Structure and function of fiddler crab communities: getting to the bottom of the burrow in Gulf Coast tidal marshes

Gwendolyn A. Murphy and Loretta L. Battaglia, School of Biological Sciences, Southern Illinois University, Carbondale, IL

Background/Question/Methods
Coastal ecosystems experience stressful soil conditions that limit plant productivity. Fiddler crab bioturbation may enhance plant growth by ameliorating stressful soil conditions, but fiddler crab-plant interactions are likely to vary with burrow density and vegetation characteristics. These relationships, and composition of the fiddler crab community itself, have not been examined in Gulf Coast tidal marshes, which have distinctive tidal and climatic patterns. Based on an initial pilot study of crab burrows, we hypothesized that burrow density would be negatively correlated with soil organic matter and soil hardness but positively correlated with redox potential, and above and below-ground plant biomass. To test these relationships and to determine fiddler crab community structure, replicate enclosures (n=6) were constructed in each of the four dominant Gulf Coast vegetation zones (salt marsh, brackish marsh, fresh marsh and salt panne) at Grand Bay National Estuarine Research Reserve, Mississippi, USA. In early 2017, crabs were baited, identified, and removed from all enclosures at the onset of the study; species and sex of fiddler crabs were documented. A specified number of crabs was added back in to half of the enclosures, such that the experiment comprised contrasting crab reduction and addition treatments. Soil and plant measurements were taken in Summer, Fall and Winter 2017. Following our final measurements, vegetation was removed, allowing for comprehensive burrow counts.
Results/Conclusions
Collectively, the results highlight variability in fiddler crab-plant interactions and suggest that bioturbation is facilitative in zones characterized by waterlogged soils, but often destructive in drier, upslope habitats. Uca longisignalis was the dominant fiddler crab species in the salt (90%) and brackish marshes (75%), where male to female ratios were highest. In contrast, fresh marsh and salt pannes had higher crab diversity and more even sex distributions. The final burrow survey revealed an inordinate number of burrows hidden under fresh marsh vegetation. While few studies acknowledge fresh marsh as fiddler crab habitat or document effects of bioturbation on fresh marsh vegetation, our findings indicate that this zone may be preferred by Gulf Coast fiddler crabs and quite possibly already acting as a habitat “bridge” for upslope migrations as sea levels rise.