Abstract
Intertidal crabs display distinct morphological traits that allow differential interactions with biotic and abiotic features of the intertidal landscape, but are also influenced by allometry and sexual selection. This study aimed to explore the influence of sexual, allometric and habitat factors on morphological variation in the intertidal mangrove crab assemblage. A standardized photographic protocol was developed using readily available, low-cost technology to capture the morphology of carapaces and claws as sets of Cartesian landmarks. Digitization errors were 1 to 2 orders of magnitude smaller than the variation among individuals. In Tubuca seismella and Tubuca signata (superfamily Ocypodoidea), species that have traditionally been studied for sexual dimorphism, standardized major claw sizes were 2.8 and 3.7, respectively, times larger for males than females. Sexual dimorphism in claw size was also observed for Metopograpsus frontalis and Parasesarma longicristatum (superfamily Grapsoidea), with the largest claw in males being 15% and 33%, respectively, larger than in females. In contrast to size, claw shape did not relate to sex, except for T. seismella. Carapace shape, although variable among individuals and displaying asymmetry, was unrelated to sex. Claw and carapace shapes displayed high correlations, with values around 0.78. Carapace shapes grouped into taxonomic families and linked to habitat preferences, while claw shapes varied along a taxonomic gradient. These results complement studies on crab morphology that focus on specific factors or species, and stress the importance of multiple, interacting factors including sexual, allometric and habitat influences as drivers of morphological trait variability.
Highlights
The present study aimed to explore the relative influence of sexual, allometric and habitat-related influences on morphological trait variation in the intertidal mangrove crab assemblage characterized by species with strong morphological sexual dimorphism
Geometric morphometric analyses have been applied to intertidal crabs (e.g. Rosenberg 2002; Hopkins and Thurman 2010; Hampton et al 2014), but a standardized protocol for the collection and assessment of data has not been described, so we developed a standardized protocol for the collection of geometric landmark sets of crab carapace and claw shapes (Online Resource 1) and applied it to the analysis of morphological variation within and among crab species in intertidal mangrove habitats
The study complements work on crab morphology that focus on specific factors or species
Summary
The position of the eyes in the families Grapsidae and Sesarmidae (superfamily Grapsoidea) at each lateral boundary of their carapace (termed ‘broad-fronted’ species) allows for well-developed stereoscopic vision (Zeil and Hemmi 2006) This allows them to gauge distances and to move efficiently through habitats with high structural complexity, such as dense mangrove roots (Zeil et al 1986). The attraction of mates using visual signals has resulted in strong sexual dimorphism, with males having one enlarged claw while their female counterparts have two smaller claws similar in size. This sexual dimorphism has made fiddler crabs the topic of several behavioural studies The essential use of visual signals in sexual communication within this genus could restrict their spatial distribution to habitats with little structural complexity, for small fiddler crabs with highly developed waving displays such as Tubuca seismella (von Hagen 1993)
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