Background
Estuaries, bays, and other Massachusetts coastal habitats are home to a variety of both native and non-native crab species.
Native crab species include:

• Blue crabs
• Cancer crabs (Jonah and Rock crab)
• Purple marsh crabs
• Fiddler crabs*

Non-native invasive species include:

• Green crabs (introduced from Europe and Africa)
• Asian shore crabs (introduced from Asia)

Although these species are different, they all require beach and marsh habitats for survival. Their distribution, therefore, is highly dependent on the availability of suitable habitat. Crabs live in close proximity to coastlines and are key organisms for connecting aquatic and terrestrial ecosystems.
Crabs are an important part of coastal ecosystems and economies. In 2013, crabs were the second most valuable fishery in the U.S., with commercial landings of approximately $700,000 (second only to salmon) ¹⁴. Blue crabs make up a majority of commercial catches and are recognized as the most important crab fishery in the U.S. ¹⁰.
Crabs are also important prey resources for many coastal species, including migratory birds, fish, and terrestrial and marine mammals.

* Note that fiddler crabs have not been historically found in Massachusetts, but have expanded their range northward due to climate change.

Native Crabs

• Blue Crab: Blue crabs occupy coastal waters from Massachusetts Bay south to the eastern coast of South America, including the Gulf of Mexico ¹⁰. Occasionally, blue crabs have been found as far north as Maine and Nova Scotia ¹⁰. They primarily occur in bays and estuaries and feed on plankton, small invertebrates, fish, and other crabs. Blue crabs mate in upper estuaries (the area farthest from the sea) and areas of low salinity during early spring to winter ¹⁰. After mating, females bury and overwinter in the mud at the mouths of bays. When waters warm in spring, females produce egg masses that they carry on their abdomen (underside of shell). Adult blue crabs typically migrate inshore to bays and upper estuaries in the summer months, and out to more saline coastal waters in the winter. Blue crabs can live up to about 3 years in age ¹⁰.
• Cancer Crab: Cancer crabs range from Nova Scotia in Canada down to Florida along the eastern U.S. coast ¹⁸. Rock and Jonah crabs, subspecies of Cancer crabs, are harvested in coastal habitats along the eastern US ⁸. These and other cancer crabs are found in habitats spanning tidal waters to the continental shelf slope, with conditions that vary across a wide range of temperatures and bottom types (rocky to sandy environments). Females spawn between April and October and early life stages are moved towards the shore by wind and tides.
• Purple Marsh Crab: In Massachusetts, purple crab populations have risen to high levels likely because of a reduction in their predators such as striped bass due to overfishing 1. Purple crabs eat marsh plants, and the crabs’ high abundance has thus led to a decrease in coastal vegetation in some areas, leaving the areas more vulnerable to erosion ^15,21.
• Fiddler Crabs: Fiddler crabs are native to the U.S. Atlantic coast, but have not been observed in Massachusetts waters until recently; they are now found in coastal New Hampshire and Massachusetts. Fiddler crabs represent a species that is benefiting from warming ocean temperatures and expanding their range northward ¹¹. A diversity of other marine (as well as terrestrial and aquatic) species are expected to move into Massachusetts as they respond to climate change and follow optimal conditions.

Non-native Invasive Crabs

• Green Crabs: Green crabs are native to coastal waters of Europe and Africa ¹⁶. Invasive European green crabs were first documented along the Atlantic coast of the U.S. in 1817, but not until 1989 on the Pacific coast ^7,16. Currently, green crabs reside in coastal habitats along the eastern U.S. from the Chesapeake Bay to Nova Scotia and feed on a wide range of mollusks, clams, mussels, and other bivalves (species with two shells hinged together) ²⁰. They live in a variety of ecosystems, ranging from rocky to sandy bottom habitats, and have been associated with other invasive species, such as the plant- smooth cordgrass. Green crabs are a nuisance in many areas to shellfish fisheries due to predation on clams, mussels, and other bivalves ^7,20. Green crabs can tolerate a wide range of salinities, which, like other invasive species, allows them to infiltrate a diversity of habitats and areas, often outcompeting native species ^3,5,7.
• Asian Shore Crabs: Asian shore crabs were first discovered in eastern U.S. in New Jersey in 1988, and have since spread as far south as North Carolina and as far north as Maine ²³. In many areas, Asian shore crabs are more abundant than all the native crabs combined ³. Non-native Asian shore crabs are generalists, feeding on mussels, snails, and algae ³. The abundance of Asian shore crabs has increased in Massachusetts in recent years with simultaneous decreases in green crab and blue mussel populations ². Asian shore crabs directly consume and outcompete native crab species, contributing to their declines. This is just one example of the negative impacts invasive species can have on native species.

Climate Impacts
Crabs are vulnerable to various aspects of climate change because they rely on salt marsh and beach habitats for survival, which are highly vulnerable to the impacts of sea level rise, storms, increased sea and air temperatures, and altered salinity ^13,22. Crabs are benthic (live on the bottom of coastal habitats) and bury themselves in the sediment for protection and overwintering. Therefore, climate-induced changes in water quality and sediments affect crabs. For example, the combination of increasing temperature, precipitation, and terrestrial runoff alters shallow and bottom habitats that crabs use. Seasonal changes in these factors may disrupt crab populations, and changes in sediment distribution can lead to higher mortality (especially for eggs, which are vulnerable to suffocation in stagnant water) ¹⁰.

Projected increases in temperature and changes in salinity will also impact crab populations, particularly during early life stages. Larval crab development has already been shown to vary with both these factors ^4,19. However, crab populations may benefit from some climate changes. Overwinter crab mortality is highest in colder areas of estuaries and lowest during mild winters ¹⁹; spawning season duration is also reduced by long periods of cold temperatures ⁶. Therefore, warming temperatures, particularly during winter and spawning seasons, could lead to increased survival. Unfortunately, non-native invasive species typically respond faster and have a greater ability to take advantage of improved conditions and may out-compete native species as these changes occur.

Similar to fiddler crabs, blue crabs have also begun to expand their range northward to coastal Massachusetts, New Hampshire, Maine, and even Nova Scotia ¹². The combination of increasing temperatures and ocean acidification, however, has generally been found to have negative impacts on crustaceans ⁹. In addition, crabs are highly vulnerable to parasites, which are expected to become more widespread with additional climate change impacts. Crab vulnerability to climate change and the resulting changes in crab populations is expected to affect the ecology and economy of Massachusetts.