Decorator crab with string

It’s time to introduce my new favorite critter at Cabrillo Marine Aquarium…the decorator crab!

In retrospect, the decorator crab should have been the feature of my Halloween post because this crab is the true master of disguise. And Halloween or not, the decorator crab has one of the best costumes ever. Decorator crabs use algae, sponges, bryozoans, anemones and whatever other bits of material might be lying around to camouflage and blend into their surroundings.

According to the book Pacific Coast Crabs and Shrimps by Gregory Jensen, there are several species of decorator crabs and some are obsessive decorators “thoroughly, fastidiously attaching material over their entire carapace and along each of their legs.” The moss crab (also called a masked crab) or Loxorhynchus crispatus at Cabrillo Marine Aquarium happens to be one of those fussy decorators. Take a close look at the photo and you’ll see she (or he?) even incorporated a tiny piece of string into the decor on the fourth left leg.

Decorator crabs are also called spider crabs because of their long, spider-like legs and the moss crab happens to be the second largest spider crab living along California’s coast. (The largest is the sheep crab or Loxorhynchus grandis.) Male moss crabs grow to be almost five inches and females about three inches, but the book doesn’t clarify if that’s the length or width, I’m guessing it’s the width.

But how do all those decorations stick to a crab’s shell? On the shell there’s tiny hooked bristles sticking up all over that hold everything together, like micro-Velcro pipe cleaners. And since crabs molt, they have to redo all the decorations for the new shell, but often they will reuse choice decorating pieces from the old shell. Once male moss crabs reach sexual maturity, they don’t bother decorating any more, probably because they are large enough to intimidate predators and no longer need camouflage.

Apparently, there’s also lazy decorator crabs, a trait captured in a great quote from Jensen’s book, “Others [lazy spider crabs] may only place a single sprig of algae on their rostrum or appear to let their roughened carapaces passively collect fouling organisms.” So there’s two categories of lazy decorating crabs, “going through the motions with a sprig of algae lazy” and “letting ocean crud stick to your shell lazy.” Maybe they’re trying to ward off predators with their shockingly bad and distasteful decorating?