Tunicates or something else?

Hi,

I'm posting these for a friend as they stumped me. Photographed in the Arabian Gulf at 10-12m depth. Described as round structures 2.5 cm in diameter. They look like they're surrounded by calcareous plates and have reddish, spongey material in the middle.

One guess is that they could be the apertures of encrusting tunicates. Although they don't look much like any tunicates I've seen.

You're right - tunicates. It's not unusual for these solitary tunicates to have bumps or nodules in the apertures of the their siphons although the ones in your friend's images have the most pronounced ones I've seen. They probably serve to reinforce the aperture when it's closed against predators. Here are a few examples

http://www.guamshellclub.org/images/marine..._061121_001.jpg

http://www.ascidians.com/families/styelida...rpacaptiosa.htm

http://www.ascidians.com/families/ascidiid...idiagemmata.htm

Thanks a lot Leslie. Very useful.