Decorator crabs encompass a diverse group of crab species, known for actively adorning their shells with materials from their surroundings. They deliberately select and attach various items to their exoskeleton. This creates a living disguise, intricately fashioned to blend with their environment.
The Art of Camouflage
Decorator crabs adorn themselves using specialized anatomical features on their carapace. Their upper shell has hooked bristles, called setae, which act like Velcro, allowing materials to adhere. Crabs use their claws (chelipeds) to snip pieces from their environment. Some species, such as the yellowline arrow crab, even tear and chew materials like seaweed to make them rougher and more likely to stick to the setae.
The crab presses and rubs the material onto its shell until it catches on the bristles. This deliberate process allows crabs to re-clothe themselves if decorations are removed. Adornments include sponges, seaweed, algae, sea anemones, bryozoans, hydroids, small shells, gravel, and non-living debris. During molting, crabs often remove decorations and reattach them to their new shell.
Survival as the Ultimate Goal
Decorating serves multiple survival purposes. Primarily, attached materials provide effective camouflage, allowing crabs to blend with surroundings and avoid predators. This deception helps them escape notice from marine animals like fish and octopuses. Remaining still makes their coverings nearly invisible, especially near predators.
Beyond concealment, some decorator crabs use adornments for chemical defense. They select and attach toxic or noxious organisms, such as sponges or stinging sea anemones. This strategy, known as aposematism, deters predators by signaling unpalatability. For instance, the longnose spider crab decorates with the brown alga Dictyota menstrualis, which deters fish. Decorations also aid in ambushing prey, with some crabs using stinging anemones to ensnare passing fish.
Habitat and Notable Species
Decorator crabs inhabit oceans globally, thriving in diverse marine environments. These include shallow coastal waters, coral reefs, kelp forests, and deeper areas, with some species found over 400 meters deep. Habitats also include rocky bottoms, muddy areas, and seagrass beds, providing materials for camouflage.
Several species exhibit this behavior. The great spider crab, Hyas araneus, found in the North Atlantic and European seas, often uses algae and sponges. It adapts by redecorating with local materials like corallines, hydrozoans, or gravel to match surroundings. The graceful decorator crab, Oregonia gracilis, is a cold-water species in the North Pacific, including the Bering Sea and Japan. It attaches algae, sponges, bryozoans, and hydroids to its carapace; juveniles and adult females decorate more than adult males.
The spider decorator crab, Camposcia retusa, found across the Indo-West Pacific, covers its body with algae, sponges, and other sessile organisms. They position decor to ensure organisms remain alive and grow, maintaining their disguise.