Sea urchins are one of the most important animal groups in the coral reefs of the Red Sea. They feed on algae and are therefore the “lawnmowers” of the reefs. They contain the spread of algae and thus create space for the settlement of beneficial larvae and slower-growing organisms, such as corals, mussels and bryozoans. Sea urchins graze on algae removing them from solid surfaces. This feeding activity leads to a strong bioerosion of the substrate that aids the development of other organisms. Particularly industrious are diadem sea urchins. As a group, diadem sea urchins consist of nine species found worldwide in warmer seas. In the Caribbean the diadem sea urchin Diadema antillarum scrapes up to 9.7 kg of calcareous sediment per square meter of coral reef every year. This creates new underwater habitats and white sandy beaches.

The disappearance led to dramatic changes in the coral reefs and significant reduction of the entire underwater biodiversity in the respective regions. Within a short time, the colourful and species-rich coral reefs were transformed into a reef landscape overgrown with algae and poor in species. In 2009, another mass mortality event took place on the East African coast, in which 65% of the diadem sea urchin species Diadema africanum died, and in 2018, 93% of all diadem sea urchins died around the Canary Islands. These die-offs have led to major changes underwater, with algae spreading vigorously.

As a native species, Diadema setosum is found in the western Pacific and along the east coast of Africa. Native populations originally restricted to the Red Sea and the Persian Gulf, have in recent years migrated into the Mediterranean as a neobiot via the Suez Canal, like many other marine animals and plants. In 2006, Diadema setosum was observed for the first time off the coast of Kaş in Turkey. It has expanded along the coasts of Greece, Lebanon, Israel, Libya and the Egyptian Mediterranean coast and is now at home throughout the Mediterranean.

Dead and dying long-spined sea urchins were first observed in July 2022 off the harbour of Kastellorizo, Greece. What initially appeared to be isolated cases developed into a mass mortality event which, much like the original migration of Diadema setosum from the Red Sea into the Mediterranean via the Suez Canal, rapidly spread in the opposite direction into the Red Sea. Prior to the mass mortality event, densities of up to 30 individuals per square metre of this crepuscular and nocturnal species could be found. Following the outbreak, the species almost completely disappeared along the coast from Aqaba and Eilat to the southern tip of the Sinai Peninsula, including the Saudi Arabian coastline, and further south into the Red Sea. Only in recent months have increasing numbers of new sightings of Diadema setosum been reported.

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