I am pretty sure these Ocellate Box Crabs (Calappa ocellata) are mating or about to mate. Not to split hairs, but one box crab carrying a smaller box crab around doesn’t necessarily mean we caught them in the act. The female mates when her shell is soft, i.e., when she has just molted. A male, sensing that the female is about to molt may grasp her and carry her around, presumably to make sure no other male sweeps her away. He may also carry her around after they have mated to lessen the opportunity for her to mate with other males. So when we see a crab in the grasp of another, we could be seeing pre-copulatory or post-copulatory mate guarding.
So how do I know these crabs were about to mate? I saw her molt – right in his grasp. I was on a night dive at the Blue Heron Bridge, with friend Elaine Blum, who was pointing out one seahorse after another of every color imaginable. I wandered away from everyone, out into the mucky channel, and saw the two crabs together, flapping rapidly. The last time I saw two crabs behaving that way, a female hermit crab was releasing her hatching eggs as the male held onto her.
I flew over to the box crabs but by the time I got close enough to see what was going on, they saw me and hunkered down. Up close, it looked like there were three crabs and I realized that the female was molting!
So maybe the flapping was her attempt to free herself from her old shell. The images in this post are all frame captures from the video. If you want to see the molting box crab in action, head over to our BlennyWatcher YouTube channel or click on the video below: