Dear Zed: I don't know enough biology to cite an example,
but there must be quite a few other such instances of
symbiosis benefiting only one of the two members. A small
fish has to find a place to hide. Any port in case of a storm.
Far more curious to me are some of the parasitic relationships. For instance, there is a fly that approaches anus of deer, and at last instant turns 180 degrees, and
injects an ovum which becomes intestinal parasite of the deer. Can you think of the scenario leading to establishment
of this behaviour?