Trim your tree – and yourself – with the graceful decorator crab
Eyes Under Puget Sound Critter of the Month
By Dany Burgess
En español
I’ve fallen hard for holiday decorating since moving to the Pacific Northwest 11 years ago. Something about stepping out of the cold and gray into a room brimming with twinkle lights and greenery fills my heart with warmth. I haven’t yet veered into Ugly Christmas Sweater territory, but I have a feeling it’s not far off.
Even in all my festive glory, I can’t hold a candle to this month’s critter — the aficionado of adornment, the guru of garnishing, the embellishment expert: the graceful decorator crab.
More is more
There are many species of decorator crabs worldwide, all belonging to one of eight families in the superfamily Majoidea. A few of these species occur in Puget Sound, but the graceful decorator crab, Oregonia gracilis, is our most flamboyant decorator.
O. gracilis occurs from the Bering Sea to Monterey, CA, gracing the intertidal zone and deeper — to 400 meters or 1,300 feet — with its cheery presence. In Puget Sound, it prefers mixed composition bottoms with plenty of rocks, sand, shells, and seaweed.
The graceful decorator crab has a triangular carapace (shell) and a long rostrum, composed of two spines, between its eyes. It can be distinguished from similar species in Puget Sound by its slender walking legs and claws and the large, curved spine behind each eye.
Tricky with the trimmings
Unlike humans, decorator crabs decorate themselves to blend in to a crowd, not to stand out. Self-decoration camouflage is a defense strategy that allows the crabs to disappear into the backdrop of their ecosystem in order to avoid predators (in this case, Pacific halibut, sea otters, and octopus).
The crabs need this trick because they aren’t great at fight or flight. Most true crabs walk sideways which allows them to quickly escape in two directions, but members of the genus Oregonia can only walk forwards in a slow crawl.
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 Go big or get eaten! Graceful decorator crabs with a variety of adornments including red algae (left, Edmonds), hydroids (center, Three Tree Pt.), and the ostrich plume hydroid (right, Ft. Casey). Photos from iNaturalist, CC BY-NC 4.0, by islandfletch (left) and Sara Thiebaud (center and right).
Greensleeves
Speaking of crawling, the graceful decorator crab is a big believer in slow, sustainable fashion and design...AND decorating on a budget. The ornaments it finds to attach to itself are all reclaimed/recycled materials from its environment, including living organisms like algae, sponges, bryozoans and hydroids. The objects are attached securely using special hooked hairs, or setae, on the crab’s legs and carapace.
The crabs develop these special hooks and start decorating immediately after transitioning from tiny floating larvae into juveniles. Interestingly, juveniles and adult females bedeck themselves to Griswold levels, while adult males have fewer hooked setae and are more humbug about the whole affair.
You’re a mean one, Mr. Grinch
Researchers at University of Washington’s Friday Harbor Labs wondered if Oregonia gracilis had a preference in decorating materials, so in a real Grinch move, they collected a few crabs and stripped them of their decorations. Fortunately, their hearts grew three sizes that day, and they offered the crabs an assortment of new decorations to replace them, and recorded the choices.
 This graceful decorator crab from Maury Island chose all the things, including yellow sponge and green algae. Photo by Zach Hawn, iNaturalist, CC BY-NC 4.0.
Most of the crabs went for bright yellow sponge bits instead of red algae or branching bryozoans, leading the researchers to hypothesize that sponge might be an easier and faster option with which to deck their halls (leaving them undecorated and vulnerable for a shorter time). Alternatively, the sponge may have offered better chemical and physical defenses, an added protective bonus that some species of decorator crabs are known for choosing.
Unexpectedly, the adult crabs seemed to be inspired to decorate more when they were in a mixed environment with multiple décor options. I can certainly relate - put me in a room with a bunch of random free decorations and they are ALL going up.
Happy holidays, everyone!
About the Critter of the Month series
Dany is a benthic taxonomist, a scientist who identifies and counts the sediment-dwelling organisms in our samples as part of our marine sediment monitoring program. We track the numbers and types of species we see to detect changes over time and understand the health of Puget Sound.
Dany shares her discoveries by bringing us a benthic Critter of the Month. These posts will give you a peek into the life of Puget Sound’s least-known inhabitants. We’ll share details on identification, habitat, life history, and the role each critter plays in the sediment community.
Can't get enough benthos? Check out our Puget Sound benthos toolbox story map for photos, taxonomic tools, and more!
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