The crab who couldn’t stop decorating

(from 2011)

Hyastenus elatus, spongy decorator crab, is a small, bright orange, vaguely triangular-shaped crab with a slow, methodical way of moving, giving the impression of a scientist-dinosaur trying to navigate an alien terrain from the careful confines of a space suit.

Mesmerized by this meticulous little creature, T and I naively decided it might make a good addition to the aquarium—with narry a thought as to reef compatibility.

This is a cautionary tale.

The decorator crab gets its name, naturally, 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 blend in. Unless startled, it will move along at a slow and deliberating pace to avoid being seen, and indeed, once appropriately decorated, visibility is greatly reduced.

Our new little friend was decoration-free when we brought him home. Once recovered from his initial introduction to the aquarium, the new decorator crab (tentatively named Arthur Deco) at first showed only mild interest in the long-hair algae growing on part of the live rock, but once he discovered the polyp colony, the task of acquiring camouflage began in earnest.

Decorator crabs will set about the patient chore of removing individual zoanthid polyps from the greater colony with all the care and precision of a zen gardener, placing these hand-selected clippings on their backs where, ideally, the zoanthid will then attach itself and continue to grow. According to other aquarium sites it takes approximately two days for a zoanthid to attach itself to a new surface.

By the end of his first night, Arthur had placed two small zoanthids upright on the lower right of his carapace and a larger one lengthwise along his pointed “snout”. We watched as he attempted to leave polyp rock in pursuit of algae, then, encountering a tight spot in the rocks, jarred loose this larger polyp. He reacted by stopping and taking the zoanthid in his claws, placing the end or root in his mouth seemingly to add adhesive, and then reaching back with a movement like scratching a shoulder blade in order to reposition the zoanthid. He readjusted it very delicately, wiggling it repeatedly as though making sure it was secure. Good, good. And then he was off to other things.

The next morning found Arthur Deco back amongst the polyps and with a few more zoanthids attached to his carapace. And 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, Arthur had managed to lose the great green polyp he had so carefully attached to his nose. And in the course of an hour I watched as he harassed one of the large algae-covered snails, chewed on the giant red, small red and small green mushrooms (causing them to contract and shrivel in horror and defense), and finally shimmied up the glass thermometer to munch long-strand algae growth.

By the time T got home from work, the decorator crab had abandoned the algae in favor of wreaking more mushroom havoc. Arthur Deco had pulled the little blue mushroom (the most shy of the five mushrooms) free of where it had been trying to root itself. No! When T attempted to intervene, the crab evaded easily, ducking into one of many crevices of the live rock, then proceeded to sit down on top of the one thriving green mushrooms, holding the blue mushroom in both claws as if eating a pie.

He left us no choice. The votes were in. With one clear mushroom casualty, three injured mushrooms, a complaintive colony of polyps, and at least one harried snail, it was clear Mr. Deco was not endearing himself to the general aquarium populace.

Hyastenus elatus, spongy decorator crab: not f*ing reef compatible.

T packed him unceremoniously into a glass jar, and I took him back to the fish store this afternoon where he was returned to his former tank and reunited with his orange-suited brethren, which were no doubt eager to debrief him following his expedition.

Meanwhile, back at the home aquarium, the disgruntled citizens of polyp rock as well as the giant red and both green mushrooms are more or less rebounded, while the small red mushroom is rehabbing in the isolation sickbox. No remains of the blue mushroom have been recovered.

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