The Pom pom crabs are of the genus Lybia, nicknamed Pom pom crabs due to their tendency to sway around holding sea anemones in their claws, as seen in this video.
The Lybias have a symbiotic relationship with these sea anemones (Schnytzer et al 2017). By swaying with the sea anemones they catch food debris floating in the water and feed off the sea anemones, the anemones get a meal as well as by the crabs leftovers. However, the amount of food intake is controlled by the crabs, thereby maintaining a small sized ‘bonsai’ anemone.
There are other crabs that show similar interspecific behavior, like the carrier crab, which has a tendency to pick up whatever it can find and carry it above its back as protection against predators, sometimes these things can be alive. Take the crab in this video as an example, carrying an unwilling sea urchin on its back.
The adaptation of utilizing other species as a shield against predators does not seem uncommon in the decapod crustaceans. However, when it comes to the Pom pom crabs, there is the specific question of where they got the anemones from, as asked by Schnytzer et al (2017). They found that these crabs actively propagate the anemones asexually by slowly splitting them in half. This behavior was observed in situations where the crab was only left with one anemone due to another crab stealing the other. Thought, the question still stands, where did they originally get the anemones from? If the species the crabs hold have not been observed in the wild, have they created a new species with the asexual reproductive chain from intraspecific competition?
Another curious aspect of the Pom pom crab was found by Karplus,et al (1998). They observed that during intraspecific competition, the crabs would not use the anemones, but rather their pereiopods to inflict harm. They noted that the anemones were used in intraspecific interactions as a method of threatening the other individual, that they would extend their claw to be in close range, but with the arm furthest away from their competitor. They had three possible explanations for this half hearted competition technique. The first being that the implications of being hurt in a fight from the serious harm these anemones can inflict outweighed the possibility of winning a fight if they were used on their competitor. The second is a bit contradictory to the first hypothesis, where they suggested that the effect of an anemone attack would not be enough to win a fight. The third possible explanation was that they were avoiding any damage to their sea anemones. Either way, this behavioral adaptation is quite unique and entertaining. Though in desperate need of more research.
Next week we will try to retrace the evolutionary history of the extreme defense mechanism where the organism ejects its internal organs on its predators.