Limpets?

[Mutton Biryani]

Never noticed them until I added some corals yesterday recently. Look like limpets from the Hitchhiker ID sticker thread.
Do these spread fast? Never saw them before and then suddenly have so many on the main rock work.

How big do they get?

 

[Shooter000]

Good ones to have, shouldn't over populate to bad, pinky nail size

 

[Mutton Biryani]

Thanks shooter.

Got a few more hitchhikers from the corals. Couple of asterina stars and this baby cerith? snail. The snail is probably 2mm big.