Taxonomic diversity of marine planktonic ‘ y-larvae’ (Crustacea: Facetotecta) from a coral reef hotspot locality (Japan, Okinawa), with a key to y-nauplii Author Olesen, Jørgen 6B569425-6BE7-4A73-B165-87E0C097715A Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, DK- 2100 Copenhagen Ø, Denmark. jolesen@snm.ku.dk&jolesen@snm.ku.dk&thecostracans@gmail.com Author Grygier, Mark J. 0A2FB16C-56B0-45E5-BB7B-61C3F0F7D46D National Museum of Marine Biology & Aquarium, 2, Houwan Rd., Checheng, Pingtung 944401, Taiwan. thecostracans@gmail.com&jolesen@snm.ku.dk&thecostracans@gmail.com text European Journal of Taxonomy 2024 2024-03-25 929 1 1 90 https://europeanjournaloftaxonomy.eu/index.php/ejt/article/download/2479/11077 journal article 10.5852/ejt.2024.929.2479 2118-9773 10876286 832192E7-A85A-4971-BA2F-D7420D299E8D Y-nauplius Type B Figs 2 , 7H–M Type B – Dreyer et al. 2023a : figs 2, 5a, c, tables s1–s2. Material examined JAPAN – Okinawa , Sesoko I. , laboratory pier, 26°38ʹ09.4ʺ N , 127°51ʹ55.2ʺ E • 1 LSN ; 1991–2005 • 3 LSN , 1 of which molted to cyprid; 2018–2019 ( Tables 1 and S1 ). Fig. 7. Last-stage nauplii of two different morphospecies of y-larvae ( Facetotecta ) from Sesoko Island (Okinawa, Japan). A–G . Type D* (“Big brown”). H–M . Type B. Shown either in life (A–B, H–J) or as slide-mounted exuviae (C–G, K–M). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible. Description LAST-STAGE NAUPLIUS (LSN) . Lecithotrophic. Body ovate in dorso-ventral view; about twice as long as wide; lateral margins tapering gradually towards caudal end with no discontinuity at posterior end of cephalic shield. Dorsum evenly curved in lateral view, with trunk axis downturned 50–55° relative to cephalic axis. Length 250–265 µm (without dorso-caudal spine), 275 µm in lateral view following body curvature; greatest width 160 µm and greatest dorso-ventral thickness 70–75 µm. Labrum weakly elevated, sub-quadrangular, approximately as long as wide; surface not divided into facets by cuticular ridges; pore pattern not examined in detail. Caudal end narrowly truncate, terminating in short (12 µm), triangular dorso-caudal spine positioned unusually far anteriorly on dorsal side and pair of longer (20 µm), narrow, sharply pointed, slightly upcurved and diverging furcal spines. CYPRID VIEWED THROUGH CUTICLE OF LSN. Body practically unpigmented, although naupliar appendages weakly brownish. Gut-like tube packed with orange-brown yolk granules. Telson small, wider than long, significantly shorter than half of thorax length. Identification and variation Easily recognizable by its unusually forward-positioned dorso-caudal spine, evenly curved (in lateral view) body and broadly triangular ventral trunk region. No distinct variation was apparent among the few examined specimens. Distribution Japan (Sesoko Island, Okinawa ).