Eyes present. Pereonite 7 reduced and hidden laterally. Pleonites 1–5 fused but with sutures. Pleonite 6 entirely fused dorsally to telson; pleotelson triangular, without statocyst. Mandible acute, with 3-articulate palp. Maxilliped with 4-articulate palp; endite present. Pereopod 7 absent on non-manca individuals. Uropodal exopod elongate triangular, tapering.

Deltanthura palpus Shiraki, Shimomura & Kakui, by original designation.

The generic name is a combination of the Greek letter delta (δέλτα), which is triangular in shape, referring to the shape of pleotelson, prefixed to anthura, from Greek anthos (a flower) and oura (a tail). The gender is feminine.

Like Pseudanthura and Paranthura, Deltanthura has an acute mandible with a 3-articulate palp, but it differs from the former two genera in having pereonite 7 reduced and in lacking pereopods 7. Deltanthura and four paranthurid genera share neotenous characters (reduced pereonite 7 and pereopods 7 lacking), but in Deltanthura the mandible is acute, with a 3-articulate palp and maxillipedal endites present, characters that are unique among the neotenous paranthurid genera (Table 1).

Deltanthura gen. nov. does not fit well within the clade (Paranthura + neotenous clade) (cf. Poore 2001), as it has a maxillipedal endite. Rather, Deltanthura seems to be more closely related to Pseudanthura, as these two share multiple character states such as a triangular pleotelson; elongate triangular, tapering uropodal exopod; and maxillipedal endite. If this latter hypothesis is correct, the neotenous condition in Paranthuridae may not have had a single origin. In addition to the five neotenous genera now known in Paranthuridae, three neotenous genera are known in anthuroid isopods (Poore 2009): the monotypic genera Exallanthura Kensley, 1980 and Leipanthura Poore, 2009 in Anthuridae Leach, 1814; and the genus Curassanthura Kensley, 1981 in Leptanthuridae Poore, 2001. We note that Poore (2009) pointed out that the Exallanthura females sensu Kensley (1980) may actually have been manca individuals.

The neotenous species Califanthura minuta Kensley & Heard, 1991 from the British West Indies may belong in Deltanthura gen. nov. It has the triangular pleotelson and tapering uropodal exopod (Kensley and Heard 1991) that occur in Deltanthura but not in other Califanthura species. However, as the shape of mandible and the presence or absence of the maxillipedal endite (characters diagnostic for Deltanthura) are unknown for C. minuta, we refrain from transferring it to Deltanthura.

Holotype. Japan • 1 female lacking oostegites, body length 7.03 mm, body width 0.91 mm; Mie prefecture, off the southern coast, Shima Spur, Stn. SM-01-(1); 34°00.83'N, 136°53.79'E to 34°01.42'N, 136°51.80'E; depth 805–852 m; 4 Mar. 2008; Tadashi Akiyama leg.; SMBL-V0645, 12 slides and 1 vial.

The specific name, palpus (Latin: palp), is a singular noun in the nominative case, referring to the presence of a mandibular palp, a unique trait in paranthurid neotenous genera.

Body relatively slender (Figs 1, 2A–C), length 7.69 times width. Head (Fig. 2A) length 1.02 times head width, roughened, irregular dorsally; rostrum protruding as much as anterolateral lobes; eyes dorsolateral, with scattered ommatidia. Pereonites 1–7 (Fig. 2A, B) roughened, irregular dorsally, with length ratio 1.00:1.02:1.26:1.19:1.08:0.78:0.18; pereonite 7 (Fig. 2B, C) reduced, hidden laterally, lacking pereopod 7. Pleonites 1–5 (Fig. 2B, C) fused but with sutures, length 0.06 times body length. Pleonite 6 entirely fused dorsally to telson (Figs 2B, C, 4H); pleotelson triangular, length 1.04 times width, with four dorsal and six apical simple setae.

Antennula (Fig. 2D) with three peduncular and six flagellar articles. Peduncular article 1 with one inner and two outer plumose sensory setae; article 2 with inner simple seta; article 3 with three inner and one outer simple setae. Flagellar articles 1 and 2 naked; article 3 with distal aesthetasc and three distal simple setae; article 4 with distal aesthetasc; article 5 with distal aesthetasc and simple seta; article 6 with four distal simple setae.

Antenna (Fig. 2E) with five peduncular and nine flagellar articles. Peduncular article 1 with outer simple seta; article 2 with two distal simple setae; article 3 with two inner simple setae; article 4 with two distal plumose sensory setae and three simple setae; article 5 with four distal plumose sensory setae and four inner simple setae. Flagellar articles 1–9 with four, four, seven, four, three, three, two, four and zero distal simple setae, respectively.

Mandible (Fig. 2F) with 3-articulate palp. Palp article 1 naked; article 2 with distal simple seta; article 3 with eleven simple setae. Molar absent. Incisor acute.

Maxilla (Fig. 2G) slender, with twelve teeth and narrow lamella.

Maxilliped (Fig. 2H) with 4-articulate palp. Palp articles 1 and 2 long, 3 and 4 short, with one, four, one, and three simple setae, respectively. Endite present, reaching middle of article 2, with distal simple seta. Epipod oval.

Pereopod 1 (Fig. 3A) subchelate, robust. Basis with three dorsal plumose sensory setae and ventrodistal seta (tip broken). Ischium with one outer and one ventrodistal simple setae. Merus with two dorsal, one outer, and one ventral simple setae. Carpus strongly protruding ventrodistally, with three inner mid-ventral spiniform setae and four simple setae. Propodus broad, with two inner proximal spiniform setae, and one outer, four dorsal, and one distal simple setae. Palm with five outer spiniform setae and seven simple setae. Dactylus and unguis fused, with two ventral and five middle simple setae.

Pereopod 2 (Fig. 3B) narrow. Basis with two dorsal plumose sensory setae, two simple setae, and dorsal seta (tip broken). Ischium with three simple setae. Merus with two dorsal and two ventrodistal simple setae. Carpus triangular, longer than wide, with three ventrodistal simple setae and ventrodistal seta (tip broken). Propodus with dorsal plumose sensory seta, three ventral spiniform setae, and two dorsal and three ventral simple setae. Dactylus with ventrodistal thick seta and three ventral, three ventrodistal, and four inner distal simple setae. Unguis naked.

Pereopod 3 (Fig. 3C) similar to pereopod 2 except in number of setae.

Pereopods 4–6 (Fig. 3D–F) narrower than pereopod 2, but similar to it except in number of setae and shape of carpus. Carpus rectangular, with ventrodistal spiniform seta.

Pleopod 1 (Fig. 4A) protopod with inner simple seta and outer seta (tip broken). Exopod operculiform, distal margin serrate, with 15 marginal plumose setae and four simple setae on surface. Endopod 0.53 times longer than exopod, with three distal simple setae.

Pleopods 2–5 (Fig. 4B–E) similar to one another. Protopod with one or two simple setae. Exopod with five to seven distal plumose setae and outer simple (pleopods 2–4) or plumose (pleopod 5) seta. Endopod with three distal plumose setae.

Uropod (Fig. 4F, G) with protopod bearing two outer and one inner plumose setae and outer simple seta. Exopod elongate triangular, tapering, with five (right) and 10 (left) simple setae. Endopod with two distal projections, five outer and three distal plumose sensory setae, and 12 distal simple setae.

Our specimen lacks oostegites and an appendix masculina, but its pereon is expanded ventrally (Fig. 1), leading us to conclude that it is a female prior to spawning, rather than a manca-stage individual. The strongly reduced pereonite 7 also supports this conclusion; this segment is not as reduced as in the mancae of non-neotenous species (cf. Frutos et al. 2011: fig. 2C, D; Wägele 1981: fig. 9M). This is the fifth neotenous genus known in Paranthuridae.

Deltanthura palpus gen. et sp. nov. differs from Califanthura minuta, which may belong in Deltanthura (see Remarks above for the genus), in the following characters (character state of C. minuta in parenthesis): body length 7.03 mm (1.60 mm); carpus of pereopod 1 strongly protruding ventrodistally (not protruding); endopod of pleopod 1 short, about half length of exopod (elongate, slightly shorter than exopod); and uropodal endopod with two distal projections (no projections).