Small crustaceans and worms that poke along near or on the seafloor are favorite eats. After being nabbed by poison-filled tentacles, the captured food is transferred to the mouth, which is attached to a long, tubular stomach extending nearly the bell’s vertical length. It isn’t luck that determines whether a jelly dines en route but distinctive, bright-red, light-sensitive ocelli (eye spots) located at the base of each tentacle that detect prey. In this case, seeing isn’t about identifying the big “E” on any eye chart but surmising light from shadow. I know the bell jelly sees something, based on its feverish pulsing to escape the flashes from my strobe. Looking under the bell, one finds organs, besides the stomach. Of note are the gonads (sex organs) — more a collection of sausage-like appendages that dangle from the under surface of the umbrella. Early taxonomists were clearly impressed enough to celebrate them by choosing Polyorchis, meaning many testicles, for the genus.
Back in the 1800s, when this bell jelly was identified, no one could have imagined its role in modern neuroscience studies. Though the jelly has long been known to bear nerve-rich tissues within its margins, not until the last couple of decades has it been understood that the tissues contain the neurotransmitter dopamine. In higher animals, neurotransmitters comprise a group of chemicals that allow nerve cells (a.k.a. neurons) to communicate with each other and, as a result, produce or inhibit actions (mental, emotional, physical). Though they don’t actually touch each other to communicate, neurons exchange information in the spaces (synapses) between where one neuron ends and another begins. When a message reaches the end of the first neuron, it triggers the release of neurotransmitter chemicals into the synapse, then these chemicals travel across the synapse and touch the beginning part of the next neuron in the chain. That touch triggers the next neuron to spread the message on down the chain of cells. Dopamine, a well-known neurotransmitter chemical, has been used in human treatment (L-dopa) for motor diseases like Parkinson’s. But neurotransmitter mechanisms can go haywire, with other brain disorder outcomes like hallucinations, paranoia, depression and other symptoms of mental illness.
The phylum Cnidaria comprises what are believed to be the most primitive organisms (jellies, anemones and corals) with a true nervous system. Dopamine is found in extracts taken from the nerve-rich tissues of the margins of this jelly (not so for other compounds in the same family like epinephrine, norepinephrine and serotonin) and is shown to be involved in the jelly’s swimming mechanism from studies carried out on contracting (crumpling) and relaxing pulses.
While dopamine is known to be floating around the tissue that surrounds the nerve cells, only recently have researchers carried out experiments that lead them to believe the nerve cells themselves might be responsible for releasing the neurotransmitter. If so, the message would be released from the neuron then relayed to the muscle to permit contraction. Whether or not the dopamine-rich tissue is the neurotransmitter link inhibiting or modulating the jelly’s central nervous system will only be known when studies definitively show that dopamine is also present inside the nerve cells, not just in the surrounding tissue, and that the cells do in fact release dopamine.
Some may wonder why time is “wasted” studying a primitive animal without a brain just because it has dopamine, when we should be putting our time and money into understanding and treating brain malfunctions? That the bell jelly has bottom-rung status evolutionarily speaks to its relatively simple mechanics (few nerve cells next to our billions of nerve cells), making it an easier model to study. Although we branched off from jellies, we still share origins, so understanding the workings of a jelly provides us with a refined starting point with which to understand ourselves. In this case, knowledge gleaned from understanding the jelly neurotransmitter mechanism helps us reconstruct the early evolution of the ways and means of neurochemical communication, since it is presumed that it was in the Cnideria, or a common ancestor, that such mechanisms first evolved.
— Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to firstname.lastname@example.org