Fiddler crab (Crustacea: Ocypodidae) distribution and the relationship between habitat occupancy and mouth appendages

Abstract

Sediment characteristics, especially grain size, are usually considered the most important variables affecting Uca distribution, mainly due to its close relationship with mouth appendage morphology. The aim of this study was to verify, from an assemblage perspective, if sediment is the most important variable affecting Uca species distribution, and if mouth appendage morphology (setae type and curvature) would be related to habitat occupancy. Niche metrics and null model approaches were used to assess and test the hypothesis. The relevance of spoon-tipped setae curvature to Uca distribution was verified for the first time. A fragmented mangrove area was divided into seven subareas, and sampling of crabs and environmental variables took place in June and November 2010. Of 10 species recorded for Brazil, seven were found in the study area: U. burgersi, U. cumulanta, U. leptodactyla, U. maracoani, U. rapax, U. thayeri and U. uruguayensis. Multivariate analysis showed that sediment grain size and the presence of vegetation were the most important variables explaining distribution, reinforcing results commonly obtained by univariate approaches. The overlap of habitat occupancy was generally low and no relationship between mouth appendages was found with breadth and overlap measures. Contrary to predictions, most non-random overlap values were lower than expected by chance, suggesting that interspecific competition might influence species distribution. Also, variables such as the presence of vegetation are important and influence crab distribution, limiting the potential distribution that would be predicted by mouth adaptations alone. Thus, the use of these adaptations as surrogates of fiddler crab distribution is not recommended.

The importance of considering small-scale variability in macrobenthic distribution: spatial segregation between two fiddler crab species (genus Leptuca) (Decapoda, Ocypodidae)

“…Our results show that fiddler crab segregation may occur at scales smaller than those commonly researched. Leptuca leptodactyla and L. uruguayensis have previously been recorded as inhabiting the same habitat and with relatively high overlap (Crane, 1975;Thurman et al, 2013;Checon & Costa, 2017). Our results show that these species may segregate at very small scales, even without a clear granulometric gradient (both subareas classified as very fine sands), variable known to segregate these species (Checon & Costa 2017).…”

“…Leptuca leptodactyla and L. uruguayensis have previously been recorded as inhabiting the same habitat and with relatively high overlap (Crane, 1975;Thurman et al, 2013;Checon & Costa, 2017). Our results show that these species may segregate at very small scales, even without a clear granulometric gradient (both subareas classified as very fine sands), variable known to segregate these species (Checon & Costa 2017). Many fiddler crab studies do not take this local spatial variability into consideration and evaluate distribution in regards to environmental variables and/or among areas with contrasting characteristics (Colpo & Negreiros-Fransozo, 2004;Bezerra et al, 2006;Benetti et al, 2007).…”

“…Thus, these co-occur over a short range, from southeastern to southern Brazil. Locally, L. leptodactyla seem more restricted, inhabiting locations with absence of vegetation and sandy sediments (Checon & Costa, 2017). In turn, L. uruguayensis have a more generalist behavior, irrespective of vegetation presence, with both species co-occuring in unvegetated areas with low content of mud and very fine sands, known to burrow on similar sediment type (Thurman et al, 2013;Checon & Costa, 2017).…”

“…Factors affecting large-scales patterns are related to larval survival during dispersal and settlement, such as water current, wind pattern and temperature change with latitudinal gradients (Bogazzi et al, 2001;Nabout et al, 2009;Wieman et al, 2014). On a local scale, variables such as sediment grain size, tidal level, vegetation cover and salinity influence species occurrence and distribution (Thurman, 1987;Thurman et al, 2013;Mohktari et al, 2015;Checon & Costa, 2017). Therefore, this suggests that the patterns observed for large scales may differ from local ones.…”

“…Locally, L. leptodactyla seem more restricted, inhabiting locations with absence of vegetation and sandy sediments (Checon & Costa, 2017). In turn, L. uruguayensis have a more generalist behavior, irrespective of vegetation presence, with both species co-occuring in unvegetated areas with low content of mud and very fine sands, known to burrow on similar sediment type (Thurman et al, 2013;Checon & Costa, 2017). They also share similarities regarding internal burrow morphology and bioturbation activity (Machado et al, 2013;Natalio et al, 2017).…”