Intraspecific variation in carapace morphology among fiddler crabs (Genus Uca) from the Atlantic coast of Brazil

Abstract

Isolation due to geographical barriers should promote genetic and morphological divergence among populations. Marine currents flowing in opposing directions along landmasses can constitute barriers that isolate populations dependent upon aquatic dispersal. The distribution of fiddler crabs (genus Uca) is regulated primarily by the oceanic transport of their planktonic larvae and by available adult habitat. Along the Brazilian coast of eastern South America, the flow of 2 major oceanic currents separates northern from southern Uca populations, which may promote intraspecific divergence in 'trans-Brazilian' species. Populations of 10 Uca species were sampled at 64 locations north and south of the Ponta do Calcanhar, Rio Grande do Norte, Brazil. Carapace shape was assessed using geometric morphometrics to analyze 12 surface landmarks in 1319 female crabs. Carapace shape differs significantly in each species. In morphospace, the carapace forms of the 10 species appear to separate into traditional subgeneric clusters. Within the 8 species exhibiting trans-Brazilian distributions, northern and southern populations show distinct carapace differences. Depending on species, either the hepatic or the branchial region is larger in northern populations. Since significant genetic variability among such populations has not been confirmed, divergence in carapace shape suggests significant ecological modulation of phenotype within each species. Apparently, environmental differences between northern and southern localities exert a greater impact on carapace morphology than impeded gene flow. The drivers underpinning diversification of carapace shape remain unknown, however.

Morphological variability in populations of the fiddler crab Leptuca uruguayensis (Nobili, 1901) (Crustacea, Decapoda, Ocypodidae) from South America

“…Another example of phenotypic plasticity in the carapace shape of fiddler crabs was reported by HAMPTON et al (2014), who found a significant morphological divergence in the carapace (variations in the hepatic or branchial regions) among the northern and southern populations of eight species living along Brazilian coast (between Amapá and Santa Catarina States). These authors attributed this divergence to the existence of a geographic barrier (Central South Equatorial Current that splits into two circulation patterns, North Brazil and South Brazil Current) for the et al (2013) larval planktonic stage of these species.…”

Contrasting Patterns of Clinal Genetic Diversity and Potential Colonization Pathways in Two Species of Western Atlantic Fiddler Crabs

“…From the results obtained in this study, it cannot be inferred at this point that the Amazon river functions as a barrier to gene flow in the two studied fiddler crabs. Yet, although Wieman et al [39] did not find evidence for significant genetic distinction, theyas well as Hampton et al [40]. detected significant morphometric differentiation that may hint at speciation commencement or phenotypic plasticity as response to ecological parameters [57].…”

“…some Jamaican individuals were much larger and had different coloration), although not yet statistically analyzed. Thus, statistical analyses on morphology are highly encouraged, including both morphometrics and trophic morphology to elucidate possible ecological adaptations and/or phenotypic plasticity as for example found in cave living crabs [82], as well as in other fiddler crabs [39, 40, 47]. Habitat variations within a broad distributional range that are not sufficiently large when related to effective dispersal, more likely result in phenotypic plasticity rather than actual local adaptation [83].…”

“…Research on other fiddler crabs along the Brazilian coast was unable to detect structuring among examined communities. However, while no significant genetic variance could be detected along the entire coastal range of Brazil, significant morphometric differences were evident for almost all local fiddler crab species [39, 40]. …”

Description of the first juvenile stage of the fiddler crab Minuca mordax (Smith, 1870) (Crustacea, Decapoda, Ocypodidae)

“…Recently, Shih et al (2016) reviewed the taxonomy of the genus and recognized all subgenera of Uca as full genera based on molecular evidence. Ten species are found in Brazil, including Studies on M. mordax have been carried out on various aspects: description of the developmental stages; density; relative growth; population dynamics and intraspecific variation (Rieger, 1997;Masunari & Dissenha, 2005;Masunari, 2006;Fransozo et al, 2009;Thurman et al, 2013;Hampton et al, 2014). However, no study on the morphology of the juvenile phase of this species has been previously undertaken.…”