Captive Critters: The Spider Crab
The spider crab has something in common with a sloth. Sloths move so slowly that the algae that grows in their fur, gives them a greenish cast which helps them hide.
Spider crabs do something similar ...
At first glance, a spider crab can look less like an animal and more like a clump of algae that learned how to walk. And that is the trick...
In the Lynnhaven River, spider crabs are masters at looking like something other than themselves. They do not rely on speed, bright colors, sharp or powerful claws to rule the bottom. Their best defense is subtler. They blend in so well and move so slow, that fish, birds, and mammals look right over them.
For this Captive Critters spotlight, we focus on the spider crab’s superpower of active camouflaging. It's not like the flounder superpower highlighted here. No, the spider has perfected the art of co-opting the underwater scenery so that they can disappear.
Spider crabs belong to a group called decorator crabs. The name fits. These crabs attach bits of the world around them to their shells and legs. Algae, sponges, hydroids, mud, and tiny pieces of plant life can all become part of the disguise.
The result is strange, beautiful and unique. One crab may look fuzzy and green. Another may look like a wad of brown seaweed. A third may seem to have a tiny garden growing on its back.
This is not just dirt or random debris. Spider crabs have small hooked hairs on their shells and legs that help hold material in place. When the crab moves through grasses, oyster shells, or algae-covered bottom, loose bits can catch on its body. The crab quite literally becomes the environment as these plants and animals take up residence on their slow moving host. In some decorator crabs, the crab actively places pieces of the environment onto its shell.
That camouflage matters in shallow estuary habitats. The Lynnhaven River has a variety of ecological niches. There are oyster reefs, sand flats, muddy bottoms, marsh grasses, dock pilings, seagrass beds, and shell hash. A blue crab stands out in almost all of those backgrounds and blue crabs rely on speed and powerful defensive claws to avoid predation. The spider crab however, moving slow, coated in local growth disappears within these environments.
This is the spider crab’s quiet genius. It does not need to look like one perfect thing. It only needs to look like not a crab.
See other Captive Critter articles below:
Oyster Toadfish, Spotfin Butterfly Fish & Grunts,
Juvenile Sheepshead, Gobies & Skilletfish,
Blue Crabs & Mud Crabs,
and other shellfish like scallops and razor clams!
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