Adaptive phenotypic plasticity of eel diadromy

Abstract

Eels are marine teleosts that have ancestrally evolved a continental growth phase during which diadromy is facultative. This migratory plasticity reflects a fitness trade-off between search for the most productive habitats and competition avoidance favoring shift to less productive areas, while migration costs (increased mortality and reduced growth) hinder movements. To cope with these conflicting selective pressures acting in heterogeneous and unpredictable environments, eels have evolved a conditional evolutionarily stable strategy (ESS) with alternative tactics as a regulation mechanism for migratory plasticity. In this model, diadromy is considered as a threshold reaction norm to an individual's energetic status. In the population, this reaction norm evolves rapidly in response to changes in the cost/benefit fitness ratio of diadromy. This conditional ESS model may explain how the collapse of freshwater eel stocks results not only from decreased recruitment but also from an adaptive drop in the proportion of the diadromous phenotype in populations. Hence, I suggest that the decline in populations of eels has been less steep in salt than in freshwater habitats.

The Genetic Consequences of Spatially Varying Selection in the Panmictic American Eel (Anguilla rostrata)

“…Indeed, the costs induced by selection on locally adaptive traits are particularly severe in the case of random mating and in the absence of habitat choice (Lenormand 2002). For eels, as for other highly fecund marine species facing huge mortality rates during larval stages, phenotypic plasticity may represent the main mechanism for coping with habitat heterogeneity (Edeline 2007), and our results suggest that differential expression of paralogous genes may be involved in this regulation. Nevertheless, the finding of locally selected mutations spreading to fixation in A. rostrata suggests that this high census size species may be regularly subject to new locally adaptive mutations.…”

A synthesis of the ecological processes influencing variation in life history and movement patterns of American eel: towards a global assessment

“…Arguably, existing evidence supports the hypothesis that eels evolved a migratory behaviour in response to the relative habitat productivity prevailing in their geographical range (Edeline 2007)-temperate eels living at higher latitudes with higher oceanic productivity exhibit a greater probability of remaining in lower reaches in brackish water than eels living at lower latitudes ) with higher freshwater productivity. But other factors must be involved since general patterns defining the variation in life history traits and movement patterns as functions of latitude (Helfman et al 1987;Tesch 2003) or ocean-freshwater productivity gradients (Jessop et al 2004) show some inconsistencies across the entire geographical range of the species (e.g., Hansen and Eversole 1984;Krueger and Oliveira 1999;Oliveira et al 2001).…”

“…The migration of American eel leptocephali to continental waters has been evolutionarily advantageous given the low nutrient content of the Sargasso Sea and its isolation from deep, nutrient rich cold waters (USFWS 2000). Migration patterns of American eel and many other fishes between freshwater and marine systems (diadromy) seem to primarily be a response to global patterns in aquatic productivity (Edeline 2007). Overall, higher oceanic productivity in temperate latitudes and higher freshwater productivity in tropical latitudes favour the evolution of anadromy (species spawned in freshwater with growth in the ocean) in temperate latitudes, and catadromy in tropical latitudes (Gross et al 1988).…”

“…Movement patterns of American eel can be explained by a trade-off between residence in the most productive habitats and shifts to less productive lower-density habitats (Edeline 2007). Highly productive aquatic habitats constitute optimal habitats for somatic growth and usually harbour high densities.…”

“…The ability of eels to colonize freshwater habitats has been associated with high locomotor activity (Edeline et al 2005). Higher energetic status would enable American eel at the extremes of its range to undertake inland migrations and gain fitness by inhabiting low-density river habitats (see also Edeline 2007) or by reaching habitats with optimal suitability. High energetic status could explain the high proportion of eel migrating to freshwater habitat as elvers in the East River, Nova Scotia (Jessop et al 2006) and their capacity to undertake the long migration to Lake Ontario (Busch et al 1998).…”