Marsh crabs build creeks!

Some very cool new research has come out of the Pennings Lab at the University of Houston last month. Huy Vu et al reports, in Ecology, that purple marsh crabs Sesarma reticulatum can be responsible for massive changes in salt marsh hydrology! Through extensive burrowing and consumption of vegetation, crabs extend, build, and move salt marsh creeks.

Photo from Vu et al 2016

photo from Vu et al 2016

photo from Vu et al 2016

Vu found that the purple marsh crab excavates a massive amount of sediment per day, sometimes almost twice as much as other marsh crabs.  In addition, purple marsh crab density was highly correlated with decreases in above and belowground biomass and plant height.  These crab activities, especially when they were concentrated in the “dead zone” at the leading edge of a marsh creek as Vu calls it, drastically change the marsh sediment.  Speaking from experience, walking in these crab infested areas is very difficult! The soil stability is very low, so its not hard to imagine how constant water flow over those areas can enlarge creek heads.  To top it off, Vu et al simulated purple marsh crab herbivory at some of these creek heads and found that creeks expanded 38% more where plants were removed in similar ways as herbivory!

The purple marsh crab has been featured on MarshLife before and this is a very cool addition to the already exciting life history of this unique crab. Across its range the PMC can cause massive erosion (New England), prey on keystone consumers (Georgia), regulate ecosystem functions (Georgia), over-graze marsh grass (Southeast US), and change the whole flow of salt marsh creeks (SE). Considering that this crab is abundant all throughout marshes up and down the Atlantic coast it begs the question, is the purple marsh crab the most important salt marsh consumer?! I say yes 🙂

PMC at night


How important are cross ecosystem interactions in a marsh?

Because of their location on the border of terrestrial and marine ecosystems, marshes act as sponges for runoff, are a factory that churns out millions of juvenile fish, crabs and shrimp, and are important buffers for storm protection. In turn, their position on the coast means that they are affected by species interactions from both terrestrial (e.g. birds, mammals) and marine (e.g. fish, crabs) habitats.  For example, in Cape Cod salt marshes, overfishing has reduced the number of marine predators, such as striped bass and cod, causing population densities of the native, herbivorous purple marsh crab, Sesarma reticulatum, to increase across its natural range. Increased above and below-ground purple marsh crab grazing from these dense populations has caused massive die-off of the foundation species Spartina alterniflora and subsequent creek bank erosion (Holdredge et al. 2009, Coverdale et al. 2012). Recent experiments indicate that the exclusion of predators increases S. reticulatum grazing and subsequent erosion of marsh creek banks in Cape Cod (Altieri et al. 2012), but the specific predators that are responsible for reducing crab densities are unknown and likely vary from marsh to marsh.

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Bird predators (above) and marine predators (below, green crab eating a tethered fiddler crab) are common in the marsh!

GC eating fid

In Folgers Marsh near our UMass-Boston Field Station on Nantucket I am currently using a combination of surveys, tethering experiments, and a manipulative field experiment to answer the following questions: (1) What are the most common predators of creek bank invertebrates in Nantucket salt marshes? and (2) What is the relative importance of shorebird and marine predators on the community structure and ecosystem functioning of Nantucket marshes?

Using tethering experiments (tying common prey species to a fishing line and limiting their access to shelter during high tide) I will be able to find out the frequency and identity of predation events. Thus far, videos (which I will post later!) have revealed that green crabs (Carcinus maenas) and blue crabs (Callinectes sapidus) may be the most important predators on the purple marsh crab and on fiddler crabs. Blue crabs are by far a superior competitor and predator than green crabs, so I would speculate that purple marsh crab numbers would respond to increases in that voracious predator!

My field experiment will help understand the relative importance of predators that come in on the high tide (i.e. blue crabs, green crabs, striped bass, ect) and predators that hunt during low tide (i.e. oystercatchers, egrets, gulls, willets, whimbrels, ect). Salt marshes creek banks are lined with organisms that have different effects on marsh functioning; fiddler crabs increase grass productivity by aerating the soil and clearing dead material off the surface, ribbed mussels bind the sediment and stabilize grasses, purple marsh crabs (at low densities) help aerate soils through intricate burrowing but can overgraze banks if their populations grow. We know that all of these species are common prey items for birds and crabs and fish, but we don’t know how each of these predators affects these important creek bank organisms and, in turn, how that affects the ability for a marsh to protect us from storms, to filter our runoff, or to provide habitat for those commercial species we care about. More often than not, large scale changes to coastal ecosystems (e.g. removal of large predators, invasions, increased pollution) have cascading effects on the structure and function of the whole food web, and manipulative field experiments are an excellent way to understand these impacts.  Check out some pictures of the experiment below!

Excl plots


Marine predator exclusion on the right and below and a marine and bird exclusion on the left (you can faintly see the fishing line and flashy tape I used to keep birds out ).  In the marine predator exclusion, nets raise up on the high tide to exclude crabs and fish (below)

Marine excl, high tide



Purple Marsh Crab Grazing…during the day…at high tide!

Well I have never seen this before…

This video features the purple marsh crab, Sesarma reticulatum. This crab is a well known marsh herbivore (actually omnivore!) that feeds on aboveground leaves and belowground roots of the dominant marsh cordgrass, Spartina alterniflora.

The purple marsh crab is almost exclusively nocturnal, coming out at night to feed on tasty Spartina (I found one grazing last week and put up the pic in the post below). Well at least I thought it was nocturnal until yesterday, when something quite strange caught my eye!

While setting up some cages in the marsh, I saw the distinctive white claws of the purple marsh crab hanging out in the water at high tide. I ran back to the lab to snag my GoPro, and caught some amazing footage:

It takes about 6 mins flat for the PMC to eat this blade of grass all the way down. You can see early on when he climbs the blade, he grips the blade tight with his sharp, strong claws, and uses his body weight to rip the grass! Genius! Its all gravy from there!

The purple marsh crab has become famous in New England as of late because, as the story goes, its natural predators have been overfished and the PMC’s densities have increased drastically throughout its native home range. These huge population booms have been the cause of large stretches of marsh die-off in Cape Cod and out here on Nantucket.

Its not the purple marsh crabs fault though. They are just doing what they do best- munching on grass!

PS- Thanks to my cousin and overall great human being Jim Kamoosi ( for help on getting the video higher quality.  Anyone who wants a perfect version can email me ( and Ill share a dropbox link