Sea stars are fascinating marine invertebrates belonging to the phylum Echinodermata, which includes sea urchins and sea cucumbers. They are not fish, lacking gills, fins, and a backbone. These organisms inhabit all the world’s oceans, from tropical reefs to abyssal depths. Their structure combines living tissue, mineralized plates, and a complex hydraulic system.
Defining the Basic Body Plan
The most recognizable feature of the sea star is its characteristic shape, which is defined by a body plan known as pentaradial symmetry. This means that the body is arranged in five parts, or multiples of five, radiating from a central point, although some species can have many more arms. The structure begins with the central disc, from which the arms, or rays, extend outwards.
The organism has two distinct surfaces: the oral and the aboral side. The oral surface is the underside, where the mouth is located at the center of the disc, and it is the side that faces the substrate. The aboral surface is the functional upper side, which may appear granular or spiny depending on the species.
A small, off-center, button-like structure called the madreporite is found on the aboral surface. This calcareous plate, often described as a sieve, serves as the entry point for water into the animal’s unique internal system. Along the underside of each arm runs a groove called the ambulacral groove, which houses the rows of tube feet.
The Internal Skeleton and Composition
The sea star’s rigidity and protection come from its endoskeleton, located just beneath the skin. The skeleton is composed of numerous small, interlocking calcium carbonate elements called ossicles.
These ossicles are embedded within the dermis, the thick connective tissue layer of the body wall, and are covered by a thin outer layer of epidermis. The mineralized plates are not solid bone but are constructed from microcrystals of calcite arranged in a three-dimensional lattice structure called a stereom. This structure gives the material a lightweight, yet tough, honeycomb-like quality.
The arrangement and shape of the ossicles vary, ranging from flat plates to granules to protective spines on the aboral surface. Specialized projections also extend from the dermal layer, including the dermal branchiae, which are thin-walled protrusions used for respiration, and pedicellariae. Pedicellariae are tiny, pincer-like structures that function to keep the starfish’s surface clean of encrusting organisms and small debris.
The Water Vascular System
A defining feature of the starfish is the specialized water vascular system, a complex network of fluid-filled canals. This hydraulic mechanism is necessary for locomotion, feeding, and gas exchange. Water is initially filtered into the system through the madreporite on the aboral surface.
From the madreporite, the water flows down a calcified tube called the stone canal to a circular ring canal that encircles the mouth. Five radial canals branch off the ring canal, with one extending down the length of each arm within the ambulacral groove. Lateral canals branch from the radial canals, each leading to a muscular sac known as an ampulla.
The ampulla sits inside the arm and connects directly to the tube foot, or podium, which protrudes outside the body. Movement is achieved when muscles contract the ampulla, forcing fluid into the attached podium and causing the tube foot to extend.
Conversely, contraction of the longitudinal muscles in the tube foot forces the fluid back into the ampulla. This action causes the podium to retract and pulls the animal forward.
The tube feet often end in suckers, which use an adhesive secretion in addition to suction to grip the substrate. This allows the starfish to move slowly and adhere firmly to surfaces. This hydraulic process enables the coordinated, slow movement of the animal across the ocean floor.