Experimental manipulation of predatory crab species identity (Panopeus obesus vs. Eurytium limosum) and size-structure and assessment of effects on invertebrate densities, sediment properties and plant biomass.

Predatory mud crabs (Panopeus obesus and Eurytium limosum) are two of the main resident infaunal predators in southeastern US salt marshes. Little is known, however, about their effects on important prey species, or their influences on sediment or plant properties. These influences are likely to be dependent on the identity of species and the size-stucture of the population. We therefore manipulated the species identity (Panopeus or Eurtium) and size-structure. The size-structure treatrment had four levels: small [9 individuals, each 18-22mm carapace diameter], medium [6 ind. 24 -28mm], large [3 ind. 32-36mm], mixed (3 small, 2 medium, 1 large, all within same cage). The numbers of crabs in each size-structure treatment were chosen to capture natural size-abundance relationships. The treatments were maintained in experimental cages (70 x 70 x 100 cm, length, width, height) in the mid-Spartina zone at Dean Creek, Sapelo Island, GA. We maintained the treatments over 4 months (July - October 2010), before assessing impacts on prey densitities (mud fiddler crabs, ribbed mussels and marsh periwinkles), and ecosystem properties (aboveground plant biomass, sediment redox potential, sediment water content).