Rowan prof working with crabsAugust 13, 2007
Goal is to determine fiddler's impact on South Jersey marshes
While crabs may be appearing this summer on the menus of neighborhood pubs and upscale restaurants, Dr. Michael Grove is pursuing another take on the crustaceans.
Grove, an associate professor of biological sciences at Rowan University, Glassboro, is focusing on the fiddler crab — a relatively tiny, burrowing saltwater creature that lives in holes along tidal mudflats — and the impact it is having on South Jersey salt marshes, particularly those in Salem and Cumberland counties and along the Delaware Bay.
According to Grove and fellow researcher Dr. Barbara McCraith, College Misericordia, Dallas, Pa., salt marshes are among the greenest places on earth, rivaling tropical rain forests in the amount of new plant material added to the ecosystem in a given area. One contributing factor to this high productivity, they said, is that after marsh plants die, much of the dead plant material is retained in the marsh, where it is broken down by bacteria, releasing nutrients into the sediment to fuel the next generation of plant growth.
Fiddler crabs dig burrows that are used as mating sites and for protection from predators, and these burrows may extend 30 cm deep into sediments. With the densities of burrows reaching up to 300 per square meter in some locations, fiddler crabs can extensively rework marsh sediments, making them less compacted and allowing for greater water flow via the burrows. Because this water brings oxygen with it, a larger population of bacteria has the opportunity to perform aerobic metabolism, leading to greater nutrient release and increased plant growth. Previous research has shown that areas of marsh from which fiddler crabs have been excluded grow significantly fewer plants than areas where crabs are allowed to be active. This increased nutrient release and plant growth in turn provide an increased food source for higher levels in the marsh food web, including blue crabs, fish and wading birds, Grove and McCraith wrote in a report on their work.
In New Jersey salt marshes, these plant communities historically have been dominated by only a few plant species, particularly the salt marsh cord grass Spartina alterniflora.
Grove, McCraith and Rowan students are trying to determine whether the fiddlers are reacting differently to a new form of the common reed Phragmites australis, introduced accidentally from Europe in the last 50 to 100 years and spreading widely, than to Spartina.
"We've known for a long time that fiddler crabs have a big effect on how productive marshes are," said Grove, who earned a B.S. in zoology from The Ohio State University, and a Ph.D. in biology from the University of South Carolina.
Grove and a team of Rowan biological science students, along with McCraith, have been conducting three experiments:
• This summer in the Port Norris and Bayside areas, they have been removing roots and plant material, including the two types of grasses, from clumps of mud. They then have been exchanging mud from one area with the mud in another to determine if the grasses themselves caused a change in the mud that physically or chemically affected the number of crabs in the samples.
• Using plastic enclosures inserted into the mud, they removed as many crabs as possible from sites and blocked them from reentering the sample areas, which feature the different types of grasses. They then introduced a set number of crabs and made fiberglass resin casts of their burrows with the goal of determining whether the grasses impact the development of burrows. "You know essentially how effectively the crabs are able to dig," Grove said.
• Next they will conduct an experiment in the lab, taking core samples of mud with the roots of the grasses intact. They'll run a CAT scan on the samples and try to develop a three-dimensional picture of the root systems, of which they will make plastic models. They'll place the models in mud, controlling it for physical and chemical characteristics, and then introduce crabs into the mud. Their goal is to determine whether the root structures impact the crabs.
Grove and McCraith indicated Phragmites might be significantly impacting the physical and chemical nature of marsh sediments and therefore the populations of blue crabs, juvenile fish and birds that utilize marshes as feeding and nesting sites.
"We're basically just asking the question if the invasion by Phragmites is detrimental to the marshes and, if so, whether we can explain why," Grove said of the studies.