I often check out my tank after the aquarium lights have been off for a while. I saw something tonight in my reef tank that I've never seen before.

One of my corals, a orange/green acanthastrea echinata, had snared a hermit crab with its stinging mesenterial filaments.

I turned on the tank lights to get take these photo's:




This coral deploys these nematocyst-laced filaments when attempting to seek and destroy any nearby corals; the filaments pack a powerful sting and can literally digest coral flesh on contact. The filiments actually had adhered to the hermit's shell and not its body, so it wasn't physically harmed but its movement was restricted.

The hermit struggled and was, little by little, able to move away from the coral.

20min later:


The hermit was able to increased its distance from the coral another couple of inches. At this point I was getting a little concerned because the hermit was extending the filaments in the immediate vicinity of some blue zoanthids. I didn't want them to be destroyed by the acan's mesenteries.

I had turned off the light while observing this and was contemplating moving the hermit away from the zoanthids, but I had just finished washing my hands with soap and didn't want to place them in the tank.

The crab kept creeping away from the acan and the filaments looked like they would snap. But then my serpent sea star approached the scene and for some reason started tapping and touching the filaments. I tried to get a photo of that but when I turned on the lights the sea star retreated into a crevice. The acan released the hermit and 'reeled in' its filaments, dragging them across the zoa's but fortunately not stinging them.