Cidaroida

Cidaroida is an order of sea urchins within the class Echinoidea. They are among the most primitive living echinoids, with a fossil record that extends back to the Paleozoic era. Modern cidaroids are distributed worldwide in a range of marine environments, from shallow coastal waters to deep-sea habitats.

Anatomy and morphology: Cidaroids have a thick, heavy test (shell) with large primary spines that attach to prominent tubercles. The spines are typically long and robust, serving defense and locomotion. The plates of the test are relatively large and the test is less specialized than in more derived echinoids; ambulacral areas are simpler, and pedicellariae are often rudimentary. Their overall appearance is rugged, reflecting a more primitive body plan compared with other sea urchins.

Ecology: They are mostly detritivores or algivores, feeding on biofilm, microalgae, and organic material on hard substrates. Cidaroids are generally slow-moving and may burrow into sediment or shelter under rocks. They inhabit a variety of substrates, including rocky bottoms, coral reefs, and soft sediments, from shallow to deep waters.

Evolution and relationships: Cidaroids represent a basal lineage of Echinoidea, offering insight into early echinoid morphology and evolution. They are less diverse today than many derived echinoid groups but remain ecologically meaningful in certain ecosystems.

Reproduction: Like other echinoids, cidaroids are predominantly broadcast spawners, releasing eggs and sperm into the water column. Fertilization is external, and the life cycle includes free-swimming larval stages typical of echinoderms.