The peculiarities of the jellyfish nervous system

Guest post from Joseph Jebelli, a PhD student at UCL

What is it like to be a jellyfish? These beautifully mesmerising creatures are so bizarre, so alien to us in so many respects that one can easily be forgiven for struggling to come up with a good answer. Biologically, jellyfish have long been thought of as simple and primitive organisms. The latter is certainly true – jellyfish have thrived on our planet for over 500 million years, making them the oldest multiorgan animals alive today. But whilst they appear simple in their basic anatomy, scientists are only now beginning to appreciate their underlying physiological complexity.

Instead of a centralised, ‘true’ brain, jellyfish possess a diffuse network of cells known as a nerve net, which spreads around the inner margin of the bell and interacts with clusters of small sensory structures called rhopalia. Some species, including the notoriously dangerous Box jellyfish, also possess a nerve ring that connects groups of rhopalia. This arrangement allows jellyfish to detect and respond to environmental stimuli from their surroundings, and for a long time it was thought that such a basic level of neuronal organisation could only be involved in creating simple reflexes.

Recently however, scientists have discovered that this neural circuitry is in fact highly sophisticated; integrating substantial sensory and motor inputs to generate complex and species typical behaviours. These include assessing salt levels and diving to avoid fresh water, as well as swimming in more elaborate patterns after capturing prey. Astonishingly, Box jellyfish have recently been shown to process visual information from an extraordinary set of 24 eyes (of four different types!) and even use terrestrial visual cues, compressed by refraction through the water surface, to navigate to a preferred habitat.

Both introductory and higher-level undergraduate textbooks have had us all believing jellyfish to be as basic and brainless as a sea sponge. Perhaps it is time to rethink our perception of an ingenious organism endowed with spatial cognition, problem solving abilities, and an intelligence that has facilitated one of the greatest evolutionary success stories of life on earth.