New research explores the intriguing ways decorator crabs camouflage themselves from the predators of Monterey Bay.
by Christian Yungert
Dec. 2, 2015—In a shallow tub filled with salt water, an artificial current ebbs back and forth. Kelp stalks and sea slugs sway like tree branches in the wind. “There is one in here. See if you can find her,” Catherine Drake challenges me. I lean close to the water’s surface, scanning for a disguised creature hidden in plain sight. She’s sitting right in front of me, a rock the size of a softball, streaked with splashes of cream and crimson. Minuscule tube worms flash in and out of their sheath-like homes. It shifts, and suddenly it has legs and claws. It was never a rock at all.
The shape of the animal emerges, the end result of countless generations spent perfecting its natural camouflage. It’s a decorator crab, bearing a costume of sponges and algae that fooled me like so many fish, otters, and sea lions before. In the adjacent tank, a male sits, naked and obvious. He has no decoration and no place to hide. However, his bare shell reveals the special ability of his species: He is covered with stiff bristles. Decorator crabs use these structures to attach plants and animals to themselves. They are scuttling collages of marine life.
Catherine Drake, a master's student at Moss Landing Marine Laboratories, is studying these peculiar creatures to find out what drives their unique behaviors. Are they consciously choosing their costumes? What do their passengers get in return—if anything?
The decorator crab, Loxorhynchus crispatus, lives in Monterey Bay and all along the California coast. Its spindly legs and bulbous body invoke a mental image of its broad classification: spider crab. Like all such crabs, the decorator crab’s special advantage comes from its setae, the body-covering bristles that act like nature’s Velcro. Its favored wardrobe consists of algae and filter-feeding sponges, bryozoans and even anemones.
These living attachments do three things for the crab. A supply of algae turns the crab into a moving pantry, with food always on hand. The costume provides camouflage, helping the crab blend into the seafloor. Moreover, some organisms covering the crab produce chemical defenses. These help protect against the crab’s many predators—especially the cabezon, a fish that can swallow young crabs whole.
The particular decorator crab that Drake studies, called the masking crab, is unique among spider crabs. In other species, the number of setae decreases as the crab ages, molts, and grows a new shell. But masking crabs maintain their setae throughout their lives, making them great study subjects, Drake says. The behaviors of the broad family of decorator crabs are well documented and seen worldwide. But the reasons for their ecological strategies remain mysterious.
Choosy Crabs or Fashion Victims?
Drake, a native of southern California, is fond of invertebrates. Childhood visits to tide pools inspired her, she says. She studied ecology and evolution at UC Irvine before deciding to pursue marine ecology more deeply. When she arrived at Moss Landing, she wasn’t yet familiar with decorators. However, Drake thought, “I always liked crabs. Wouldn’t it be cool if I could study them?” A year ago, she was given an assignment to study the living members of a micro-environment. Drake chose the back of a decorator crab—and she was hooked.
Her master’s research focuses on two aspects of the masking crabs’ behavior. First, she is examining whether the organisms on each crab’s shell are different than the creatures it naturally lives with. This question will clue Drake into the crabs' decision-making processes. Are they simply mimicking their habitat by wearing anything that’s available? Or are they making selective choices about what to carry on their shells? Despite having a large and varied habitat range, decorators tend to wear the same types of organisms regardless of where they live. Drake is curious about whether the crabs are just picking clothes off the rack or looking for stripes when solids are on sale.
To find out, she is comparing the colorful arrays of creatures worn by the crabs to the composition of their environment. At four locations, ranging from north of Cannery Row in Monterey down to Carmel, Drake searches for her specimens. She and a diving partner photograph, determine the sex, and measure the size of each crab they find. They analyze the photos with a computer program to assess the specific elements in the crab’s wardrobe.
Drake uses a technique called random-point contact, which examines the amount of body space covered and the mixture of species on random parts of each crab’s shell. If crabs closely resemble their habitat, Drake can conclude that they’re just attaching whatever’s available. But if the crabs look markedly different, they may have preferences for what they wear. At one site, Drake has seen a tantalizing clue: “Rhodymenia and Chondrocanthus [red algaes] are almost equally available to them, but they’re not covering themselves in Chondrocanthus.”
The second part of Drake’s thesis concerns mutualism: the partnerships between masking crabs and the organisms they wear. Mutualism is a voluntary relationship between organisms in which both benefit from each other. The crabs clearly benefit from the plants and animals on their backs, receiving camouflage and protection. But what do the filter-feeders on the crabs earn? Drake aims to find out.
Filter-feeders like the sponges, bryozoans, anemones, and tunicates that cover the crabs are stationary animals. They eat by using a series of siphons or tiny tentacles to pull food particles out of the water. This dining approach leaves them at the mercy of the ocean’s currents. However, living on a moving object that produces its own currents may give the filter-feeders distinct advantages.
Drake is exploring this through experiments in Monterey Bay. Her hypothesis is that the crabs’ travels give their passengers greater access to food, helping them grow big and healthy. She plans to control the crabs’ movement to test this idea. Some crabs will roam freely; cages will restrict others. After a month, Drake will measure how much each set of filter-feeders has grown. If crabs are mobile buffet lines, the results will be clear.
Drake knows her projects may not have widespread impacts beyond the field of spider crab studies. But in an area where research has been spotty, she is helping scientists better understand the bay’s natural history. She thinks of science as a brick house where every study or brick reinforces the whole structure. “The big bricks have been laid for my field. I like to think my brick will be an addition,” Drake says.
Decorate or Die?
Still, the burgeoning scientist may tread new ground. Ecologists have noted that as spider crabs grow, they decorate less. Larger crabs are better at protecting themselves from predators, so they don’t need as much camouflage. Studies also have shown that females decorate more than males. Drake has not yet seen those trends, but her early sample size is small.
These observations are pulling Drake toward larger questions about making a living in the ocean. Is covering themselves a choice by crabs, which ecologists call active decoration? Or does it result from a crowded environment, making it passive decoration? Stationary filter feeders require hard surfaces to grow. In the ocean, competition is fierce and hard patches like rocks or the backs of crustaceans are prime real estate, so organisms will land wherever they can. Setae-covered crab shells are a perfect target.
There are many ecological forces at work, Drake says. A lack of available living space and the threat of being eaten could explain why she sees males decorating as much as females and adults decorating as much as juveniles. If high levels of predation drive crabs of both sexes and all ages to heavily decorate, it’s an active strategy for living longer. Alternatively, more competition for space among filter-feeders might make crab shells the best places to settle—leading to passive decoration. In that scenario, the crabs are homes of convenience.
The challenge facing Drake is how to measure and quantify such phenomena. Active and passive decoration will be hard to test, she admits. It’s impossible to know the intentions of the crabs, and it’s highly unprofessional to assume you do. But on occasion, the difference between active and passive is clear: “If it’s a piece of broken-up algae, I know the crab has physically broken it up and put it on its body,” says Drake. But in other cases, she doesn’t know what to conclude. She recalls seeing a large male crab with a meter-long kelp plant attached and growing on his back. “Obviously he didn’t put it there. The spore landed and grew,” she says. She concedes she may never know.
Drake is learning to refrain from imbuing these creatures with feelings and intentions. But it’s difficult when studying creatures that seem to have unique personalities. “I think there is an individualistic aspect to it,” she says. “I haven’t really ever seen one crab that resembles another. I still keep finding new things.” Regardless of why the crabs do what they do, Catherine Drake is bringing us a step closer to knowing more about a local neighbor.
Christian Yungert majored in biology at UC Santa Cruz. He wrote this story for SCIC 160: Introduction to Science Writing.