Ilyodromus intermedius, Smith, Robin J., Janz, Horst & Okubo, Ichiro, 2011

Smith, Robin J., Janz, Horst & Okubo, Ichiro, 2011, Recent Cyprididae and Ilyocyprididae (Crustacea: Ostracoda) from Lake Biwa, Japan, including a summary of the lake’s ostracod fauna, Zootaxa 2874, pp. 1-37 : 13-16

Ilyodromus intermedius n. sp. Okubo

( Figs 5 I–L, 7 & 8)

1989 Ilyodromus smaragdinus Sars, 1894 —Okubo & Ida: 106, pl. 2j.

1990b Ilyodromus smaragdinus Sars, 1894 —Okubo: 44–45, figs 2j–m.

2000 Ilyodromus smaragdinus Sars, 1894 —Okubo: 110, figs 15a–d.

nomen nudum 2004 Ilyodromus intermedia sp. nov. —Okubo: 32—33, figs 16j–m. Synonymy herein.

Type locality. Lake Biwa, southern basin, N35º 02' 59.1", E135º 53' 45.4". Mud substrate at depth of 4.6 m, locality 29 on Fig. 1 .

Type material. Holotype, dissected female ( LBM 1430003561 from loc. 29). Paratypes, three dissected females ( LBM 1430003562 from loc. 29, LBM 1430003563 from loc. 25 and LBM 1430003558 from loc. 25) and two whole dried females ( LBM 1430003560 from loc. 25 and LBM 1430003559 from loc. 25).

Derivation of name. Name derived from intermediate, to highlight the intermediate characters of this species compared with other Ilyodromus species.

Diagnosis. Dorsal margin slightly curved, posterior margin evenly rounded, anterior margin with maximum curvature below mid-height. Left valve overlaps right valve along all margins. Inner calcified lamella wide posteriorly, narrow anteriorly. Right valve with prominent selvage on posterior margin. Left valve with inner list along anterior margin. Rome organ large and two-segmented. Antenna natatory setae partly reduced, not reaching ends of terminal claws. Alpha seta of mandible long, with wide triangular base tapering to slender middle section, and thinning further to very thin distal section. Seta e of walking leg very long.

Description. Female. Carapace length 1051–1154 µm, height 500–564 µm. Carapace elongate, dorsal and ventral margins sub-parallel, dorsal margin slightly curved ( Figs 7 A & B). Posterior margin evenly rounded. Anterior margin more tightly curved than posterior margin, apex of curve below mid-height. Left valve overlapping right valve along all margins. Right valve with prominent selvage on posterior margin. Left valve with inner list along anterior margin. Dorsal view with maximum width at mid-length, anterior margin slightly more drawn out than posterior margin. Calcified inner lamella narrow posteriorly, wider anteriorly. Surface of valves smooth. Colour white to pale green.

An1 ( Fig. 7 C) with eight segments, first two of which fused together forming one large base. Second segment with one dorsal seta and two long, ventral-apical setae. Third segment quadrate with long, slender, two-segmented Rome organ, first segment of which wider and slightly longer than second. Dorsal-apical edge of third segment with long, slender seta, reaching beyond end of fourth segment. Fourth segment elongate, with one relatively long, slender dorsal-apical seta, and one shorter ventral-apical seta. Fifth and sixth segments both quadrate, each with two long dorsal-apical setae and two short, stout ventral-apical setae. Seventh segment with four long, apical setae. Final segment with two medium length, stout setae, one longer, slender seta and aesthetasc ya.

An2 ( Figs 7 D & E) exopodite with three setae, longest of which relatively short, not reaching to end of third segment. Aesthetasc Y short and stumpy. Natatory setae partly reduced, not reaching to end of longest claws. Claw G2 long, equal in length to claw G1 and slightly longer and stouter and claw G3. Claw Gm slender, reaching to approximately 60% of length of GM. Bifurcated aesthetasc y3 with stout, claw-like setal part, almost equal in size to claw Gm.

Md ( Figs 7 F–H) alpha seta long, proximal third with wide sub-triangular base, tapering to slender, rod-like middle third, and with final third very slender. Beta seta very stout and wide, tapering distally to point, and sprouting numerous long, stiff setules. Gamma seta with wide base, tapering to point, with setules on outer edge.

Mx ( Fig. 7 I) endopodite two-segmented, first segment with group of five apical setae on outer edge and one sub-apical seta at mid-width of segment. Second segment wider than long, apically with two claw-like setae and four shorter setae. Zahnborsten of third endite smooth.

L5 ( Fig. 7 J) endopodite with one long and two shorter terminal setae. Branchial plate with six rays. Basis with two a setae on anterior edge, and hirsute b seta towards inner edge. Seta d hirsute, relatively long and stout. Endite with approximately 14 apical setae of differing lengths.

L6 ( Fig. 8 A) robust, five-segmented, with setae d1 and d2 equal in length. Second segment with long e seta, considerably longer than third segment. Seta f of third segment approximately same length as fourth segment. Final segment with relatively long h3 seta. Claw h2 approximately 20% longer than lengths of segments three and four combined.

L7 ( Fig. 8 B) with relatively short dp and d2 setae, not reaching to end of second segment. Seta e of second segment relatively stout and long. Seta f of third segment short, not reaching to end of segment. Distal end of limb with well-developed pincer arrangement.

CR ( Fig. 8 D) robust and stout, slightly curved. Claw Gp approximately 90% size of Ga. Seta Sa short and slen- der. Sp short, approximately 50% length of Gp, but robust and claw like. CR attachment ( Fig. 8 C) with slightly bent main stem, distally bifurcating into two long branches.

Remarks. No males or evidence of males (e.g. spermatozoa in females) were found. This species was first described by Okubo (2004), but as there was no explicit fixation of a holotype or syntypes, and no statement pertaining to which collection the type material has been deposited and where that collection is located (contravening sections 16.4.1 and 16.4.2 of the ICZN code), it was not a valid species and hence has to be considered as a nomen nudum. Herein it is formally described to correct these omissions.

This species is similar to Ilyodromus smaragdinus Sars 1894 in the overall shape of the carapace and lack of striations on the surface of the valves ( Sars 1894). However, I. smaragdinus is larger (1.5 mm in length), with a more tightly curved posterior margin compared with I. intermedius Okubo n. sp. Additionally, I. smaragdinus has a straight hinge, which is parallel with the ventral margin, while that of I. intermedius Okubo n. sp. is curved towards the anterior and slopes down towards the posterior margin. The colour is also different, with I. smaragdinus reported as a vivid emerald-green clouded with darker shades, whereas I. intermedius Okubo n. sp. is white to pale green. The appendages of I. smaragdinus were not figured by Sars (1894) with the exception of the CR, and this has proportionally shorter claws and setae compared with that of I. intermedius Okubo n. sp. ( Table 1 ). Ilyodromus cf. smaragdinus of McKenzie (1971) is also similar, but differs in the following: a tighter curved posterior margin, a larger carapace size (length 1.6 mm), longer and more slender Y aesthetasc, serrated Zahnborsten (smooth in I. intermedius Okubo n. sp.), a proportionally longer claw on the L6 (67% longer than the length of segments three and four combined), a much shorter e seta on the L6, and proportionally shorter Gp and Sp on the CR ( Table 1 ) compared with those of I. intermedius Okubo n. sp.

In addition to the type locality, Ilydromus intermedius Okubo n. sp. has been found in small numbers along the shore of the Karasuma Peninsula. It has also been found in Kusatsu City, Shiga Prefecture ( Okubo 2004), Gunma, and Okayama Prefectures (the last two as Ilyodromus smaragdinus ) ( Okubo 1990b; Okubo & Ida 1989). Localities ( Fig. 1 ): 25, 29.

* = full length not drawn.

Figures

FIGURE 1. Location of Lake Biwa and sampling localities that yielded Cyprididae and Ilyocyprididae species.

FIGURE 7. Ilyodromus intermedius Okubo n. sp. A- right valve, internal view (LBM 1430003563). B—left valve, internal view (LBM 1430003563). C—antennule (LBM 1430003561). D—antenna (LBM 1430003561). E—detail of antenna (LBM 1430003561). F—mandibular palp (LBM 1430003561). G—alpha, beta and gamma setae of mandible (LBM 1430003561). H—mandibular coxa (LBM 1430003561). I—maxillula palp and endites (setae on endites not drawn) (LBM 1430003561). J—fifth limb (LBM 1430003562). li = list, se = selvage.

FIGURE 8. Ilyodromus intermedius Okubo n. sp. A—sixth limb (LBM 1430003561). B—seventh limb (LBM 1430003561). C—caudal ramus attachment (LBM 1430003561). D—caudal ramus (LBM 1430003561).

FIGURE 5. A & B Cypretta seurati. A—right lateral view of whole carapace. B—dorsal view of whole carapace, anterior to right (LBM 1430003550). C & D Stenocypris malayica. C—right lateral view of whole carapace (LBM 1430003551). D—left valve, internal view (LBM 1430003552). E & F Stenocypris hislopi. E—right lateral view of whole carapace (LBM 1430003554). F—left valve, internal view (LBM 1430003555). G & H Chrissia formosa. G—right valve, external view (LBM 1430003557). H—left valve, internal view (LBM 1430003557). I – L Ilyodromus intermedius Okubo n. sp. I—right lateral view of whole carapace (LBM 1430003560). J—left valve, internal view (LBM 1430003558). K—right valve, internal view (LBM 1430003558). L—dorsal view of whole carapace, anterior to right (LBM 1430003559). Scale bar = 335 µm for A & B, 652 µm for C & D, 769 µm for E & F, 675 µm for G & H, and 574 µm for I – L.

Tables

TABLE 1. Length ratios of caudal rami characters. Data for I. smaragdinus and I. cf. smaragdinus taken from Sars (1894) and McKenzie (1971) respectively.

Ga/ramus	I. intermedius 0.47	I. smaragdinus 0.42	I. cf. smaragdinus *
Gp/Ga	0.81	0.71	0.60
Sp/Gp Sp/Ga	0.45 0.36	0.60 0.43	0.36 0.22