Exclusion Process
Crawfish often cluster in large aggregations on the rocky seabed. Some conspecifics, however, are left out when cuddling in groups - they are simply cut. The reason for this unfriendly behavior: the outsiders are bad for your he alth.
Actually, the Caribbean crayfish Panulirus argus have a pronounced preference for physical contact with other crayfish. Every autumn, for example, they embark on long processions – lined up one behind the other, antennae placed on the carapace of the person in front, they march in single file up to 130 kilometers along the sandy seabed. But the crabs also prefer the company of their conspecifics in their native regions and share underwater caves or rocky outcrops.
However, some young Caribbean crayfish lead extremely solitary lives. Hardly anyone wants to approach her, nobody wants to share the cave with her. The reason for the involuntary seclusion: The still tiny cancer is sick - infected by a deadly virus that will kill him after only 80 days. Only about 7 percent of infected crawfish, discovered Donald Behringer and Jeffrey Shields of Old Dominion University and Mark Butler of the Virginia Institute of Marine Science, find any companionship in the wild - most of those doomed to death only share nocturnal shelters during their short lives with friend Plankton.
So do the he althy Caribbean crayfish avoid their sick fellows? Behringer and his colleagues devised a laboratory experiment to test the presumed selective asociality of the crustaceans: they set up a mesocosm in several large tubs that corresponded to the ecological living conditions of the crustaceans. If a he althy lobster subject was settled there, he found two possible sleeping places: one cave was empty, in the other there was a conspecific tied to it.
When the tethered crab was he althy, the newly arrived crawfish kept it company over 60 percent of the time. However, if the prospective roommate was infected with the deadly virus, most of the newcomers avoided sharing accommodation and retreated to the empty cave - a good decision, because the Panulirus argus virus 1 is extremely contagious and is not only transmitted in young crayfish via physical contact, but also partly through the sea water surrounding the sick animals.
However, if a he althy lobster wants to protect itself effectively against disease, it must be able to recognize sick fellow crayfish even if they are not yet infectious and therefore life-threatening. The first symptoms of the virus infection appear in sick Caribbean crayfish after about six weeks. Two weeks later they are contagious. Consequently, as the evaluation of the laboratory experiment revealed, diseased crayfish in these two stages were completely avoided by their he althy fellows. But after just four weeks of incubation, infected crabs found fewer roommates - over 60 percent of the he althy subjects preferred the security of an empty dwelling.
Behringer and his colleagues suspect that the Caribbean crayfish owe their special feeling for the illness of their counterpart to their fine sense of smell. After all, the animals not only sniff out their food, but also their mating partners. Even ranking recognizes a crawfish by smell.
Interestingly, the sick crabs are not choosy when it comes to choosing their company: In the experiment, they consistently preferred their own kind. They didn't give a damn whether he was sick or he althy. However, it is still unclear whether the reason for this is the lack of smell or simple loneliness.
