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9/23/05 - Defender of the reef
by David Nichols, State of Hawaii, DLNR, HIHWNMS

One of the two new genus of coral guard crabs that have been recorded by Scott Godwin of Bishop Museum. Photo by
Scott Godwin.

We have finished with Maro Reef and are transiting to Pearl and Hermes Atoll today. Transit days give the scientists time to organize and evaluate some of the data and organisms that have been collected. Invertebrate specialist Scott Godwin of Bishop Museum spends most of his free time in the vessel’s wet lab analyzing some of the specimens he and assistant, Holly Bolick collect during an REA dive.

Today he shows me a coral guard crab (Family: Trapeziidae). These crabs live in association with various species of branching coral throughout tropical coral reef systems. Mated pairs actually live deep within the coral branches and are completely dependent on the coral for food and shelter. In fact, these crabs use their massive claws to ward off intruders that threaten their host coral. If a coral-eating predator (i.e. crown-of-thorns starfish) settles on their coral head, then the guard crab will begin to snip at its tube feet causing it to retreat. They claim that even a diver can get a pinch if he places his hand over a branch of coral with these crabs.

There are twelve genera of coral guard crab throughout the world and only one genera, Trapezia, has ever been recorded from the Hawaiian Archipelago. Two genera and species of coral guard crabs have been recorded for the first time from the Hawaiian Islands during this cruise by Scott from Maro Reef and French Frigate Shoals. These species, Tetralia glaberrima and Tetraloides heterodactyla, represent two new Hawaiian Archipelago records for the family Trapeziidae. These two species are obligate symbionts to the coral genus Acropora and the Northwestern Hawaiian Islands is the only location in the Hawaiian Archipelago in which this coral exists.

Here we have a crab (so small you could carry it around on a dime) protecting his own little piece of coral. The picturesque Tetralia glaberrima gets my vote for the “unofficial NWHI poster critter”. The coral guard crab - defender of the reef!

Holly Bolick of Bishop Museum surveys the coral at Maro Reef
for invertebrates.

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