The life of a hermit crab is full of difficult decisions. Living in empty conch shells gives them advantages, such as protection against predators, water currents, if they are marine, and drying out if they are terrestrial. However, dragging a shell around all day is a significant energy drain, so it is crucial for any hermit to select the best possible one. This has made them develop quite surprising cognitive abilities for a small crab. A recently published review by Robert W. Elwood, collects the most relevant studies that have been done with these animals to date.
When hermit crabs spot an interesting shell, they first assess it with their eyes. This is how they obtain information about the type of conch, the size and the color. If they like it, they come closer to do a better analysis. With their legs and tweezers, they explore the new shell inside and out. To make sure it is the right size, they measure the shell well using the tweezers as a ruler. All hermits have one claw larger than another. Normally, they prefer to measure with the large caliper, but if they are not inhabiting any shells while doing the analysis, use the small clampand so they have the big one to defend themselves.
Before making the change, it is important to clean the shell well. When the hermits find shells face up, which have loose sand inside, they turn them clockwise, getting the sand out. This happens because most of the shells are right-handed, that is, placed face up, the opening is to the right. Therefore, if they were turned in the other direction, the sand would go to the bottom of the spiral. A study he wanted to find out if this behavior was stereotyped or if the crabs had some flexibility. The researchers gave them left-handed dirty shells, and in most cases, the hermits switched by turning them counterclockwise, thus successfully scooping out the sand.
Once the shell is clean and well inspected, the hermit decides if the change is worth it, which is not always easy. The doubt comes when the difference between both shells is not obvious. It is possible that, once inside the new shell, it does not convince them and they return to the previous one without needing to analyze it again. For hermit crabs, having a good memory that allows them to remember the characteristics of the shells would be an advantage, because they would save work. Is demonstrated that, in effect, they remember shells that they have previously inhabited or inspected because they spend less time analyzing them compared to new ones.
They also take into account external factors for the choice of shell, such as the type of predator What’s near. The shell-peeling crabArenaeus mexicanus) actively seeks out hermits in order to devour them. Against this predator, the best the prey can do is take shelter inside the shell. However, the strategy changes when the hermits fall victim to another crab, the shell crusher (Eriphia squamata), which, since it attacks by ambush, it is better to run away. When the hermits detect chemical indicators of the sheller, they choose large shells to hide better, but in the face of the sheller they prefer small shells that allow them to move faster.
Another important external factor for hermits are the obstacles that can be found in the environment. In A study, the crabs were given a choice between two artificially made shells, one with spines on the outside and the other with spines on the inside. Evidently, the shell with thorns on the outside was the favorite, since, except for one individual, it was chosen by all. Interestingly, the choice changed when they were locked in a container whose only way out was a small hole. In this case, a third of the crabs chose the shell with the spines inside, since it was the only one that could fit through the hole. Although they were facing the problem for the first time, a considerable number of hermits managed to solve it and escape. The authors of the experiment attribute individual differences to different cognitive abilities. In other words, some are smarter than others.
Lastly, shells are a limited resource and the hermit may have no choice but to improve his current house by taking it from another individual. In this case, the attacker approaches the inhabited shell that he wants to steal and hooks it with his legs and claws. Then the activity called “shell beating” begins. Without letting go, the attacker strikes his shell against the defender’s shell repeatedly with short, quick blows. The defender tries to break free of the hold, but if he finally gives up, he signals to the attacker by pawing at him. On signal, the attacker hooks and pulls one of the defender’s pincers, which relaxes the defender’s abdomen allowing it to be extracted from the shell. This behavior can be observed in a bbc video.
The defender will most likely take the attacker’s shell, but this need not always be a disadvantage. in an experiment made with sea hermits, they were forced to inhabit shells that had sand glued inside, which is not particularly welcoming. When another hermit, unaware of the shell’s defect, approached to take it from him, the defender was quite willing to meet him and surrendered earlier than usual. It turns out that the defenders also evaluate the shells of the attackers, and if the trade suits them, it happens faster.
In the terrestrial hermits the curious chains of exchange take place, also recorded by the BBC. If a hermit crab comes across a shell that is too large, he will reject it, but he won’t go far. Other crabs may also find the shell and do the same. The moment a crab large enough for the shell arrives, those waiting are sorted by size. If the first crab performs the exchange, the exchange begins. chain reaction and everyone gets a better shell. The interesting thing is that, with this behavior, the crabs seem to predict that there will be new shells available and organize themselves in such a way that everyone can change shells.
The brain of hermits has barely begun to be studied, but early studies have found differences with other crabs. For example, the areas responsible for spatial perception and exploration are more developed. According to the review’s authors, several of the behaviors we’ve discussed are evidence of some form of consciousness, which is surprising given how different our brains are. It is curious how important shells are in the marine world. Some animals like octopuses have become smarter for losing it and hermits for finding it.
Laura Camon it is biologist, primatologist and science communicator.
You can follow MATTER in Facebook, Twitter and Instagramor sign up here to receive our weekly newsletter.
#Hermit #crabs #art #selecting #shell