(Cover illustration by Christina Zhao)
Out of the millions of creatures that occupy Earth’s animal kingdom, zebrafish are not the first to come to mind when we think of humans’ closest relatives. Typically, humans do not live underwater, have fins and gills, nor are described as clear-headed, zebra-striped, and minuscule. However, these traits perfectly summarize a few petite but fascinating marine organisms—zebrafish and their clear-headed buddies, glassfish. Allowing scientists to look inside their minds without extensive diving, the physical appearance of glassfish and zebrafish is essential to researchers as both fish species have transparent heads. You might be wondering how exactly these minute creatures are relevant to humans other than their coexistence on the same planet. In actuality, zebrafish possess nearly 70% of the same genes as humans. About 84% of genes known to cause diseases in humans have a respective twin in the marine organism’s genome. Professor Lovett-Baron of UC San Diego’s Department of Neurobiology researches the adaptability of zebrafish and glassfish brains by observing changes in their behavior after inducing various stimuli such as movement and light. Much like how humans display innate behaviors when they encounter external stimuli on a day-to-day basis, these model fishes act similarly. Professor Lovett-Baron contextualizes comparable intuitive actions in humans and many other vertebrate species.
How do researchers in the Lovett-Baron lab observe and categorize zebrafish and glassfish behavior?
To understand the specific behaviors and reactions zebrafish and glassfish have to various stimuli, researchers in the Lovett-Baron lab use fish larvae rather than fully grown fish because they are smaller and easier to control. Interestingly, the method of inducing stimuli can be compared to a virtual reality simulator. Researchers surround the fish with screens they control to display an image or video of their choice. To see fight or flight instincts at play, researchers imitate the movement of other organisms by sharing an image of a large dot and slowly bringing it closer to the fish. Placing the larvae into a contained area allows scientists to monitor all movement and behaviors with a camera or brain-wide activity imaging. The imaging technique displays eye movement– an indicator of the fish’s sensitivity to motion– and brain activity, highlighting active regions in their brain during a fight or flight response. In addition to observed behaviors, the Lovett-Baron lab uses fluorescence in situ hybridization to link activity to gene expression. In situ hybridization is the process of identifying specific nucleic acid sequences within fixed tissues or cells, and using this technique, researchers can locate where a specific gene sequence is expressed, directly connecting zebrafish genetics to brain activity. The fish’s translucent skulls allow scientists to track brain activity with fluorescence, making zebrafish an especially applicable model animal when evaluating brain activity patterns. Scientists genetically engineer zebrafish larvae to express Green Fluorescence Proteins (GFPs) at key neural locations to serve as visual indicators of neuron activity.
Aside from individual behavior, the Lovett-Baron lab studies schooling behaviors of zebrafish and glassfish, characterized by the synchronized swimming or aggregation of fish (usually forming a group of the same species). Similar to the individual larvae experiment, an external stimulus, such as a large moving dot, is placed in an area of visibility facing the school of larvae. In response to the stimuli, researchers observed changes in movement, positioning, and brain activity. Image-based tracking, video footage, and 3-dimensional imaging help researchers quantify the behaviors from simple movement patterns into comparable data. A detailed image displaying data of brain activity is then developed, and lit-up regions present important active neural regions during social interactions.
What conclusions have been drawn about schooling and group behavior?
“Understanding collective behavior through neurobiology,”, a paper co-authored by Professor Lovett-Baron and other researchers in 2024, highlights the neuronal pathways involved in schooling and social interactions. Neuronal pathways are the complex system of neurons and synapses responsible for transmitting signals to various nervous system sites in the body. Communication and decision-making processes in some species occur through simple signaling mechanisms such as sounds, visual signals, motion, and pheromones—chemical signals released for communication. For example, bats utilize a communication system of echolocation, which relies on their sense of sound to enable seamless spatial awareness and signaling over short and long distances. With zebrafish, a visual cue, such as a moving dot, causes the activation of multiple brain regions, which in turn leads the fish to display schooling or shoaling behavior. Shoaling is recognized when a school of fish maintains a group formation for social reasons such as hunting or protection. Additionally, the positioning of an individual within a group is often an indication of status or rank. The reasoning behind group positioning is been linked to the hippocampus, a small brain region responsible for learning and memory. Researchers put together pieces of the visual and chemical puzzle to create a greater understanding of the components of the brain that contribute to social interactions.
Why does this apply to human social interaction and thought processing?
The overall goal of researching neuronal pathways using model animals like zebrafish extends beyond uncovering the hidden communication methods of fish. Animal biology has contextual applications in human psychology, animal behavior, and more. Studying animal behavior leads researchers to draw parallels between various species and the evolution of humans. As the neurobiological processes in the brains of tiny model organisms are largely consistent across many vertebrates, psychologists may explore similar areas to gain a deeper insight into human interactions with the world. Using smaller and less complex model organisms such as zebrafish and glassfish allows researchers to picture and dissect the intricate reactions occurring in organ systems of vertebrates.
Additionally, the Lovett-Baron lab directly contributes to the growing neurobiological field within marine biology. The technique of studying group behavior has applications in marine biology, specifically in the schooling behaviors of saltwater fish species. The intersectionality of fields across biology allows for insight into organisms separated by a large distance on the phylogeny of life. Even though humans might not have fins nor live entirely underwater, we can still search for hints from our knowledge of the creatures we share our planet with.