Table S1. Samples used in the phylogenetic tree, including site location (if known), sequence number used in present study, species name, source of data and GenBank number. Outgroup species and their GenBank numbers are also provided. – indicates no information or not applicable.

Site Location Sequence numbers Species Source Genbank numbers

Seychelles Sainte Anne SY1-SY06, SY14, SY16-SY17 Labroides JCU KC151242- KC151250 Island dimidiatus Cocos-Keeling - CK21-CK22, CK24-CK26 JCU KC151134- KC151138 Island Heron Island Wistari Reef HIW27, HIW29-HIW36 Labroides JCU KC151163- KC151171 dimidiatus Heron Island Harry’s Bommie HIH47-HIH56 Labroides JCU KC151153- KC151162 dimidiatus PNG Kimbe Bay PNG68-PNG76, PNG78 Labroides JCU KC151228- KC151237 dimidiatus Japan Murote Island JP89-JP98, JP102-JP103, Labroides Present study KC151172- KC151184 JP107 dimidiatus Heron Island - HI109-HI111, HI113, HI115- Labroides Present study KC151139- KC151152 HI116, HI119-HI126 dimidiatus Lizard Island - LI127-LI143, LI145-LI146 Labroides Present study KC151189- KC151207 dimidiatus Maldives - ML147-ML149, ML151, Labroides Present study KC151212- KC151217 ML154, ML158, ML161, dimidiatus ML165 Christmas - CI181-CI187 Labroides Present study KC151127- KC151133 Island dimidiatus Indonesia Krakatau IN195-IN196 Labroides Drew et al. (2008) EU256902-EU256903 dimidiatus West Papua Teluk WP197-WP208 Labroides Drew et al. (2008) EU256890-EU256901 Cenderwasih dimidiatus Fiji Yasawas FI209 Labroides Drew et al. (2008) EU256875, EU256889 dimidiatus Fiji Viti Levu FI210-FI213 Labroides Drew et al. (2008) EU256885-EU256888 dimidiatus Fiji Taveuni FI214-FI215 Labroides Drew et al. (2008) EU256883- EU256884 dimidiatus Fiji Vanua Levu FI216-FI220, FI222 Labroides Drew et al. (2008) EU256876- EU256882 dimidiatus Red Sea Thuwal RS224- RS227 Labroides Present study KC151238- KC151241 dimidiatus PNG Kimbe Bay PNG228-PNG237 Labroides Present study KC151218- KC151227 dimidiatus Christmas - LP01-LP04 Labroides pectoralis Present Study KC151208- KC151211 Island Seychelles Sainte Anne LB01-LB02, LB09-LB10 Labroides bicolor JCU KC151185- KC151188 Island - - - Coris julis Genbank HQ917553, HQ917564- HQ917566, HQ917568, HQ917570, HQ917572, HQ917576, HQ917585, HQ917590- HQ917591, HQ917600, HQ917606, HQ917613 - - - Halichoeres Genbank HM043314, HM043324, bivittatus HM043328, HM043337, HM043343-HM043347, HM043358, HM043363, HM043384-HM043385 - - - Thalassoma Genbank DQ913907, DQ913910, bifasciatum DQ913912, DQ913973, DQ913986, DQ914008, DQ914020, DQ914023, DQ914029, DQ914036 - - - Thalassoma Genbank AY329799-AY329800 sanctaehelenae - - - Thalassoma Genbank AY329671 ascensionis - - - Thalassoma pavo Genbank AY329766, AY329779

Table S2: Loading values for the first three principle component (PC) scores of the principle component analysis (PCA). Italicised values are the original variables that contributed the most variance to the three PC scores, which are also shown in the PCA scatterplots as arrows (see Fig. 2a, b). Only the Peduncle Top width measure is not well represented in the variance of the three PC scores.

Original variable PC1 PC2 PC3 Head Top 0.04195880 -0.10186511 0.435905108 Head Bottom -0.18266640 -0.49638823 -0.260256762 Head Stripe 0.05199123 0.28941570 -0.419213126 Anal Fin Top 0.08773735 -0.07626424 0.471021551 Anal Fin Bottom -0.67847059 -0.38227559 -0.220998630 Anal Fin Stripe 0.24968633 0.27093190 -0.503161278 Peduncle Top 0.03922514 0.02099529 0.001177751 Peduncle Bottom -0.51415470 0.42025367 0.055914844 Peduncle Stripe 0.40732326 -0.50700284 -0.198777980