Overview

Starfish, commonly called sea stars, are marine invertebrates belonging to the echinoderm group. They live on the sea floor from shallow intertidal zones to the deep ocean and show a wide range of sizes, colors and arm counts. There are more than 1,500 known species, many of which are important benthic predators or scavengers. Notable examples include very large sun‑shaped species and the coral‑feeding crown‑of‑thorns.

Anatomy and physiology

Sea stars typically have a central disc and radiating arms (commonly five, but some species have many more). Their bodies are supported by an internal network of calcareous plates called ossicles, which together form an endoskeleton. They lack a brain and true blood: instead, a water vascular system powered by seawater moves nutrients and operates hundreds to thousands of tube feet used for locomotion and handling prey. A porous plate called the madreporite connects this system to the surrounding water. Gas exchange occurs across thin skin gills or papulae, and their digestive system can include an eversible stomach that is pushed out to envelop and externally digest prey.

Behavior, diet and ecology

Most starfish are carnivorous, feeding on bivalves (such as mussels and clams), barnacles and other slow-moving animals; some species are scavengers or feed on detritus. Many catch shellfish by prying shells open with their tube feet and everting their stomachs to digest the soft tissues. In some ecosystems certain sea stars act as keystone predators, shaping the structure of intertidal communities by controlling prey populations. Conversely, outbreaks of coral‑eating species can significantly damage reefs.

Life cycle and reproduction

Reproduction is typically sexual, with separate sexes releasing eggs and sperm into the water where fertilization occurs. Larvae are planktonic for a time and undergo several developmental stages before settling and metamorphosing into the adult form. Some species can also reproduce asexually by fragmentation or by regenerating lost arms; regeneration is a widespread and well‑known ability among sea stars.

Human significance and conservation

Sea stars are of interest to scientists studying regeneration, development and marine ecology. They influence fisheries and reef health, sometimes positively by controlling prey species and sometimes negatively when population explosions of coral predators occur. Habitat loss, pollution and disease have led to declines in some populations; conservation efforts and monitoring help track and protect vulnerable species.

Notable facts and distinctions

  • Starfish are echinoderms with radial symmetry as adults, related to sea urchins and sea cucumbers.
  • They do not have a centralized brain; instead they have a nerve ring and radial nerves coordinating movement.
  • Their water vascular system replaces the need for a blood circulation system, using coelomic fluid and seawater to transport materials.
  • Regeneration allows some individuals to regrow entire arms and, in certain cases, whole bodies from a fragment.