2.1 Collections, rearing, imaging & digital sorting
About 11,000 y-larvae were collected with 75-µm-mesh plankton nets from surface waters (0-5m) at three sites: the University of the Ryukyus Sesoko Laboratory on Sesoko Island, Okinawa, Japan (JA) and Gongguan Harbor on Green Island and WangHaiXiang Fishing Harbor in Keelung, both in Taiwan (TA) (Olesen et al., 2022; Fig. 1; Table S1); 35 of these were sequenced and included in this work. Supplementary material came from Tioman Island (Malaysia; MAL), Piscinas do Pesquerio in Ponta Delgada, Azores, Portugal (AZ, n=10), and waters off the White Sea Marine Biological Station, Russia (RU; n=14) between 2017 and 2021. Larvae were sorted from plankton samples with glass Pasteur pipettes under an Olympus stereomicroscope and either (1) photographed and video-recorded live with a Nikon ECLIPSE 80i compound microscope (JA) or either an Olympus IX70 inverted compound microscope or a Zeiss AX10 light microscope (TA), both of the latter being equipped with Nomarsky (DIC) optics and a Canon EOS 5D Mark IV digital camera, or (2) bulk-fixed for subsequent molecular work (TA, RU, AZ). All images were sorted digitally to identify groups of morphologically similar specimens (i.e., morphotypes; Olesen et al., 2022), so that at least 2-3 specimens per group were used for Sanger sequencing.