(Left) Researchers have discovered a spatial organization of neuronal activation as the jellyfish coordinates its behavior; (Right) The jellyfish bends the right side of its body to bring a tiny brine shrimp to its mouth. (Image credit: B. Weissburd)
Despite their lack of a brain, jellyfish are capable of performing surprisingly complex actions using their rudimentary nervous system. Now, by experimenting with jellyfish genes, scientists have developed a method for observing the internal processes of these animals.
In the new study, the team created a model based on the jellyfish species Clytia hemisphaerica, a transparent, umbrella-shaped jellyfish with a tube-shaped mouth in the center. This small jellyfish is only 0.4 inches (1 centimeter) in diameter, which allowed the team to place it under a microscope and observe its entire nervous system at once.
While the human brain functions as a centralized control center for the body, jellyfish do not have this structure in their nervous systems. Instead, many jellyfish have a diffuse “network” of nerves that spread out symmetrically from the center of their bodies, and they also have a nerve ring that encircles the bottom of the bell, the crescent-shaped part of the jellyfish. Some jellyfish lack nerve nets and only have nerve rings, but C. hemisphaerica has both, according to a 2013 report in Current Biology.
The big question is how these little jellyfish, without any central control over their movements, are able to perform coordinated actions? For example, how do these bubbly creatures snatch shrimp from the water column and then fold in half to pull the snacks into their tube-shaped mouths?
To find out, the team grew a batch of C. hemisphaerica with a genetic modification encoding a protein called GCaMP, which glows green when it interacts with calcium.
The special glowing protein was engineered into a specific location in the jellyfish genome so that it glowed only in active neurons, said first author Brandon Weisburd, a postdoctoral fellow in biology and bioengineering at the California Institute of Technology. “When neurons are active, the amount of calcium inside the neurons increases, so GCaMP becomes more fluorescent. This means that neural activity shows up as flickering,” Weisburd told Live Science in an email.
But jellyfish are naturally luminescent. So to better visualize their artificial glow, the team used CRISPR to remove a specific gene that produced another fluorescent protein that obscured the GCaMP they had inserted, he noted.
Having turned the jellyfish into miniature light shows, the team ran a series of experiments to determine which neurons were activated during their typical feeding behavior. They found that when the jellyfish were attached to brine shrimp or came into contact with the team’s “shrimp extract,” a group of neurons close to the shrimp suddenly fired.
This activation did not spread throughout the jellyfish, like a rock dropped into a puddle, causing ripples across the surface. Instead, only neurons in a well-defined wedge-shaped region of the bell lit up in response to the shrimp. This wedge of active neurons resembled a single slice of pizza inside a round pie, according to the statement. The team found that the neurons closest to the shrimp fired first, followed by strobe lights illuminating the rest of the slice.
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