Jellyfish, with their ancient lineage and ethereal, pulsating forms, often lead to questions about their internal workings. Lacking a brain or complex organs, these gelatinous inhabitants of the ocean do possess a unique way of sensing and responding to their world.
The Jellyfish Nervous System
Jellyfish possess a nervous system, though it differs significantly from the centralized brain and spinal cord found in many other animals. They feature a decentralized network of neurons known as a “nerve net.” This diffuse system is spread throughout their body, particularly concentrated under the bell, allowing for widespread communication. Unlike a brain, which acts as a central processing unit, the nerve net allows signals to spread across the entire organism.
This arrangement enables jellyfish to react to stimuli from any direction. The nerve net includes a motor nerve net (MNN) and a diffuse nerve net (DNN). The MNN, extending over the subumbrella, consists of large neurons with two neurites and transmits signals at speeds between 45 cm/s and 1 m/s. This decentralized structure ensures that even if part of the bell is damaged, the remaining sections can continue to function.
How Jellyfish Sense Their Environment
Jellyfish sense their surroundings through specialized sensory structures and receptors. Along the margin of their bell, they have small sensory organs called rhopalia. These rhopalia often house ocelli, which are simple eyespots capable of detecting light, and statocysts, which are balance organs that aid in orientation and balance.
The nerve net processes signals from these rhopalia, which are integrated into the overall nervous system. Beyond light and balance, jellyfish also utilize chemoreceptors to detect chemical cues in the water, such as the presence of food or predators. Mechanoreceptors allow them to sense touch and vibrations, providing information about water currents or contact with other objects. These sensory inputs allow jellyfish to perceive environmental changes like temperature shifts, water salinity, oxygen concentration, and currents.
Movement and Predation
The nerve net coordinates the rhythmic movements for jellyfish locomotion and feeding behaviors. The motor nerve net generates impulses that cause the bell to contract, propelling the jellyfish through the water. This pulsing motion is coordinated by pacemakers located in the rhopalia, which fire action potentials that propagate across the motor nerve net. Different delays between nerve net activations can lead to varied and controlled movements, including turning, where asymmetric bell contractions are generated by the initial firing of pacemakers on the leading edge of the turn.
The nerve net controls the precise movements of the tentacles, which are equipped with stinging cells called nematocysts. When prey is detected through sensory input, the nerve net coordinates the deployment of these nematocysts to capture the prey. After capture, the nerve net also directs the tentacles to move the food towards the mouth. This coordination of sensory input and motor output allows for actions like swimming and feeding to occur simultaneously.