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 293299 10.5852/ejt.2024.929.2479 f7dd08c6-e206-4d97-8c9d-7432c5df72b2 2118-9773 10876286 832192E7-A85A-4971-BA2F-D7420D299E8D Y-nauplius Type AL Figs 2 , 18A–F Material examined JAPAN Okinawa , Sesoko I. , laboratory pier, 26°38ʹ09.4ʺ N , 127°51ʹ55.2ʺ E 3 LSN , 1 of which molted to cyprid; 2018–2019 ( Tables 1 and S1 ). Description LAST-STAGE NAUPLIUS (LSN). Lecithotrophic. Body narrowly ovate in dorso-ventral view; about 1.9 times as long as wide; relative length of trunk region quite variable; lateral margins with very slight narrowing in body outline between cephalic shield and trunk. In lateral view, longitudinal axes of cephalon and trunk in approximately same plane (not examined). Length 340 µm (ventral view in life, without dorso-caudal spine), greatest width ca 180 µm, dorso-ventral thickness not measured but body perhaps significantly flattened. Labrum produced as triangular elevation, about as long as wide, with surface divided into facets by cuticular ridges and bearing at least four pores: two in posterior midline and additional pair of pores rather posteriorly along left and right labral margins. Caudal end broadly acute, terminating in 10 µm long dorso-caudal spine with one or two annulate ridges, this spine being flanked ventro-laterally by pair of tiny, pointed furcal spines, these being situated slightly ventral to, and forward from, base of dorso-caudal spine. CYPRID VIEWED THROUGH CUTICLE OF LSN. Body pigmentation varying among examined specimens: one dark brown overall except in marginal areas, and telson with distinct, reddish-brown pigmentation; in another, only telson brown. Telson about as long as wide and half as long as thorax. Identification and variation Easy recognizable by its narrow-ovate form, dorso-ventrally flattened appearance and tiny furcal spines. Some variation was seen in the size of the furcal and dorso-caudal spines and in the pigmentation of the cyprid inside the LSN (see above). Distribution Japan (Sesoko Island, Okinawa ).