Oysters as a Keystone Species in the Chesapeake Bay

an oyster reef at low tide
An intertidal reef of the eastern oyster Crassostrea virginica. (Virginia Institute of Marine Science)

There was once a massive reef in the Chesapeake Bay. Oysters grew on top of one another to form elaborate, three-dimensional, skyscraper-like reef structures that functioned together like cities, providing habitat for a wide range of fishes and other coastal and marine organisms. Fish species like the naked goby hid among the oyster reef’s crevices while larger predators like sunfish surveyed the scene from above, searching for their next meal. Mussels nestled in and shrimp scurried among the oysters while numerous jellyfish, eels, crabs, sea anemone, and other species used the reef as a source of habitat and food. Several types of coastal birds, snakes, and turtles dove down into the reef to feed and bring their catch back to the surface. But today the scene is much different. The vast majority of the oysters are gone. 
The Chesapeake Bay is the largest estuary in the United States and eastern oysters (Crassostrea virginica) historically served as a keystone species in the bay, providing the foundation for the entire ecosystem in a manner similar to coral reefs, kelp forests, and mangroves.  Their reefs have also been known to help protect the coasts from the effects of sea level rise and marine erosion by buffering wave-action. And, oysters clean the water as a result of their filter-feeding lifestyle. In fact, it is often stated that a single oyster can filter up to 50 gallons of water a day. 

Preview Jelly in murky water
The Bay Nettle is a species of jellyfish native to the Chesapeake that visits oyster reefs in order to feed. (Alex Jansen)

Oysters are also a traditional food source for people, and for thousands of years humans and oysters coexisted in the Chesapeake Bay. Heaps of empty shells and other remains discovered in archaeological trash piles (known as shell middens) can be found across the region showing that Native Americans harvested oysters and other coastal resources for at least 3,200 years. With the arrival of European settlers, however, new technological innovations such as dredging and tonging were introduced. This allowed oysters to be more intensively harvested to support the massive commercial fishery that took place in the following centuries. As in the rest of the world, oysters were intensively fished and traded throughout the Chesapeake  using massive ships. It was a booming industry that supported numerous canneries and other processing facilities throughout the region. 
By the late 1800s, oyster populations were significantly reduced in the Chesapeake because of overfishing and overexploitation caused by these new technologies. Additionally, these technologies knocked-down the three-dimensional reef structure, which damaged overall reef habitat, health, and ecology, and made oysters further vulnerable to other human-induced factors  such as introduced oyster diseases, increased sedimentation caused by development, and pollution. As a result, oysters are now thought to be at around 1-2% of their historical levels. 
While oysters are still far from their population level that existed prior to the arrival of Europeans, significant efforts are now underway to restore them in the bay. Throughout the Chesapeake, many federal, state, and local government organizations, conservation groups, and non-profits are  working together to restore oyster reefs to help bring back their essential ecosystem functions. This includes replacing the hard surfaces that oyster larvae settle on with new three-dimensional structures, including concrete “reef balls” as well as the use of recycled oyster shells obtained from restaurants that have been seeded with baby oysters to mimic their natural environment. Urban waterways are perfect places for restoration efforts because of their close proximity to population centers and ability to engage people about their local waterways. Despite continued challenges to restoration efforts like on-going pollution, development, and other water quality issues, oyster populations have seen significant improvement. 

Preview a fish on the bottom of the seafloor
Striped Blenny and White-Fingered Mud Crab are two species that call oyster reefs home. (Alex Jansen)

In the Baltimore Harbor, a large seaport connected to the City of Baltimore and located in the Chesapeake watershed, many of the species known to rely on oyster reefs still utilize its waters despite its history as a former industrial waterfront and the loss of oysters in the area. Restoration efforts have improved environmental health and water quality and are helping bring sea life back. Throughout the waterfront, fish species such as naked goby (Gobiosoma bosc), American eel (Anguilla rostrata), northern pipefish (Syngnathus fuscus) and striped blenny (Chasmodes bosquianus) call oyster reefs home and can be found hiding within the harbor’s cracks and crevices. Skilletfish (Gobiesox strumosus) have a unique frying pan-like shape and suction-cup like disk located underneath their bodies that allow them to cling to both oysters and concrete. Mummichog (Fundulus heteroclitus) are a small, minnow-like fish species with brown, green, and yellow colorations and silver stripes that forage on oyster reefs while pumpkinseed sunfish (Lepomis gibbosus) with its large, round body, consume a number of smaller animals and vegetation. The existence of these species in the harbor also attracts the many predators that are known to utilize oyster reefs as a food source, including striped bass (Morone saxatilis), white perch (Morone americana), and Atlantic blue crab (Callinectes sapidus). 
Fish aren’t the only sea life that call the harbor home. Many invertebrate species common to oyster reefs can also still be found in the harbor. With their tan and sometimes translucent-like bodies, common grass shrimp (Palaemonetes pugio) primarily feed on other marine organisms and vegetation that can be seen clinging to the sides of harbor’s marinas and piers. White-fingered mud crabs (Rhithropanopeus harrisii) use their small size to their advantage, finding crevices in the harbor’s structures that are no bigger than a quarter. Ghost anemone (Diadumene leucolena), a species of sea anemone that uses stinging tentacles like jellyfish, can be found clinging to oyster shells and other hard surfaces throughout the harbor. A wide variety of other filter-feeding type organisms, including lacy crust bryozoans, hooked mussels, and barnacles can also be seen throughout the water’s edges. Similar to oysters, these organisms also clean the water and can enhance the overall benefits of oyster reefs. In fact, recent studies have stated that hooked mussels can double the overall filtering capacity of oyster reefs due to their filtering abilities. 
The presence of all these species in the harbor brings hope. The fact that these organisms have adapted to life in the harbor and can now be seen throughout its waters demonstrates restoring oyster reefs in the Chesapeake and other keystone species throughout the world is possible.  Both species native to Chesapeake oyster reefs as well as oceanic, estuarine, and migratory animals such as eels, sea anemone, pipefish, and jellyfish are thriving in urban waterways showing that the ocean is one global, interconnected system that is capable of significant perseverance and recovery even in the face of human influence.

August 2023