Vicki Clark, Virginia Sea Grant, Virginia Institute of Marine Science
1.5 - 2 hrs.
The blue crab's native range extends along the Atlantic and Gulf Coasts from Nova Scotia through South America. Historically, the Chesapeake region was the source of at least half of the United States' annual blue crab catch. But recently the Chesapeake Bay's harvest of blue crabs has been in decline. According to the Chesapeake Bay Stock Assessment Committee's 2018 Blue Crab Advisory report, blue crab abundance is currently below target levels despite measures implemented by fisheries scientists and resource managers. Check out the latest statistics on the Chesapeake Bay blue crab stock.
In order to effectively monitor, manage, and conserve the blue crab in the Chesapeake Bay, we must understand the crab's life cycle and how it utilizes its habitat. The Bay is a large and variable aquatic environment, and animals that inhabit it must be adapted to near constant change. Along the Bay's north-south range, salinities can range from close to zero parts per thousand to full ocean salinity. Tides, storms, and droughts are a few of the natural occurrences that regularly alter the water chemistry and aquatic landscape. Some human activities have degraded the water quality and destroyed important habitat areas. But the blue crab is a hardy organism, and it is uniquely adapted to take advantage of the variety of habitats offered by estuaries such as the Chesapeake Bay.
The Cycle of Life
The blue crab's cycle of reproductive activity begins in early spring. All winter, the females have remained on the bottom of the Bay, the mature males have been buried in the sediments of the estuaries, and the juveniles have been sheltered in shallow-water habitats. With the arrival of warmer temperatures, the male and female crabs move away from their wintering grounds to look for food and seek out a mate. Male and female crabs mate in the greatest numbers from spring to summer in the mid-salinity areas of the Chesapeake Bay and its tributaries. A female will mate only once in her life, but from this single mating she may produce two or more fertilized egg masses over her 2 - 2 ½ year reproductive lifetime.
Larval Growth and Development
After mating, the male blue crab remains in the middle to upper Bay or its tributaries and continues to mate with other females. The inseminated female leaves the male and moves toward higher salinity waters near the mouth of the Chesapeake Bay. As she migrates, her ovaries produce eggs that are eventually fertilized by the stored sperm forming an egg mass, the sponge, that may contain from 750,000 to as many as 8 million eggs. Only a tiny fraction of these eggs will result in a mature adult.
The larvae take about two weeks to develop inside the egg. During that time, the female completes her migration toward the high salinity waters at the mouth of the Chesapeake Bay. The eggs generally must hatch in water that is between 66 - 84 degrees Fahrenheit with a salinity of 23 to 35 parts per thousand. The larval crabs, called zoeae, hatch out of their eggs during an ebb tide, and are swept away from the mouth of the Chesapeake out into the plankton-rich waters of the Atlantic Ocean's inner continental shelf. Here they spend about 45 days in this nursery area, drifting with the currents, feeding on zooplankton, and growing rapidly, molting seven to eight times. After its final molt, the zoea undergoes a dramatic metamorphosis and takes on a more crab-like shape. It is now called a megalopa. Blue crab megalopae are transported by currents, tides and their own movements back into into the Chesapeake Bay primarily during late summer and early fall. Megalopae find their way to seagrass beds or other habitats that will provide food and shelter. Here the megalopae settle onto the bottom, and molt into the "first crab" stage. These juvenile crabs may molt 18 to 20 more times over 14 to 18 months before becoming mature adults. If they survive to maturity, they will seek out mates and begin the cycle of life again.
The Virginia Institute of Marine Science (VIMS) has conducted a monthly trawl survey of finfish and blue crabs in 60 stations throughout the Chesapeake Bay since 1955. This activity uses blue crab data from 13 of the VIMS trawl stations. Students will determine which areas of the Chesapeake Bay are being used by blue crabs during different life stages.
Divide your class into four groups. Print out the four blue crab data sets and corresponding worksheets below and give each group one data set.
Access the map of the Chesapeake Bay. If possible, print a copy of the map on an overhead transparency for each group. Alternatively, give each group a paper copy of the map and then print one map on an overhead transparency to which all groups will transfer their answers later. Note that the map shows thirteen trawl survey stations.
Each data set has thirteen data tables, one for each of the thirteen trawl stations. The data tables record the temperature, salinity and the average number of crabs found at that station each season. Depending on the life stage of the data set, the data may be separated by size category and/or sex. Additional habitat information is provided in the juvenile survey data.
Using the worksheets, have your students analyze the 13 tables in their data set to determine where in the Chesapeake Bay their life stage is most likely to be found. Have them indicate on the map which stations had the highest abundances of blue crabs. (Keep in mind that each individual data point should not be recorded, only the major trends.) Have them construct a hypothesis which relates the location of their crab life stage to a specific variable such as salinity, season, etc.
Special thanks to Jacques van Montfrans and the Crustacean Ecology Group at VIMS for providing data and information.