Traditionally, it is thought that tropical species diversity and abundance decrease with an increase in latitude, responding to a gradient of drivers related to climate or biotic interactions. To investigate this, potential abiotic (soil, water, and biogeographic parameters) and biotic (brachyuran diversity, gastropod density, tree density, meiofauna density and food availability) drivers of fiddler crabs along the east coast of South Africa (26°S - 34°S) were assessed in multivariate models of fiddler crab presence/absence and abundance. Overall fiddler crab presence/absence was most strongly influenced by biotic variables (i.e. brachyuran diversity, gastropod density, and tree density), while abundance was driven largely by sediment grain size and canopy cover (abiotic variables). Specifically, Austruca occidentalis did not have any strong drivers aside from tree density, both Paraleptuca chlorophthalmus and Tubuca urvillei were influenced strongly by sediment grain size, tree density and crab diversity while Gelasimus hesperiae was influenced by a change in latitude, sediment grain size and meiofauna density. Distribution is not always correlated with latitude or latitudinally-driven variables. Biotic factors seem to play a large role in determining whether or not a species inhabits a specific mangrove habitat while, once established, abiotic factors drive density. This study highlights the complexity of drivers affecting species at their distributional limits, an important assessment considering that these populations would likely demonstrate range shifts in response to climate shifts or changes.

Biophysical drivers of fiddler crab species distribution at a latitudinal limit

Three species of fiddler crabs, Uca minax, U. pugnax, and U. pugilator, are commonly found in estuaries along the Atlantic coast, each with distinct adult habitats differing in salinity and sediment grain size. Prior research has found evidence for larvae exhibiting selective settlement; however, the degree to which and the method by which they choose their speciesappropriate habitat to settle in is still unknown. Additionally, a recent study determined that chemical cues from adult crabs stimulate molting in field-caught fiddler crab megalopae, as previously determined in lab-reared megalopae; however, in 35 ppt seawater, few U. minax molted. This study tested the hypotheses that 1) chemical cues from adult fiddler crabs would stimulate molting of conspecific megalopae in 10 ppt seawater, and 2) that at 10 ppt, more U. minax would molt than at 35 ppt. Adult chemical cues accelerated molting in field-caught megalopae of U. pugilator and U. pugnax, which both molted significantly earlier in all odor water treatments than in the control water, and fastest in conspecific water. U. pugilator megalopae were stimulated to molt in all treatments, whereas U. pugnax molted mostly in conspecific odor. However, even at 10 ppt, few U. minax molted and there were no differences in molting frequency or time among treatments. The few U. minax molts suggest that these experimental conditions may still not be ideal, and other factors may be responsible for stimulating and accelerating their molting and settlement site selection.

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Effect of Adult Chemical Cues on Molting of Fiddler Crab Megalopae in Low Salinity Seawater

https://cupola.gettysburg.edu/cgi/viewcontent.cgi?article=1594&context=student_scholarship

The Role of Adult Fiddler Crab Environmental Acoustic Cues and Chemical Cues in Stimulating Molting of Field-Caught Megalopae

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Emily E. Waddell

Papers

Chemical cues from adult fiddler crabs stimulate molting of conspecific megalopae: evidence from field-caught individuals

The Role of Adult Fiddler Crab Environmental Acoustic Cues and Chemical Cues in Stimulating Molting of Field-Caught Megalopae