by Anna D’Aleo
As a Marylander born and raised by the Chesapeake Bay, I am very familiar with the cultural significance of the Blue Crab. Milestones, birthdays, holidays, and any other excuse to gather as a group is an excuse to eat crabs. They’ve become a state symbol, like the palmetto for South Carolina. Their importance goes much further than the bay, however. In fact, they’re a key aspect in ecosystems throughout southeastern marshes and play a major role when talking about the runaway predation happening in these habitats by the key invertebrate grazer, Littorina littorea – the Common Periwinkle – as a result of the predation on blue crabs.
Previously, scientists thought that the successful balance of a marsh ecosystem was reliant on the supply of nutrients available. In other words, the marsh grasses were what led to the success of populations living in them; “studies assumed that dying and senescing marsh plants…attracted invertebrate grazers and did not test the alternate hypothesis that invertebrate grazers generated these patterns [depleting marsh health]” (Silliman, Bertness, 2002). Brian Reed Silliman and Mark Bertness from Brown University’s department of ecology decided to test if invertebrate grazers were responsible for the depletion of marsh grasslands, rather than existing as a result of dying plants. When examining the relationships between invertebrate grazers and their environment, the Blue Crab turns out to be a bigger player than anticipated. While blue crabs are not the ones directly killing the salt marshes, the overfishing of them may indirectly be the culprit. If you’ve ever worked a job that has a change in management you’ll understand; as soon as you remove the boss, everything starts to fall apart. Actually, they’re one of the main predators of Common Periwinkles.
These snails may look slow but – Did you know that they typically lay around 100 eggs per clutch? Or that in ideal conditions, reproduction can occur once a month throughout the year? I certainly did not. One female snail feeding on grasses can very quickly turn into over a thousand without predation. Periwinkle snails do not lay as many eggs, but their grazing can quickly “convert one of the most productive grasslands in the world into a barren mudflat within 8 months” (Silliman & Bertness, 2002).
The data above, from Silliman and Bertness’s study, compares tethered snails and untethered snails and how the predation on them changes their environment. After 22 months, the high predation rate of tethered snails, shown in graph A, produced a drastically different density of biomass compared to the untethered, and less preyed upon, snails. The graphs support the idea of a trophic cascade within southeastern marshland ecosystems, displayed by the graphic on the right.
You get it, overfishing of blue crabs allows for runaway grazing by snails that destroy the marshlands. Why does that matter? I live in the lowcountry of South Carolina, which has more salt marsh than any other state on the east coast. Not only does the marsh provide the familiar views that attract so many tourists, they’re vital for our ecosystems to function. Many of the local fish we enjoy begin their lives hidden in the safety of the marshes, so do birds! They protect the coast from both flooding and hurricane damages, as well as filter runoff and protect water quality. “Hurrications” may be fun, but without our marshes, we wouldn’t have a city to come back to. Locals have already seen how Morris Street in downtown Charleston becomes The Morris River with a summer rainstorm – imagine the alternative. The health of our marshes also directly impacts the health of surrounding ecosystems. If one suffers, so do the others.
How can we best support our marshes so that they can continue to support us? Silliman and Bertness say that “the high plant production on southeastern marshes is ultimately realized through a trophic cascade,” meaning that “the overexploitation of a major predator may indirectly alter the structure and function of intertidal marsh habitats.” To best support the success of plant growth, we have to support the predators that keep grazing organisms in check. In Maryland specifically, this meant limitations on crabbing in order to raise population numbers. It can also look like enforcing proper boating practices to prevent damage to habitat or cleaning up trash. Maybe Charleston could even start a new social tradition to rival crab feasts – Periwinkle Parties? Bon Appetit.
Silliman, B. R., and M. D. Bertness. 2002. A Trophic Cascade Regulates Salt Marsh Primary Production. PNAS. https://www.pnas.org/doi/epdf/10.1073/pnas.162366599