Parvocythere gracilis, Higashi, Ryouichi & Tsukagoshi, Akira, 2012

Higashi, Ryouichi & Tsukagoshi, Akira, 2012, Two new species of the interstitial genus Parvocythere (Crustacea, Ostracoda, Cytheroidea) from Japan: an example of morphological variation, ZooKeys 193, pp. 27-48 : 34-39

Parvocythere gracilis sp. n. Figs 7-11

Type series.

Holotype: adult male (SUM-CO-2050), right valve length 170 µm, height 75 µm, left valve length 169 µm, height 73 µm, appendages mounted on slide and valves preserved in a cardboard cell slide, Paratypes: 7 adult males (SUM-CO-2051-2057) and 8 adult females (SUM-CO-2058-2065). All illustrated specimens were collected from interstitial pore-water at the type locality on April 16, 2010.

Type locality.

Kozu, Odawara City, Kanagawa Prefecture, Pacific coast of central Japan, 35°16.73'N, 139°12.75'E in sediments at approximately 20 cm depth. The sediment is mainly composed of clastic granules and pebbles.

Etymology.

The Latin gracilis (slender) refers to the slender aspect of the carapace of this species.

Diagnosis.

Carapace elongated and bean-shaped in lateral view. Anterior margin slightly extending like thin plate in ventral and dorsal views. Middle of posterior margin of right valve slightly pointed in lateral view. Forty-two pore systems per valve. Six and two marginal pores along anterior and posterior margins, respectively. Marginal infold narrow in anterior and ventral, and very narrow in posterior. Hingement modified pentodont type. No suture on middle of fourth podomere of antennula. Antenna with only one stout claw and one tiny seta on distal end. Male copulatory organ asymmetry: left organ larger than right one. Left organ bearing long L-shaped copulatory duct, dorsal ramus with two small projections, blunt two-pronged ventral ramus, and well-developed crescent-shaped distal lobe with one conspicuous seta on its centre; right organ reduced and bearing extremely reduced copulatory duct and well-developed crescent-shaped distal lobe with thick seta on proximal part.

Description.

Carapace (Figs 7, 8). Carapace elongated and bean-shaped in lateral view. Anterior margin gently rounded in lateral view. Anterior part slightly extending like thin plate in ventral and dorsal views. Posterior margin gently rounded in left valve and slightly pointed at middle height of right valve. Dorsal margin slightly rounded. Ventral margin almost straight. Carapace surface smooth. All pore-systems of simple type and 42 per valve. Six and two marginal pores along anterior and posterior margins, respectively. Marginal infold narrow in anterior and ventral, and very narrow in posterior area. Vestibula occupying large part of marginal infold. Inner surface covered with numerous pits. Hingement modified pentodont type. Right valve slightly overlapping left valve along hinge line. Four adductor muscle scars in oblique row. Two separated mandibular scars visible beneath frontal scar.

Antennula (Fig. 9A). Five articulated podomeres. First podomere bare and short. Second podomere three times as long as first podomere and bare. Third podomere half as long as second podomere, with one medium seta on antero-distal end. Fourth podomere eleven-tenths as long as third podomere, with two very long setae on antero-distal end and one long seta on postero-distal end. Fifth podomere three-eighths as long as fourth podomere, with two long and one long spatula-like setae on distal end.

Antenna (Fig. 9B). Four articulated podomeres. First podomere with indistinct two-segmented spinneret (exopodite) on distal end. Second podomere half as long as first podomere, with bunch of fine setae at middle of anterior margin and one short seta on postero-distal end. Third podomere five-halves as long as second podomere, with numerous fine setae along antero-proximal margin, one short seta on middle of anterior margin, one medium seta on middle of posterior margin, and one short and thick seta on postero-distal end. Fifth podomere two-seventh as long as fourth podomere, with one stout claw and one very short seta on distal end.

Mandibula (Fig. 9C1, C2, C3). Coxa (Fig. 9C1) elongated, with one seta on antero-ventral part, two very short setae on postero-ventral part, and six coxal endites. Palp (Fig. 9C2) consisting of four articulated podomere. First podomere (basis) with bifurcated lamella (exopodite; Fig. 9C3) on middle of dorsal margin. Second podomere twice as long as first podomere, with three long setae on outside of distal margin. Third podomere two-thirds as long as second podomere, with one short seta on middle of dorsal margin, one medium seta on outside of distal end and two short setae on ventro-distal end. Fourth podomere four-sevenths as long as third podomere, with one short and three medium setae on distal end.

Maxillula (Fig. 9D). Thin branchial plate (exopodite) with approximately six plumose setae. Basal podomere with one palp and three endites. Palp consisting of two articulated podomeres: first podomere with one long and one medium setae on antero-distal end; second podomere as long as first podomere, with one medium seta on middle of posterior margin and two medium setae on distal end. Endites with three setae at the distal end.

Fifth limb (Fig. 9E). Four articulated podomeres. First podomere with one short seta on antero-distal end. Second podomere four-thirds as long as first podomere, with one very short seta on antero-distal end. Third podomere nine-tenths as long as second podomere. Fourth podomere eleven-tenths as long as third podomere, with long distal claw.

Sixth limb (Fig. 9F). Four articulated podomeres. First podomere with one short seta on antero-distal end. Second podomere two-thirds as long as first podomere, with one medium seta on antero-distal end. Third podomere nine-tenths as long as second podomere. Fourth podomere as long as third podomere with well-developed stout distal claw.

Seventh limb. Absent.

Male copulatory organ (Fig. 10). Asymmetric. Right organ smaller than left, with reduced components: square capsule with weakened framework; copulatory duct (Cd) extremely reduced; and distal lobe (Dl) thin and crescent-shaped with thick seta on proximal part. Left male copulatory organ bearing L-shaped long copulatory duct (Cd), dorsal ramus (Dr) with two projections, two-pronged blunt ventral ramus (Vr), and crescent-shaped and well-developed blunt-tipped distal lobe (Dl) with thick seta on centre.

Genitalia of female (Fig. 11). Symmetric and consisting of rounded frame-work, sigmoid duct connected with strongly sclerotised opening. Duct opening on ventral side.

Eye. Absent.

Dimensions.

See Table 2.

Occurrences.

Type locality and Daio-zaki, Shima City, Mie Prefecture, Pacific coast of central Japan, (34°16.59'N, 136°53.83'E).

Remarks.

Parvocythere gracilis sp. n. resembles Parvocythere galapagoensis Gottwald, 1983, Parvocythere schmidti Gottwald, 1983 and Parvocythere subterranea Gottwald, 1983 in the shape of carapace. The new species, however, is slenderer than the other species, and can be distinguished from them by the following reduced characters: no rib posterior to adductor muscle scars; no suture on middle of antennular fourth podomere; only one developed distal claw of antenna; and asymmetric male copulatory organ. Although Parvocythere gracilis sp. n. also resembles Parvocythere elongata Hartmann, 1959 and Parvocythere supralitoralis Gottwald, 1983 in the shape of carapace and the reductive characters of antennula and antenna, only the new species has asymmetric male copulatory organ. Moreover, Parvocythere gracilis sp. n. is also similar to Parvocythere dimorpha Hartmann, 1974 in the shape of the carapace, the many characters of appendages and an asymmetric male copulatory organ but the new species differs from other species in the number of antennal distal claw, the thickness of sixth limb and the form of copulatory duct.