Health/Sci-TechLifestyleVOLUME 18 ISSUE # 45

No brain, no problem for jellyfish

When it comes to learned behavior, even the simplest minds are capable of advanced thought. The Caribbean box jellyfish (Tripedalia cystophora), which doesn’t even have a brain, can alter its behavior based on past experiences, new research reveals.

Scientists believe the creature uses this learning ability along with its astoundingly complex visual system to navigate the murky mangrove swamps it calls home. Scientists have known for some time that animals in the phylum Cnidaria—which includes jellyfish, corals, and sea anemones—are capable of basic forms of learning when repeatedly presented with a stimulus in their environment. They will either respond to it less (what’s known as habituation) or more (known as sensitization). Humans do this all the time—if you hear a sound over and over again, your ears might tune it out. In other instances, the sound may become difficult to ignore.

Habituation and sensitization are types of so-called nonassociative learning, as they don’t require connections between two different stimulus types in order to generate a response. In contrast, associative learning, which involves linking different types of stimuli together to then modify behavior, is regarded as a more advanced kind of learning.

Until recently, scientists believed that only animals with advanced nervous systems—such as humans and other mammals—could do it. Because a jellyfish’s nervous system is dispersed throughout its body, with no centralized brainlike structure, these animals have been viewed as incapable of associative learning. Even among other brainless animals, T. cystophora stands out. Whereas most Cnidarians can only vaguely detect light sources, this fingernail-size, translucent creature has a whopping 24 eyes, arranged in clusters around the body to allow it to perceive the world with impressive visual detail and navigate around underwater mangrove roots in the Caribbean Sea and Central Indo-Pacific region.

Anders Garm, a marine biologist at the University of Copenhagen, suspected T. cystophora might also be capable of associative learning. Research from other labs had found some evidence of the phenomenon in sea anemones, but Garm and his colleagues were skeptical. In one of these previous studies, researchers subjected anemones to electric shocks and bright flashes of light.

Over time, the anemones shrank away from the flashes, but because neither stimulus occurs in the animals’ natural habitat, it was difficult to know whether the anemones were truly exhibiting an association between the light and the electric shock, or they had just become highly sensitized to the stimuli. “It’s like judging a fish by the ability to climb a tree,” says the study’s lead author, Jan Bielecki, a neurobiologist at Kiel University.