Hermit crabs are exciting creatures with a unique way to protect themselves from predators. Instead of developing an exoskeleton, like most crustaceans, hermit crabs are soft crabs that would be defenseless without the protection of shells dropped by another animal. But math and science are involved in choosing a shell much more than scientists could have imagined. The process of replacing shells can also involve a complex event that requires the cooperation of many hermit crabs working together in what scientists call the holiday chain. Hermit crabs roam the ocean floor at different depths, eating rotten animals and plants, keeping the ocean clean and limiting the growth of bacteria. They provide a valuable service to all who inhabit the ocean. When they eat, hermit crabs grow and eventually become large enough to no longer be able to completely hide their vulnerable bodies in their current shell. When the shell no longer provides proper coverage, the crab will look for more. If he finds an empty shell and fits well, the exchange will happen quickly, because the crab abandons the old and integrates into the new. The abdomen folds inward to hold the shell in place, and the crab goes on. The hermit crab knows that the perfect shell will allow it to grow a little and that it also can’t be too heavy. The ideal weight ratio is the shell, which is about 60% of the crab’s body weight. If the shell is heavier than this, its transportation requires metabolic costs, and the crab grows more slowly. Where this is a serious shortcoming, it is attracting females. In competition, the largest crab gets a daughter, and this advantage of reproductive rights is crucial. The crab will choose its new shell based on its size as well as its weight or density. It becomes even more interesting when a hermit crab encounters a good sink that is too big for him. He will wait patiently near the new building until eight o’clock. Other crabs will come to inspect the empty shell. They can also reject the shell if it is too large, and they line up near the first crab that is waiting. This can be continued until the chain of waiting hermit crabs reaches 20 individuals. Unbelievable, but they all line up in size in the so-called chain of vacancies. When a crab arrives, receives and settles in a sink that remains vacant, it will leave its old shell. If his old shell is more suitable for the arrival of the first crab, the waiting crab will change quickly, and others will quickly follow. Crabs get into new premises very quickly, which is very important to protect against predators during this process. Then everyone will move on, eating and growing, until the next time they need a new shell. Unfortunately, for some hermit crabs, they can be expelled from a very desirable shell. Crabs will combine into a male with the perfect shell, and they will work together to pull it out. They will then fight for dominance with the new shell as a prize. It is known that when there are not enough shells, hermit crabs are used to cover debris or even leaves. If they climb inside the containers and cannot get out through the slippery surface, they die. The dead crab emits an odor that attracts other hermit crabs, signaling that the shell is available due to the death of the previous owner. This can be fatal for other crabs in the process, which will also climb inside and die. Hermit crabs are interesting and more complex than we ever thought. Who would have thought that they would need to use math skills to determine the best home for their needs ?!