Whats stuck on this Urchin?

As I watched this little growth, attachment or maybe attacker for awhile I noticed that the "ball" would seem to deflate every so often. What is it?

That's the madreporite. Echinoderms have an internal hydraulic system which extends from the madreporite (water intake) through the body and into the tube feet. The inflation & deflation you noticed is the process of sucking in water. It's only this noticeable on some species.

Wow, thanks! Where we go into the water here there are literally thousands of urchins underfoot and this is the first one I have seen against the seawall which has the two color spines. Is the madreporite always extended like that or is it a temporary extrusion to refill the internal system with water?

They expand & contract and aren't always out. Lucky you to have seen it - nearly everything I see is preserved and colorless.....

They expand & contract and aren't always out. Lucky you to have seen it - nearly everything I see is preserved and colorless.....

We really have to get you out on a trip, Leslie.

We really have to get you out on a trip, Leslie.

Here's to that!

Leslie,

My wife and I went out for an evening swim last night and she found this urchin for you!

thanks! Now if you can find a couple of nice worms (that aren't fireworms or dusters) that would really make my day!