in their urine to try to attract females. “He literally pees at her,” Maruska says, describing how a male African cichlid courts a female. Maruska and her lab members are studying the brain of the female African cichlid fish to determine how she is integrating all of this disparate information in her brain and what specific brain regions are involved.
“In the wild, the males of this species have clustered territories that females come in and visit, so that the female has the opportunity to sample many different males and determine which one is the fittest to spawn with,” Maruska said.
Ultimately, studying the brain of the African cichlid fish and which neurons fire under different social environment conditions can potentially tell us a lot about how human brains function in various contexts. “These fish have brain regions very similar to those in all other vertebrates, including humans,” Maruska said. “There are a lot of links that can be made to biomedical research, which always really interest my undergraduate research students who want to pursue careers in medicine. For example, in the case of our mouthbrooding female fish, we are looking at how the brain might be curbing the motivation to eat before and after the baby fish emerge from the female’s mouth. This has biomedical applications for things like eating disorders in humans. If we can figure out how the brain is controlling a female’s urge to eat, then that’s important for understanding disorders like anorexia or bulimia, because the same brain structures might be involved in fish.”
Who knew fish braiiinnnssss controlled such complicated social behaviors? Follow Karen Maruska’s lab on Twitter, @MaruskaLab, to learn more about the African cichlid and its brain.