Abstract
ABSTRACTThe habitat stability hypothesis states that species in spring-like habitats have little reason to disperse compared to species in temporary habitats. Planarians commonly inhabit springs around the world and they have long been considered poor dispersers. Recently, however, genetic analyses have shown contradictory results on the dispersal of planarians. Asexual planarians that can establish a new population by colonization of a single individual showed little genetic differentiation between sites separated by hundreds of kilometers, whereas species inhabiting springs showed deep differentiation between sites separated by hundreds of meters. The latter results are consistent with the habitat stability hypothesis. We used the cytochrome oxidase subunit I gene from 468 individuals of Polycelis coronata, an asexual species, collected from 50 sites, nested in 26 tributaries, in 4 catchments of the Wasatch Mountains of Utah, USA, to explore the dispersal capabilities of P. coronata. The longest distance between sites was 66 km. Despite this small spatial extent, we found that 77% of the 130 haplotypes were collected from a single site and 89% from a single catchment. FST values between local populations in the same tributary (0.221, 0.266, 0.389) were similar to the average FST values in different catchments for other headwater taxa. Also, variation among individuals accounted for the majority of genetic structuring with little differentiation beyond the scale of a single site. Dispersal is very slow in this species which is consistent with the habitat stability hypothesis. However, we suggest that other explanations also warrant consideration. We also identified two potential cryptic species suggesting a high degree of hidden variation at the level of species in this genus.
Highlights
Planariidae possess several traits that may limit their ability to disperse within stream ecosystems
77% of the haplotypes were confined to a single site and 89% were restricted to a single catchment
Average FST values for P. coronata between local populations in the same tributary (0.221, 0.266, 0.389) were similar to the average FST values in different catchments (0.17, 0.318, 0.38, 0.39, 0.57) for other headwater taxa (Blephariceridae, Simuliidae, shrimp) that poorly disperse (Hughes et al 1995, 1996; Wishart & Hughes 2001; Finn & Adler 2006, Finn & Adler 2006). These data show that dispersal is rare in P. coronata even between sites in the same tributary
Summary
Planariidae possess several traits that may limit their ability to disperse within stream ecosystems. They are obligate aquatic species without a terrestrial phase in their life cycle, which eliminates active, overland dispersal. They may rarely encounter passive vectors (e.g. birds) because the adults are nocturnal and cryptic, and if they produce resting eggs, their cases are firmly attached to the undersides of rocks (Walter 1907; Kolasa et al 2009). Biologists have long thought that planarians have a poor dispersal capability We still have much to learn about dispersal in flatworms because
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