Why I love trapezia crabs

[FuEl]

"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