Abstract
Two large (ca. 0.5 mm long), rare, and probably closely related species of Facetotecta (y-larvae), Hansenocaris cristalabri sp. nov. and Hansenocaris aquila sp. nov., are described on the basis of last-stage lecithotrophic nauplii reared from plankton at Sesoko Island, Okinawa, Japan. The two species resemble each other in having a labrum with a row of spines and a long, attenuate trunk region that terminates in a long, heavily spinose dorsocaudal spine. The labrum of H. cristalabri sp. nov. has an enormous, cockscomb-like ventral process that bears a row of distally directed, dagger-like spines along its anterior side, while the spine-bearing keel of the labrum of H. aquila sp. nov. extends posteriorly into a robust, eagle-like beak. The labral “crest” of H. cristalabri sp. nov. has no equivalent in any other described y-larva, nor in any other crustacean nauplius; its possible functions are discussed. Another diagnostic feature of H. cristalabri sp. nov., absent in H. aquila sp. nov., is a pair of shallow, rounded notches bounded by sharp spinules on the far posteriolateral margins of the cephalic shield. Both new species have longitudinal spine rows on the trunk dorsum, two rows in H. cristalabri sp. nov. and four in H. aquila sp. nov., something not previously documented for y-nauplii. The plate arrangement of the cephalic shield in H. cristalabri sp. nov. is described in detail, with an attempt to homologize the pattern with that of other y-nauplii (especially Hansenocaris furcifera Itô, 1989). The body surface of H. cristalabri sp. nov. has fewer setae and pores than any other late- or last-stage facetotectan nauplius described to date, suggesting paedomorphic development. A formal diagnosis is presented for the family-group taxon Hansenocarididae fam. nov.; this name, while already in use, has until now been nomenclaturally unavailable.
