The phrase “tasty beautiful swimmer” for an animal living in the St. Johns River probably would not lead you to properly identify this well-known inhabitant. But the scientific name of the blue crab, Callinectes sapidus, means just that.
Growing up in the Norfolk, Va., area on the banks of the Chesapeake Bay, I thought the correct name was Chesapeake Bay Blue Crab. But after I left home to go to college in North Carolina, I soon learned that the blue crab is claimed by many and often identified by the body of water from which it was caught. I think I have eaten blue crabs from almost every state south of the Mason-Dixon Line, including our St. Johns River blue crabs.
My master’s degree research was on the neurophysiology of the swimming leg musculature. That experience gave me a great appreciation of this elegant swimmer that is so well adapted to living in the ever-changing estuary. The body is sturdy and well designed for swimming. It is a decapod, or 10-legged animal, with the back pair of legs paddle-like for a very effective propulsion system. The front pair of legs are pincher claws with one, the crusher, being larger and heavier than the other slightly smaller and sharper cutter. As the names imply, they serve two different functions.
The crab must shed its shell, or molt, in order to grow. When it molts, we get the soft-shelled crabs so prized by many seafood lovers. The male crab can sense when the female is about to molt and will actually pick her up and carry her around so they can reproduce when she does molt. We call these crabs “doublers.” You can identify the adult male by turning it over and seeing the Washington Monument, whereas the female has what appears to be the Capital Dome on her underside. The juvenile crab has a triangular apron.
The female needs higher salinity to release her eggs. Thus, you find more females near the mouth of the estuary and more males farther upstream. A female with eggs is a “sponge crab” with initially bright orange yolk. As the eggs develop, the sponge turns dark gray as the larval crabs develop their black eyes. She will lie on her back and stroke the sponge with her legs to release the young crabs, or zoea. The zoea are swept out to the ocean where they will go through several molts before moving back into the river.
The zoea move into the river by swimming toward the surface on an incoming tide and moving down to the bottom on the outgoing time. This up and down movement will gradually allow these tiny zoea to move into the estuary. These larval crabs are a very important part of the estuarine food web. In fact, of the approximately 2 million eggs each female produces, only a few will reach adulthood. The rest become food for other animals and even the adults are prey for predators like fish, octopus and humans.
So the next time you eat steamed crabs or crab cakes, take a moment to appreciate this beautiful swimmer, and its fascinating life history.
ASK RIVER LIFE
My child really loves the river and ocean, so how can I get them more involved?
Opportunities are all around to get your child engaged and actively learning about the river. The Museum of Science and History has exhibits on science, local marine mammals and the river. The Jacksonville Zoo also has aquatic exhibits. At Jacksonville University, we’ll have a Marine Science Summer Camp for teens the last two weeks in June. You can visit the JU MSRI website for details.
About the author: E-mail A. Quinton White, executive director of Jacksonville University’s Marine Science Research Institute, with questions about our waterways at firstname.lastname@example.org. For more on the MSRI, visit ju.edu/msri.