A Surprise in a Hole

Sometimes a place on the rock or coral looks different, sometimes its a spot of bright color—this time a slight movement back in a hole caught Bill’s eye and he swam in for a closer look. It was a rhythmic sweeping motion just barely discernible in the dark hole. He called me over and we both strained to see what it was inside. We both had an idea what it was but could not quite see well enough to confirm it so we just shot pictures from every angle that we could. Bill was able to get his camera down in the hole and blindly clicked away and shot some video for good measure. Thank goodness for digital!!

When we got back to the boat most of the pictures were out of focus but Bill got a lucky break with a couple and the video was quite amazing. The motion we saw was made by the mouth appendages (maxillipeds) of a Blue Porcelain Crab (Petrolisthes caribensis). The crab sweeps these appendages through the water to catch food particles on the long, fine hairs. The video demonstrates why they are also called Semaphore Crabs – the motion is similar to someone signaling with flags.

Porcelain Crabs are in the family Porcellanidae and are flat crabs with a rounded carapace and long antennae. They are common world-wide and many are brightly colored; but they are secretive, hiding under rocks and in coral. In late summer we have seen swarms of the distinctive looking Porcellanid zoea larvae and megalops post-larvae in Narragansett Bay.

We saw this crab in a hole on the top of a Star Coral head near Key Largo, Florida.

Video file size is large.