‘Purple Forest’
Decorator Crab (Microphrys bicornuta) on Asparagopsis taxiformis algae
Music, Video, and Aquarium
2010 Morphologic Studios

This week’s video features an aquascape comprised of the beautiful purple macro algae  Asparagopsis taxiformis. However, if you pay close attention to the left 1/3 of the screen, you’ll notice something… moving with claws… Nestled amongst the algae is a perfectly camouflaged decorator crab (Microphrys bicornuta).  Keep paying attention… at 26 seconds into the clip you’ll notice a tiny isopod crustacean float by in the current and descend helicopter-style right onto the crab’s back. The unsuspecting isopod has no idea that it has landed upon an algae covered beast. Furthermore, it appears that the crab is not aware of the unexpected visitor until the isopod begins to explore its decorated exoskeleton.  50 seconds into the clip the isopod meets its fate with a few swift snatches of the crab’s claws.  Without missing a beat, the crab continues scavenging amongst the rocks and algae.  And life on the reef goes on…

Decorator crabs are amazing creatures in that they pick up pieces of their surrounding habitat and place them on their carapace (back, exoskeleton) in order to blend into their surroundings.  Decorator crabs that live amongst sponges decorate with sponges, those that live amongst zoanthids use zoanthids, and so on. This instinctual logic is truly remarkable.  The  crab in the video has attached small pieces of the Asparagopsis upon itself, and as a result is all but indistinguishable from its surroundings.

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The spongy decorator crab (Marcocoeloma trispinosum) takes expert care in snipping off pieces of living sponge and attaching them to its carapace (exoskeleton shell).  Detritus and debris are added for additional camouflaging effect. This particular crab has taken the decoration to the next level by including some spectacular zoanthids (Zoanthus sp.) into its design. As you can imagine, the crab was living amongst a colony of the same zoanthid morph, rendering it nearly impossible to detect.  Furthermore, decorator crabs move very slowly and deliberately, quite unlike the unpredictable scurry of most other crabs.  The purposeful addition of  camouflaging marine life to the body of this crab highlights the evolution and subconscious intelligence of  ‘tool-use’ at such a ‘primitive’ level in the animal kingdom.