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 X Figs 2 , 13A–E Type X – Dreyer et al. 2023a : figs 4, 5a, c, tables s1–s2. — Olesen et al. 2024 : fig. 3d–e. En levende nauplius-y – Høeg 2005: 18 , unnumbered LM photograph. Not Type X – Itô 1987a: 913 , 916–917, fig. 2a. — Kikuchi et al . 1991: 69 , 72–73, tables 1, 3, fig. 2b. — Watanabe et al. 2000: 7 , 9, fig. 5. — Kolbasov & Høeg 2003: 3 , 13. Material examined JAPAN – Okinawa , Sesoko I. , laboratory pier, 26°38ʹ09.4ʺ N , 127°51ʹ55.2ʺ E • 1 LSN ; 1991–2005 • 9 LSN , 3 of which molted to cyprids; 2018–2019 ( Tables 1 and S1 ). Description LAST-STAGE NAUPLIUS (LSN) . Lecithotrophic. Body elongate-ovate in dorso-vental view, i.e., racket- or snowshoe-shaped, about 1.7 times as long as wide (excluding dorso-caudal spine); cephalic shield ovate/ellipsoidal, with no discontinuity in lateral body outline between shield and trunk. In lateral view, longitudinal axes of cephalon, trunk and dorso-caudal spine all approximately in same plane. Length 330–360 µm (ventral view in life, without dorso-caudal spine), greatest width ca 220 µm, dorso-ventral thickness not measured. Labrum produced as low, nearly square elevation, posterior two-thirds of which trapeZoidal owing to lateral insertions of first and second antennae; surface weakly divided into facets by cuticular ridges; pore pattern not studied in detail. Caudal end attenuate/acuminate, terminating without marked discontinuity in broad-based ca 100 µm long, multi-annulate and heavily spinose dorso-caudal spine accompanied ventrally by pair of triangular furcal spines ca 10 µm long. CYPRID VIEWED THROUGH CUTICLE OF LSN. Body overall transparent but with brownish pigmentation appearing later in LSN phase. In early-phase LSN, lateral margins of cyprid’s cephalon with clusters of crystal-like particles and gut-like tube packed with brown/white yolk granules, those in telson being orange in some specimens. Telson about as wide as long and half as long as thorax during early phase of LSN, becoming more elongate later. Identification and variation Easily recognizable by its large size and snowshoe- or racket-shaped body. Little variation is apparent among LSN specimens. Distribution Japan (Sesoko Island, Okinawa ).