Sea Urchin (hitch hiker)

[kay-bee19]

I have a hitch-hiker sea urchin in my tank (came along with some live rock I acquired).

When I first discovered it about 7 months ago it had the diameter of a plain m&m. Now it has the approximate diameter of an acorn.

Any idea what type it is and how large it will actually get?:

 

[graveyardworm]

It appears a common rock boring urchin, Echinometra Mathaei. ID taken from picture in Reef Invertebrates by Calfo/Fenner. Here's a little more infohttp://saltcorner.com/sections/zoo/inverts/echinoderms/urchins/Emathaei.htm

 

[kay-bee19]

That looks like it.

Thanks!