DOI: 10.25338/b8c60z
title: Colour dimorphism in labrid fishes as an adaptation to life on coral reefs

description: 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.
description: 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: <strong>TableS1.xlsx</strong>). 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
 (<strong>deposited in the Dryad Digital Repository:
 https://doi.org/10.25338/B8GC91</strong>; 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 <strong>deposited
 in the Dryad Digital
 Repository: https://doi.org/10.25338/B8GC91</strong> (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: <strong>RandomSample_121TimeTrees_89spp.nex</strong>). We also compiled species-specific data on minimum, midpoint and maximum depths from IUCN species pages and FishBase (file: <strong>DepthData.csv</strong>). 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.  
description: <strong>Table S1.</strong> 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.
 <strong>RandomSample_121TimeTrees_89spp.nex.</strong> 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. <strong>DepthData.csv. </strong>Depths are reported in metres.