Top of the oyster

[Europhyllia]

You know how they say it's important to put bivalves that open to the side exactly as they grew on the reef? Well that makes perfect sense of course! On the big thorny oyster it's easy to tell.
But I got a little oyster (or clam) as a gift and can't tell what's up and down. The only thing growing on it is a couple of small tunicates which just poke straight out so don't give any clues either.
Does anybody know any fail proof ways to figure out what's up/down?

[BSJF]

Hum, never even thought about that before. Is one side darker than the other side?

[Europhyllia]

I looked closely at the oyster and it seems the thorny oyster has most thorns on top, so I turned the little oyster clam with most scales on top as well. The tunicate seems to make more sense that way too.

http://www.dominopads.com/tunicate.jpg

[justahobby]

I'm glad you posted this. I didn't know it was an issue. Now if I can just retain and recall this if/ when I get one.....

[Europhyllia]

ah well nevermind. I'm all wrong. An urchin knocked my clam off and while I picked it up and looked at it I noticed sponge growth on one side. So I decided that would be most likely the underside.
Turned it around and the tunicate's siphons are now both horizontal. I think this is how the oyster was sitting in nature and the amount of spikes may have nothing to do with top or bottom.
http://www.dominopads.com/tunicateclam.jpg

[Europhyllia]

I should pluck that little tuft of hair algae, shouldn't I lol