Plasticity and habitat choice match colour to function in an ambush bug

Multispecies colour polymorphisms associated with contrasting microhabitats in two Mediterranean wrasse radiations

Intraspecific colour polymorphisms (CPs) present unique opportunities to study fundamental evolutionary questions, such as the link between ecology and phenotype, mechanisms maintaining genetic diversity and their putative role in speciation. Wrasses are highly diverse in ecology and morphology and harbour a variety of colour‐polymorphic species. In the Mediterranean Sea, wrasses of the tribe Labrini evolved two species radiations each harbouring several species with a brown and a green morph. The colour morphs occur in complete sympatry in mosaic habitats with rocky outcrops and Neptune grass patches. Morph‐specific differences had not been characterized yet and the evolutionary forces maintaining them remained unknown. With genome‐wide data for almost all Labrini species, we show that species with CPs are distributed across the phylogeny, but show evidence of hybridization. This suggests that the colour morphs are either ancient and have been lost repeatedly, that they have evolved repeatedly or have been shared via hybridization. Focusing on two polymorphic species, we find that each colour morph is more common in the microhabitat providing the best colour match and that the morphs exhibit additional behavioural and morphological differences further improving crypsis in their respective microhabitats. We find little evidence for genetic differentiation between the morphs in either species. Therefore, we propose that these colour morphs represent a multi‐niche polymorphism as an adaptation to the highly heterogeneous habitat. Our study highlights how colour polymorphism (CP) can be advantageous in mosaic habitats and that Mediterranean wrasses are an ideal system to study trans‐species polymorphisms, i.e. polymorphisms maintained across several species, in adaptive radiations.

Habitat choice promotes and constrains phenotypic plasticity

Habitat choice can either speed up or slow rates of phenotypic evolution, depending on which trait is measured. We suggest that habitat choice plays an analogous, and generally overlooked, role in shaping patterns of phenotypic plasticity. Using our work with an amphibious fish, we discuss two case studies that demonstrate how habitat choice can both promote and constrain expression of plasticity. First, habitat choice during the dry season accentuates adaptive metabolic plasticity and minimizes maladaptive changes to muscle, ultimately increasing survival time out of water. Second, a trade-off between water- and air-breathing drives matching habitat choice, resulting in positive feedback that reinforces respiratory specialization and environmental preference. Overall, these case studies demonstrate that we must consider the interactions between plasticity and habitat choice to fully understand how animals survive in the face of environmental change. Without considering both processes simultaneously, the performance of animals in challenging conditions can be either under- or over-estimated. Finally, because habitat choice shapes the frequency and predictability of environmental changes that animals experience, feedback between habitat choice and expressions of phenotypic plasticity may be an important factor that influences how plasticity evolves.

Relationship between body colour and microhabitat breadth in an orb-web spider

Habitat use often differs among intraspecific individuals, and the degree to which individual animals use specific habitats, i.e. microhabitat breadth, can also vary. Variation in body colour sometimes emerges as dark vs. bright coloration, which can be related to habitat selectivity. The aim of this study was to examine whether darker animals prefer shady sites to avoid overheating from direct sunlight exposure, whereas brighter animals would use both shady and open sites. Orb-web spiders, Cyclosa argenteoalba, have a silver dorsal abdomen with black markings; the proportion of these black markings varies between 20 and 100% among individuals. In summer, there was less variation in the duration of direct sunlight hitting the webs of darker spiders compared with that of brighter spiders. This indicated a narrower microhabitat range for darker spiders, which preferred shady sites. This pattern was not observed in spring and autumn, when thermal conditions were less severe. These results are consistent with the hypothesis that when the temperature is high, darker animals are microhabitat specialists, whereas brighter animals are generalists. A previous study found that darker spiders capture more prey than brighter spiders, and the amount of black markings is considered to be a trade-off between foraging success and microhabitat availability.

A sit-and-wait predator, but not an active-pursuit predator, alters pollinator-mediated selection on floral traits

Indirect species interactions are ubiquitous in nature, often outnumbering direct species interactions. Yet despite evidence that indirect interactions have strong ecological effects, relatively little is known about whether they can shape adaptive evolution by altering the strength and/or direction of natural selection. We tested whether indirect interactions affect the strength and direction of pollinator-mediated selection on floral traits of the bumble-bee pollinated wildflower Lobelia siphilitica. We estimated the indirect effects of two pollinator predators with contrasting hunting modes: dragonflies (Aeshnidae and Corduliidae) and ambush bugs (Phymata americana, Reduviidae). Because dragonflies are active pursuit predators, we hypothesized that they would strengthen pollinator-mediated selection by weakening plant-pollinator interactions (i.e., a density-mediated indirect effect). In contrast, because ambush bugs are sit-and-wait predators, we hypothesized that they would weaken or reverse the direction of pollinator-mediated selection by altering pollinator foraging behavior (i.e., a trait-mediated indirect effect). Specifically, if ambush bugs hunt from plants with traits that attract pollinators (i.e., prey), then pollinators will spend less time visiting those plants, weakening or reversing the direction of selection on attractive floral traits. We did not find evidence that high dragonfly abundance strengthened selection on floral traits via a density-mediated indirect effect: neither pollen limitation (a proxy for the strength of plant-pollinator interactions) nor directional selection on floral traits of L. siphilitica differed significantly between high- and low-dragonfly abundance treatments. In contrast, we did find evidence that ambush bug presence affected selection on floral traits via a trait-mediated indirect effect: ambush bugs hunted from L. siphilitica plants with larger daily floral displays, reversing the direction of pollinator-mediated selection on daily display size. These results suggest that indirect species interactions have the potential to shape adaptive evolution by altering natural selection.

Experimental evidence that matching habitat choice drives local adaptation in a wild population

Matching habitat choice is a unique, flexible form of habitat choice based on self-assessment of local performance. This mechanism is thought to play an important role in adaptation and population persistence in variable environments. Nevertheless, the operation of matching habitat choice in natural populations remains to be unequivocally demonstrated. We investigated the association between body colour and substrate use by ground-perching grasshoppers (Sphingonotus azurescens) in an urban mosaic of dark and pale pavements, and then performed a colour manipulation experiment to test for matching habitat choice based on camouflage through background matching. Naturally, dark and pale grasshoppers occurred mostly on pavements that provided matching backgrounds. Colour-manipulated individuals recapitulated this pattern, such that black-painted and white-painted grasshoppers recaptured after the treatment aggregated together on the dark asphalt and pale pavement, respectively. Our study demonstrates that grasshoppers adjust their movement patterns to choose the substrate that confers an apparent improvement in camouflage given their individual-specific colour. More generally, our study provides unique experimental evidence of matching habitat choice as a driver of phenotype-environment correlations in natural populations and, furthermore, suggests that performance-based habitat choice might act as a mechanism of adaptation to changing environments, including human-modified (urban) landscapes.