Kategorie: News
In search of the last diadem sea urchins
The Citizen Science project Dive4Diadema is in search of the last Diadem sea urchins in the Mediterranean and the Red Sea. With your help, we want to investigate the disappearance of the sea urchin Diadema setosum and document its presence or absence in the Mediterranean and the Red Sea.
Mass mortality of Diadema sea urchins
Dead and dying Diadem sea urchins were first observed in July 2022 off the port of Kastellorizo in Greece. In the meantime, the individual cases have turned into a mass mortality that, like the immigration of the diadem sea urchins from the Red Sea via the Suez Canal into the Mediterranean, is now continuing in the opposite direction into the Red Sea at great speed. Until recently, up to 30 animals per square metre of the crepuscular and nocturnal species could be found. In the meantime, they have almost completely disappeared along the coast from Aqaba, Eilat to the southern tip of the Sinai Peninsula and also on the Saudi Arabian side. Between Hurghada and further south, no more Diadema setosum have been seen in recent weeks either.
What impact this will have on the coral reefs in the Red Sea is not yet entirely clear. However, experience from the Caribbean and other regions suggests that the coral reefs, already so stressed by climate change and warmer water temperatures, will change.
"Lawnmowers" of the reefs
Sea urchins are one of the most important groups of animals 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 larvae and slower-growing organisms, such as corals, mussels and bryozoans. By gnawing off the algae, a strong bioerosion of the substrate also takes place at the same time. Particularly industrious are the sea urchins from the group of diadem sea urchins, of which there are nine species worldwide, which occur in all warmer seas.
Mass extinction
Repeated mass mortality events are known from the Caribbean species Diadema antillarum, which peaked in the early 1980s due to a waterborne pathogen. In the Caribbean, 98 % of all diadem sea urchins disappeared at that time. The survivors have slowly recovered in recent years, but by 2022, about 95 % of all sea urchins had died again. Scientists were able to identify the protozoan Philaster apodigitiformis as the pathogen in the Caribbean, which is already a known parasite in fish. In other regions, however, the bacterium Vibrio alginolyticus seems to be the culprit, which together with high water temperatures led to the mass mortality events. Both pathogens lead to cell death and the typical loss of spines in the diadem sea urchin, which is usually noticed first.
Dramatic changes in the coral reefs
The disappearance led to dramatic changes in the coral reefs and the entire underwater biodiversity in the respective regions. Within a short period of 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 off East Africa, 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. Again, this has led to a major change underwater, with algae spreading vigorously.
How can I get involved?
We are all encouraged to report our observations during their dives using the free, multilingual app for iPhone, iPad and Android smartphones and tablets. Every observation of live and dead diadem sea urchins helps us to learn more about the current mass mortality. Even if no urchins are sighted on a dive, this is important information to report.
Dive4Diadema is a joint effort of dive centres, tour operators, training organisations and scientific institutions. Only together can we succeed in finding and saving the last occurrences of the black, long-spined diadem sea urchin.
Join us and participate in the Citizen Science project Dive4Diadema!
More Information:
https://dive4diadema.org
Google Playstore:
https://play.google.com/store/...diadema
Apple App Store:
https://apps.apple.com/de/app/dive4diadema