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 AH* Figs 2 , 9I–N Type AH* – Dreyer et al. 2023a : figs 4, 5a, c, tables s1–s2. Material examined JAPAN – Okinawa , Sesoko I. , laboratory pier, 26°38ʹ09.4ʺ N , 127°51ʹ55.2ʺ E • 11 LSN ; 1991–2005 • 31 LSN , 28 of which molted to cyprids; 2018–2019 ( Tables 1 and S1 ). Description LAST-STAGE NAUPLIUS (LSN) . Lecithotrophic. Body spoon-shaped in dorso-ventral view; about twice as long as wide; cephalic shield widely ovate, with sharp discontinuity in body outline leading into trunk. In lateral view, trunk axis bent downwards 30–35° with respect to cephalic axis. Length 240–270 µm (ventral view in life, without dorso-caudal spine), greatest width 175 µm, greatest dorso-ventral thickness ca 110 µm. Labrum with goblet-shaped outline in ventral view, its free, rounded posterior margin being extended into medium-sized (15 µm long) medial spine (‘stem’ of ‘goblet’); labral surface divided into facets by cuticular ridges; pore pattern not examined. Caudal end attenuate, terminating in robust, 90 µm long dorso-caudal spine armed with smaller spines and accompanied ventrally at base by pair of triangular furcal spines ca 10 µm long; axis of dorso-caudal spine bent upwards at 30° with respect to trunk axis. CYPRID VIEWED THROUGH CUTICLE OF LSN. Body weakly brownish with many yolk granules/lipid vesicles concentrated dorsally, one typically being situated in midline anterior to nauplius eye, and with clusters of small vesicles posterior to compound eyes. Telson about as long as wide, about half as long as thorax. Identification and variation Recognizable by the combination of its general shape, the large, spinose dorso-caudal spine and the large, goblet-shaped labrum with a free posterior extension continuing into a median spine overhanging the mouth region. Little morphological variation is seen between specimens, most or all of which have internal lipid vesicles/yolk granules distributed as described above. Molecular diversity revealed by sequencing 13 specimens ( Dreyer et al. 2023a ) suggests that this naupliar morphospecies comprises three cryptic species. Distribution Japan (Sesoko Island, Okinawa ).