id?
 

the small white semi-circular things...possibly baby snails? they're about 2mm wide, use the tube worm for comparison. there are about 20-30 of these scattered on the rocks.

http://invigor.sasktelwebsite.net/snails.jpg

Limpet

http://www.woodbridge.tased.edu.au/m...er/limpets.htm

Limpet. Probably Acmaea sp.

hmm, cool. by the sounds of them they sound helpful moreso than harmful?

Hard to say. Not sure what species you got.
Just reading up on them with a book in front of me here (Calfo & Fenner).
"Voracious algae grazers that are not always useful for the mixed invert aquarium."
"Most are herbivorous, some are predatory..." [Coral grazers. Carribean Orange (Lucapina) and Black Limpet (Scutus)].
Also, this...."Even among the algae grazing species, they can be devastating to desirable algae forms like corallines...."

I'd say keep 'em, but monitor what they are up to! :eek:

Cheers,

PS Apparently there are claims of them causing abrasion to acrylic tanks, too.

yup limpets. i have a bunch in my tank, on my crab and snail shells.