Starfish, also known as sea stars, are diverse marine invertebrates belonging to the class Asteroidea. These creatures inhabit all the world’s oceans, from shallow intertidal zones to abyssal depths. With over 1,900 living species, starfish exhibit a wide array of shapes, colors, and sizes.
Common Arm Count
The most common arm count for a starfish is five, radiating from a central disc. This five-part radial symmetry is characteristic of echinoderms, the larger group to which starfish belong. Many familiar species, such as the common starfish (Asterias rubens) and the blue linckia starfish, typically display this five-arm configuration.
Despite this commonality, the number of arms varies significantly among different species. Some starfish naturally possess six or seven arms, while others can have between 10 and 15. The number of arms can even exceed fifty in certain species, such as the Antarctic Labidiaster annulatus, also known as the wolftrap starfish.
How Arm Numbers Can Change
Starfish arm numbers vary due to species differences and their ability to regenerate lost limbs. Some species, like the sunflower sea star (Pycnopodia helianthoides), can have many arms, sometimes up to 24. This species starts with five arms and adds more as it grows, potentially reaching over 50 arms. The crown-of-thorns starfish (Acanthaster planci) also has a high arm count, typically ranging from 10 to 21.
Starfish are known for their regenerative capabilities, allowing them to regrow lost arms. This process, called autotomy, involves the voluntary shedding of an arm, often as a defense mechanism. A new arm can then regenerate, a process that takes several weeks to months.
During regeneration, a new arm grows from the injury site. This can lead to temporary variations in arm count, as a regenerating arm might appear smaller. In some tropical species, an entire new starfish can develop from just a severed arm, provided a portion of the central disc is attached. This ability to regenerate from a single arm is a form of asexual reproduction.
What Starfish Arms Do
Starfish arms serve several functions, primarily for movement and feeding. The underside of each arm contains thousands of tiny, tubular projections called tube feet. These tube feet operate using a hydraulic water vascular system, allowing the starfish to move across surfaces by extending and retracting them. They adhere to surfaces using a chemical adhesive, not suction, enabling the starfish to cling to rocks and navigate various terrains.
Starfish also use their arms and tube feet for feeding. Many species are predators, using their arms to grasp and manipulate prey such as clams and mussels. The starfish will pull apart the bivalve’s shell, then evert its stomach into the small gap to digest the prey externally. Each arm also contains sensory capabilities, including light-sensitive eyespots at their tips that detect changes in light intensity. Sensory tentacles on the underside of their arms possess chemoreceptors, helping them detect chemical signals from food sources.