The geographic structure of morphological variation in eight species of fiddler crabs (Ocypodidae: genus Uca) from the eastern United States and Mexico

Abstract

Species with larger geographic distributions are more likely to encounter a greater variety of environmental conditions and barriers to gene flow than geographically-restricted species. Thus, even closely-related species with similar life-history strategies might vary in degree and geographic structure of variation if they differ in geographic range size. In the present study, we investigated this using samples collected across the geographic ranges of eight species of fiddler crabs (Crustacea: Uca) from the Atlantic and Gulf coasts of North America. Morphological variation in the carapace was assessed using geometric morphometric analysis of 945 specimens. Although the eight Uca species exhibit different degrees of intraspecific variation, widespread species do not necessarily exhibit more intraspecific or geographic variation in carapace morphology. Instead, species with more intraspecific variation show stronger morphological divergence among populations. This morphological divergence is partly a result of allometric growth coupled with differences in maximum body size among populations. On average, 10% of total within-species variation is attributable to allometry. Possible drivers of the remaining morphological differences among populations include gene flow mediated by ocean currents and plastic responses to various environmental stimuli, with isolation-by-distance playing a less important role. The results obtained indicate that morphological divergence among populations can occur over shorter distances than expected based on dispersal potential.

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

“…These findings contrast with a study from the east coast of Africa in which Silva et al (2010) Silva et al (2010) allowed analysis of carapace perimeter but not of the dorsal surface. However, the findings most likely differ due to the spatial scale assessed: the coastlines sampled by Hopkins & Thurman (2010), and here, are much longer (∼13 500 and ∼7500 km, respectively) than the African coastline (∼3300 km) sampled by Silva et al (2010). Divergent carapace morphology in the 8 transBrazilian species suggests that the Ponta do Calcanhar may be a disruptive geographical barrier.…”

“…3). Orientation and digitization error was assessed by repeatedly mounting and digitizing a single random specimen for each species analyzed (Hopkins & Thurman 2010). A single investigator (M.J.H.)…”

“…Thus, enlarging the branchial chambers would likely serve as a safeguard against desiccation (Jones 1941). Fiddler crab species from arid regions in the western Gulf of Mexico show similar adaptations (Thurman 1998, Hopkins & Thurman 2010. A detailed examination of the relationship between various environmental factors and carapace shape in several species is currently in progress.…”

“…Silva et al (2010) found the widespread species U. annulipes (Milne-Edwards, 1837) from southeast Africa to exhibit very little morphological or genetic structure, suggesting that populations distributed over 3300 km between Mikindani, Kenya, and Mlalazi, Republic of South Africa, are sufficiently connected by high larval transport to maintain panmixia. In contrast, along 13 500 km of coastline in the USA and Mexico, several endemic Uca species show detectable morphological variation (Hopkins & Thurman 2010) even though the widerspread species do not necessarily exhibit greater variation than species with smaller ranges.…”

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

“…The comprehension of morphological variations have been crucial to answer fundamental questions in evolutionary biology (HOPKINS; THURMAN, 2010;LEZCANO et al, 2012). Along its geographical distribution, a species may face distinct environmental and selective pressures that act upon morphology, reproductive patterns, growth rates and mortality (HOFFMANN;SHIRRIFFS, 2002).…”