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
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 🙂
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 (http://www.mooseherd.com/) for help on getting the video higher quality. Anyone who wants a perfect version can email me (firstname.lastname@example.org) and Ill share a dropbox link
Look who I found eating some grass out at Folgers Marsh outside UMass Boston’s Field Station on Nantucket! Thats right, your friendly neighborhood purple marsh crab. Some great nighttime #marshlife