Quantifying phenotype-environment matching in the protected Kerry spotted slug (Mollusca: Gastropoda) using digital photography: exposure to UV radiation determines cryptic colour morphs

Abstract

BackgroundAnimal colours and patterns commonly play a role in reducing detection by predators, social signalling or increasing survival in response to some other environmental pressure. Different colour morphs can evolve within populations exposed to different levels of predation or environmental stress and in some cases can arise within the lifetime of an individual as the result of phenotypic plasticity. Skin pigmentation is variable for many terrestrial slugs (Mollusca: Gastropoda), both between and within species. The Kerry spotted slug Geomalacus maculosus Allman, an EU protected species, exhibits two distinct phenotypes: brown individuals occur in forested habitats whereas black animals live in open habitats such as blanket bog. Both colour forms are spotted and each type strongly resembles the substrate of their habitat, suggesting that G. maculosus possesses camouflage.ResultsAnalysis of digital images of wild slugs demonstrated that each colour morph is strongly and positively correlated with the colour properties of the background in each habitat but not with the substrate of the alternative habitats, suggesting habitat-specific crypsis. Experiments were undertaken on laboratory-reared juvenile slugs to investigate whether ultraviolet (UV) radiation or diet could induce colour change. Exposure to UV radiation induced the black (bog) phenotype whereas slugs reared in darkness did not change colour. Diet had no effect on juvenile colouration. Examination of skin tissue from specimens exposed to either UV or dark treatments demonstrated that UV-exposed slugs had significantly higher concentrations of black pigment in their epithelium.ConclusionsThese results suggest that colour dimorphism in G. maculosus is an example of phenotypic plasticity which is explained by differential exposure to UV radiation. Each resulting colour morph provides incidental camouflage against the different coloured substrate of each habitat. This, to our knowledge, is the first documented example of colour change in response to UV radiation in a terrestrial mollusc. Pigmentation appears to be correlated with a number of behavioural traits in G. maculosus, and we suggest that understanding melanisation in other terrestrial molluscs may be useful in the study of pestiferous and invasive species. The implications of colour change for G. maculosus conservation are also discussed.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-017-0218-9) contains supplementary material, which is available to authorized users.

Terrestrial slugs (Mollusca: Gastropoda) share common anti‐predator defence mechanisms but their expression differs among species

“…Conglobation could be an adaptation to avoid predation by birds, as it is only expressed when the slugs are completely removed from their substrate and may, in combination with the secretion of slippery body mucus, render the slugs difficult for a bird to handle. Additionally, G. maculosus possesses background matching camouflage (O'Hanlon et al, 2017) and L. marginata also closely resembles the visual properties of tree trunks (i.e. it is brown with dark lateral bands and mottling) which is consistent with evolution by selection from visual predators such as birds.…”

“…Additionally, G . maculosus possesses background matching camouflage (O'Hanlon et al ., ) and L . marginata also closely resembles the visual properties of tree trunks (i.e.…”

Feeling sluggish: The extreme semislugs of Australia (Stylommatophora,Helicarionidae)

“…In extreme semislugs, as in slugs, species identification presents increased challenges due to the reduction of the shell and therefore the loss of many potentially useful diagnostic characters (Nitz, Heim, Schneppat, Hyman, & Haszprunar, 2009). Body color is often highly variable in slugs and can be influenced by environmental factors (O'Hanlon, Feeney, Dockery, & Gormally, 2017; Reise, 1997). Even where consistent differences are observed in live animals, colors fade very quickly in preservation, making preserved material difficult to identify by this means.…”

Background matching, disruptive coloration, and differential use of microhabitats in two neotropical grasshoppers with sexual dichromatism

“…In order to test the color background matching for females and males of the two grasshopper species, we performed major axis linear regressions between the color channels' reflectance of males and females of each species and their respective backgrounds. Because a high correlation and slopes near 1 between grasshoppers and their background RGB values, would denote background color matching (O'Hanlon, Feeney, Dockery, & Gormally, ), we compared if the slopes differed from 1 for those traits where the major axis regression was significant. We performed the slope comparations using a likelihood ratio test (details in Warton, Wright, Falster, & Westoby, ) in smart 3 R package (Warton, Duursma, Falster, & Taskinen, ).…”