Bring on the Blues

The blue crab, Callinectes sapidus, commonly found along the East Coast of the United States, particularly in the Chesapeake Bay, is known for its distinct blue claws and savory meat. The name comes from their distinctive blue claws. Their carapace (shell) is usually a mottled brownish, providing camouflage in their natural habitats. Adult blue crabs typically measure 10 to 22 cm (4-9 inches) across the carapace. Males are generally larger than females (called jimmies and sooks, respectively, in the local vernacular). They are aggressive crabs known for pinching at anyone or anything that gets close.


Blue crabs are fascinating creatures, a lifeline for fishermen, and a cornerstone of coastal economies. The dockside value of the Maryland blue crab harvest typically ranges from $50 million to $100 million per year, depending on the health of the crab population and market conditions. This thriving industry supports thousands of jobs in crabbing, processing, distribution, and retail, underscoring the vital role these creatures play in our communities.


During the crabbing season, which typically runs from April to December, stringent regulations on size limits, daily catch limits, and gear restrictions are in place. These measures are not just rules but are designed to ensure the sustainability of the blue crab population. In addition to commercial crabbing, recreational crabbing is popular, with regulations to ensure it does not negatively impact crab populations, providing a sense of security for the future of this creature and a commitment to the sustainable.


Blue crabs undergo a truly fascinating growth process. They molt their exoskeleton, a process known as ecdysis. This critical phase in their growth leaves them vulnerable until their new shell hardens.


Blue crabs have a varied diet, feeding on plants, small fish, bivalves, and detritus. They are opportunistic predators and scavengers. They are bottom-dwellers, often found in seagrass beds and muddy or sandy bottoms. They thrive in the brackish waters of estuaries, bays, and lagoons. However, females migrate to higher salinity waters to spawn. They carry the fertilized eggs on their abdomens until they hatch.


The hatched larvae, known as Zoea, spend 30 to 50 days in the plankton. They alter their morphology to megalopa, which is almost, but not quite, their true crab form. These spend some time in their plankton and then settle to the bottom. These very tiny crabs go through another metamorphosis and now take on the adult Blue Crab shape, but they are still very tiny. 


                                                Figure 1. Blue Crab Zoeae. Photo by Stephen Tomasetti, 

                                                from the Tomasetti Coastal Environmental Science Lab 

                                                

Figure 2. Blue crab megalops. Source Smithsonian.



                                            Figure 3. Female Blue Crab carrying eggs. Source Smithsonian.

                                        Figure 4. Blue Crab Life Cycle. The adults are great-looking crab.

                                         I, for one, welcome our crab overlords. Source Smithsonian.



Observations of Blue Crabs are increasing in Maine waters. This is primarily due to rising ocean temperatures methinks. By spending up to 50 days as plankton, the crabs have a convenient dispersal mechanism that periodically gets them to Maine. As the water warms, the crabs will set up shop and breed. Near Wells Maiine, crabs have been considered a now-and-then species in traps for years. However, since about 2019, they have become an expected species. In 2022, a breeding population was indicated in southern Maine.  


This could be an economic boom for fisheries hampered by reduced catches due to overfishing and environmental changes; Groundfish stocks, cod, haddock, and flounder declined many years ago. The bait industry, primarily herring, has slowed. Lobster catches, once the hallmark of a well-regulated sustainable fishery, have recently fallen.


Will the Blue crabs become a major predator of shellfish? Unsure.


Will they compete with the Rock and Jonah crabs? Unlikely.


Will they displace Green Crabs? Probably.

They will also compete with the newly arrived Asian Shore Crabs.


Let’s find out for sure. Bring on the Blues!




Sources and Further readings:


The folks at Manomet are watching the crabs.

https://www.manomet.org/publication/understanding-blue-crabs-gulf-of-maine/

See the data and report a sighting:

https://survey123.arcgis.com/share/c21e7f55ff6e4fa29e81a024745b643b


Smithsonian Environmental Research Center. Learning lab image collection.

https://learninglab.si.edu/collections/serc-blue-crab-images/uMCL3iRJHVOw9hW7






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