Flaccisagitta hexaptera ( d'Orbigny , 1836)

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

Flaccisagitta hexaptera ( d'Orbigny, 1836)

Fig. 15A-E

Sagitta hexaptera : Conant, 1896: 213 p.; Aida 1897: 14 p., fig. 3; Fowler 1906: 70 p., figs 30-33; Ritter-Záhony 1908: 9-10 p., figs 3, 3A, 3B; Ritter-Záhony 1909: 789-790 p.; Ritter-Záhony 1911: 2-3 p.; Burfield and Harvey 1926: 95-96 p., figs 6-9; Tokioka 1959: 382-383 p., fig. 21; Alvariño 1967: 27-29 p., fig. 16A-I; Srinivasan 1979: 14-16 p., fig. 7A-G; Michel 1984: 21p., fig. 25.

Flaccisagitta hexaptera : McLelland, 1989: 159-160 p., figs 7B, C and 12A.

Material examined.

Korea Strait (33°24.504'N, 127°54.600'E), 0-50 m depth, oblique towing with MOCNESS, Nov 2019, NIBRIV0000895298 (one specimen) .

Description.

Total body length ranged between 15 and 49 mm. Tail 19-24% of body length. Hooks 4-11. Anterior teeth 2-4 and posterior teeth 2-9, respectively. Large and translucent body (Fig. 15 ). Intestinal diverticula absent (Fig. 15B ). Collarette absent (Fig. 15A ). Eyes “D” shaped with “T” shaped eye pigments (Fig. 15B ). Anterior fins short, beginning at middle of body between ventral ganglion and caudal septum, round-shaped and partially rayed (Fig. 15A, D ). Posterior fins well-separated from anterior fins and partially rayed (Fig. 15A, E ). Caudal fin roundish triangular-shaped and completely rayed (Fig. 15A, C ). Seminal vesicles spherical with anterolateral edge opening (Fig. 15C ). Seminal vesicle touching or close to tail fin and well-separated from lateral fins (Fig. 15C ). Eggs reaching forward end of anterior fins. Corona ciliata beginning in front of eyes and reaching neck (Fig. 15B ).

Distribution.

This cosmopolitan species is found in the epipelagic (0-200 m depth) and mesopelagic zones (200-500 m depth) of the Pacific, Indian and Atlantic Oceans ( Pierrot-Bults and Nair 1991), in the Indian coast ( George 1952), the Tosa Bay in Japan ( Ohnishi et al. 2014), the Korea Strait and south of East Sea ( Park 1970). In this study, it was found in the epipelagic zone (0-50 m depth) of the Korea Strait (Fig. 1 : station KS07).

Ecology.

This species is considered as an indicator species of the Kuroshio warm current and fully-grown adults inhabit depths of <200 m ( Park 1970). In this study, individuals under Stage 3 were mainly found between the Jeju Straits (0-100 m depth). The temperature range in the sampling stations was 16.52-20.57 °C and salinity range was 34.48-34.61 psu.

Remarks.

Though mainly immature individuals were reported in previous studies from Korea, in this study, adults longer than 40 mm were collected for the first time. The Korean species was consistent with those found in previous studies by Alvariño (1967) and Michel (1984) in terms of body size, length and shape of the egg, presence of small and round anterior fins in the middle of the body and the absence of bridge connecting the anterior and posterior fins. As the adult Flaccisagitta hexaptera is large (> 20 mm), it can be difficult to distinguish the adult Flaccisagitta enflata from the immature F. hexaptera . However, it is possible when noting that the eggs of F. hexaptera are long and thin, whereas those of F. enflata are large and short. Another feature relevant for the diagnosis of F. hexaptera is the number of conspicuous anterior teeth which never exceed four, while it is eight in F. enflata . No specific pattern was observed through CBE staining on the body surface.

Figures

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

Figure 15. A Flaccisagitta hexaptera (dorsal view) B head C tail D anterior fin E posterior fin. Abbreviations: AF = anterior fin; CC = corona ciliata; E = eye; IN = intestine; PF = posterior fin; SV = seminal vesicle.