Larval Tolerance, Gene Flow, and the Northern Geographic Range Limit of Fiddler Crabs

Abstract

Despite growing interest in species' range shifts, little is known about the ecological and evolutionary factors that control geographic range boundaries. We investigated the processes that maintain the northern range limit of the mud fiddler crab (Uca pugnax) at North Scituate, Massachusetts, USA (42 degrees 14' N), located approximately 60 km north of Cape Cod. Larvae from five populations in Massachusetts were reared under controlled temperatures to test whether cooler water near the edge of this species' range inhibits planktonic development. Few larvae completed development at temperatures < 18 degrees C, a threshold that larvae would regularly encounter north of Cape Cod. Extensive salt marshes are present north of the current range boundary, and a transplant experiment using field enclosures confirmed that benthic fiddler crabs can survive severe winter conditions in this northern habitat. Taken with oceanographic data, these results suggest that the range boundary of fiddler crabs is likely maintained by the influence of cooler water temperatures on the larval phase. Analyses of mitochondrial DNA sequences from a neutral marker (COI) indicate high gene flow among U. pugnax populations in Massachusetts with little differentiation across Cape Cod. Consistent with predictions regarding the homogenizing influence of gene flow, larvae from source populations north and south of Cape Cod shared a common lower threshold for development. However, larvae produced near the range edge had faster growth rates than those from the south side of Cape Cod (typically reaching the final megalopal stage 1.0-5.5 d faster at 18 degrees C). Additional studies are needed to determine the mechanism underlying this counter-gradient variation in development time. We hypothesize that dispersal into cooler water on the north side of Cape Cod may act as a selection filter that sieves out slow developers from the larval pool by increasing planktonic duration and exposure to associated sources of mortality. Thus while high gene flow may prevent the evolution of greater cold tolerance in northern populations, recurrent selection on existing variation may lead to an unexpected concentration of favorable adaptations at the edge of the range. Such a pattern could permit edge populations to play a dominant and unrecognized role in future range extensions.

The effect of salinity on larval development of <em>Uca tangeri</em> (Eydoux, 1835) (Brachyura: Ocypodidae) and new findings of the zoeal morphology

“…Mortality associated with an increase in duration of development at low temperatures and the latitudinal temperature diminution could limit the larval dispersal of this species, and consequently its northern distribution. also, the northern limit of Uca pugnax in the northwestern atlantic is likely maintained by the influence of cooler water temperatures on the larval phase, as reported by Sanford et al (2006), who found that there are suitable northern habitats for this species (extensive salt marshes) and that transplanted adult crabs could resist severe winter conditions. However, few larvae completed development at temperatures that they would regularly encounter north of their range boundary.…”

Restricted gene flow in Chilean barnacles reflects an oceanographic and biogeographic transition zone

“…Comparative and integrated analysis of ecological transitions (Connolly et al 2001, Navarrete et al 2008, changes in behavior associated with these transitions (Sanford et al 2003), genetic patterns concordant with these transitions (Petersen 2007), and experimental evaluation of fitness (Sanford et al 2006) at all life stages of coastal invertebrates will ultimately provide significant insights into the function of coastal communities and the diversity they harbor. Currents and their effect on dispersal are clearly responsible for some of these observed patterns, but the environmental changes in climate and topographic conditions, which usually modulate the changes in coastal hydrography, might exert unsuspected and poorly studied selective pressures.…”

Supply-side biogeography: geographic patterns of settlement and early mortality for a barnacle approaching its range limit

“…Mar Ecol Prog Ser 412: [141][142][143][144][145][146][147][148][149][150] 2010 although these supply-side processes are often highly variable in space and time (see Underwood & Fairweather 1989, Caley et al 1996, Hunt & Scheibling 1997, they sometimes reflect latitudinal gradients of temperature, currents, wave exposure and nutrient availability in the ocean (Connolly & Roughgarden 1998, Connolly et al 2001, Zacherl et al 2003, Sanford et al 2006. Consequently, the geographic distribution of benthic marine invertebrates, particularly as a species approaches its range limit, may be strongly influenced by latitudinal gradients in the supply and early life history of larvae.…”