The sea star, commonly known as a starfish, is a marine invertebrate belonging to the phylum Echinodermata, a group recognized for its spiny skin and unique body architecture. This organism is not a fish, but an animal with a distinct structure adapted for life on the ocean floor. To understand what a sea star looks like requires examining its body plan, from its signature star shape to the minute structures covering its surface.
Radial Symmetry and Arm Structure
The most defining visual feature of a sea star is its body plan, which exhibits pentaradial symmetry. This means the body parts are arranged around a central axis, typically in multiples of five, giving the animal its characteristic star shape. The body is composed of a central disc from which five rays, or arms, radiate outward.
While five arms are the most common configuration, some species can possess many more, exhibiting between 10 and 50 arms. These arms are extensions of the central body, containing extensions of the animal’s organ systems. The arms are generally broad at the base and taper toward the tip, which often bears a light-sensitive eyespot that can distinguish between light and dark.
The radial symmetry allows the sea star to move in any direction without a distinct front or back. Each arm functions in movement and sensory perception, coordinating with the others through a nerve ring in the central disc. This adaptation helps the animal navigate its environment and respond to stimuli from all sides.
External Surface Details: Skin, Spines, and Coloration
The aboral, or upper, surface of the sea star is rarely smooth. This texture results from a complex endoskeleton made of tiny, calcified plates called ossicles embedded beneath the skin. These ossicles can take the form of granules, bumps, or sharp spines, giving the animal its “spiny-skinned” classification.
These spines provide defense against predators and vary widely in size and density across different species. Scattered among the spines are tiny projections called papulae, which function as gills for gas exchange. The madreporite is also visible on the aboral surface.
The madreporite is a small, sieve-like plate, often pale or reddish-orange, located slightly off-center on the central disc. This structure is an external component of the animal’s water vascular system, filtering seawater into its internal canals. Sea stars display a range of coloration, including bright reds, blues, oranges, and purples, which can serve as a warning sign or as camouflage.
The Oral Surface: Tube Feet and Feeding
Flipping the sea star over reveals the oral surface, the underside where the mouth is located. A distinct feature is the presence of ambulacral grooves, which run from the central disc down the underside of each arm. These open furrows are lined with rows of small, tubular projections known as tube feet.
The tube feet are translucent and operate using hydraulic pressure, extending and contracting to facilitate movement and grip. They often possess a suction cup or adhesive tip, allowing the sea star to cling firmly to rocks or exert force to open bivalve prey. These hundreds of moving feet give the appearance of a slow, creeping motion.
The mouth is situated centrally on the oral surface, surrounded by the bases of the arms. It is a simple opening from which the animal can evert its stomach to digest prey externally. This feeding mechanism, combined with the gripping power of the tube feet, allows the sea star to consume food items much larger than its mouth might suggest.