"In studies of the ubiquitous Hawaiian branching stony coral
Pocillopora damicornis, Dr. John Stimson, of the University of
Hawaii, found that the presence of Trapezia sp. crabs,
actually stimulated the coral to produce fat bodies, which were
clearly visible as small, whitish globs in the tentacles of the
polyps. These were then fed upon by the crabs. How this occurs is not
clear. They could be eating the tentacles directly, they could
stimulate the polyp in some way to cause it to release the fat
bodies, or the fat may be exuded in coral mucus. When the crabs are
removed, the fat bodies get larger for about a week, but then grow
smaller. Dr. Stimson has recently discovered that if the crabs are
fed, they readily release ammonia, but if they are not fed, they
don't. This raises the possibility that when corals feed their crabs
lipids, the crabs become living fertilizers, moving amongst the
branches, releasing nitrogen-rich ammonia to the zooxanthellae. This
may explain why polyps in the lower branches retain high vitality
rates despite being exposed to less light. When crabs are removed,
the lower branch polyps often die-off, and the lower branches become
covered with algae, sponges and tunicates. It has also been
speculated that the movements of the crabs lower down in the colony
increase water circulation and prevents detritus from accumulating.
They may also stimulate an increase in mucus production by the coral,
further contributing to keeping the branches clean."
From http://www2.hawaii.edu/~delbeek/afmjan97.html