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 U* Figs 2 , 12A–D Type U* – Dreyer et al. 2023a : figs 4, 5a, c, tables s1–s2. — Olesen et al. 2024 : fig. 2f–g. Material examined JAPAN – Okinawa , Sesoko I. , laboratory pier, 26°38ʹ09.4ʺ N , 127°51ʹ55.2ʺ E • 1 LSN ; 1991–2005 • 1 LSN ; 2018–2019 ( Tables 1 and S1 ). Description LAST-STAGE NAUPLIUS (LSN) . Lecithotrophic. Body widely ovate in dorso-ventral view; about 1.4 times as long as wide; cephalic shield nearly circular, with minor discontinuity in body outline leading into trunk. In lateral view, longitudinal axes of cephalon, trunk and dorso-caudal spine all in approximately same plane. Length 460 µm (ventral view in life, without dorso-caudal spine), greatest width ca 330 µm, greatest dorso-ventral thickness ca 30 µm. Labrum shorter than wide, a nearly rectangular low elevation weakly divided into facets by cuticular ridges; bearing four pores, two in midline posteriorly and pair situated outside of left and right ridge-defined labral margins. Lateral margins of trunk bearing spine rows. Caudal end obtuse, terminating in 75 µm long dorso-caudal spine with at least three annular ridges bearing several subsidiary spines, accompanied ventrally and slightly forward of its base by pair of tiny, vestigial furcal spines. CYPRID VIEWED THROUGH CUTICLE OF LSN. Occupying only central part of LSN near midline, leaving large fraction of outer portions of disc ‘empty’. Lacking distinct pigmentation, except for some small dark pigment spots in telson. Yolk granules of gut-like central tube mostly concentrated in telson. Lipid vesicles present anteriorly and laterally in cephalon. Telson about as long as wide and about half as long as thorax. Identification and variation Easily recogniZable by its flat ‘disc-shaped’ form (the broadest of the lecithotrophic y-nauplii encountered at Sesoko Island) and the central position of the developing cyprid within. Distribution Japan (Sesoko Island, Okinawa ); Taiwan ( Keelung ). Remarks Itô (1987b : figs 1–3) described three forms of wide, flat y-nauplii ( Types VIIIa-c) from Tanabe Bay ( Japan : Wakayama Prefecture ) that are the closest described forms in the literature to Type U* from Okinawa . Although they differ from Type U* in having antero-lateral indentations in the body margin, they might be earlier instars of either Type U* or something closely related to it. Because detailed morphological and molecular comparison with Itô’s material has not been done, we do not treat any of these forms as synonymous here.