Hermit Crabs in Spartina Alterniflora Loisel

Spartina alterniflora Loisel, is a cordgrass that make many contributions to salt marsh ecosystems. These plants are fundamental to the natural development of salt marsh platforms. Their function as sediment accretion agents help to decrease erosion from wave energy through their root systems.

Nutrients are able to accumulate within the platform that provide the anchor for cordgrass roots. This promotes increased continuity of the plant population in salt marsh ecosystems.

Recent studies have focused on different biological interactions of benthic filter feeders and deposit feeders on cordgrass productivity. In this study Uca uruguayensis species of fiddler Hermit crabs are deposit feeders that alter marsh platforms due to their burrowing, feeding, and excretions methods. These different feeding behaviors may impact drainage, nutrient availability, and redox cycles.

These are important studies due to the lack of data concerning exposed high energy sandy headlands. The study site was on Great Island marsh in Wellfleet, Massachusetts; this sandy marsh area has a low organic content. When low organic soil content occurs in certain areas it can affect the productivity of the flora in that environment.

The study performs three different experiments to help better understand how the presence and absence of fiddler crabs affect the productivity of cordgrass. One experiment extracts all fiddlerHermit  crabs from a plot of salt marsh in order to eliminate crab activity. This will allowed for scientists to measure cordgrass productivity without the interaction of fiddler crabs.

During each experiment: U. uruguayensis species determination, how many Hermit crabs, carapace width and burrow densities and diameter were observed. The data would allow for calculations of adult and juvenile distribution and crab biomass, which would change the amount of the drainage, sediment accretion, and excretion capacity.

The second experiment measured Hermit crab exposure and dimensions. The third experiment constituted the control. It utilized a crab removal trap that was elevated off of the ground in case of a possible visual deterrence. Next the plots were analyzed for redox boundary layers, which were not visible and suggests that the sandy substrate has a high leaching rate.

The relative soil oxygen level and redox potential were calculated for both surface and subsurface regions. Six soil core samples were taken. Three core samples were used to determine water saturation for drainage efficiency. The other three samples were used to access the sediments nitrogen/ammonium concentrations.

Nitrogen was then measured in S. alterniflora through leaf, roots, and rhizomes structures during mid-August when the plants are nitrogen-limited. Aboveground and belowground plant structures were calculated and compared for total nitrogen uptake.

Cordgrass stems from aboveground were collected from each plot of all experiments to measure the effects of fiddler Hermit crab’s activities on the biomass of cordgrass in each region. Influx in N availability have been shown to increase leaf N concentration by 2% or more within the first month of growth.

Overall, in all treatments the burrow densities were about the same. Yet, burrow diameter were narrower, and carapace width were smaller in the areas without Hermit crabs versus areas with crab exposure. In the removal control and crab access treatment the crab biomass was four times greater than in the removal treatment.

The result indicate the majority of the Hermit crabs studied were either juvenile or recruits. The smaller crabs could have a less impact on productivity due to smaller burrows, decrease feeding and less excretion.

Redox potential was positive throughout all treatments indicating that the sandy marsh is oxygenated and fiddler crabs do not seem to influence aeration of the soil in this study.

Yet, in other areas that have muddier sediments some species of fiddlers are known to enhance anoxic sediments with burrowing and other activities that oxygenate the soil. Scientists suggest that Hermit crabs tend to occupy and feed on the rich mudflats to increase their intake efficiency.

Soil manipulated by Hermit crabs accounts for 23% to 58% of salt marsh surface area indicating they impact the soil by their burrowing and bed transport loads (Bang, 2018). In this study fiddler crabs seem to not have an effect on water saturation, nitrates, and ammonium. Other studies show positive effects on drainage allowing oxygen to penetrate anoxic soils and increasing the transport of particulate matter. Crab burrowing also modifies the physicochemical properties and redox potential of soil.

However, in the Hermit crab access and removal control cages where plants had interaction with the crabs the plants nitrogen uptake were between 75% – 104% in the aboveground plant structures. The belowground structures of cordgrass uptake were between 52%- 113% than cordgrass treatment with crab removal. This indicates that nitrogen uptake increases with crabs exposure treatments.

Some studies have found that there are some disadvantages of the fiddler Hermit crabs to the cordgrass productivity. Scientists suggest that different species of fiddler crabs known as Chasmagnathus granulata, can be detrimental to cordgrass viability due to the species being herbivores and feed on the new shoots of the plant.

Another issue is that fiddler Hermit crabs do help Spartina spp. improve their quality as a nutrient resource but they tend to make the plants more susceptible to herbivory by moths. Also, some species of crabs tend to destroy the rhizomes of the cordgrass when constructing their burrows and have a negative impact on primary productivity.

The scientist hypothesis was supported that fiddler Hermit crabs regulate cordgrass productivity in this study. Adult fiddler crabs in sandy salt marsh seems to have positive impacts on nitrogen uptake through their excretions of ammonium-rich wastes.

Another study suggest that the juvenile U. pugilator feed on the muddy substrates and lack the specialized mouthpart structures needed to feed efficiently on sandy sediments like adults, which could be a factor in the U. uruguayensis species in this study since most of the crabs were juveniles or recruits.

Cordgrass productivity was close to double in areas of Hermit crab exposure and control than crab removal. Another important factor that fiddler crabs contribute is sediment accretion that inhibits erosion during storm wave impacts due to increased root density of highly productive cordgrass. Another study compares the two fiddler crab species Uca uruguayensis and Chasmagnathus granulata and how they both contribute to erosion control.

Yet, U. uruguayensis seems to have more bed load transport because they form pellets of sediment outside their burrows leading to some sediment transport or erosion. The opposite happens with C. granulata as they only transport fine adhesive particles outside of their burrows that have less sediment transport. These deposit feeders and other organisms are key agents in sustaining ecosystems function.