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

Abstract

Defining the appropriate scale is important when trying to understand distribution patterns in community studies. Fiddler crabs are among the most common organisms inhabiting estuarine environments, and despite having a wide latitudinal distribution, are limited by shifts in temperature, current and wind pattern. Thus, many co-occur at a local scale, where their distribution is influenced by variables such as mean sediment grain diameter, salinity and tidal level. Our goal was to test intra and interspecific segregation in two similar and commonly co-occurring fiddler crabs species (Leptuca leptodactyla and Leptuca uruguayensis) at a small scale (10 m²). Interspecific segregation was observed, with L. leptodactyla occurring mainly at the upper level in relation to the water line and L. uruguayensis, at the lower. However, this pattern was irrespective of sex and developmental stage, as no intraspecific segregation was seen. Possible impacts of tidal level, soil silt/clay content and competition on interspecific segregation are discussed. Although L. leptodactyla and L. uruguayensis overlap in their occurrence when intertidal zonation is disregarded, they segregate when tidal levels are individually sampled as habitats. This highlights the importance of small-scale studies to identify patterns unobserved at regional scales, even when no environmental gradient is readily apparent.KEYWORDS. Fiddler crabs, estuarine beach, small-scale, Leptuca, allometric growth. RESUMO.A importância de considerar a variabilidade em pequena escala na distribuição macrobêntica: segregação espacial entre duas espécies de caranguejo chama-maré (gênero Leptuca) (Decapoda, Ocypodidae). Definir a escala apropriada de um estudo é importante quando tentamos entender os padrões de distribuição em estudos de comunidade. Caranguejos chama-maré estão entre os organismos mais comuns que habitam os ambientes estuarinos, e apesar de possuírem uma ampla distribuição latitudinal, são limitados por alterações em temperatura, correntes e padrões dos ventos. Dessa forma, muitas espécies co-ocorrem em escala local, onde suas distribuições são influenciadas por variáveis como diâmetro do grão de sedimento, salinidade e nível de maré. Nosso objetivo foi testar a segregação intra-e interespecífica em duas espécies similares de caranguejos chamamaré, que comumente são encontradas em co-ocorrência (Leptuca leptodactyla e Leptuca uruguayensis), em uma pequena escala (10 m²). Foi observada segregação interespecífica, com L. leptodactyla ocorrendo primariamente no nível superior em relação à linha de maré, e L. uruguayensis no inferior. Entretanto, esse padrão foi independente de gênero e estágio de desenvolvimento, uma vez que segregações intraespecíficas não foram registradas. Possíveis influências do nível de maré, teor de silte/argila e competição na segregação interespecífica são discutidas. Apesar de L. leptodactyla e L. uruguayensis terem sobreposição em ocorrência quando a zonação do entremarés é desconsiderada, elas segregam quando os níveis de maré são...

Sharing is living: The role of habitat heterogeneity in the coexistence of closely related species

“…In addition, the proportion of L. leptodactyla was reduced only for the 80% of shading level treatment, the one that also had lower temperatures and higher organic matter content in the sediment. Although our findings for both species are accordingly to the descriptive approaches that evaluate crabs' distribution considering shading as a binary factor (i.e., shaded or nonshaded; Checon & Costa, 2017 , 2018 ; Thurman et al, 2013 ), we observed that the degree of shade is also important in this context, as there is a limit of shading that can alter the community (in this case from 80% of shading). Finally, despite the crabs' distribution being also related to the shade only when this affected sediment organic matter, it is known that L. uruguayensis did not show any preference for specific organic matter content (De Grande et al, 2018 ).…”

“…Habitat heterogeneity determines species abundance and richness (Erdős et al, 2018 ; Leviten & Kohn, 1980 ; Tews et al, 2004 ), and the coexistence of sympatric species is explained by habitat partition (Jorgensen, 2004 ; Liu et al, 2017 ; Palmer, 2003 ). Fiddler crabs are arguably a diverse patch‐specialist group with species inhabiting a great range of different microhabitats (Checon & Costa, 2017 , 2018 ; Thurman et al, 2013 ; Weis & Weis, 2004 ). This favors the coexistence of so many species of fiddler crabs in the same ecosystem since it can be costly to live in mixed assemblies (Sanches et al, 2018 ).…”

“…Groups of species that are phylogenetically close may become shade‐patch specialists, so a competitive congener species can share the same ecosystem (e.g., mangrove forests, salt‐marshes fields) since they occur in different microhabitats (e.g., vegetated or nonvegetated) (Nobbs, 2003 ; Nobbs & Blamires, 2015 ). In this sense, restrictions in thermal tolerances may constrain some species to occupy the shaded areas in the vegetation or under the shade of natural or artificial structures (e.g., rocks, bridges, buildings), while other species tend to predominate in nonvegetated clearings at the edges of the forest or in sandbars (Checon & Costa, 2017 , 2018 ; Lorda & Lafferty, 2012 ; Thurman et al, 2013 ). Therefore, these distributions of species can be linked to their intrinsic physiological limitations, influencing the decrease of interspecific competitive pressure in mixed‐species assemblies.…”

“…For example, the fiddler crabs Leptuca leptodactyla and Leptuca uruguayensis are closely related species that share sandy sediment banks along with mangrove forests from Southwestern Atlantic (Fogo et al, 2019 ; Machado et al, 2013 ; Thurman et al, 2013 ). While L. leptodactyla tends to predominate in nonvegetated clearings at the edges of the forest or in sandbars, L. uruguayensis tends to occupy the shaded areas from the interior of the vegetation (Checon & Costa, 2017 , 2018 ; Thurman et al, 2013 ) or under the shade of artificial structures (e.g., bridges, buildings; personal observation). Both species can occur in mixed assemblies for which it has been reported that males compete extensively for space and resources (Fogo et al, 2019 ).…”