One type of symbiosis is called commensalism. Here, the advantage goes only to one member of the association, and the other is unaffected. The advantage may be shelter, food, transport, support, or even a combination of two or more of these. For example, an epiphytic ("upon plant") orchid lives its entire life in a fork or branch of a tree. In tropical forests where such orchids live, the forest floor is so dark that the orchid requires a base of attachment nearer the forest canopy. The host tree seems to be unaffected by the commensal orchids. But what about birds that use trees for the support of nests? Is this a symbiosis as well? The birds benefit from the association (by keeping nests away from ground-dwelling predators), but the association is usually temporary. 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. If not, the symbiosis might be considered parasitism (defined below), rather than commensalism.

• Examples: intertidal marine organisms.
• Examine a specimen of the Bat star, Patiria miniata. Notice that it has on its oral side (the side with the mouth opening, which you can see by turning the starfish upside down) five deep grooves radiating out from the mouth, one for each arm of the starfish. A segmented worm, Ophiodromus pugettensis, can frequently be found living in these grooves if you prod very gently with a clean probe. The association is a facultative commensalism; the worms readily leave one starfish for another, and many worms evidently are free-living and never use the shelter of the starfish 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. Evidently most species of slipper snails are obligate commensals; that is, they are not found living apart from their host shell.

• 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.
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