Great info on these Ilyanassa obsoleta snails

http://reefkeeping.com/issues/2006-11/mg/index.php

hope this helps. Helped me understand the diff.

Well I went cheap and bought an order of these. I have about 200 in my display tank. Contrary to the article, they do a pretty good job of cleaning off the glass and haven't being dying off (maybe good or bad).  I have noticed that my cerith snails have disappeared-possibly due to predation? Also, my copepod/amphipod diversity is less than it had been. Mostly all I have now are larger amphipods- hardly any of the little white dot kind. I thought maybe they were being eaten my the larger amphipods, but now i recognize the snails might be the problem. However,  I don't have any of the obsoleta in my 125g sump, so I should have a continuous source of copepods for the display tank. I think part of the problem midht be that I have the same substrate in top and bottom tanks. I added a frag tank and am going to use a coarser substrate-hopefully will pay off with more pod diversity. I'll see how that works out before blaming the snails. for now, they don't seem to be suffering unduly and don't really appear to be causing any problems.