Starfish, also known as sea stars, are marine invertebrates that lack the complex, centralized sensory organs found in vertebrates, including ears. These echinoderms are characterized by radial symmetry, meaning their body parts are arranged around a central axis. They do not possess a brain or traditional eyes. Instead of relying on a few highly specialized organs, the starfish uses a distributed network of cells and structures across its body to interact with its aquatic environment. This unique anatomy allows them to perceive light, detect movement, and locate food.
The Starfish Nervous System
The starfish nervous system is organized around its radial body plan, which explains the absence of a distinct head or brain. A nerve ring encircles the mouth at the center of the animal’s body, serving as the main coordination point for the organism. Extending from this central ring are radial nerves that run down the length of each arm.
The nervous system relies on a simpler, interconnected network, rather than a hierarchical structure, to regulate the animal’s functions. The radial nerves act somewhat autonomously, with one arm often taking the lead and directing movement when a stimulus is detected. Each arm has a degree of independence, but they coordinate their actions through the nerve ring to achieve locomotion and other behaviors.
How Starfish Perceive Light
Despite not having eyes in the traditional sense, starfish perceive their surroundings using specialized light-sensing organs called ocelli, or eyespots. A single eyespot is located at the very tip of each arm, often protected by a small spine. These structures are compound eyes, sometimes containing hundreds of light-sensitive units called ommatidia.
The ocelli are connected directly to the radial nerve in that arm, allowing for immediate signaling. They are highly effective at detecting changes in light intensity and perceiving large stationary objects, though they do not form sharp, detailed images. This low-resolution vision, with a spatial resolution estimated to be between 8 and 16 degrees in some species, is sufficient for navigation. For example, a starfish can use its eyespots to distinguish between the bright open water and the dark outline of a coral reef structure.
Sensing Movement and Chemical Cues
While starfish lack ears to hear sound waves, they are highly sensitive to physical disturbances and vibrations in the water, which serves a similar function for avoiding predators and finding prey. This mechanoreception is primarily handled by sensory cells distributed across the epidermis and, most significantly, by the hundreds of tube feet lining the grooves on the underside of each arm.
These small, elongated projections detect minute pressure changes and water currents, allowing the starfish to sense the movement of nearby animals or potential threats. The tube feet are also crucial for chemoreception, the starfish’s version of taste and smell. Receptors on the tube feet and the arm tips detect dissolved chemicals in the seawater. This chemical sensitivity allows the starfish to track the scent trail of prey, such as bivalves, or to locate a potential mate, making the tube feet the most versatile sensory organs on their body.