How do echinoderms regrow lost arms or other body parts?

Echinoderms possess a highly developed ability to regenerate lost or destroyed body parts, a process that is essential for their survival and, in some species, serves as a method of asexual reproduction.1

The process generally requires the animal to seal the wound effectively after an injury or fragmentation. For many species, such as starfishes and brittle stars, successful regeneration depends on the presence of a portion of the central disk; if an arm is lost, a new one can grow as long as some part of the disk remains associated with it.1

Specific regenerative capabilities vary by group:
* Asteroidea (starfishes) and Ophiuroidea (brittle stars): Can regenerate new arms if existing ones are broken off.1
* Holothuroidea (sea cucumbers): Can regenerate internal organs after expelling them (autoevisceration) and can also undergo transverse division, where significant tissue reorganization occurs to form two complete individuals.1
* Echinoidea (sea urchins): Readily regenerate lost spines, pedicellariae, and small areas of their internal skeleton, or test.1

In some cases, fragmentation is a deliberate reproductive strategy where the body tears into pieces, each of which then regenerates the missing components to form a complete animal.1