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 F Figs 2 , 8A–D Type F – Dreyer et al. 2023a : figs 3, 5a, c, tables s1–s2. Material examined JAPAN – Okinawa , Sesoko I. , laboratory pier, 26°38ʹ09.4ʺ N , 127°51ʹ55.2ʺ E • 2 LSN ; 1991–2005 • 5 LSN , 1 of which molted to cyprid; 2018–2019 ( Tables 1 and S1 ). Description LAST-STAGE NAUPLIUS (LSN) . Lecithotrophic. Body ovate in dorso-ventral view; about 1.4 times as long as wide; lateral margins tapering gradually towards caudal end with no discontinuity at posterior end of cephalic shield. Dorsum evenly but strongly curved in lateral view, with trunk axis downturned 65° relative to cephalic axis. Length ca 300 µm (anteroventral view, without dorso-caudal spine), 310 µm in lateral view following curvature of body, greatest width 220 µm, greatest dorso-ventral thickness ca 70 µm. Labrum elevated, pentagonal, about as long as wide, with distinct and free posterior margin; other details unknown. Body narrowly rounded, terminating in short, conical dorso-caudal spine and pair of longer (25 µm), curved, robust furcal spines situated ventrally slightly forward from caudal end. CYPRID VIEWED THROUGH CUTICLE OF LSN. Body overall weakly brownish, but pigment most heavily concentrated in telson and in naupliar appendages. Central-dorsal gut-like tube devoid of distinct yolk granules except for some orange granules in trunk. Telson much shorter than wide, distinctly less than half as long as thorax. About six lipid vesicles present along anterior margin of cephalon and four along posterior margin of labrum. Identification and variation Easily recognizable by the combination of its distinctly downturned trunk region, short, conical dorso-caudal spine and robust but still unremarkably long, curved furcal spines positioned forward from the caudal end. The last feature makes this a ‘short-talon’ morphospecies in comparison to the next two described below. Fig. 8. Last-stage nauplii of three different morphospecies of y-larvae ( Facetotecta ) from Sesoko Island (Okinawa, Japan). A–D . Type F. E–G . Type G. H–J . Type H. Shown either in life (A–B, E–F, H–I) or as slide-mounted exuviae (C–D, G, J). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible. Distribution Japan (Sesoko Island, Okinawa ).