Cool hitchhiker!
I do know of certain fishes which sleeps at same spot most of the time once used to tank/system Rcks/ currents but never noticed such behavior from gastrpods. Their predators are usually stars and/or other gastropods.
Athough imported uncured/cured rocks for many yrs and saved many UFOs and rare specimen, this pic does not ring a bell.
Although not easy to take pics, post more shots from differnt angles if and when possible.
I am curious to what this maybe.
Are you sure it is sponge on its back?
If so, how long did you have this specimen and have you notice any growth of what you assumed to be sponge?
btw RFG, arent you the one who set up plant tank recently.
How did first shipment of algae do in new set up?
I'm fairly certain it was a sponge, but I could be wrong. I posted him on several website and could never get an answer as to what he was, but most people agreed it was a sponge on his back. Not sure if it just HAPPENED to grow there on his shell, or he was actually some kind of symbiotic creature.
I had him for a year and a half or so, and when I took the tank down and gave it to my friend as far as I know he was still alive and coming out periodically a couple years later

.
The friend I gave him to is actually a marine biologist and he agreed that it appeared to be a sponge, but had not seen similar. He had trouble IDing what the snail must have been since a shell was not visible and the top half of him was covered.
Actually as far as growth, while I had him I didn't notice any, but when he was moved to my friend's tank the "sponge" seemed to die off somewhat during transfer. It grew back into about the same shape/size it was previously after awhile.
These are the only other shots of him I have uploaded. I will have to look through my hard drive and see if I have more. If I find them I will PM them to you.
Yes I am the one that recently set up a planted tank. Most of the algaes seem to be doing well but my mermaids shot glass seems to have died off (there is still stem left) and not sure if it will come back or not.
I also got some kind of macro (the penny macro from reef cleaners where they send you a random one) but I have no clue what it is. It was really slimy and kinda looks like pieces of light green rotting lettuce. I will have to take a pic. I thought it all died off and decayed, but it appears it just floated around the tank and some has attached to a shell and started growing.
I ended up with a piece of manatee grass on the rock my cactus halimeda came on and it appears to be growing well.
Also got some chaeto in the bags of copepods I ordered to seed the tank and it's started growing well.
Overall I ended up with:
Cactus halimeda
Laurencia
Pencil cap
Flame algae
Red titan algae
Sargassia
Chaeto
Whatever the penny macro was, lol
and that one piece of manatee grass
All are small at the moment, and not much growth yet. I haven't posted pics since I have a brown algae outbreak probably due to cycling, but it is dying down and I'm going to do a water change tonight or tomorrow which should start to kill off the rest. So I will post pics in my thread soon.
It would help if my nerites would do their job and eat some algae, but they appear to me more interested in laying eggs at the moment!