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 AG* Figs 2 , 10G–K Type AG* – 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 • 13 LSN ; 1991–2005 • 27 LSN , 21 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 1.7 times as long as wide in ventral view; cephalic shield widely ovate, with sharp discontinuity in body outline leading into trunk. In lateral view, trunk axis bent downwards ca 35° with respect to cephalic axis. Length 270–280 µm (without dorso-caudal spine), greatest width ca 150 µm, greatest dorso-ventral thickness ca 100 µm. Labrum with round-cornered rectangular outline in ventral view; surface divided into facets by cuticular ridges and with midline keel extending posteriorly into 20 µm long median spine; pore pattern not examined in detail, but at least with pair of postero-lateral pores. Caudal end attenuate, terminating in ca 65 µm long, slightly dorsally curved dorso-caudal spine with weak sculpturing of minutely spinose cuticular rings; spine upturned 35° with respect to body axis and accompanied ventrally at base by pair of triangular furcal spines ca 5 µm long. CYPRID VIEWED THROUGH CUTICLE OF LSN. Body unpigmented. Cephalon typically with one or two pairs of distinct yolk granules/vesicles close to anterior margin. Gut-like tube typically with two to four yolk granules in posterior part, one of which most often distinctly orange. Telson longer than wide and about half as long as thorax. Fig. 10. Last-stage nauplii of two different morphospecies of y-larvae ( Facetotecta ) from Sesoko Island (Okinawa, Japan). A–F . Y-nauplius Type E*. G–K . Y-nauplius Type AG*. Shown either in life (A–B, G–H) or as slide-mounted exuviae (C–F, I–K). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible. Identification and variation Recognizable by the combination of its general body shape, the rounded-quadrangular labral outline with a midline keel terminating in a small spine and the cyprid’s relatively slender telson with orange yolk granules in its gut-like tube. Little variation is seen among specimens. The molecular diversity of eight sequenced specimens ( Dreyer et al. 2023a ) suggests that this naupliar morphospecies includes two distinct species. Distribution Japan (Sesoko Island, Okinawa ).