Seaside Subterfuge

A shaggy, ten-legged figure advances through a Monterey tidepool. Her four-inch-long, oval outline is fuzzy—but not because of the murk of the spring low tides. Every part of her body is enveloped in overlapping sponges, sea squirts, and bryozoans. It is difficult to tell where creature ends and tidepool begins. 

Nearing some algae, the animal tears off a piece with her pincers. Bringing it to her mouth, she chews it. Then, as if clipping a barrette to her hair, she reaches up and fastens the roughed-up algae to the small, Velcro-like hooks near her pointy snout. Thus adorned, she continues scuttling across the surf grass, adding to the living garden on her back as she goes. 

This is a moss crab (Loxorhynchus crispatus), a species of decorator crab in the superfamily Majoidea. A rare tidepool sight, she is a master of disguise, using marine life from her surroundings to bamboozle potential predators. More than 800 species of decorator crabs exist worldwide, ranging in size from marble to large softball, according to Kristin Hultgren, a biology professor at Seattle University. A dozen or so of these species live in the Bay Area. 

“These crabs are actively choosing what is on them,” says Jay Stachowicz, a UC Davis ecologist whose research on marine communities includes decorator crab ecology and evolution. “They’re quite specific and intentional about what they’re choosing, and they’re doing so in a way to minimize the chance that they’re going to be found.” Some crabs, like the moss crab and the graceful decorator crab (Oregonia gracilis), which is more often found farther north, decorate their entire bodies until they look like they’re wearing a crab-shaped onesie. Others, like the more common cryptic kelp crab (Pugettia richii) and the northern kelp crab (Pugettia producta), only decorate small sections of their bodies. Some species decorate just their snouts, perhaps to hide their twitching antennae or to mimic a flickering blade of kelp buffeted by the waves. 

This decoration can include sea squirts, sponges, seaweed, bryozoans, sea anemones, hydroids, and more. The assemblage serves as a self-made disguise as well as a “walking snack supply” for the crab, says Sarah Berke, a biology professor at Siena University who studied decorator crabs as a grad student. At times, crabs will even reuse their decorative assortment between molts, plucking items off their just-shed exoskeletons and fixing them to their new shells.

Though the crab chooses most of its decorations—based on what the tidepool provides and what fits their tiny hooks—some decorations accumulate by chance, settling on the crabs’ backs after drifting through the water. “From the perspective of the organisms being used as decoration, this [crab] is just any hard surface,” says Stachowicz. 

From the perspective of the crabs, though, their decorations mean the difference between life and death. Decorating evolved millions of years ago in crabs, perhaps initially as a way to store food. But decoration comes at a steep cost. Bulbous sponges and billowing, feather-like algae make it harder for crabs living—and moving—at the whims of a temperamental surf. Some decorator crabs have been found carrying nearly their entire body weight’s worth of added decorations, says Berke. “If you go swimming with clothes on, it is much, much harder.” The same is true for these crabs. In lab experiments, Berke found that decorated crabs lost weight faster than undecorated crabs under the
same conditions. 

And a sea squirt suit doesn’t always help you hide. “Where’s Waldo is hard to find,” says Stachowicz. “But if everybody was dressed like Where’s Waldo, you could find one Waldo no problem.” The more common the disguise, the more likely it is that predators catch on. “Most cryptic or camouflaged species are not super common for exactly that reason,” Stachowicz notes.

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Given the risks, decoration is mostly a young crab’s game. When they’re small, decorator crabs can be picked up and eaten “like popcorn” by fish and other predators, says Berke. So anything that helps them hide is worth it, regardless of cost. And some species that stay small decorate for life, spending all their days skulking in the shadows.

Some medium-size crabs will also decorate for life—particularly females, since the added concealment can help safeguard the next generation of crabs. Meanwhile males of the same species might go disguiseless, perhaps investing their resources into bigger claws instead. Large species of decorator crabs, like moss crabs, often decorate less as adults, with some species losing their specialized hooks entirely as they age. By then, they’ve outgrown the maws that snapped at their younger selves—and thus the need to disappear.

If you visit the low tides this spring, keep an eye—or a hand—out for decorator crabs: they are sometimes easier to feel than see, and the young ones don’t pinch too hard, says Hultgren. Kelp crab youngsters tend to hide under algae in the intertidal zone; moss crabs emerge farther out and along docks and wharfs. With luck, you might find a decorator crab curating its own costume, snipping away just at the water’s edge.