Rats are famously skilled at navigating mazes in the labs—a sign that they are capable of what scientists call “spatial learning,” or the ability to find your way around an environment. Spatial learning has, in fact, been well demonstrated in many vertebrates and a few invertebrates, like honey bees. But a new study in Biology Letters highlights the cognitive abilities of an understudied species: crabs, which, as it turns out, is pretty good at completing mazes, too.
You might not expect that crabs would be able to perform this relatively complex task; after all, “[c]rustaceans have a brain roughly 10 times less than the size of a bee’s in terms of neuronal count,” study co-author Edward Pope, a marine biologist at Swansea University in Wales, tells Layal Liverpool of New Scientist. But as the researchers note in the study, crustaceans live in dynamic underwater habitats, and “[l]earning the location of, and routes to, resources should, therefore, be an adaptive trait.”
Mazes were a perfect way to put the crawling critters’ spatial learning to the test. The researchers scooped up 12 European shore crabs (Carcinus maenas) from two aquatic locations in South Wales and brought them back to the lab, where the animals were kept individually in tanks. After an acclimation period, the team popped the crabs into an underwater maze, with a single crushed mussel—a tasty snack for crabs—at its end point. There was only one correct path through the maze, which required five changes in direction and included three dead ends.
The crabs were tested once a week for four weeks. All of the animals were initially able to complete the maze within 25 minutes, but during the first week, none made it to the end of the maze without taking wrong turns, reports Veronique Greenwood of the New York Times. As the experiment progressed, the crabs were able to complete the task more quickly, and with fewer mistakes. By week three, the study authors write, some of the crabs were able to navigate the maze without taking any wrong turns.
The smell of food was “undoubtedly” important in helping the crabs navigate the maze, the researchers note, so the team next sought to determine how crabs would fare without olfactory cues to guide them. The researchers waited two weeks after the first phase of the experiment had concluded, then placed the crabs back into the maze—this time without a snack waiting at the end. All of the crabs zipped to the end of the maze within eight minutes, presumably expecting food, which in turn suggests that they had “some memory of the maze,” the study authors write. For comparison’s sake, the researchers also tested 12 crabs that had never seen the maze before. Without food to attract them, they struggled to find their way out, and only seven actually completed the task.
“[W]e know that insects, especially ants and bees, have some impressive mental abilities but we haven’t really looked for them in their aquatic counterparts,” Pope says. “The fact that crabs show a similar ability to insects is, in some ways, not that surprising but it is great to be able to show it so clearly.”
Understanding crustaceans’ spatial learning abilities not only fosters our appreciation for these creatures, but also helps scientists gauge how they might respond to the changing ocean environment. For instance, as Greenwood notes, researchers can study how crabs’ maze skills are affected by water that mimics the warmer and more acidic oceans that are predicted to become a reality by 2100.
“Gaining a baseline understanding of the lives of the animals that are going to actually be impacted by the changes in our future oceans is really important,” says study co-author Mary Gagen, a geographer at Swansea University who specializes in climate change. “That doesn't just mean the big charismatic animals, it means things like crabs that are so important for the food chain.”
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