Background
Shellfish beds in Massachusetts are made up of bivalve mollusks such as oysters, sea scallops, razor clams, soft-shell clams, blue mussels, and northern Quahogs in estuarine, bay, and ocean habitats ¹⁰. Shellfish beds are unique habitats and provide a number of ecosystem services including sediment stabilization, habitat for marine organisms, food for marine wildlife and humans, carbon sequestration, water filtration and nutrient cycling ^1,4. Because shellfish beds anchor sediments, they act as buffers against ocean wave and current impacts and can alter flow and turbulence in intertidal channels ¹⁶. Oyster beds also help reduce erosion from storm surges and wave energy in inland areas ².

Shellfish are filter feeders and remove very small particles from the water column, such as plankton, suspended organic matter, and even contaminants ⁴. This function improves water clarity, light penetration, and overall water quality. Shellfish beds also provide excellent habitat for many marine invertebrates, such as worms, crabs, snails, and sea stars ^1,4. Shellfish beds occur in various types of estuarine and marine habitats, with some species such as soft-shell clams, occurring near the mouths of creeks and in slack waters near sand bars or islands, while others such as sea scallops occur in deeper marine waters along the continental shelf ⁸. Shellfish are excellent indicators of environmental conditions and because of their good preservation in fossil records and wide distribution, they can serve as natural recorders of historical change in coastal areas ⁶.

Some shellfish, such as blue mussels, have strong flexible strings known as "byssal threads" that are secreted from glands in their foot. This feature enables them to anchor to hard surfaces, including other shells, and they are often found in large groups known as “clumps”. Other shellfish, such as scallops and razor clams, do not attach to each other, but anchor as individuals to the bottom. Byssal threads also allow some degree of movement, facilitating reposition relative to currents, as well as within mussel clumps ¹³. Species, such as oysters and softshell clams, can’t move and therefore rely heavily on early life phases to settle new areas and maintain populations ^12,15.

Shellfish are currently threatened by urbanization, excess nutrients and contaminants, and fishing gears that come into contact and physically damage bottom habitats or re-suspend sediments ¹⁴. Sediment altering activities such as dredging, along with jetty and seawall construction, negatively impact shellfish populations through suffocation or changing the availability and distribution of preferred sediment. Nearshore shellfish beds are at the highest risk from human activities. In many places, shellfish have declined due to increasing disease and parasite outbreaks, which have become more widespread due to reduced water quality and increased salinity.

Climate Impacts
Shellfish beds will be impacted by climate change in multiple ways, including through increased temperatures, precipitation and runoff, as well as ocean acidification from increased CO2 and consequent changes in ocean pH. Increased water temperatures are expected to increase outbreaks of disease and parasites, which contribute to the decline of many shellfish species ⁴. This is worsened by reduced water quality and increases in salinity. Expected increases in precipitation and storm events will likely influence sediment distribution patterns, which will also impact shellfish communities. Shellfish that burrow, such as clams, require a specific sediment size for burrowing, which can be disrupted by human activities as well as climate change impacts ⁵.

Ocean acidification is projected to significantly increase in the coming centuries as a result of CO2 absorption by the ocean ³ and have some of the most negative impacts on calcifying marine organisms including shellfish, corals, and crustaceans ^7,9. Global losses of shellfish production due to ocean acidification are estimated around 100 billion dollars ¹¹. Sea level rise will also impact shellfish beds. For instance, vertical oyster reef growth depends on salinity and the amount of time they are exposed to air (emerge) during a tidal cycle. For mature intertidal reefs (oyster reefs at least 10 years old), areas that showed the highest mean growth were exposed to air 20-40% of the daily tidal cycle (i.e. at low tide). If oyster reefs are located in areas that receive less than 10% exposure to air time during tidal cycles, little to no growth will occur. In these areas, increasing rates of sea level rise will likely outpace vertical reef accretion and contribute to loss of habitat, especially along developed shorelines where the ability of reefs to migrate inland is limited ¹⁷. Some species may be able to cope with changes in water levels by migrating to different areas with more optimal conditions.