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

Abstract 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.

Different forms of balancing selection can maintain a CP such as negative frequency-dependent selection, heterozygote advantage or fitness variation in time and space (Gray & McKinnon, 2006;Jamie & Meier, 2020;Llaurens et al., 2017). Negative frequencydependent natural selection may be driven by predator avoidance or competition for mates or territories (Gray & McKinnon, 2006).
CP can reduce visual predation if predators form search images for the most common morph, leading to a fitness advantage of rare morphs (Gray & McKinnon, 2006;Karpestam et al., 2016;Maynard Smith, 1962;Sinervo & Svensson, 2002). Sexual selection may also be negative frequency-dependent with females preferring rare male morphs (O'Donald & Majerus, 1988), males being more aggressive to common male morphs or their own morph (Seehausen & Schluter, 2004), or polymorphisms linked to alternative mating strategies (Sinervo & Lively, 1996). Such CPs maintained by sexual selection or sexual conflict are typically limited to one sex (Ajuria Ibarra & Reader, 2014;Kingston et al., 2003;Outomuro et al., 2014).
Alternatively, CPs can be maintained due to heterozygote advantage (Gemmell & Slate, 2006;Kellenberger et al., 2019) or if contrasting microhabitats favour different morphs (multi-niche polymorphisms, Gray & McKinnon, 2006;Maynard Smith, 1962;Rueffler et al., 2006;Skulason & Smith, 1995). In such multi-niche polymorphisms, balancing selection arises from competition within each niche, leading to increased fitness of a morph that is rarer relative to the carrying capacity of the niche in which it is favoured.
Distinct colour morphs are not necessarily genetically determined, but may be induced by environmental cues or depend on the developmental stage of an organism (so-called polyphenisms, Mayr, 1963;Pfenning et al., 2010). In such cases, there is no balancing selection. However, as plasticity is expected to be costly, the alleles underlying a polyphenism should only persist if the ability to express alternative phenotypes is advantageous (Scheiner, 1993).
In many cases, colour morphs also differ in other traits such as life history, behavioural or morphological traits (Gray & McKinnon, 2006;Svensson, 2017). A nearly perfect match between colour morphs and associated traits may exist due to plastic responses of some or all traits to the same environmental cues, pleiotropic effects of a single locus, or a supergene with multiple tightly linked genes (Gray & McKinnon, 2006;Jamie & Meier, 2020;Svensson, 2017).
Alternatively, the association can be due to the presence of closely related taxa in a species radiation, allowing the (re-)introduction of advantageous alleles underlying the morphs via gene flow (Jamie & Meier, 2020). In order to understand the relationship between intraspecific polymorphisms and adaptive radiation in a specific study system, it is important to test if the polymorphism represents an ancestral condition, if it evolved independently in several species or if it has been transferred between species by introgression. This requires a robust phylogeny, information of the presence or absence of a CP in each species and tests for introgression.
Wrasses (family Labridae) provide an ideal opportunity to investigate the complex interaction of ultimate drivers and proximate mechanisms maintaining a CP. In the Mediterranean Sea, wrasses of the tribe Labrini have undergone two species radiations in the genera Symphodus and Labrus (Hanel et al., 2002), with several species in each radiation displaying two distinct morphs of either brown or green colouration (Louisy et al., 2015). All polymorphic wrasses are found in the Mediterranean Sea, whereas Atlantic wrasses do not harbour colour morphs. The colour morphs may thus have evolved as an adaptation to the habitat specific to the Mediterranean Sea.
Due to the absence of strong tidal action, the shallow parts of the Mediterranean Sea are covered by mosaics of Neptune grass (Posidonia oceanica) patches and rocky substrate. Little is known about the selective forces or the genetic mechanisms underlying the colour morphs in Mediterranean wrasses (Zupo & Stuebing, 2010).

Recent research focusing on the ballan wrasse (Labrus bergylta)
revealed genome-wide differentiation between sympatric plain and spotted morphs in some parts of the distribution range, consistent with early stage speciation (Casas et al., 2021;Quintela et al., 2016).
Although no study has investigated genomic differences between green and brown morphs in wrasses so far, aquarium experiments by Arigoni et al. (2002) showed that the CP in Symphodus ocellatus and Symphodus roissali is plastic to some extent with colour changes within several weeks to months, but not plastic in Symphodus rostratus. Intermediately coloured individuals exist within several of these polymorphic species, but it is unclear whether they represent a transitional colour stage (as a result of plasticity) or if they are in fact genetically intermediate.
Here, we first assess the phylogenomic relationships of Labrini species. Next, focusing on the two most common Symphodus species, S. roissali and S. rostratus, we ask whether green and brown colour morphs differ in traits other than colouration and what evolutionary mechanisms are likely to maintain the polymorphism. We find that the colour-polymorphic Symphodus species are not each other's closest relatives indicating repeated origins or losses of the CP, whereas in Labrus, the three polymorphic species form a monophyletic clade with possibly ancestral CP. In both focal species, we find that the green morph is tightly associated with P. oceanica, and the brown one with rocks covered in brown algae (Cystoseira spp.) forests. Furthermore, the morphs also differ in morphology, aggression and anti-predator behaviours, all of which are consistent with adaptation to contrasting selection pressures in the two microhabitats. We find no association of colouration with sex or age in either species, and morphs show little, if any, genome-wide differentiation.
Our findings are thus consistent with maintenance of the CP as a multi-niche polymorphism that is an adaptation to the highly heterogeneous mosaic habitat of rocky shores in the Mediterranean Sea.
The many colour-polymorphic species in both Labrini radiations are thus a great study system to investigate the role of polymorphisms in adaptive radiations.
2 | ME THODS 2.1 | Phylogenetic tree based on genome-wide data 2.1.1 | RAD sequencing To infer the phylogenetic relationships of Labrini species, we obtained 101 samples covering 21 of the 24 known species (see Tables S1 and S2). For our focal species, we collected a total of 41 S. rostratus and 12 S. roissali samples for population genomic analyses (Table S3). DNA was extracted from fin clips or muscle tissues stored in 100% ethanol or RNA later using a phenol chloroform protocol (Sambrook & Russell, 2001). We prepared six restriction-site associated DNA (RAD) libraries following Baird et al. (2008) with some modifications. We used 400 ng of genomic DNA per sample. Restriction digestion was performed overnight using the restriction endonuclease High-Fidelity (HF) SbfI (New England Biolabs). We multiplexed about 35 individuals per library after the ligation step using TruSeq P1 adapters and custom barcodes of 5-8 bp length. The libraries were sheared for 2 min on a COVARIS M220 Focused-ultrasonicator. In the first size selection step, sheared fragments between 300 and 700 bp were size selected on a SageELF machine and in a second size selection step by using Agencourt AMPure XP beads. All libraries were singleend sequenced on a single Illumina HiSeq 2500 lane at the Centre of Integrative Genomics (CIG), University of Lausanne. We added 12.5% bacteriophage PhiX genomic DNA to each library to increase sequence complexity.
The reads were demultiplexed and trimmed to 90 bp length using the process_radtags script from Stacks (Catchen et al., 2013), correcting single errors in the barcode and the restriction site, and discarding reads with incomplete restriction sites. Using the FastX-toolkit (http://hanno nlab.cshl.edu/fastx_toolk it/index.html by Hannon Lab), all reads with a Phred quality score below 10, and reads with more than 5% of bases with a quality score below 30 were removed. The reads of each individual were aligned to the Symphodus melops reference genome of Mattingsdal et al. (2018) consisting of 5,040 contigs (N50 = 461 652 bp) and a total size of 614 Mbp. Alignment was performed end-to-end with Bowtie2 (Langmead & Salzberg, 2012) using default parameters. Base score recalibration was performed using PhiX reads, as described in  (Danecek et al., 2011). Single nucleotide polymorphisms (SNPs) were required to have a minimum quality value of 30, and genotypes were required to have a minimum quality value of 30, and a minimum sequencing depth of 10 reads. Indels, sites within 5 bp of an indel and multiallelic SNPs were removed. Additionally, we used VCFtools to remove sites with more than 25% missing data, as well as sites with a mean genotype depth greater than median ×1.5 (>83×), as these are expected to be enriched for paralogs. For the VCF file containing the genomic dataset of all species used for the phylogenetic reconstruction, monomorphic sites were kept, whereas for all other analyses, only SNPs were used.

| Phylogenetic tree reconstruction
The genomic dataset, including monomorphic sites, of 21 species was converted into a phylip file with a custom script (https://github. com/joana m/scrip ts/raw/maste r/vcf2p hylip.py). We estimated the best substitution model using ModelFinder implemented in IQ-TREE v2 (Minh et al., 2020). Next, we conducted a maximum-likelihood tree inference with 1000 rapid bootstraps with the best-fit substitution model according to BIC (K3Pu+F+R4) in IQ-TREE. To obtain a balanced tree, we used only the five individuals with least missing data each of S. roissali and S. rostratus. The tree was subsequently mid-point rooted using Figtree (http://tree.bio.ed.ac.uk/softw are/ figtr ee/).

| Microhabitat association
To assess if colour morphs are associated with different microhabitats, we performed observational studies of our two focal species, S. roissali and S. rostratus, in Elba and Corsica in summer and autumn of 2017, respectively (Table S1). Our data encompass observations of a total of 86 individuals of S. roissali (75 on Elba and 11 on Corsica) and 23 individuals of S. rostratus (13 on Elba and 10 on Corsica, Table   S3). Sampling locations and sampling effort per species and site are shown in Figure 1. Standard length of individuals was estimated with an underwater ruler on site, or, if video footage was analysed (Corsica, 2017), from a reference in their environment, such as the width of a fully grown Neptune grass leaf. To avoid pseudo-replication, we considered individual scale patterns, marks or scars to distinguish between individuals of the same colour morph. Colour morphs were assigned qualitatively as green, brown or intermediate in colouration by SNF and OS independently. Where available, pictures or video footage was used to confirm this classification (photo/video confirmation available for 77% of all S. roissali and for 87% of all S. rostratus observations) and the final classification was agreed upon by both SNF and OS. The fish were classified into green, if fins and most of the body patterning were green, whereas they were classified into brown when the patterning was mainly brown/greyish and the fins orange/yellow. Intermediates were difficult to assign due to high variability. If some of the patterning was olive/brown and the fins green, the fish was assigned as intermediate.  as Fauteux et al. (2012). To evaluate the model and relative importance of each independent variable (i.e., the contribution of each factor to the percent variation in our data explained), an analysis of variance (ANOVA) type II was performed (using the "drop1" function of the stats package in R), testing subsequently for one main effect after the other under F I G U R E 1 Sampling sites and morph frequencies among fish with behavioural data. The map shows the sampling sites on Elba (site 1) and Corsica (sites 2-5) and the proportion of green, brown and intermediate individuals for which we obtained behavioural data in Symphodus rostratus (a) and Symphodus roissali (b). The number of fish observed is reflected in the size of the pie charts. Drawings by SNF

| Interactions
For the more common species, S. roissali, we also tested for differences in behavioural interactions between the brown and green morphs in summer 2017 in Elba (Table S1). We recorded behaviours during all observed encounters with other wrasse individuals.
Interactions were classified as either conspecific or heterospecific, and the nature of the behaviour was categorized into aggressive (chasing, biting, hitting) and non-aggressive behaviour (swimming close-by, feeding nearby) (Tables S5 and S6

| Morphological differences between colour morphs
We tested for differences in standard length between colour morphs (Table S4) (Table S1) to check for potential sex-associated colour differences.
Next, we assessed body shape variation between the morphs in both species. Following Claverie and Wainwright (2014), we computed elongation (body depth divided by standard length).
We set 10 digital landmarks ( Figure S2) on images of fish collected but mostly released again between 2011 and spring 2018 in Calvi, Corsica (Table S1) from further analysis because they were too few to be analysed on their own. Images were converted to tps files by using tpsUtil (32bit version) and landmarks were placed in tpsDig2 (32-bit version) (software by Rohlf, 2015). The measurement between the tip of the premaxillary and the hypural bones (commonly used to infer standard length) was affected by bending of the tail. Therefore, we used the beginning of the caudal peduncle (middle position between the end of the dorsal and anal fin base) instead of the hypural bones as the caudal point ( Figure S2). The distance between landmarks (2) and (7) (body depth at the first spine of the dorsal fin) was used to infer body depth.
To calculate elongation, the tps file was loaded into R, scaled for size, and linear measurements were extracted between landmarks (1) and (4) for standard length and between landmarks (2) and (7) for body depth. Body depth was divided by standard length to obtain elongation. Landmarks other than 1/4 and 2/7 were used to correct for potential body displacements (e.g., mouth opening, bending). We calculated a linear model to test for differences in elongation between colour morphs (lm(Elongation ~ Colour, data = Morpho_data)). Next, we tested for genome-wide differentiation between colour morphs within S. rostratus (we had too few green S. roissali in our dataset to perform the same test for this species). F ST between green and brown morphs was estimated with Arlequin version 3.5 (Excoffier & Lischer, 2010) accounting for intra-individual variation and allowing for maximum 20% missing data per site. We tested for significance with 100 permutations at a significance level of 5%. The matrix of genetic distances with F ST estimates per site was plotted using a sliding window approach with a minimum of five SNPs per window and a window step size of two SNPs.

| Hybridization analyses
To test for gene flow between species of the Symphodus radiation, we tested for excess allele sharing with the f-branch statistic (Malinsky et al., 2021). We pruned the phylogeny to one individual per spe-    analyses repeated only with those individuals for which photos or video footage was available corroborated the microhabitat association differences between morphs found with the full dataset (Results S1).

| Inter-and intraspecific interactions of S. roissali
Among the observed individuals of green and brown S. roissali, we  (Tables S5 and S6). However, the proportion of interactions classified as aggressive was higher in the green morph than in the brown morph (linear model, p = 0.00011, Figure 4).

Symphodus roissali
During our field observations, we noticed morph-specific preda-

Symphodus rostratus
The green morph of S. rostratus displayed a "vertical hovering" be-

| Morphological data
We found female and male individuals among both colour morphs in S. rostratus, as depicted in Figure S3A, with a total of eight females (one green, one intermediate and six brown) and ten males (four green, one intermediate and five brown) among our preserved samples from Corsica (spring 2018). Due to lack of green individuals in S. roissali in our preserved collection from spring 2018, and the difficulty of sexing fish reliably from photos, we were unable to confirm both sexes in the green morph of this species. However, we were able to confirm brown individuals of both sexes ( Figure S3B).

| Standard length
The mean standard length of green and intermediately col-

| Body elongation
The linear model revealed that the green and brown morphs of S. rostratus differed in body elongation (p < 0.001). The brown morph generally had a deeper body compared to the green morph ( Figure 5d, Table S7). In contrast, we found no significant difference in elongation between the morphs of S. roissali (p = 0.85) (Figure 5c, Table S8).

| Genomic differentiation between colour morphs of S. rostratus and S. roissali
The genomic dataset of S. roissali consisted of 21 278 SNPs including three green and eight brown individuals. In the genomic PCA, there is no separation by colour morph ( Figure S4A). For S. rostratus, the genomic dataset consisted of 11 973 SNPs with eight green, six intermediates and 15 brown individuals. The PCA does not separate the individuals by colour morph ( Figure S4B). For S. rostratus, the overall weighted F ST between S. rostratus colour morphs was not different from 0 (F ST = 0.0000081, p = 0.52). When computing F ST values in windows of 5 SNPs shifting by 2, we discovered a single window exceeding F ST estimates of permuted datasets (highest permuted F ST = 0.23). This outlier window has an F ST estimate of 0.25 and is located on scaffold SYMME_00023611 highlighted with a red arrow in Figure S5. In S. roissali, we did not have enough wellsequenced individuals to test for genomic differentiation between morphs. Similarly, in the Labrus radiation, we find evidence for excess allele sharing between L. Merula with L. Bergylta and with L. Mixtus ( Figure S7, Table S9). In addition, we find evidence for excess allele sharing deep in the phylogeny ( Figure S7, Table S9). The Symphodus radiation shows excess allele sharing with T. adspersus and with the Labrus radiation. The different radiation members show the same excess allele sharing patterns, indicating that gene flow affected the base of the radiations.

| Phylogeny of Labrini suggests colour polymorphism is phylogenetically widespread
To our knowledge, our phylogenomic tree is the most robust and complete phylogenetic reconstruction of the Labrini tribe including 21 of 24 described species. High bootstrap values for all nodes confirm that there are two separate radiations in the Labrini tribe. The CP is found in multiple species across the radiation, implying that multiple species either have standing variation for alleles underlying different morphs or the genetic basis for plasticity in colouration.
The polymorphic Labrus species form a single monophyletic clade suggesting a polymorphic ancestor in that clade. In contrast, the polymorphic Symphodus species are dispersed within the Symphodus clade. It is possible that the CP is also ancestral in the Given that the colour polymorphism (CP) is found in multiple species in the two adaptive radiations but not in any of the genera that have not radiated, the CP could also play a role in adaptive radiation. The CP could lead to increased speciation rates if the colour morphs represent precursors of incipient species or increase population size by widening the niche available to colour-polymorphic species (Gray & McKinnon, 2006;Hugall & Stuart-Fox, 2012;Jamie & Meier, 2020). As the genera Symphodus and Labrus represent two young radiations (3.5 Mya and 135 000 years, respectively, Hanel et al., 2002) with sympatric species that show evidence of current or past hybridization ( Figures S6 and S7, Table S9), it is also possible that the presence of closely related species in the adaptive radiations facilitates the maintenance of CP (Jamie & Meier, 2020). We find that all colour-morphic Symphodus species show evidence of F I G U R E 5 Standard length and body elongation in Symphodus roissali and Symphodus rostratus morphs. (a) The brown morph of Symphodus roissali with a mean standard length of 7.7 cm (±1.22) differs in size from the green morph with a mean standard length of 6.72 cm (±1.27). Most of the observed fishes range between 6 and 9 cm, which is considered a mature size. Green and intermediately coloured individuals each contain a single outlier of 3 cm standard length, which likely represent subadults. (b) There is no significant size difference between morphs of Symphodus rostratus (with mean standard lengths of 9  Figure S7).
Therefore, it is possible that if an advantageous morph evolves in one species, the underlying allele might spread across multiple species via gene flow, or if a beneficial allele is lost in one species, it might be reintroduced via gene flow from another species. Given that we find evidence for gene flow between the Symphodus and the Labrus radiation, it is possible that they share highly similar colour morphs due to introgression. Whole genome sequencing would be required to identify the genes underlying the CP in the different species and to test if the alleles share a common descent or have introgressed between species.

| Contrasting microhabitat association of colour morphs suggests background matching
In both species, colour is the best predictor for microhabitat association, whereby the green morph shows strong association with P. oceanica, and the brown morph with rocky substrate. This finding is consistent with theory predicting that in heterogeneous environments (i.e., a patchy mosaic of microhabitats), differently coloured individuals should also behaviourally choose backgrounds that best match their appearance (Michalis et al., 2017;Svensson, 2017). In S. roissali and S. rostratus, the colour polymorphism and the background-matching behaviour may represent adaptations to reduce predation in both microhabitats (Bond, 2007;Duarte et al., 2016;Gray & McKinnon, 2006;Michalis et al., 2017).

| Species-specific morph differences point to alternative strategies to enhance crypsis
The morphs do not only differ in colour but also in morphology and behaviour. In S. roissali, we found that green and intermediately coloured individuals are smaller than brown ones. Even though an association between colour and size could in principle indicate ontogenetic colour changes (Duarte et al., 2016;Sanmartín-Villar et al., 2016;Stevens et al., 2014;Wilson et al., 2007) Posidonia leaf and may also reduce detection by visual predators. As high body depth is associated with high predation pressures in other fish species (Domenici et al., 2007), the deeper body of the brown morph could be associated with higher pressure of gape-limited predators over rocky substrate.
In conclusion, despite harbouring a similar CP, S. roissali and S. rostratus morphs display different behaviours and morphological differences that may represent alternative strategies to escape visual predation by enhancing crypsis in their preferred microhabitats.

| Morph-specific territorial behaviour
While green individuals of S. roissali frequently displayed aggressive behaviour to both conspecifics and heterospecifics, brown individuals showed little aggression, suggesting that the green morph is more aggressive than the brown morph. Considering that Posidonia is more patchily distributed and less abundant (i.e., lower in connectivity and availability) than rocky substrate in the depth range of S. roissali, the microhabitat may qualify as a limited resource. The higher aggression observed in the green morph is thus in line with findings linking territorial behaviour to the defence of a critical resource (Mayr & Berger, 1992) which functions as a protective cover that reduces the risk of visual predation (Johnsson et al., 2004).

| Multivariate differences between morphs contrast with absence of genomic differentiation
Given the multivariate differences between morphs, one might expect the evolution of assortative mating preventing mismatched combinations of colour, behaviour and morphology. However, we find no evidence of genome-wide differentiation between our eight green and 15 brown individuals in S. rostratus. Given that this result is based on 21,278 SNPs, we should have enough power to detect genome-wide differentiation, unless it is very weak Willing et al., 2012). Also in S. roissali, we find no clustering by colour in the PCA in line with random mating. However, weak differentiation between morphs in this species could remain undetected due to insufficient sample size (three green and eight brown individuals).
It is possible that some of the colour, morphological and behavioural traits are plastic and respond to the same environmental or developmental cue, preventing mismatched combinations. Such interaction of plastic colouration and habitat choice or additional traits has been observed, for instance, in the shrimp Hippolyte obliquimanus (Duarte et al., 2016) or in the ambush bug (Phymata americana, Boyle & Start, 2020). While Arigoni et al. (2002) showed that colour can change plastically in S. roisalli and S. ocellatus within weeks to months, they found no plasticity in colouration in S. rostratus. Common garden experiments would be required to identify the extent of plasticity of the different traits associated with each colour morph.
If more than one trait is genetically determined, genetic linkage must somehow be maintained among the genes coding for the many traits that differ between the morphs. If colour and the associated morphological and behavioural traits are controlled by different genes on multiple chromosomes, random segregation of chromosomes is expected to produce disadvantageous trait combinations (e.g., green individual flipping to its side over brown rocks when facing a predator) (Funk et al., 2021;Sinervo & Svensson, 2002;Svennsson, 2017). Physical linkage of the genes underlying the different traits, such as in a chromosomal inversion or a supergene, could prevent a mismatch between genetically determined traits once the variants have accumulated in a supergene region (Thompson & Jiggins, 2014). Only a single window of five SNPs showed a significantly elevated F ST estimate between colour morphs in S. rostratus, but a relatively small supergene could remain undetected with sparse RAD sequencing and our relatively low number of individuals. In S. roissali, we cannot test for genomic regions of high differentiation due to too low sample size of green individuals that we obtained. Alternatively, there could be a single pleiotropic gene that affects colour, morphology and behaviour. Pleiotropy affecting such diverse traits may seem unlikely, but a comparable scenario has been found in threespine sticklebacks, where the Eda gene affects body armour and schooling behaviour (Greenwood et al., 2016).

| CON CLUS IONS
The green and brown morphs shared across multiple species in the Symphodus and Labrus radiations of Mediterranean wrasses likely represent a multi-niche polymorphism ( Figure 6) as an adaptation to the mosaic habitat of rocky substrate and Posidonia sea grass patches that is widespread in the Mediterranean Sea.
These wrasses are thus an ideal system to study the role of polymorphisms in mosaic habitats and how they interact with adaptive radiation.

ACK N OWLED G EM ENTS
We would like to thank Salome Mwaiko, Marcel Häsler and Anna Feller for their assistance with lab work, and Elio Herzog for assistance during field work on Elba. Furthermore, we thank the HYDRA Institute for

CONFLICT OF INTEREST
The authors have no conflict of interest to declare.

AUTH O R CO NTR I B UTI O N S
OS, JIM and SNF conceived the study with suggestions from HKK and SHA. OS, SNF and SG performed behavioural observations. HKK, SHA, SNF and SG collected samples. SNF carried out the F I G U R E 6 Summary of the multi-niche polymorphism in Symphodus roissali and Symphodus rostratus. Green and brown morphs of both species likely occupy alternative fitness peaks, Posidonia oceanica and rocky substrate, respectively. (a) In Symphodus roissali, the small, green morph hiding in Posidonia contrasts with the larger brown morph, exhibiting lateral tilting display as anti-predator behaviour. (b) In Symphodus rostratus, the green morph is more elongated and has been observed swimming nose-down in Posidonia, whereas the brown morph is more deep-bodied. Drawings by SNF lab work. SNF and JIM analysed the data. JIM and SNF wrote the first draft of the manuscript and OS, HKK and SHA contributed to writing.

PEER R E V I E W
The peer review history for this article is available at https://publo ns.com/publo n/10.1111/jeb.13999.

DATA AVA I L A B I L I T Y S TAT E M E N T
RAD sequencing data are available on the National Center for