We know them from holidays by the sea: beach crabs are simple crustaceansewho just run away, you might think. But are these crustaceans really that simple? They also have considerable cognitive abilities, according to an experimental study: Crabs know where to go. she can memorize the optimal path through a labyrinth. According to the scientists, they have complex learning abilities and a spatial memory.
We learn and then we can use our knowledge – the basic principle of this concept is widespread in the animal kingdom: Even a worm can learn to react to stimuli with a certain behavior. More complex forms such as spatial learning, on the other hand, have so far been demonstrated primarily in vertebrates – but not only: Studies in recent years have increasingly revealed the amazing cognitive performance of some insects. Little is known about the abilities of aquatic arthropods. A comparison of the brain structures suggests that crustaceans “have less” than insects: They have significantly fewer nerve cells. But studies have shown that there is not always a direct relationship between brain size and the complexity of behavior. The following can also apply to nerve structures: class instead of mass.
Crabs find their way through a labyrinth
As part of their study, the researchers led by Ross Davies from Swansea University in the UK have now investigated the extent to which a species of crab is capable of spatial learning that many people are sure to know from beach holidays: the beach crab (Carcinus maenas). It seems plausible that spatial learning skills can be useful for these animals: Beach crabs often live in complex habitats with three-dimensional and angled structures. The researchers have experimentally investigated to what extent they can actually memorize optimal paths to a goal.
They used a water-flooded labyrinth made of black Plexiglas. From a starting chamber there was an optimal path about two meters in length to a destination. There were three dead ends that led astray. To get through optimally, five correct direction decisions were necessary, explain the scientists. At the destination, a reward was waiting for the crabs: a broken shell. During the experiments, the respective test animal was placed in the starting chamber and was then free to find its way through the labyrinth. His movements were recorded by a camera and later evaluated by the scientists. Each animal was put back into the labyrinth at certain time intervals to check whether it could find its way to the goal faster by learning the optimal path.
Evidence for a spatial long-term memory
It turned out that with each new attempt the crabs found their way to the food at the end of the maze faster. They also got lost less and less, the evaluations of the video recordings showed: They made increasingly correct decisions at the junctions until they were finally able to navigate perfectly through the labyrinth. They didn’t just follow the scent of the delicious mussel: Even if there was no tidbit waiting at the end of the labyrinth, they crawled on the fastest way to the goal they had learned, the experiments showed.
Apparently, the animals were also able to memorize the path sustainably: in tests after two weeks, the crabs still found their way around the familiar maze. They reached the goal in less than eight minutes, the scientists report. Control animals that were confronted with the labyrinth for the first time, on the other hand, took much longer: about half were still wandering around the system of branches after an hour.
According to Davies and his team, these results show that beach crabs also have complex learning skills and spatial memory. So far, however, it remains unclear what the animals orientate themselves by or what concept they use when learning, say the researchers. A sequential learning would be possible: turn right, ignore two openings, then turn left…. Through further experiments, the scientists now want to gain more insight into what is going on in the crabs’ “heads”.