"Why are the Crabs Dirty?"

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“Why are the crabs dirty?” I heard behind me in the Marine Exhibit one day. Thrown off by the visitor’s question, I turned around and my eyes focused on the crab they were pointing to in the wall tank. It all made sense when I saw the graceful decorator crab scuttling along among the abalone, away from the sides of the tank, almost as if it knew it had been found. I jumped into educator mode right away, and answered, “That’s a great question.”

A (dirty?) graceful decorator crab (Oregonia Gracilis) hanging out

on the inflow of the small wall tank. Photo by Juhi LaFuente

The most wonderful aspect our Marine Exhibit is the chance to gain new knowledge about Salish Sea fish and invertebrates. When I first started at the Port Townsend Marine Science Center, I knew very little about specific species of crabs. Aside from what I had learned in my college invertebrate zoology class, I hadn’t thought about them much. However, once I started feeding individual crabs, and interacting with them in tanks every day, they became one of my favorite animals in our Marine Exhibit. As a person who is recovering from a phobia of spiders and spider-adjacent creatures, it means a lot when I say that I think the way these arthropods walk around the tanks and move their little maxillipeds (mouth parts) is quite cute. I love inspiring others to see them the way I do now. So, when I explain how decorator crabs got their name, and see others’ eyes light up with fascination, it is a truly fulfilling experience. I hope I can light your eyes up, too.

Decorator crabs consist of many different species, all belonging to part of the superfamily Majoidea, known as spider crabs. We have many spider crabs in our exhibit, but not all are decorators. The decorator crab in question was the graceful decorator, Oregonia gracilis, a naturally abundant Puget Sound species. If you can spot them, graceful decorator crabs can be found in the small wall tank, against the back wall of the Marine Exhibit. In our huge piling tank, we have another type of decorator crab, called the longhorn, Chorilia longipes, which is usually hanging out against the tank walls, looking at the passers-by. These crabs’ natural ecosystem includes kelp forests and the intertidal zone, ranging from Alaska all the way down to Mexico, but they’re particularly abundant in the Pacific Northwest. In nature and in our tanks, the decorators are covered in foreign material (hence their dirty appearance) and found behind rocks and kelp, hiding from predators like fish, sea otters, and octopus.

Longhorn decorator crab (Chorilia longipes) hanging on the pilings. Photo by Wendy Feltham (Checkout our Instagram to see a video of this crab with a filter-feeding hitchhiker on its arm!)

The decorator crab namesake refers to the way they adorn their bodies with all kinds of ornaments. The decorations include almost any materials found in their environment. The crab’s hard outer shell, or exoskeleton, has tiny hooks all over called hooked setae. Hooked setae act as a sort of Velcro on which the crab can stick materials it picks up from its environment. Their adornments act as a camouflage, but also as a symbiotic, mutually beneficial relationship. The “materials” picked up by the crab can include some bryozoans, hydroids, algae, sponges, and small anemones. Invertebrates gain access to parts of the water column and food they wouldn’t usually have access to by hitching a ride on the back of the crab. In turn, the crab gets an easy disguise, and when resources are limited, also an easy source of food. If a crab finds an accessory like chemically-defended plants or stinging anemones, the crab is even less likely to be predated upon. Larger crabs need fewer decorations because they can defend themselves, but they’ll often still wind up with some hitchhikers on their backs.

The decorating doesn’t really stop after covering the exoskeleton canvas; a crab is always working. Crabs’ exoskeletons do not grow with them. As they outgrow their armor, they grow a new shell underneath the old, effectively forcing the old exoskeleton off. After disrobing, the crab will hide until its new exoskeleton hardens, and then the decorating process starts all over again. They often recycle old bits and pieces of material from their discarded shell (a great example of environmental stewardship!).  In captivity, decorator crabs have been known to disrobe and grow a new shell when moved to a different tank and new environment, effectively creating a blank slate for its new camouflage masterpiece.

A graceful decorator crab (Oregonia gracilis) waving at you through the acrylic. This crab is decorated in red algae. Photo by Wendy Feltham

Our decorator crabs at PTMSC interact with guests and staff all the time. In nature, humans and decorator crabs have very little direct interaction, although there are some negative human impacts on their ecosystems. Surface runoff of oil, pesticides, sewage, and other chemicals can contribute to pollution of ocean ecosystems, truly making the crabs’ environment “dirty.” (Runoff pollution contributes to larger issues like ocean acidification.) As environmental stewards, we can all make eco-friendlier choices to reduce negative impact on the Salish Sea. The crabs might thank you for that.

Next time you visit our Marine Exhibit, I hope you take the time to search the tanks for our decorator crabs. I hope you experience the revelation that the rock you were staring at underneath the kelp, barnacles, and sponges is actually a crab, as it slowly crawls away to munch on some food or find some new artistic materials. I also hope you become so enamored that you sound just as silly as I do when I have one-sided conversations with them through the acrylic. While cleaning or feeding, I compliment them on their attention to detail in design. I like to think they appreciate the compliments on their labor, because much like an environmental steward, a crab’s work is never done.

Find out how you can "Go Blue" to protect the crabs and become an environmental steward on our site! ptmsc.org/about/go-blue-initiative

Follow us on other platforms to see more cool crabs! Instagram (@ ptmarinescictr) Facebook (Port Townsend Marine Science Center) Newsletter (Octopress Online)

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BROOKE ASKEY is the Citizen Science Educator and AmeriCorps member serving at the Port Townsend Marine Science Center