Aidanosagitta regularis (Aida, 1897)

Figs 6A, 7A, 10A, 20A-E

Sagitta regularis: Aida, 1897: 17-18 p., fig. 8; Doncaster 1902: 211 p., fig. 7; Burfield and Harvey 1926: 100 p., figs 31-32; Thomson 1947: 18-19 p.; Alvariño 1967: 72-75 p., fig. 48A-D; Srinivasan 1979: 31-32 p., fig. 17A-F; Nair et al. 2008: 110 p., table 2.

Material examined.

Korea Strait (33°33.600'N, 127°34.002'E), 0-96 m depth, oblique towing with conical net, Feb 2020, NIBRIV0000895302 (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, NIBRIV0000895301 (one specimen) ; northern East China Sea (32°30.000'N, 127°5.100'E), 0-120 m depth, oblique towing with conical net, Feb 2020 (one specimen) .

Description.

Total body length ranged within 6.1 and 6.7 mm. Tail 30.5-31.7% of body length. Hooks 6. Anterior teeth 3-4 and posterior teeth 5-6, respectively. Rigid and opaque body (Fig. 20). Head small, triangular shaped (Fig. 20B). Round eyes with “B” -shaped eye pigments (Fig. 20B). Collarette expanding to seminal vesicles (Fig. 20A). Corona ciliata beginning from neck to anterior of ventral ganglion (Fig. 10A). Anterior fins 12.3% of body length. Anterior fins beginning at posterior of ventral ganglion. Starting points of anterior fins 37.3% and ending points of anterior fins 50.9% of body length, respectively (Fig. 20D). Posterior fins 22.0% of body length and 1.8 times longer than anterior fins. Starting points of posterior fins 62.9% and ending points of posterior fins 84.7% of body length, respectively. Posterior fins not connected to anterior fins, beginning in front of caudal septum and both anterior fins and posterior fins completely rayed (Fig. 20D, E). Caudal fin fully rayed and roughly round or triangle-shaped (Fig. 20C). Intestinal diverticula present (Figs 7A, 20B). Seminal vesicles longitudinally elongated along body (Fig. 20C). Eggs reaching posterior of ventral ganglion.

Distribution.

This species is found in the epipelagic (0-200 m depth) and mesopelagic zones (200-500 m depth) of Pacific and Indian Oceans (Pierrot-Bults and Nair 1991) and the epipelagic zone (0-100 m depth) of the Tosa Bay in Japan (Ohnishi et al. 2014). In this study, it was found in the epipelagic zone (0-120 m depth) of the Korea Strait and northern East China Sea (Fig. 1, stations KS06, nECS01 and nECS02).

Ecology.

This species is considered a warm-water Indo-Pacific indicator species because many individuals appear in high-temperature and high-salinity water and the distribution range is limited to the areas affected by warm currents (Park 1970).

Remarks.

Amongst the Korean specimens, the collarette was differently inflated, thick and covered the body surface. However, the position and shape of the corona ciliata and fins were consistent with the original description (Aida 1897). Aidanosagitta regularis is similar to A. bedfordii; however, these two species can be distinguished by the morphological difference in the eye pigments (elongated vs. roundish). We observed two specimens for CBE staining pattern: dorsomedian line, 28 dots; dorsolateral line, 34 dots; lateral line, 44 dots; receptors on the lateral fin, 3 dots; anterolateral receptors on the caudal fin, 2 dots; posterior receptors on the caudal fin, not observed due to damage.