Serratosagitta pacifica (Tokioka, 1940)

Choo, Seohwi, Jeong, Man-Ki & Soh, Ho Young, 2022, Taxonomic reassessment of chaetognaths (Chaetognatha, Sagittoidea, Aphragmophora) from Korean waters, ZooKeys 1106, pp. 165-211 : 165

Serratosagitta pacifica (Tokioka, 1940)

Figs 6F , 8C , 10C , 13A-E

Sagitta serratodentata pacifica : Tokioka, 1959: 72-80 p., fig. 10, table 10; Park et al. 1990: 52-54 p. figs 31, 32.

Sagitta pacifica : Alvariño, 1961: 71 p., fig. A, B, table 2; Alvariño 1967: 36-39 p., fig. 22A-D; Pierrot-Bults 1974: 221-222 p., fig. 6; Francisco 1977: 226-229 p., plate 1; Srinivasan 1979: 27-29 p., fig. 15A-G; Kim 1987: 18-20 p., plate 3.

Serratosagitta pacifica : Tokioka, 1965: 345-346 p.; Lutschinger 1993: 30-31 p., fig. 15 A-B.

Material examined.

Korea Strait (33°24.504'N, 127°54.600'E), 0-50 m depth, oblique towing with MOCNESS, May 2019, NIBRIV0000895311 (two specimens) ; Korea Strait (33°33.600'N, 127°34.002'E), 0-96 m depth, oblique towing with conical net, Feb 2020 (one specimen); northern East China Sea (32°33.000'N, 126°30.000 E), 0-100 m depth, oblique towing with conical net, Feb 2020, NIBRIV0000895310 (three specimens); northern East China Sea (32°00.000'N, 127°4.098'E), 0-120 m depth, oblique towing with conical net, Feb 2020, two specimens .

Description.

Total body length ranged within 11.8 and 13.7 mm. Tail 23.4-24.9% of body length. Hooks 6-7. Anterior 10-13 and posterior teeth 16-25. Rigid and opaque body (Fig. 13A ). Small head (Fig. 13A, B ). Grasping spines serrated on edge (Fig. 8C ). Collarette absent (Fig. 10C , 13A ). Rectangular eyes with “T” shaped eye pigments (Fig. 13B ). Intestinal diverticula absent (Fig. 10C ). Anterior fins spanned 21.9% of body length. Anterior fins completely rayed beginning between ventral ganglion and caudal septum. Starting point of anterior fins 34.6% and ending points of anterior fins 55.1% of body length, respectively (Fig. 13A, D ). Posterior fins 26.2% of body length and 1.2 times longer than anterior fins. Starting points of posterior fins 63.7% and ending points of posterior fins 89.7% of body length, respectively. Posterior fins well-separated from anterior fins (Fig. 13A, E ). Caudal fin triangular shaped (Fig. 13A, C ). Seminal vesicles touched or closed to lateral fins and well-separated from tail fin (Fig. 13C ) with elongated knob facing obliquely forward and teeth-like appendages forming 5-10 distal protrusions. Eggs reached anterior of anterior fins. Collarette beginning in front of eyes and extended over neck (Fig. 10C , 13A ).

Distribution.

This species is found in the epipelagic (0-200 m depth) and mesopelagic zones (200-500 m depth) of the Pacific and Indian Oceans ( Alvariño 1967; Pierrot-Bults and Nair 1991), the epipelagic zone of Red Sea, Californian waters ( Pierrot-Bults 1976) and the Tosa Bay in Japan ( Ohnishi et al. 2014). In this study, it was distributed in the epipelagic zone (0-50 m depth) of the Korea Strait and northern East China Sea (Fig. 1 : stations KS06, KS07, nECS01 and nECS04).

Ecology.

This species mainly inhabits Indo-pacific warm-water masses ( Bieri 1959). In the Pacific Ocean, it is a known indicator species of the Kuroshio Water Mass ( Kim 1987). In this study, the temperature range of sampling locations was 16.37-20.57 °C and salinity ranged between 34.48-34.61 psu.

Remarks.

The seminal vesicles are used as an important morphological feature to identify the genus Serratosagitta . S. serratodentata has thick collarette tissue in front of the seminal vesicles and two projections at the anterior-lateral corner. The seminal vesicles touch the end of posterior fins ( Alvariño 1961). The seminal vesicles of S. pseudoserratodentata have one projection at the front corner, with small teeth at the anterior end. The seminal vesicles are well-separated from the posterior fins and caudal fin ( Alvariño 1961). In the S. pacifica (nine specimens), the number of protrusions vary between 5 and 10. The inner serrated row of the grasping spine and the "teeth cells" forming protrusions at the anterior margin of the seminal vesicles were consistent with previous records ( Alvariño 1967; Pierrot-Bults 1976). We observed three specimens for CBE staining pattern: dorsomedian line 43 dots; dorsolateral line, 54-69 dots; lateral line, 24 dots; receptors on the lateral fin, 10 dots; anterolateral receptors on the caudal fin, 2 dots; posterior receptors on the caudal fin, 3-4 dots. The dorsomedian dots are patterned as small spots that cross the centre of the body and larger symmetrical spots on dorsolateral line dots.

Figures

Figure 1. Sampling stations in Korean waters. Empty circles, sampling stations; filled circles, the stations where the chaetognaths were secured.

Figure 6. Shape and position of seminal vesicles A Aidanosagitta regularis (elongated) B Ferosagitta robusta (pear shape) C Pseudosagitta lyra (spherical shape) D Zonosagitta nagae (conical shape) E Flaccisagitta enflata (spherical shape) F Serratosagitta pacifica (seminal shape with chitinous teeth).

Figure 8. Grasping spine A abruptly curved hooks (Krohnitta subtilis, Krohnittidae) B gently curved and not serrated hooks (Ferosagitta robusta, Sagittidae) C gently curved and serrated hooks (Serratosagitta pacifica, Sagittidae).

Figure 10. Position and shape of corona ciliata A Aidanosagitta regularis B Pterosagitta draco C Serratosagitta pacifica D Flaccisagitta enflata.

Figure 13. A Serratosagitta pacifica (dorsal view) B head C tail D anterior fin E posterior fin. Abbreviations: AF = anterior fin; AT = anterior teeth; CC = corona ciliata; COL = collarette; E = eye; PF = posterior fin; PT = posterior teeth; SV = seminal vesicle.