Mazes aren’t just for mice: European shore crabs exhibit spatial learning

Davies, R., Gagen, M. H., Bull, J. C., & Pope, E. C. (2019). Maze learning and memory in a decapod crustacean. Biology Letters, 15(10), 20190407.

Crabs spend their time scrambling along the sea floor, a complex three-dimensional environment that they scour in search of food. European shore crabs (Carcinus maenas) can be found in intertidal zones and shallow seas, where they feed on just about anything they can find. Given the complexity of their environment, it makes sense that a crab may need to remember the different paths it took in order to find various resources, like food. Mazes used in scientific studies may have multiple turns and dead-ends and are thus a good representation of the sorts of difficult natural arenas that crabs must make their way through to find food on a daily basis.

A European shore crab (Carcinus maenas) on a beach. Source: John Haslam

For the most part, studies using some of these more complex mazes with multiple turns and dead-ends have been used to study vertebrates, and particularly lab mice. Vertebrates are usually considered the more advanced group when it comes to learning, and especially the challenges of spatial learning. But based on a recent test of crab performance in a complex maze, it appears that crustaceans should be added to this list of maze-learning animals.

A recreation of the maze used in the study.

To conduct their experiments, researchers placed food (a crushed mussel) at one end of the maze and crabs at the other, enticing the crabs to find their way through to the end. After placing the crab in the maze, everybody exited the room, leaving behind a video camera to record the crab’s journey. The researchers did this for twelve crabs over the course of four weeks and saw that the time it took crabs to find the end of the maze and the number of wrong turns they took decreased with experience. It appeared as though crabs were remembering how they had navigated in the past.

Blue mussels (Mytilus edulis) are a preferred prey of European shore crabs, and were placed at the end of the maze to reward crabs. Image credit: Emőke Dénes

The final stage of this study took place six weeks after the first crabs were tested. During this final step, a new set of crabs in addition to the ones which had previously spent four weeks learning the maze were both tested, but this time without a food reward placed at the end. Although the experienced crabs performed worse than they had two weeks previously when there was food at the end of the maze, they still performed better than the naïve crabs which were making their way through the maze for the first time. Since the only difference between the two groups of crabs was their experience in the maze, this is likely what caused a different outcome in their maze performance. Even without a food reward, the experienced crabs still performed better, an indication that some kind of learning had occurred to cause that difference.

Crab maze learning is an exciting development, but it also raises a number of interesting questions. Why did the crabs perform worse two weeks later? Was it because they forgot how to navigate the maze, because they no longer had a food reward to motivate them, or because of some other difference we haven’t thought of? How were the crabs actually learning and remembering how to get through the maze? How do these mazes relate to the crabs’ natural habitat? How long does a crab’s memory of its environment last and how does that relate to the rapid change of coastal environments from waves, weather, and tides?

Studies regarding these sorts of intangible processes, like learning, are particularly difficult since we can’t always see the change occurring in an animal. But studies like this also remind us how little we know about the cognitive processes of other animals, particularly complex invertebrates like crabs, and it is an exciting prospect to think about what direction studies will take us next.

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