Guard Crab, Paladin of the Reef
by Owen James Burke
A guard or Trapezia crab (of the Trapeziidae family) defends a coral reef from predatory sea stars in Thailands Similan Islands
Researchers on Moorea Island in French Polynesia have discovered that various members of the guard crab genus (Trapezia) provide their own respective services as keepers of the gate for their coral reef habitats from destructive crown-of-thorn sea stars which decimate reefs by consuming them at menacing speeds.
Crown-of-thorn sea stars (Ancanthaster planci) can grow to the size of a trashcan lid. The specimen above is halfway through consuming an entire coral bommie off Fiji. (Photo via Omnilexica)
That’s not to say that crown-of-thorns don’t have their own functions within coral reef systems — they do: when coral reefs become overgrown, crown-of-thorns perform something like an undersea slash-and-burn to clear space for new life. But since 2008 in the waters surrounding Moorea, there has been a population boom, and the predatory sea stars are threatening the livelihood of the coral reefs. Fortunately this one resident is so happily lodged within the corals — it feeds on fats found in the tentacles of the corals’ polyps, which, by the way, also helps to keep them clean — that it’s gracious enough to keep the villainous sea stars at bay. Ah, symbiosis.