The green sea urchin occurs there in large numbers and is considered the second most abundant urchin species in the rocky reefs of the Galápagos.
Studies of sea urchin and damselfish populations of yellowtail damselfish Stegastes arcifrons, which maintain algal turf in their territories, showed that the damselfish immediately attacked the sea urchin species Eucidaris galapagensis and Lytechinus semituberculatus strongly when they were moved onto the fish's algal turf.
Damselfish generally attacked the invaders singly or in pairs, with the exception of pencil urchins in farms where it was common for 3-4 damselfish to attack the invader. When attacked, the invaders almost always landed in rock crevices or on sand, often leaving the urchins upside down.
Introduced sea urchins such as Thais melones or rocks were attacked significantly less by the damselfishes.
The aggressive behavior of the perch, which defended their algae turf against the hungry urchins, made it clear that the mass development of the urchins could be limited, at least in parts of the reefs.
You can read more about the experiment here: https://besjournals.onlinelibrary.wiley.com/doi/10.1111/j.1365-2745.2008.01467.x
Anapesus semituberculatus (Valenciennes in L. Agassiz, 1846)
Echinus (Psammechinus) semituberculatus Valenciennes in L. Agassiz, 1846
Psammechinus semituberculatus Valenciennes in L. Agassiz, 1846
Schizechinus semituberculatus (Valenciennes in L. Agassiz, 1846)
Toxopneustes semituberculatus (Valenciennes in L. Agassiz, 1846)