Asteroidea

Asteroidea, commonly known as starfish or sea stars, is a class of marine echinoderms. Adults typically display pentaradial symmetry with a central disk and multiple radiating arms. They have a calcareous endoskeleton of ossicles beneath a mutable dermis, often with spines and pedicellariae. The water vascular system, derived from the coelom, drives tube feet along the oral and aboral surfaces for locomotion, attachment, and feeding. The nervous system consists of a circumoral nerve ring with radial nerves in each arm; sensory structures detect touch, chemicals, and light. Most species are dioecious with external fertilization; development proceeds through free-swimming larvae such as bipinnaria and brachiolaria before metamorphosis into a juvenile starfish. Some species reproduce asexually by fragmentation or arm regeneration.

Habitat and ecology: Starfish occur worldwide in marine environments from tidal pools to deep sea, on rocky shores, sandy bottoms, coral reefs, and seagrass beds. Diets are diverse, but many are predators of mollusks and barnacles, using tube feet and sometimes an everted stomach to digest prey externally. They play important roles as predators and scavengers, influencing benthic communities. Reproduction yields a planktonic larval phase before settlement. Starfish are known for regenerative abilities, with some species regrowing lost arms and, in exceptional cases, regenerating from a single arm with part of the central disk. Some populations have declined due to disease such as wasting syndromes, highlighting ecological sensitivity.