Interspecific and intraspecific interactions between fiddler crabs Minuca pugnax (mud fiddler) and Leptuca pugilator (sand fiddler) influence species' burrowing behavior

Abstract

Species occupying similar habitat and functional niches will necessarily have some external force driving differentiation between them, thus creating complementarity, or otherwise face high competition for resources. In order to fully understand the role of each species in an ecosystem and their contributions to ecosystem function, it is important to understand the nature of these interactions. However, for species that occur in the same spatial niche and appear to occupy redundant functional niches, it is often difficult to distinguish between complementarity and redundancy and therefore to designate the functional role each species plays. We used two co-occurring fiddler crab species that are presumed to be functionally similar, Leptuca pugilator (sand fiddler crab) and Minuca pugnax (mud fiddler crab), to explore how species interactions may influence burrowing behavior. We ran manipulative mesocosm experiments to assess potential effects of species interactions on the number and location of burrows as well as the burrowing behavior of the crabs. Overall, sand fiddler crabs dug more burrows on average than mud fiddlers across all experiments while mud fiddlers were choosier about the location and sediment type of their burrows, suggesting complementarity in burrowing between the two species. Burrow counts in heterospecific treatments suggest that neither species interfered with nor enhanced burrow creation by the other species. However, increased burrow occupancy by mud fiddlers in the presence of sand fiddlers and aggressive behavior by mud fiddlers toward sand fiddlers suggest that mud fiddler crabs may receive refuge benefits from sand fiddler crabs through antagonistic rather than facilitative interactions. Thus, species-specific habitat use and interspecific interactions are likely influencing the role each species plays as a burrower and marsh ecosystem engineer.