What are they.

Question
Todd, I have a saltwater aquarium appx. 10 yrs. old.  Lately I have had something growing that I have never seen before.  Round objects like different size marbles, dark grey in color.  Exterior is a plastic like substance.  When squeezed they pop and are full of water.  Any idea what they are?
Thanks,
Rob  

Answer

Rob,

without actually seeing these guys they may (and I am guessing) some type of sponge.  Did you add any live rock to your tank recently.  Some times these guys hitchhike on
live rock.  Maybe you can shoot me over a picture.  My email
is tfilicicchia@ymcamalden.org.

Thanks
Todd