Unidentia kiku, Korshunova & Fletcher & Martynov, 2025

Unidentia kiku sp.nov.

( Figs 1, 3)

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Etymology: From Japanese kiku (キク), chrysanthemum, to denote the external similarity of this beautiful new species to the flower that is iconic in Japanese culture.

Holotype: KM1077 , L = 13 mm (live), Japan, Okinawa, Sakimotobu beach, 3 m depth, 31 October 2021, coll. Kaoru Imagawa.

Description

External morphology ( Fig. 3A–D) Body elongate. Rhinophores similar in size to oral tentacles, smooth. Cerata notably long, thin, mostly finger-shaped, some partly fusiform, placed on several distinctly elongated elevations with ceratal clusters along dorsal edges. Apices of cerata pointed, with elongated cnidosacs. Distinct notal edge absent. Digestive gland diverticulum fills significant volume of the cerata. Anal opening pleuroproctic toward acleioproctic position on right side below second large ceratal cluster. Reproductive openings lateral, below first anterior cluster of cerata.

Colour ( Fig. 3C, D) General colour translucent dark orange, with thin purple middorsal line. Oral tentacles irregularly covered in purple pigment nearly half its length, basal half of oral tentacles and further toward tip covered with irregular white spots. Rhinophores covered with similar irregular white spots, apical part unevenly purple. Cerata variegated due to encrustation of white specks and small spots, dark pinkish to light brownish branches of the digestive gland show through. Subapical purple rings on cerata, apices yellowish.

Jaws ( Fig. 3E, F) Jaws broadly triangular, masticatory borders with unique elaborate long denticles in clusters of up to about 70.

Radula ( Fig. 3G, H) Radula formula 30 × 0.1.0. Central tooth with up to at least eight distinct denticles. Central cusp smooth, widened in the middle and distinctly narrowed toward the tip; no small denticles at the base of the posterior and middle teeth. No lateral teeth.

Reproductive system ( Fig. 3I, J) Ampulla voluminous, bent. Prostate distinct, thick. Vas deferens short, penial sheath oval, conical penis armed with hollow stylet. Two proximal receptaculum seminis.

Ecology: Shallow water environment.

Distribution: So far known from Okinawa, Japan, potentially may be found in other tropical waters.

Remarks: The new species Unidentia kiku sp. nov. ( Fig. 1) clustered together with Unidentia aliciae , Unidentia sp. , Unidentia nihonrossija , and U. sandramillenae , constituting a highly supported clade (PP = 1, BS = 100) according to the molecular phylogenetic data of the present study. The COI minimal uncorrected p -distances between the U. kiku sp. nov. clade and Unidentia aliciae , Unidentia sp. , Unidentia nihonrossija , and U. sandramillenae clades are 15.5%, 17.0%, 16.0%, and 15.5%, respectively. Externally, U. kiku sp. nov. immediately differs from U. sandramillenae by its translucent dark orange ground and variegated ( Fig. 3) and not whitish or light purple ground colour and generally more uniform coloration. Internally, the general pattern of the radular teeth of U. sandramillenae readily differs from Unidentia kiku sp. nov. by having a more ‘pectinate’ appearance of the central teeth [compare U. sandramillenae from fig. 43H, I in Korshunova et al. (2017a) and U. kiku sp. nov. from Fig. 3G, H, in the present study]. Unidentia aliciae Korshunova et al. 2019 ( Fig. 2), distantly related according to the present molecular phylogenetic analysis, well differs from U. kiku sp. nov. by its creamy-yellowish ground coloration and details of the reproductive system, with considerably larger proximal receptaculum seminis ( Korshunova et al. 2019b). Finally, the only described species of Unidentia from Japan so far, U. nihonrossija Korshunova et al., 2017 (but from the Pacific side of the main Japanese Island Honshu) immediately differs from U. kiku sp. nov. by its whitish background colour and reddish-orange, largely non-variegated coloration and details of its radular patterns (see: Korshunova et al. 2017a: fig. 42 and present study, Fig. 3), and significant molecular phylogenetic distance ( Fig. 2). Unidentia angelvaldesi Millen and Hermosillo, 2012 , with no molecular data available, readily differs morphologically from U. kiku sp. nov. by the inclusion of forms with more uniform reddish coloration and the presence of only a single receptaculum seminis [compare Millen and Hermosillo (2012: fig. 7F) and the present study, Fig. 3].