Starfish, properly known as sea stars, are marine invertebrates belonging to the class Asteroidea. They do not have a face in the traditional sense, as they lack the centralized brain and cluster of forward-facing sensory organs that define a head. Instead, their unique body plan distributes sensory functions across their entire structure. This decentralized system enables them to navigate, find food, and avoid threats without needing a forward-facing orientation.
Anatomy of Symmetry Why Starfish Lack a Central Head
The fundamental reason a starfish cannot possess a face lies in its unique structural organization, which is based on radial symmetry. Most complex animals exhibit bilateral symmetry, meaning they can be divided into two mirror-image halves along a single plane. This bilateral design is what concentrates the nervous system and primary senses into a single, centralized head.
Adult sea stars, however, typically display pentaradial symmetry, where body parts are arranged around a central axis in five equal sections. Because of this five-point arrangement, there is no single anterior end to house a brain or a cluster of sensory inputs. The animal can move in any direction, effectively making every arm a potential “front.”
The nervous system reflects this symmetry, consisting of a nerve ring encircling the mouth and a radial nerve extending down the middle of each arm. This distributed network allows each arm to act somewhat independently, with the central ring coordinating the overall movement. Without a centralized brain, the sea star’s movements and reactions are a coordinated response from multiple semi-autonomous units.
How Starfish See The Function of Eyespots (Ocelli)
Despite lacking complex eyes, sea stars possess a form of vision through specialized structures called eyespots, or ocelli, located at the tip of each arm. These rudimentary organs are often visible as small red or black dots when the arm tip is curled upward. A five-armed sea star therefore has five visual sensors, each providing a limited field of view.
These eyespots are composed of simple pigment cups containing photoreceptor cells. The ocelli do not form sharp, detailed images, but rather act as light detectors. Their primary function is to distinguish between light and shadow, and to perceive large, stationary objects in their environment.
In certain species, such as the blue sea star, the visual information gathered by the ocelli is sufficient for important navigational tasks. Researchers have shown that these animals use their eyespots to locate large environmental features, like the dark outline of a coral reef against the brighter surrounding water. This simple visual input is enough to guide the sea star toward a suitable habitat or feeding ground from a distance of up to two meters.
Sensing the Environment Chemoreception and Tube Feet
Beyond their simple vision, sea stars rely heavily on chemical and tactile senses to navigate and interact with their surroundings. The tube feet are small, hydraulic appendages lining the grooves on the underside of each arm. They are the primary tools for both movement and sensing, functioning for locomotion, adhesion, and as sophisticated sensory organs.
The tips of the tube feet are rich in sensory cells sensitive to touch, allowing the sea star to feel the texture and contours of the substrate it is moving over. This mechanoreception helps them maintain orientation and balance. Additionally, the entire surface of the sea star, particularly the tube feet, is covered with receptors responsible for chemoreception.
Chemoreception is essentially the sense of smell and taste in the water. These receptors detect dissolved chemical cues, enabling the animal to track food sources, such as mollusks, or detect the presence of predators. By sensing the chemical gradient in the water, a sea star can effectively follow an “odor trail” to its next meal without ever seeing it.