Starfish, also known as sea stars, are invertebrates found in all the world’s oceans, from tropical to polar regions, and from shallow waters to abyssal depths. These marine creatures, part of the Echinodermata phylum, typically feature a central disc with radiating arms, though the number of arms can vary significantly among the nearly 2,000 known species. Starfish possess unique biological features and extraordinary adaptations.
Remarkable Regeneration
Starfish are widely recognized for their ability to regenerate lost or damaged arms. Many species can regrow a severed limb, and some can even regenerate an entire body from a single arm, provided a portion of the central disc remains attached. This regenerative capacity is a significant survival mechanism, allowing them to escape predators by intentionally shedding an arm, a process known as autotomy. The process involves wound healing and the formation of new tissues, often taking several months for full reformation.
Unusual Dining Habits
Starfish employ a distinctive feeding strategy that involves everting their stomach to consume prey. When feeding on bivalves, the starfish uses its tube feet to grip and pry open the shell. Once a gap appears, the starfish pushes its cardiac stomach out through its mouth and into the shell, enveloping the prey’s soft tissues. Potent digestive enzymes are then secreted externally, breaking down the prey before the partially digested material is drawn back into the body for final processing. This external digestion allows them to consume organisms much larger than their small mouth opening.
A Unique Internal System
Starfish lack a centralized brain and a traditional blood circulatory system. Their nervous system is decentralized, featuring a nerve ring around the mouth and radial nerves extending into each arm. This distributed network coordinates movements and sensory functions without a single command center. For internal transport, starfish utilize a unique water vascular system, circulating seawater throughout their body via a network of canals. This hydraulic system facilitates the distribution of nutrients and gases, effectively replacing the functions of blood.
How They Move
Starfish move across various surfaces using hundreds to thousands of tiny tube feet located on the underside of their arms. These tube feet operate through hydraulic pressure, a key function of their water vascular system. Water enters this system and is channeled into the tube feet, causing them to extend and create suction for grip. Muscles within the tube feet then contract, retracting them and pulling the starfish forward in a coordinated, slow-paced motion. This allows them to cling firmly to rocks and navigate their environment.
Simple Sight, Big World
At the tip of each starfish arm, there is a simple light-sensing structure known as an eyespot. These eyespots are not complex eyes like those of humans; instead, they consist of clusters of photoreceptor cells capable of detecting changes in light and darkness. This basic form of vision allows starfish to perceive general shapes and shadows, which assists with navigation and avoiding potential threats. While they cannot form detailed images, this simple visual capability aids in finding shelter or food.