diadema

Diadema is a genus of tropical sea urchins in the family Diadematidae, commonly known as long-spined urchins. The genus name derives from the Greek diádēma, meaning a band or diadem, a reference to the animals’ prominent spines and crown-like appearance. Diadema species are found in warm oceans around the world, with many representatives in the Indo-Pacific and a Caribbean species in the Atlantic.

The genus comprises several species, including Diadema setosum, Diadema paucispinum, Diadema savignyi, and, in the Atlantic, Diadema antillarum. Some species have been reported outside their native ranges due to human activities, and in certain regions non-native Diadema populations may behave as invasives.

Diadema urchins are notable for their very long, slender spines that radiate from a relatively small, rounded test. Spines are often dark in color and can inflict painful punctures if handled. The body’s main support and mobility come from tube feet and their ambulacral system, while feeding occurs via grazing on algae and biofilms on reef substrates.

Ecologically, Diadema are important herbivores on coral reefs, especially active at night. Their grazing helps regulate algal growth, which can facilitate coral recruitment and contribute to reef resilience. Reproduction is by external fertilization, with free-swimming larvae that eventually settle as juvenile urchins.

Threats to Diadema populations include habitat degradation, overharvesting for the aquarium trade, disease, and climate-related stress. Notably, mass mortality events of D. antillarum in the Caribbean during the early 1980s had profound effects on reef community structure by removing a key algal-control force. Some species may also expand or shift ranges when introduced to new regions.