Evol Ecol Res 20: 639–656 (2019)     Full PDF if your library subscribes.

Urbanization drives phenotypic evolution in mosquitofish

Joshua R. Mays^1,2, Thomas J. DeWitt³, Prarthana Dharampal^1,4, C. Fred T. Andrus⁵ and Robert H. Findlay¹

¹Department of Biological Sciences, The University of Alabama, Tuscaloosa, Alabama, USA, ²Department of Science, Gaston College, Dallas, North Carolina, USA, ³Department of Ecology and Conservation Biology, Texas A&M University, College Station, Texas, USA, ⁴Department of Entomology, University of Wisconsin-Madison, Madison, Wisconsin, USA and ⁵Department of Geological Sciences, The University of Alabama, Tuscaloosa, Alabama, USA

Correspondence: T.J. DeWitt, Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX 77843, USA. email: tdewitt@tamu.edu

ABSTRACT

Background: Forced or chosen habitat changes drive the evolution of strategies by which organisms tolerate or mitigate functional trade-offs among habitats. Phenotypic plasticity is a powerful adaptation to mitigate the effects of habitat change but it may be limited by the constraint of prior phenotypes developed in former environments. Addressing this question in animals has been difficult, as it is hard to quantify time spent in different habitats over several years of life in natural environments. Recent advances in otolith isotope analysis, however, have made possible such environmental reconstruction.

Study system: Bluegill sunfish (Lepomis macrochirus) from three habitats (main river channel, oxbow lake, beaver pond) in the Sipsey River watershed in Alabama, USA.

Goals: We address the following questions: (1) Do bluegills move between habitats in the study system? (2) Does higher fecundity of fish inhabiting floodplain lakes result in higher recruitment within or migration to the system as a whole? (3) Do fish in the alternative habitats evince differences in diet? (4) Has previously observed morphological differentiation persisted? (5) If habitat-associated ecophenotypy is observed, is it due to phenotypic plasticity? (6) If morphological differentiation is evident and due to plasticity, is there a residual of natal ecophenotypy in adults that migrated to new habitats?

Methods: We collected fish from the three focal environments and assessed body shape as reflected by the positions of 12 landmarks of fish external morphology. We measured morphometry and inferred natal habitat by isotope discriminant patterns in recently-laid bone (otolith edges) compared with first-laid bone (otolith core). We assessed the trophic positions of fish in different habitats using amino-acid specific differentials in ¹⁵N isotopes. We estimated diet composition using ¹³C, ¹⁸O, and ¹⁵N isotopes. Multivariate shape analysis was used to define body shapes associated with habitat of capture and isotope-inferred residual morphology due to natal habitat.

Results: (1) Sixty-five percent of fish were captured in a habitat differing from their natal habitat. (2) Habitat-specific fecundity differentials did not drive measurable differences in recruitment among habitats. (3) Isotope-inferred diets differed among habitats but did not result in differences in summative trophic levels. (4) Fish morphology varied with habitat of capture: individuals from dammed ponds had deeper mid-bodies, elongated snouts, and larger heads. Fish from the oxbow lake had larger eyes, disciform bodies, posteriorly shifted pelvic fins, short caudal peduncles, and short dorsal fin bases. (5) Fish that changed habitats had differing morphology than those not changing, indicating phenotypic plasticity. (6) Natal habitats left residual morphological signatures very similar to current-environment effects. We discerned that residual morphology would impose functional costs and thus implies a limit to the evolution of plasticity.

Keywords: costs and limits of phenotypic plasticity, environmental change, geometric morphometrics, habitat choice, habitat migration, stable isotope analysis.

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