A New, Tiny Species of “Y-Larva” (Pancrustacea: Thecostraca: Facetotecta) from Okinawa, Japan, Described with SEM on the Basis of Laboratory-Reared Last Naupliar and Cyprid Stages

As part of a survey of the remarkable diversity of Facetotecta (“y-larvae”) at an Okinawan (Japan) hotspot locality, a very small and relatively abundant species of this group, Hansenocaris lenticula sp. nov. (formerly designated as “Type K”), is described based on last-stage lecithotrophic nauplii and cyprids reared from the plankton. Among the now 18 formally described species of Facetotecta, this is the fourth for which both nauplii and cyprids are known, and it has the smallest lecithotrophic nauplii yet reported for this group. The nauplii of H. lenticula sp. nov. are distinguished by their small size (approximately 0.2 mm long), flat, trapezoidal labrum, very small antennal and mandibular endopods, and significantly dorso-ventrally bent body with a conically tapered trunk region ending in small, conical furcal and dorsocaudal spines, the latter being slightly smaller than the former. The cyprids can be recognized by their small size, unique labral armature (three similarly sized distal hooks), relatively low number of plates/facets on the telson (three to seven plates per row) and reduced setation of the furcal rami (two very unequal blade-like setae). Potentially diagnostic aspects of the cuticular ornamentation of both larval stages (pores, setae, ridge-bounded plates/facets) are discussed, as well as potential commonalities with other y-larvae, but a detailed comparison is postponed until more facetotectan species have been described in detail. This study reconfirms our earlier assertion that lecithotrophic y-nauplii from Okinawa are relatively easy to distinguish from each other, while the discrimination of their cyprids requires more detailed microscopy, including SEM.