While scanning the Acro tank in our LFS we noticed this overgrowing the base of a colony. It’s an ascidian, a colonial tunicate, possibly Botrylloides leachii, which has spread over this Indonesian maricultured Acropora in a matter of just weeks. Tunicates are harmless filter feeders which reproduce sexually before settling larvae on submerged surfaces. The larvae metamorphose into zooids which then produce more zooids via budding and the colony grows. The zooids can often become chainlike as seen on this one, but colors can vary hugely from blue and black to green orange, yellow and purple. Each colony contains a number of inhalant and exhalant siphons which it uses to capture food particles, and they are interesting organisms when viewed close up.
This one decided to land on a coral base however and is beginning to overgrow the coral itself, including its CITES plastic identification label. It was also spotted growing separate colonies on the tank glass and the plastic egg crate the corals were sitting on. The remedy for this coral is to cut the unaffected branches and remount them as separate frags. Attempts to scrape it off will prove futile as it can regenerate from just a tiny piece. Just like sponges, colonial tunicates make up an interesting part of reef ecology, but try to keep them out of your reef tank for the sake of your corals.