This is a Pencil Sea Urchin. The body is a bit
larger than a tennis ball, with 2 inch spikes. He mostly eats
algae, and uses the pencils as protection (who would want to try
to eat that?). Here he is from above:
Here he is from the side. The lighting is not
all that good, but you can see his teeth a bit.
Dental closeup. A relative of the star fish,
the urchin has five-way symmetry, and thus five teeth. He holds
onto rocks and glass with suction cups on thin tentacles, a few
of which you can see attached to the glass (upper middle and
upper left). Those teeth make you wonder if he only likes algae.
Still, a snail could out-run him...