Anybody who has visited one of the sandy beaches in California has probably seen kids running around digging up mole crabs (Emerita analoga). These crabs live in the swash zone at around the depth where the waves would be breaking over your ankles, moving up and down with the tide. They are bizarre little creatures, burrowing backwards into the sand with just their eyestalks and first antennae reaching up into the water.
Although it’s called a mole crab, Emerita‘s external anatomy isn’t very similar to that of other crabs. For one thing, it doesn’t have claws. In fact, its legs are quite unlike the legs that you’d see in a typical crab. Check out Emerita‘s appendages:
The crab’s head faces to the left in this diagram. The real surprise that these little crabs hide is the nature of the second antennae. Usually the crab keeps these long, delicate antennae protected under its outer (third) pair of maxillipeds. This is why you don’t see them when you dig up a mole crab.
You do see them when the crabs are feeding. As a wave washes over the crab, it extends the second antennae and pivots them them around on ball-and-socket joints. The feathery antennae catch particles in the water, then are drawn underneath the maxillipeds so the food can be slurped off and eaten.
Here’s a top-down view of two Emerita feeding. The purple-grayish thing in the field of view is a sand dollar (Dendraster excentricus).
This side view gives a better angle of what’s going on:
I find these little crabs quite captivating. I love how they rise up when I put food into their tank. Watching them feed always makes me smile.