Jellyfish may be brainless, yet they can do surprisingly complex things with their simplistic nervous systems. Now, by fiddling with the genes of jellyfish, researchers have devised a way to spy on the animals' inner workings.
In the new study, the researchers created a model using the jellyfish species Clytia hemisphaerica, a transparent, umbrella-shaped jellyfish with a tube-like mouth at its center. The teeny jellyfish grows to be only 0.4 inches (1 centimeter) in diameter, meaning the team could place the whole jellyfish under the microscope and observe its entire nervous system at once.
While the human brain serves as a centralized control center for the body, jellyfish have no such structure in their nervous systems. Instead, many jellyfish carry a diffuse "net" of nerves that radiates symmetrically from the center of their bodies; in addition, they have a nerve ring that runs around the bottom of the bell — the half-moon-shaped portion of the jellyfish. Some jellyfish lack nerve nets and have only nerve rings, according to a 2013 report in the journal Current Biology, but C. hemisphaerica has both of these structures.
The big question is, with no centralized control over their movements, how do these teensy jellyfish perform coordinated behaviors? For instance, how do the blobby critters snatch shrimp from the water column and then fold in half to pull the snacks toward their tubular mouths?
The special glowing protein was inserted into a location in the jellyfish genome so that it only lit up in active neurons, said first author Brandon Weissbourd, a postdoctoral scholar in biology and biological engineering at the California Institute of Technology. "When neurons are active, the amount of calcium [inside the neurons] goes up, so GCaMP becomes more fluorescent. This means that neural activity looks like flashing," Weissbourd told Live Science in an email.
But jellyfish are naturally luminescent. So to see their engineered flashing more clearly, the team used CRISPR to snip out a specific gene that makes a different fluorescent protein, one that kept outshining the GCaMP they had inserted, he said.
With their jellyfish thus transformed into miniature light shows, the team ran a number of experiments to see which neurons lit up during their typical feeding behaviors. They found that, when the jellyfish latched onto a brine shrimp, or came into contact with a "shrimp extract" made by the team, a group of neurons physically near the shrimp suddenly lit up.
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This activation didn't ripple through the entire jellyfish, like how a stone plopped in a puddle would send ripples across its entire surface. Rather, only neurons within a well-defined, wedge-shaped region of the bell lit up in response to the shrimpy snack. This wedge of active neurons was shaped like like a single pizza slice within a circular pie, according to a statement. The neurons that were closest to the shrimp lit up first, the team found, and then a slew of strobe lights would illuminate the rest of the slice.
So for example, if a shrimp was placed at the far edge of the pizza slice, onto its "crust," the crust would light up first, followed by the rest of the slice. This ripple effect coincided with the jellyfish folding up in the corner of its bell, in order to bring the shrimp to its mouth.
The team didn't expect to observe this level of organization within the seemingly unstructured nerve net, Weissbourd said. "The finding of an intrinsic structure within the network was certainly surprising," he said.
Looking forward, the team plans to investigate how jellyfish exert control over all their behaviors, not just feeding, and they plan to study different species of jellyfish, which perform different behaviors to C. hemisphaerica, Weissbourd said. For instance, while some jellyfish perform a similar food-passing behavior as C. hemisphaerica, others instead use long-reaching mouthparts to pluck food from their tentacles. "Given the diversity of jellyfish, and that so many of them are small and transparent, I think they could provide an exciting platform in the future for understanding how nervous systems evolve."
These studies of strobing jellyfish could also shed light on basic principles that govern all nervous systems, from the most simplistic to the most complex. "The idea is to develop experimental and theoretical approaches towards understanding how simpler nervous systems work as a step towards understanding the human brain, which is orders of magnitude more complex," Weissbourd told Live Science.
The team published their findings Nov. 24 in the journal Cell.
Originally published on Live Science.