The Speedy Little Sand Crab
The Pacific sand crab, (Emerita analoga), aka mole crab , is a small invertebrate decapod crustacean — meaning it lacks a backbone, but has five pairs of legs, ten feet, three pairs of swimmerets and is in the same phylum as lobsters and barnacles. It has eye stalks to peek out from under sand and 2 pairs of antennae, all on a 1.5” creature
The sand crab lives in the sand and moves up and down the beach with the breaking waves. It eats tiny plankton and is eaten, itself, by some birds and fish.
Unlike its crabby relatives, the sand crab can only move backwards, and is reputed to have, in its tail, the largest sensory neurons known to any animal. Good thing because that ability to burrow extra fast is its defense. The Monterey Bay Aquarium website notes that laboratories use sand crabs in neurological studies because of that unique capacity. (Sensory neurons convert external stimuli into internal electrical impulses.)
Its hard shell is an exoskeleton, typical of a crustacean, made of protein and calcium carbonate. The shell must be changed from time to time to permit the crab to grow – a molt. Mass quantities of pieces and parts are seen on the beach, a result of the molting process and a good indicator of a healthy population. Often seen are the carapace-the upper shell-and the telson-the tail end. Whole crabs are also found, victims of birds who don’t find them particularly meaty or tasty or crabs that have been traumatized from all that bouncing around in the surf.
The female can be twice the size of the male and is sometimes found with orange eggs attached. Sand crabs can live up to three years. They reproduce the first year with an egg clutch of up to 45,000 eggs which will hatch in 30 days. The eggs are released into the water and float out into the ocean where they develop into “recruits”, as they are called when they land on a beach, generally far from where they were born.
Arriving on the beach, the tiny sand crabs who find appropriate soft sand and safety will begin filter feeding using their feather-like antennae and live on to produce the next generation. But it is a rough and tumble world for them.