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Colour dimorphism in labrid fishes as an adaptation to life on coral reefs

Citation

Hodge, Jennifer; Santini, Francesco; Wainwright, Peter (2020), Colour dimorphism in labrid fishes as an adaptation to life on coral reefs, Dryad, Dataset, https://doi.org/10.25338/B8C60Z

Abstract

Conspicuous colouration displayed by animals that express sexual colour dimorphism is generally explained as an adaptation to sexual selection, yet the interactions and relative effects of selective forces influencing colour dimorphism are largely unknown. Qualitatively, colour dimorphism appears more pronounced in marine fishes that live on coral reefs where traits associated with strong sexual selection are purportedly more common. Using phylogenetic comparative analysis, we show that wrasses and parrotfishes exclusive to coral reefs are the most colour-dimorphic, but surprisingly, the effect of habitat is not influenced by traits associated with strong sexual selection. Rather, habitat-specific selective forces, including clear water and structural refuge, promote the evolution of pronounced colour dimorphism that manifests colours less likely to be displayed in other habitats. Our results demonstrate that environmental context ultimately determines the evolution of conspicuous colouration in colour-dimorphic labrid fishes, despite other influential selective forces.

Methods

We quantified colour dimorphism by scoring seven different body and fin regions as either primarily the same or different in colour between initial and terminal phase fishes using photographs from online repositories and scholarly identification guides (file: TableS1.xlsx). Specifically, we scored the head, flank, and the pectoral, pelvic, dorsal, anal, and caudal fins, as either primarily the same or different in colour between photographs of living and preserved initial and terminal phase fishes. When available, multiple photographs of each species and phase were scored. When it was unclear whether a region was primarily the same or different in colour no score was recorded for that region. This produced a binary matrix that we reduced to a primary axis of colour-dimorphic variation using logistic principal component analysis implemented in the R package logisticPCA. We used cross validation to determine the value used to approximate the natural parameters from the saturated model and we reduced the matrix to two dimensions. We considered the first principal component as the primary axis of variation in colour dimorphism (see Table S2 for the variable loadings) and performed all further analyses on the resultant PC1 scores.

We also compiled data on sex allocation, mating system and habitat association from the literature and online repositories (see Table 1 for definitions of each character state). We classified species as either coral reef exclusive, coral reef associated, or non-coral reef based on occurrence data catalogued in FishBase and the IUCN Red List of Threatened Species. We used an existing dataset of species-specific sex allocation and mating systems (deposited in the Dryad Digital Repository: https://doi.org/10.25338/B8GC91; Hodge et al. 2020). Classifications focused on the mating systems reported for terminal phase males and did not consider the mating strategies of initial phase males. Sex allocation data were restricted to accounts of protogyny that were distinguishable as either monandric or diandric based on gonad histology, population demographics or both. The complete dataset includes 89 labrid species.

The sample of 1000 phylogenetic trees used in the trait correlation analyses have already been deposited in the Dryad Digital Repository: https://doi.org/10.25338/B8GC91 (Hodge et al. 2020). The random sub-sample of 121 trees (drawn from the set of 1,000 trees) that were used for stochastic character mapping are provided here (file: RandomSample_121TimeTrees_89spp.nex).

We also compiled species-specific data on minimum, midpoint and maximum depths from IUCN species pages and FishBase (file: DepthData.csv). For each species, the shallowest minimum depth and deepest maximum depth values were used; substituted by the most common depths when available. The midpoint was calculated as half of the depth range plus the minimum depth.  

Usage Notes

Table S1. Trait data, colour dimorphism, and colour data by species. For colour dimorphism, '0' indicates colour similarity and '1' indicates colour difference between initial and terminal phase fish. The 'Proportion colour dimorphic' column summarises colour dimorphism data by indicating the proportion of body regions that were scored colour dimorphic relative to the total number of body regions considered. For colour group expression, '0' indicates absence and '1' indicates expression of the colour group by initial or terminal phase fish.

RandomSample_121TimeTrees_89spp.nex. The first tree in this file is the maximum clade credibility tree. The 58th tree in this file was excluded from the OUwie analyses due to issues with the topology, branch lengths and/or character mapping.

DepthData.csv. Depths are reported in metres.

Funding

National Science Foundation, Award: DBI-1523934

National Science Foundation, Award: DEB-0717009

National Science Foundation, Award: DEB-1061981