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