Hermit crab Eupagurus bernhardus on a hollow stone
Photo: E. Hensel
About a sea creature who carries his house around with him and often moves: the hermit crab
This aquarium tank differs significantly in its appearance from the usual aquarium tanks, because instead of leisurely swimming fish, crabs scurry over the floor and sometimes even climb around on the hollowed stone in the middle.It is the hermit crab Eupagurus bernhardus, whose “trademark” is a snail shell that it carries around with it for almost its entire life.
Only for molting and reproduction does it voluntarily leave its protective housing for a short time. If it becomes too tight for the growing crab, it has to look for a larger one, into which it then switches over at lightning speed. Occasionally, however, it also happens that a hermit crab takes possession of another’s housing by simply dragging it out of its mobile accommodation. The weaker then has to look for a new place to stay.
Such a pulling out is actually supposed to be prevented by a barb at the end of the body, which has evolved from the “tail fan” in the course of evolution. But not only the hook is a special adaptation to the extraordinary dwelling, but also the special design of the rear body: This one is – unlike other crabs – soft and twisted, so that it fits into the shell. The two rear pairs of legs are greatly shortened; they hold the bowl and press against the inside of the bowl when tried to pull the animals out. The larger of the two scissors is also used to close the opening in the housing when the crab has retreated completely into the housing in case of danger. In order to bring these scissors into a suitable shape, the animal, which is still soft after molting, pulls back into the housing and presses the scissors into the housing opening, where it then hardens in the appropriate shape.
Are there preferences for certain housing shapes or sizes? In fact, smaller hermit crabs prefer the shells of the periwinkle or the gyro snail and are therefore more common in the intertidal zone where these snails live. If they need larger shells, they migrate to deeper water areas, where they usually acquire the shell of the whelk.
In order to test possible shape preferences, scientists from the University of Cologne offered to a group of hermit crabs differently twisted glass housings as possible new homes. Only one hermit crab found the glass case that was most similar to the natural one to be suitable and attractive. After he and his new dwelling had been put back into a show tank, he preferred the “old building” and changed back to a real snail shell.
Hermit crab in the glass house
Photo: E. Hensel
Sometimes the crustaceans not only have to carry the weight of their housing when they migrate through the tank, but also have a smaller conspecific in tow. This is held firmly to the housing with one of the two scissors, thus preventing it’s escape, and defending it vehemently against competitors. Why does the animals undergo this additional stress? Presumably due to reproduction, because it can only take place in connection with molting. And in order to “secure” the appropriate sexual mate in good time, it is practically “adopted” after certain fragrances have been spread, which play an important role in the reproduction of the hermit crabs.
Two hermit crabs in tow
Photo: E. Hensel
However, some specimens also fall victim to other voracious colleagues after molting, as they are no longer protected by their housing for a short time.
The tank of the hermit crabs also houses rock shrimp of the species Palaemon serratus (see photo with the hollow stone), all of which invite you to exciting observations into the world of crabs.
And if you come across a fast-moving snail shell on the beach, then it is probably the mobile living unit with its resident, the hermit crab …
Author: Dr. Emanuel Hensel