Hermid Crabs Interact With Spartina Alterniflora and Alter Primary Productivity
“Smooth Cordgrass” (Spartina alterniflora) is a salt tolerant perennial deciduous grass that inhabits the intertidal zone and makes many contributions to salt marsh ecosystem. 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.
This allows organic material to accumulate within the platform and provide and anchor for additional cordgrass roots. This promotes increased continuity of the plant population in salt marsh ecosystems. In addition to natural erosion control, S. alterniflora increases water quality through filtration of pollutants in stormwater runoff (nonpoint source pollution).
S. alterniflora provides habitat for marine organisms that forage, breed, and nurse in salt marsh territory at some point in their life cycle. This habitat has a bottom- up trophic level cascade with decomposers, detritivores, and predators that impacts the entire food web. One such organism inhabiting the salt marsh ecosystem is the “Hermit Crab” (Uca).
They have an integral role within the S. alterniflora environment. The health of their population can directly impact S. alterniflora. For example, in Rhode Island the food web has experienced a top-down effect due to overfishing of striped bass and other fish that consume the hermit crabs creating over populated areas that negatively affect salt marsh. Over abundance of crabs leads to “swiss cheese” marsh that can alter root systems of marsh flora.
In the past, studies involving interactions between Uca and S. Alterniflora have focused on wave protected coasts. The need for further research on exposed coasts and the different biological interactions of benthic filter feeders and deposit feeders on cordgrass productivity led to the following experiment.
The focus of this study was Uca uruguayensis species of hermit crabs, which are deposit feeders that alter marsh platforms due to their burrowing, feeding, and excretions method. These different feeding behaviors may impact drainage, nutrient availability, and redox cycles. This study is important due to the lack of data concerning exposed high energy sandy headlands.
The location of the study was on Great Island marsh in Wellfleet, Massachusetts, which is a sandy marsh area that has 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 hermit crabs affect the productivity of cordgrass. One experiment extracts all hermit crabs from a plot of salt marsh in order to eliminate crab activity. This allowed for scientists to measure cordgrass productivity without the interaction of hermit crabs.
During each experiment, data was recorded for U. uruguayensis species determination that included quantity of crabs, carapace width, burrow densities, and diameters of the local hermit crab habitat. The data allowed for calculations of adult-juvenile distribution and crab biomass, which could 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 for the effects of visual stimuli and utilized a non-functional hermit crab removal trap that was elevated off of the ground. 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 hermit crab’s activities on the biomass of cordgrass in each region. Other studies have shown an influx in N availability have been shown to increase leaf N concentration by 2% or more within the first month of growth.
Overall, the study indicated that burrow densities were about the same. Yet, burrow diameter was narrower, and carapace width was smaller in the areas without crabs versus areas with crab exposure. In the hermit crab removal control and access experiment, the crab biomass was four times greater than in the removal experiment.
This result indicates the majority of the hermit crabs studied were either juvenile or recruits. The smaller hermit crabs could have less impact on productivity due to smaller burrows and decreased feeding leading to less excretion.
Redox potential was positive throughout all treatments indicating that the sandy marsh is oxygenated and hermit crabs do not seem to influence aeration of the soil in this study. Yet, in other areas that have muddier sediments some species of hermit crabs 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 that they impact the soil by their burrowing and bed transport loads. In this study hermit crabs seem to have no 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.
Hermit crab burrowing also modifies the physicochemical properties and redox potential of soil (Fanjul , 2007). In the hermit crab access and removal control experiment where the plants had interaction with the hermit crabs, the plant’s nitrogen uptake was between 75% – 104% greater in the aboveground plant structures.
The belowground structures of cordgrass uptake were between 52%- 113% greater than cordgrass treatment with hermit crab removal. This indicates that nitrogen uptake increases with hermit crab exposure and interaction.
Some studies have found that there are some disadvantages of the hermit crabs to the cordgrass productivity. Scientists suggest that different species of hermit crabs known as Chasmagnathus granulata, can be detrimental to cordgrass viability due to the species being herbivorous and feeding on the new shoots of the plant.
In addition, hermit crabs do help S. alterniflora improve the quality of nutrient uptake, 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 can have a negative impact on primary productivity.
The study’s hypothesis that hermit crabs regulate cordgrass productivity was supported by the experimental results. Adult hermit crabs in exposed sandy salt marsh seem 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 because they lack the specialized mouthpart structures needed to feed efficiently on sandy sediments. Specialized mouthparts that have not fully matured could be a factor in the U. uruguayensis species in this study because most of the crabs were juveniles or recruits.
Cordgrass productivity was close to double in the hermit crab exposure and control experiment than in crab removal experiment. An important factor that hermit crabs contribute is sediment accretion that inhibits erosion during storm wave impacts due to increased root density of highly productive cordgrass.
A different study compares the two hermit 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. S. alterniflora and other plant species are vital to the buffer zone in salt marsh habitat. It is crucial to understand the interactions that promote primary production of these filtrating plants to maintain a healthy environment.