Starfish are marine invertebrates recognized for their distinctive star-like body plan. Classified within the phylum Echinodermata, they inhabit diverse marine environments across the globe, from shallow coastal waters to the deep sea.
Naming Starfish Arms
What are commonly referred to as “starfish arms” are scientifically known as rays. Most starfish species possess five rays that extend outwards from a central body disc. This arrangement, known as pentaradial symmetry, organizes body parts in five equal sections around a central axis. While five rays are characteristic, the number can vary significantly among different species. Some starfish may have six or seven rays, while others, like the sunflower sea star, can develop up to 24 rays as they mature.
What Starfish Arms Do
The rays of a starfish perform several functions, primarily locomotion and feeding. Thousands of tiny, suction-tipped tube feet, also called podia, line the underside of each ray. These tube feet operate through a hydraulic water vascular system, a network of fluid-filled canals enabling slow movement across surfaces. Water enters this system through a sieve-like plate on the starfish’s upper surface, called the madreporite, and travels through a series of canals, including a ring canal and radial canals that extend into each ray. When ampullae, small muscular sacs connected to the tube feet, contract, they force water into the podia, causing them to extend and attach to the substrate, enabling movement.
Beyond movement, these rays are important for prey capture, particularly for species that feed on bivalves like clams and mussels. The starfish uses its powerful tube feet to pry open the shells of its prey just enough to evert its stomach through its mouth and into the bivalve. Digestive glands, known as pyloric caeca, are located within each ray and produce enzymes that begin digesting the prey externally. After partial external digestion, the liquefied food is absorbed back into the starfish’s body. Simple light-sensing eyespots, located at the tip of each ray, allow the starfish to detect changes in light and shadow, assisting in navigation and predator avoidance.
Beyond the Arms: Other Starfish Adaptations
Beyond their specialized rays, starfish exhibit other adaptations. The central disc, from which the rays radiate, houses vital organs. The madreporite, a small, often visible plate on the upper surface of the central disc, is important for regulating water pressure within the water vascular system, which is essential for movement and other physiological processes.
Starfish are known for their capacity for regeneration. Many species can regrow a lost or damaged ray, a process that can take a year or longer. In some instances, an entire new starfish can regenerate from a single ray, provided that a portion of the central disc remains attached. This regenerative ability serves as a defense mechanism, allowing a starfish to shed an arm to escape a predator and later regrow the lost limb.