River Life: The fascinating story of mole crabs

November 14, 2022

Let’s move from the river to the beach for this month’s column to look at one of my favorite beach critters. Some longtime readers may remember that my favorite marine species is the blue crab. But the mole crab is a close second.

Emerita talpoida is the scientific name of the mole crab, beach or sand flea. They are a popular bait when surf fishing for things like pompano. In my case, it was the subject of my doctoral dissertation: Rhythmic aspects of the behavior of an intertidal sandy beach organism: Emerita talpoida.

Mole crabs are fascinating creatures and are not exactly what they seem. The entire populations moves up and down with the tide in what is known as the swash zone. That is the face of the beach where the waves wash up and down with the tides. In fact, that was the basis for my doctoral research. Was the activity driven solely by the tide or was there an internal “clock” that cued the crabs to move?

We humans have a sort of circadian rhythm in our physiology. If you put us in a room with no outside light/dark cycles, or clocks, and let us be active or sleep based on when we want to eat, rest, or work, we fall into a cycle that is about 24 hours long That is where the name circadian is derived. Circa means about and dian is a day.

It turns out that mole crabs also have an internal activity trigger too. When placed in an aquarium with no tides, they still move with a sort of tidal pattern, or circatidal.

That isn’t the only thing unique and interesting about mole crabs either. The population in a given area of the beach is sorted, with the larger animals in the lower portion of the swash zone and the smaller ones higher on the beach. When waves break onto a beach, there is more energy and motion in the water than higher up. Wave energy is expended as it flows up the beach.

The larger animals are almost all females. And if you lift the telson, or tail flap, and if she has eggs, you will often find a very small mole crab attached to the egg mass. That is the male. He will eventually outgrow that location but will remain small or about half the size of the female.

Mole crabs migrate up and down the beach face as the swash zones moves. They have a set of feathery antennae they use to feed. You can watch them on the beach, position themselves headfirst up the beach and use their antennae, alternating one with the other, to filter sand and algae. They pass their antennae across their mouth to remove what has been filtered.

If you examine their stomach, it is full of sand, but they get enough nutrients from the mixture to survive. You will often see shore birds feeding on them as well.

So enjoy a walk in the swash zone and look for the mole crabs swimming up and down with the tides. And don’t worry, they are tough and not harmed as you step on their sandy home.

Glad you asked River Life.

Is the St. Johns River higher than normal and how much longer will it last?

According to the National Oceanic and Atmospheric Administration (NOAA), the tides in the lower St. Johns River are higher than normal by about 6 inches. This is due to the huge amount of water that fell during Hurricane Ian, especially in the head waters of the river. It will take weeks, if not months, for the water to flow north and into the Atlantic Ocean. Because the State of Florida and therefore the St. Johns River is so flat, that flow is very, very slow.

River Life runs the first Tuesday of the month in The Florida Times-Union. E-mail A. Quinton White, executive director of Jacksonville University’s Marine Science Research Institute, with questions about our waterways at qwhite@ju.edu. For more on the MSRI, visit ju.edu/msri.


Dr. Quinton White


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