INTRODUCTION

The genus Labidocera Lubbock, 1853 presently comprising 56 species, mainly inhabit in surface waters (0- 30 cm layer) of tropical and warm temperate regions. In general, they are considered as good indicators of water masses (Sherman 1963, 1964;Matsuo and Marumo 1982;Jeong et al. 2009;Razouls et al. 2005-2022). The genus is the largest in the family Pontellidae, divided into five species-groups such as detruncata, kröyeri, minuta, pectinata and unassigned group on the basis of morphological characters (Fleminger et al. 1982;Othman 1986;Mulyadi 1997, 2002). In the detruncata group, L. detruncata, L. pavo, and L. sinilobata occur in Korean waters and the last one is the first record. Since the historial descriptions of the latter two species were often not given in sufficient detail (e.g., Sewell 1932;Mori 1937, 1964;Wilson 1950;Zheng et al. 1965, 1982;Silas and Pillai 1973;Kim 1985), we provide their redescriptions with morphological characteristics of the detruncata group and notes of their zoogeography.

MATERIALS AND METHODS

Zooplankton were collected from the southwestern waters of Korea (Fig. 1) on August, 2003 and July, 2019, using a Norpac net (mesh size 0.2 mm) and fixed with 5% neutralized formalin/seawater immediately after capture. Specimens were dissected and mounted in CMC-10 (Masters Company Inc., USA) and illustrated with an optical microscope (Nikon 80i, Japan) equipped with a drawing tube. The rostrum and genital compound somite of the females, and male fifth leg were also examined with a scanning electron microscope (Hitachi S-3000N, Japan). Morphological terminology followed Huys and Boxshall (1991). We directly confirmed references and rearranged the synonymies. Mouthparts and swimming legs 1-4 are not described because they do not differ from those reported by Jeong et al. (2009). All specimens were deposited in the National Marine Biodiversity Institute of Korea (MABIK).

RESULTS

Order Calanoida G. O. Sars, 1902

Family Pontellidae Dana, 1853

Genus Labidocera Lubbock, 1853

Labidocera sinilobataShen and Lee, 1963 (Figs. 2-4)

Labidocera sinilobataShen and Lee, 1963: 580, figs. 20- 25 (type locality: Zhanjiang, China); Chen and Zhang, 1965: 100, pl. 42, figs. 8-14; Zheng et al., 1982: pl. 42; Mulyadi, 2002: 82, fig. 4.

Materials examined. Nine females and seven males were collected from the northeastern area of the East China Sea on 20 August, 2003 (34°48ʹ54ʺN, 127°24ʹ39ʺE) and 31 July, 2019 (34°26ʹN, 127°34ʹE), of which two females and both males were dissected and closely examined. Five females (MABIK CR00252958) and five males (MABIK CR00252959) from the same locality on 31 July, 2019 were used for length measurement.

Females. Body length 2.42-2.65 mm (n=9). Prosome (Fig. 2A) cylindrical without lateral hooks: cephalosome and first pedigerous somite completely separated; posterior corners of prosome asymmetrical, right corner longer than left, with pointed triangular-shaped processes (Fig. 2A, B). Cephalosome bluntly rounded with pair of dorsal lenses (Fig. 2A): rostrum bifid, gap between rostral rami narrow (Fig. 4A). Urosome 2-segmented: genital compound somite elongated with lateral lump on the left side (Figs. 2A-C, 4B, C); anal somite short with tumor-like protrusion on distal margin ventrally; caudal rami fanshaped, with 2 inner setae (seta I, II) swollen proximally, and much thicker than others (seta III to V). Fifth leg (Fig. 2D) asymmetrical, left leg longer than the right: endopod absent; exopod 1-segmented and elongate having 2 lateral and 3 apical spinous processes.

Males. Body length 1.94-2.14 mm (n=7). Prosome more compact than female: cephalosome and first pedigerous somite completely separated; fourth and fifth pedigerous somites completely fused; posterior corners of prosome asymmetrical with sharp process posteriorly (Fig. 3A, B). Urosome 5-segmented: genital somite swollen laterally; caudal rami symmetrical, each ramus with 6 setae. Right antennule geniculate, indistinctly 15-segmented, posterior margin of second to fifth segments fringed with fine hairs (Fig. 3C). Fusion pattern and setal formula as follows: I-1, II-IV-4+ae, V-IX-10+3ae, X-XI-4+ae, XII-XIV-6+2ae, XV-XVI-4+2ae, XVII-2+ae, XVIII- 2+ae, XIX-1+p (process)+ae, XX-1+p+ae, XXIXXIII- 3+p+ae, XXIV-1+1+p, XXV-1+1+ae, XXVI- 2, XXVII-XXVIII-4+ae. Segment XIX with lateral round projection tapering hooked process; segment XX and compound segments XXI to XXIII with toothed ridge possessing comb-like denticles; segment XXIV with finger-like process extended beyond the next segment (Fig. 3C, D). Fifth leg (Fig. 3E) uniramous, asymmetrical: right exopod 2-segmented, first segment comprising palm with 2 setae, semi-circular process near base of thumb, and elongated thumb, second segment long and finger-shaped, with 2 medial and 2 apical setae; left exopod 2-segmented, first segment with small distolateral process, second segment (Fig. 4D) bulb-shaped bearing 1 stout process, 3 unequal round-tipped process, distinct protrusions, and inner hirsute border.

Remarks.Labidocera sinilobata is easily distinguished by posterior corners of prosome asymmetrical of both sexes, the absence of endopod in female fifth leg, and the structure of right antennule in male. The present specimens are different from the original description: in the female of latter, 1) genital segment concave in right margin, and 2) anal somite with a semicircle plate; in the male, 1) fingerlike process of ancestral segment XXIV in right antennule does not extend to the next segment, and 2) distal exopodal segment of left fifth leg with 1 small seta and 2 elongated setae. Korean specimens are well coinciding with illustrations of Chen and Zhang (1965) and Zheng et al. (1982), but those of Mulyadi (2002) were different from the female urosome, the right antennule and the fifth leg of male.

Labidocera pavoGiesbrecht 1889 (Figs. 5, 6)

Labidocera pavoGiesbrecht, 1889: 27 (type locality: Red Sea); 1893: 446, 460, 773, pl. 25, fig. 34, pl. 41, figs. 18, 38; Sewell, 1914: 234, pl. 21, figs. 1-3;Brodskii, 1950: 409, fig. 290; Wilson, 1950: 248, fig. 363; Tanaka, 1964: 255; Chen and Zhang, 1965, 98: pl. 401, figs. 1-4;Silas & Pillai, 1973 (1976): 804, fig. 14; Mulyadi, 2002, 79: fig. 26; Othman & Toda, 2006: 312, figs. 12-13; Abo-Taleb, 2019: figs. 2-5.

Materials examined. Three females and one male were collected from northeastern area of the East China Sea (34°30ʹN, 128°E) on 19 August 2002. All specimens were dissected and closely examined.

Females. Body length 2.35-2.52 mm (n=3). Prosome cylindrical without lateral cephalic hooks (Fig. 5A, B): posterior corners of prosome slightly asymmetrical, rounded in lateral view and with small pointed subterminal process on each side. Cephalosome bluntly rounded anteriorly and pair dorsal lenses. Urosome 2-segmented (Fig. 5C): genital compound somite asymmetrical, right margin with 1 conical lobe, left margin with rounded knob-like process; anal somite short; caudal rami broad and symmetrical with short and swollen proximally caudal setae. Fifth leg (Fig. 5D) nearly symmetrical: coxae and intercoxal sclerite completely fused; exopod uni-segmented, with 2 outer processes and 3 unequal apical processes, middle one longest; endopod uni-segmented and conical, right longer than left.

Males. Body length 1.62 mm (n=1). Prosome similar to females except for well-developed pair of dorsal lenses (Fig. 6A, B). Urosome 5-segmented, without any processes on genital somite: caudal rami short and distally widened. Right antennule geniculate, denticulate ridges on the inner surface of segment XIX and XX and compound segments XXI to XXIII (Fig. 6C, D). Fifth leg (Fig. 6E) uniramous, asymmetrical: right exopod 2-segmented, first segment bearing large proximal bifid process and distal seta on inner margin, second segment with 2 inner setae, 3 thin terminal processes, and 1 subterminal outer seta; left exopod 2-segmented, first segment with distolateral spine; second segment with 1 medial spine, 3 spine-like apical processes and inner hirsute border.

Remarks.Labidocera pavo most resemble L. bataviae and L. madurae in detruncata group, but are distinguished by the morphology of the female genital segment and structure of fifth legs of both genders (Table 1): in female 1) right side of genital segment with prominent conical-shaped lobe, and 2) endopod of fifth leg asymmetrical, left endopod longer than right one; in male 1) distal exopodal segment of left fifth leg with outer marginal seta and 3 apical equal processes, 2) first exopodal segment of right fifth leg with conical thumb and small lobe bearing seta at tip, and 3) second exopodal segment of right fifth leg with 4 setae and trifid apex.

DISCUSSION

Labidocera sinilobata was first described by Shen and Lee (1963) from the estuary of Zaikong River, southern China. The second record was made from two estuaries in China adjacent to the East China Sea and the Yellow Sea, respectively (Chen and Zhang 1965;Zheng et al. 1982). In Korea, this species occurs from the estuary off Beolgyo adjacent to Suncheon Bay and the northeastern area of the East China Sea. L. pavo was first described based female specimens in the Red Sea by Giesbrecht (1893) and the male was done from Gulf of Mannar by Sewell (1914). The distribution is extended to a narrow coastal distribution of the Indo-West Pacific: the East Sea, the Yellow Sea, the East China Sea, the South China Sea, the Eastern Australia, the Eastern Tropical Pacific (Central America, Galapagos, Northern Peru), the Gulf of Thailand, Madagascar, the Eastern South Africa, the Arabian Sea, the Persian Sea, Bay of Bengal, the Andaman Sea, the Strait of Malaca, Mediterranean Sea, and Black Sea (Razouls et al. 2005-2022). Distribution of brackish and neritic species of Tortanus and Pseudodiaptomus in the Far East may correspond to the East Asian initial endemic element originated from the ancient East China Sea which might be a huge gulf of low salinity water from the Middle Miocene to the Pleistocene (Nishimura 1981). Among the species, T. dextrilobatus and P. poplecia show an analogy in distributional patterns with Labidocera sinilobata, which may have been recently introduced (after the Jômonian transgression) from the (southern) Chinese population by ships, migratory birds or other dispersal mechanism (see Ohtsuka et al. 1992;Ohtsuka and Reid 1998;Soh et al. 2001). On the other hand, L. detruncata species group show a restricted distribution in tropical to temperate waters of the Indo-Pacific as being well referred Hirabayashi and Ohtsuka (2014). Also, Hirabayashi and Ohtsuka (2014) suggest that such restricted distribution within this species complex may be a parallel speciation due to the isolation mechanism by the existence of Sundaland during the glacial periods (cf. Fleminger 1986).