Although the most plentiful creatures on Earth are invertebrates, not all species of spineless creatures are abundant. In fact, some are endangered. The U.S. Fish & Wildlife Service lists 105 U.S. species and subspecies of snails, clams, crustaceans, insects, and arachnids as endangered, and 25 more as threatened.
Many are threatened by habitat destruction, including the California freshwater shrimp, a small crustacean that has declined after streamside vegetation was removed by livestock grazing and erosion clouded the stream waters in which it lives. School children have been instrumental in trying to save this species, raising money and public awareness, and working with farmers to replant some of the native plants lost from the shrimps' remaining hideaways.
What lives in the Chesapeake Bay and was decribed by author William Warner as a “beautiful swimmer”? The answer to this would have to be the blue crab, Callinectes sapidus. The blue crab and its Chesapeake Bay home are the subjects of an exhibit at the Invertebrate Exhibit.
The Zoo's exhibit focuses on the biology and ecology of the blue crab, one of our area’s greatest seafood interests and source of recent conservation efforts. This new exhibit serves to educate the visitor on conservation issues associated with the Chesapeake Bay watershed, the largest estuary in North America.
Blue crab means “beautiful and tasty swimmer” in Greek. This unique crustacean (related to other crabs, shrimps, and lobsters to name a few) ranges along the Atlantic coast from Maine to Florida, along the coast of the Gulf of Mexico, and into the Caribbean Sea. These crabs are truly a colorful species, with shades of red and opal blue shell coloring developing as they grow.
As they age, crabs undergo a process known as molting, whereby they shed their old shell in order to make room for the growth of a new shell forming underneath. After a crab has molted and shed its shell, or exoskeleton, it is termed a “soft-shelled” crab, and is vulnerable to everything from larger marine predators including fishes and other crabs, to human dinner plates, as they are prized at this point in their growth cycle as a valued food source.
These crabs, which live up to about three years, undergo numerous molts throughout their lives, after starting out in seven different larval, or floating planktonic body forms, until they progress into miniature versions of their adult form.
Right around the period of achieving this first true crab stage, the juvenile crabs seek out the shelter of sea grass and plant beds throughout the Chesapeake Bay or other estuary grounds where they are found. These beds of vegetation are referred to as SAV, or submerged aquatic vegetation, another source of conservation measure in the bay’s ecosystem.
Upon maturing, which occurs at roughly 12 to 18 months of age, males and females mate. Then, males typically head for the lower salinity waters of the northern Chesapeake Bay, and females l head for the more salty waters in the south where the Bay flows into the Atlantic Ocean. Once the females are in these higher salinity waters, they lay an egg mass otherwise known as an “egg sponge,” which harbors up to about eight million eggs. As these eggs hatch out, the process begins again.
From a conservation standpoint, blue crabs are being fished to their capacity in the Chesapeake Bay ecosystem. In some years, too many females were caught in the harvesting of crab pots, and over the long period of time, the population was unable to replenish itself to its previous numbers. There have been numerous crabbing regulations organized by local state governments and advisory committees which aim to curtail these population declines so this amazing animal will be around for years to come.
Regulations do not just apply to the crab itself, however. There are several major areas in need of further conservation efforts with regards to the entire Chesapeake Bay watershed. As earlier mentioned, the submerged aquatic vegetation that provides shelter for juvenile crabs has been under great decline this past century as a result of lower water quality and murkier waters as a result of soil and agricultural runoff, which spur the growth of unwanted algal blooms, and thus do not allow enough sunlight to be available to the growing aquatic plants. As the SAV declines, so does the capacity for the Chesapeake Bay to deal with nutrient input from these sources, but this also leads to dramatic declines in the amount of dissolved oxygen supporting other life in the bay waters.
Riparian buffer zones are under increasing threat as urban development continues. As more trees are removed from areas around the bay watershed, the prevailing soil becomes looser and less stabilized, thus causing greater runoff into streams and ultimately into the bay.
This exhibit at the Zoo’s Invertebrate Exhibit enlightens the visitor about the unique nature of our local neighbor the blue crab, and inspires our guests to take action with any one of our cooperative environmental partners in this exhibit, of which there are several.
There are 3,600 plants and animal species that call the Chesapeake Bay home. The bay itself constitutes a watershed about 64,000 square miles in size, ranging from New York to Virginia. We all benefit from the health of this expansive and valuable ecosystem. Stop by the Invertebrate Exhibit, learn a little something, and find out what you can do to help conserve our local environment.
For additional information, see the following websites:
Research Center www.serc.si.edu
Alliance for the Chesapeake Bay
Chesapeake Bay Foundation
Maryland Department of Natural Resources www.dnr.state.md.us
Virginia Institute of Marine Science