"Wayne Sallee" wrote in message k.net...
I would not count on any urchin not eating rock,
though some eat it more than others. Live rock +
urchin = Live sand :-)

I had maybe 3 tuxedo urchins over the years and
have not noticed them eating rocks... They eat
sometimes surface layers of coraline algae, but
not rocks.

I was talking about rock boring urchins as they
literaly bore a hole in the rock for themselves
and sit in the hole they made grazing on the
algae on the hole surface. This is their way of
avoiding predation: they are protected with rock.

Check this out:
http://www.jal-foundation.or.jp/html/coral/lesson4.htm