Aidanosagitta crassa (Tokioka, 1939)

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

Aidanosagitta crassa (Tokioka, 1939)

Figs 3C , 5B , 19A-F

Aidanosagitta crassa f. naikaiensis : Tokioka, 1959: 376-377 p., fig. 16, table 15.

Sagitta crassa : Tokioka, 1939: 349-352 p., figs 1-8

Material examined.

Type C (collarette type): East Sea (37°33.198'N, 131°14.598'E), 0-100 m depth, oblique towing with conical net, Feb 2020, NIBRIV0000895304 (one specimen) ; Yellow Sea (35°22.550'N, 126°5.366'E), 0-16.5 m depth, oblique towing with conical net, Jul 2020, five specimens ; Korea Strait (34°13.698'N, 127°35.400'E), 0-28 m depth, oblique towing with conical net, Feb 2020 (one specimen) . Type N (naked type): Yellow Sea (35°17.316'N, 126°10.483'E), 0-6.4 m depth, oblique towing with conical net, Jul 2020, NIBRIV0000895303 (three specimens).

Description.

Type C: total body length ranged within 9.9 and 11.2 mm. Tail 26.6-29.8% of body length. Rigid and opaque body (Fig. 19 ). Head small. Collarette beginning at neck and reaching middle of body (Fig. 19A ). Round eyes star-shaped eye pigments. Corona ciliata beginning from neck, elongated to anterior of ventral ganglion (Fig. 19A, B ). Intestinal diverticula present (Fig. 19C ). Two pairs of lateral fins completely rayed (Fig. 19E, F ). Anterior fins 18.1% of body length. Anterior fins beginning at posterior of ventral ganglion. Starting points of anterior fins 29.6% and ending points of anterior fins 46.6% of body length, respectively (Fig. 19D ). Posterior fins 23.0% of body length and 1.3 times longer than anterior fins. Starting of posterior fins 62.0% and ending points of posterior fins 86.9% of body length, respectively. Posterior fins well separated from anterior fins beginning at middle of body (Fig. 19A, D ). Seminal vesicles longitudinally elongated (Fig. 19D ). Eggs reaching posterior of ventral ganglion.

Type N: total body length ranged between 8.1 and8.2 mm. Tail 27.0-30.0% of body length (Fig. 19B ). Collarette beginning at anterior of ventral ganglion and reaching posterior of ventral ganglion (Fig. 19B ). Anterior fins 19.1% of body length. Anterior fins beginning at posterior of ventral ganglion. Starting points of anterior fins 34.0% and end points of anterior fins 54.6% of body length, respectively (Fig. 19B ). Posterior fins 24.6% of body length and 1.3 times longer than anterior fins. Starting points of posterior head 60.9% and ending points of posterior fins 88.2% of body length, respectively. Posterior fins well-separated from anterior fins beginning at middle of body (Fig. 19A, B ).

Distribution.

This species is found in the neritic water of the Pacific Ocean ( Pierrot-Bults and Nair 1991), the neritic coastal water of Hong Kong ( Tse et al. 2007) and the Tosa Bay of Japan ( Ohnishi et al. 2014). In this study, it was found in the epipelagic zone (0-100 m depth) of the East Sea, Korea Strait and Yellow Sea (Fig. 1 , stations ES01, YS01, YS02 and KS01).

Ecology.

Aidanosagitta crassa appears in high abundance throughout the year in the relatively low saline waters of the Yellow Sea and coast of Jeju ( Park 1970). This species rarely appears in the summer warm waters of southern Korea. The body length varies according to the season and it has been reported that they are large (type C) in winter and small in summer (type N) ( Park 1970). Specimens of type C die after spawning and those of type N dominate the new generation ( Park 1970). We obtained specimens from the East Sea in winter and the Yellow Sea in summer.

Remarks.

Previous researchers classified Aidanosagitta crassa into three types according to the distribution of the collarette: C type, covers from the neck to the body; N type, covers only the ventral ganglion; and I type, covers the ventral ganglion and partially covers the body ( Park 1970). In this study, specimens of types C and N were collected and the Korean type C from the Yellow Sea had the same morphological characteristics of the collarette as the original description of this species reported by Tokioka (1939). Similarly, type N, which appeared together with type C at other stations of the Yellow Sea, was consistent with the morphological characteristics of the collarette of A. crassa and Aidanosagitta crassa f. naikaiensis ( Tokioka 1939). It has been reported that the three types of A. crassa appear at different periods depending on environmental factors (water temperature and salinity) of the specific sea area; however, in this study, both types appeared simultaneously in the Yellow Sea. A more detailed ecological investigation of the impact of environment factors on the succession of the three types of A. crassa is necessary. We observed four specimens for type C of A. crassa CBE staining pattern: dorsomedian line, 14 dots; dorsolateral line,> 100 dots; lateral line, 8 dots; receptors on the lateral fin, 8 dots; anterolateral receptors on the caudal fin, 4 dots; posterior receptors on the caudal fin, 6 dots. The pattern of dorsomedian dots was small spots that crossed the centre of the body and larger symmetrical spots on dorsolateral line dots. In addition, we observed three specimens for type N of A. crassa CBE staining pattern: dorsomedian line, 35 dots; dorsolateral line, 34 dots; lateral line, 12 dots; receptors on the lateral fin, not observed; anterolateral receptors on the caudal fin, 2 dots; posterior receptors on the caudal fin, 4 dots.

Figures

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

Figure 3. Appearance of body A transparent and flaccid body (Flaccisagitta enflata) B translucent body (Pterosagitta draco) C opaque and rigid body (Aidanosagitta crassa).

Figure 5. Presence and absence of rayless zone on lateral fins A black arrow shows rayless zone on posterior fin (Zonosagitta nagae) B completely rayed fin (Aidanosagitta crassa).

Figure 19. A Aidanosagitta crassa Type C (dorsal view) B Aidanosagitta crassa Type N (dorsal view) C head and neck D tail E anterior fin F posterior fin. Abbreviations: AF = anterior fin; CC = corona ciliata; COL = collarette; E = eye; IN = intestine; ID = intestine diverticula; PF = posterior fin; SV = seminal vesicle.