Of Snappers and Urchins

I am on a bit of a roll when it comes to observing unusual & interesting fish behavior. Two days ago, at night in a seagrass area at 5 meters off Negros Island in the Philippines I observed two juvenile red emperor snappers, Lutjanus sebae, hiding among the spines of two venomous fire urchins. The urchins were moving around at a considerable speed (for urchins), and the snappers stayed with them occasionally darting out to feed.

It’s quite obvious what the advantage for the snappers is, protection, but I am not sure if there is any advantage the urchins get from this symbiosis, and what it would be.

Here is the video with the observation: