One afternoon, my wife Georgina and I were walking along the sandy beach in San Josef Bay when I notice a pair of antennae sticking out of the sand just at the water’s edge, as a wave rolled over them, it uncovered a cool little creature called a mole crab. I was fascinated by these animals as I had never seen them before. But there were many of them on our beaches that year, you could run your hand through the sand at the water’s edge and find them in your hand, these are the little ones, there were also some fairly large females with eggs, some as big as the palm of my hand.
Pacific Mole Crabs, (Emerita analoga) live along sandy beaches on the west coast of North America, usually buried in the sand with only the breathing antennae sticking out. They are common along the California Coast but do show up on our West coast Vancouver Island sandy beaches occasionally, they usually arrive as larvae by riding the northern currents. Although there is the distinct possibility that the water has warmed up enough that local populations have begun to procreate, and we are now seeing the end result of this. I have seen several females with eggs and this seems to support this theory.
Mole crabs are herbivores, they feed on dinoflagellates and diatoms, they bury themselves in the sand, at the ocean edge and when a wave goes out, it curls its large, feathery second antennae backward and uses them as nets to capture the phytoplankton. The food is scraped from the antennae by specialized appendages and delivered to the mouth. They can bury themselves in less than 2 seconds, they will use the waves to move up or down the beach by letting go just as a wave rolls over them and then quickly bury themselves again in the surf zone.
On our beaches here on the island, there are years when you will see none, and other years when they are everywhere, From 1958 to 1960, our west coast beaches were awash with them, but since then they have only shown up in large invasions occasionally. In 2016, at San Josep Bay, we observed many and some were quite large, one must have been close to 10 cm long. It was quite a sight to see. It was my first time seeing them, I am always on the look for them now and once in a while, I might find one or two specimens.
There are many predators that feed on these little crabs, seabirds are quick to grab one if it becomes unburied and fish will feed on them at the water’s edge. Fishermen will use them as bait when they are surf fishing.
Sand crabs can carry the parasite known as the spiny-headed worm. These parasites are passed onto the predators. Sea otters and birds can eat large quantities of these crabs per day, and sometimes they will consume enough of the spiny-headed worms that it kills them.