The crab who couldn’t stop decorating

Marcocoeloma trispinosum, commonly known as the decorator crab: I had my first encounter with one of these alien-like creatures last Sunday at the fish store. This bright orange crab has an unusual triangular body, and a slow, meticulous way of moving. I commented to a friend that it looks like a cross between a dinosaur and a spaceman.

This is a cautionary tale that research should be done prior to purchase and introduction of such a creature to your home aquarium.

The decorator crab gets its name from its habit of attaching items such as individual zoanthid polyps, corals, algae and small shells to its carapace. It
is quite adept at understanding its environment and how to camouflage itself thus to match. Unless startled, it moves at a deliberately slow pace to avoid being seen, and indeed, once appropriately decorated, visibility is greatly reduced.

Once recovered from his initial introduction to the aquarium, our decorator crab at first showed some interest in the long-hair algae growing on some of the live rock, but once he discovered the polyp colony, he set immediately to the task of acquiring camouflage.

Decorator crabs will very delicately remove individual zoanthid polyps from the greater colony and place them on their backs where ideally the zoanthid will attach itself and continue to grow. According to other aquarium sites it takes approximately two days for one of the polyps to attach itself to a new surface.

By the end of the first night Crabby had placed two small zoanthids upright on the lower right of his carapace and a larger one lengthwise along his pointed “snout”. We watched when he left the polyp rock, climbed after some algae, encountered a tight spot in the rocks and jarred loose this larger polyp. He reacted by stopping and taking the zoanthid in his claws, placing the end or root in his mouth and then reaching back as though to scratch a shoulder blade to reposition the polyp. He readjusted it very delicately, wiggling it repeatedly as though making sure it was firmly attached this time. A very patient animal, this one.

The next morning found Crabby back amongst the polyps and with a few more attached to his carapace, and for most of the afternoon he sat completely still in the middle of polyp rock pretending be part of the colony.

However, by evening he went roaming. When I checked on him he had lost the big green polyp he had attached so carefully to his nose, and in the course of an hour he harassed one of the large algae-covered snails, chewed on the giant red, small red and small green mushrooms and shimmied up the glass thermometer to munch long-strand algae growth.

By the time T came home, Crabby had abandoned the algae in favor of wreaking more mushroom havoc. He had pulled the little blue mushroom free of where it had been trying to root itself. No! When T attempted to intervene, Crabby ducked into one of the rocky areas, sat down on the one healthy green mushroom, holding the blue mushroom in both claws as if eating a pie.

He left us no choice. With one clear mushroom casualty, three injured mushrooms and a discontented colony of polyps, it was clear Crabby was incompatible with the general aquarium populace. T packed him into a glass jar prison, and I took him back to the fish store this afternoon where he was unceremoniously returned to the tank with his decorator crab brethren.

The giant red and small green mushrooms rebounded reasonably quick, while the small red mushroom is recovering in the isolation sick box. No remains of the blue mushroom have been recovered.