Thus many biologists would not consider this a symbiosis, but the matter is not clear cut. Another ambiguity is whether the orchid or bird really does leave the tree unharmed. A segmented worm, Ophiodromus pugettensis , can frequently be found living in these grooves. Also examine the image of a slipper snail, Crepidula . There are several local species of slipper snail, all with the habit of attaching to the shell of another, larger snail.
Most snails pull in their food by scraping or rasping algal film off the substrate.
However, adult slipper snails do not rasp the surface of their host shells, so that is not the advantage they derive from the association. Instead, with modified gills, they filter small particles from the water as they ride about attached to their host.