Bald Sea Urchin Disease

Bald Sea Urchin Disease is a bacterial disease known to affect several species of sea urchins in the Mediterranean Sea, North Atlantic and along the California coastline. The disease was first described in the red sea urchin (Mesocentrotus franciscanus)[1].

Research suggests two pathogens are responsible for the disease, Listonella anguillarum and Aeromonas salmonicida[2].

Infection generally occurs at the site of an existing physical injury. The affected area turns green and spines and other appendages are lost. Urchins also lose control of their tube feet, which they use to walk.

Spine loss is the key characteristic of the disease. If the lesion remains shallow and covers less than 30% of the animal's surface area, the animal tends to survive and eventually regenerates any lost tissue. However, if the damage is more extensive or so deep that the hard inner test is perforated, the disease is fatal.

In the 1980s the near disappearance of a keystone herbivore, the long-spined black sea urchin (Diadema antillarum) due to disease of unknown etiology resulted in a massive ecological phase shift in the Caribbean Sea from coral cover to uncontrolled algal growth on the reefs.

At the beginning of 2023, researchers spotted the first signs of the urchin plague in the Mediterranean Sea, when an invasive species of urchin, the black sea urchin (Diadema setosum), began falling sick in waters around Greece and Turkey. From there, the disease appears to have spread southward through the Suez Canal to the Red Sea.

The epidemic looks set to wipe out all of the Mediterranean and Red Sea’s urchins, and possibly their coral reefs too.

"It's a fast and violent death: within just two days a healthy sea urchin becomes a skeleton with massive tissue loss," Omri Bronstein, a senior lecturer in Zoology at Tel Aviv University, said in a statement. "While some corpses are washed ashore, most sea urchins are devoured while they are dying and unable to defend themselves, which could speed up contagion by the fish who prey on them."

It is unknown why sea urchins are suddenly vulnerable to these bacterial predators. Maybe it is a result of climate change or maybe one of these bacteria mutated and shared some mutated genes with another bacterial species.

[1] Johnson: Studies on Diseased Urchins from Point Loma in Annual Report Kelp Habitat Improvement Project – 1971
[2] Shaw et al: Bald sea urchin disease shifts the surface microbiome on purple sea urchins in an aquarium in Pathogens and disease – 2023. See here.