Composetia tokashikiensis Sato, 2020

Sato, Masanori, Ebihara, Takumi, Satake, Kiyoshi, Kojima, Shigeaki, Fukumori, Hiroaki & Angsupanich, Saowapa, 2020, Distributions and Variations of Two Estuarine Species of Composetia (Annelida: Nereididae) in the Ryukyu Islands, Southern Japan, with a New Record of Composetia tokashikiensis from Thailand, Species Diversity 25 (1), pp. 25-38 : 34-35

Composetia tokashikiensis Sato, 2020

( Fig. 8 )

Composetia tokashikiensis Sato, 2020: 17–20 , figs 4B, 6–8. Composetia sp. A: Sato and Sakaguchi 2016: 85.

Composetia sp. 1: Sato 2017: 483.

Materials examined. One single female (PMBC- 29970), the lower reaches of Songkhla Lagoon (7°10′37.4″N, 100°32′26.2″E), southern Thailand, 19 November 2008, coll . S . Angsupanich ( Fig. 1 ; Table 1 ).

Description. Complete individual, 18 mm BL, 1.0 mm BW, with 57 chaetigers. Colour in preserved specimen whitish cream with brownish pigmentation in anterior dorsum.

Prostomium pear-shaped ( Fig. 8A ). Antennae short, tapered. Palps with massive palpophores and short subconical palpostyles. Two pairs of eyes arranged trapezoidally, anterior pair reniform, more separated and larger than posterior pair; posterior pair round. Mid-longitudinal white slit present on dorsal anterior surface of prostomium.

Apodous segment (peristomium) with four pairs of tentacular cirri of unequal length; posterior dorsal tentacular cirri longest, reaching back to chaetigers 11.

Proboscis with pair of amber jaws, each with eight marked teeth. Brown paragnaths present only on maxillary ring ( Fig. 8A, B ). Paragnath numbers on each area as follows: area I: 0; area II: 13 on left and 16 on right in two arched rows, total 29; area III: 21 in wide patch; area IV: 14 on each side in triangular patch, total 28. Oral ring greatly enlarged into trapezoidal shape in full-everted proboscis, without any paragnaths or papillae.

Sub-biramous parapodia of first 2 chaetigers with thin notoacicula ( Fig. 8C ). Notopodial dorsal ligule conical with tapering tip throughout. Both notopodial prechaetal lobe and notoacicular process absent throughout ( Fig. 8 D–G). Notopodial ventral ligule conical with tapering tip throughout, almost subequal to notopodial dorsal ligule. Dorsal cirri slender in most chaetigers (relatively thick in chaetiger 5), tapering, as long as or shorter than notopodial dorsal ligule throughout ( Fig. 8 C–G). Three (rarely two) whitish glandular patches present on dorsal edge of notopodia; distalmost glandular patch larger than others, covering whole conical projection of notopodial dorsal ligule throughout ( Fig. 8 D–G).

Neuropodial postchaetal lobe with tapering tip present in first 18 chaetigers ( Fig. 8C, D ), absent in following chaetigers ( Fig. 8F, G ). Superior lobe in acicular ligule absent throughout. Inferior lobe conical in anterior parapodia, diminishing in middle parapodia, and absent in posterior parapodia. Ventral ligule conical with tapering tip throughout. Ventral cirrus slender with tapering tip.

Notochaetae all homogomph spinigers, having long blades with finely serrated edge; up to 8 spinigers present.

Upper neurochaetae at superior/anterior position in anterior chaetigers (around first 20 chaetigers) consisting of heterogomph spinigers with short finely-serrated blades (3, 7, and 0 spinigers present in chaetigers 1, 5, and 20, respectively), and few heterogomph falcigers with short finely-serrated blades ( Fig. 8J ) (1, 0, and 2 falcigers present in chaetigers 1, 5, and 20, respectively). Upper neurochaetae at superior/anterior position in posterior chaetigers consisting of only few heterogomph falcigers (0, 2, and 2 falcigers present in chaetigers 44, 46, and 48, respectively), lacking heterogomph spinigers.

Upper neurochaetae at posterior position consisting of homogomph spinigers with long finely-serrated blades ( Fig. 8H ) throughout (up to 6 spinigers present).

Lower neurochaetae in anterior chaetigers consisting of many heterogomph spinigers with finely-serrated blades (12, 13 and 4 spinigers present in chaetigers 1, 5 and 20, respectively), and few heterogomph falcigers with short serrated blades (0, 1 and 3 falcigers present in chaetigers 1, 5 and 20, respectively); spinigers with long blades located at posterior position; spinigers with short blades ( Fig. 8I ) and falcigers located at inferior/anterior position. Lower neurochaetae in posterior chaetigers consisting of several heterogomph spinigers at posterior position (5, 5 and 4 spinigers present in chaetigers 44, 46 and 48, respectively), and few heterogomph falcigers at inferior/anterior position (0, 1 and 1 falcigers present in chaetigers 44, 46 and 48, respectively).

The coelom filled with oocytes of 180–200 µm in diameter.

Habitat. Shallow subtidal bottom in the coast of the lower reaches of Songkhla Lagoon, where salinity widely ranges from 1 to 33 psu depending on variable seasonal rainfall; in general, about 1–10 psu in November to March, about 10–20 psu in April to July, about 20–33 in August to October ( Angsupanich and Rakkheaw 1997; S. Angsupanich, unpublished data).

Geographical distribution. The middle and southern Ryukyu Islands in southern Japan (north to Amami-oshima, south to Yonagni-jima), the coast of South China Sea in southern Thailand ( Fig. 1 ).

Remarks. The morphological characteristics of the present Thailand specimen agreed well with those of the original description of C. tokashikiensis by Sato (2020) and also of many additional non-type specimens of this species collected from the Ryukyu Islands, southern Japan in the present study. All of the numerical characteristics (BW, BL, numbers of total chaetigers, anterior chaetigers, and paragnaths) of the Thailand specimen were within the ranges of their variations, which were mentioned above for the Japanese populations ( Figs 6 , 7 ; Table 3 ).

However , the notoacicular process was absent in the Thailand specimen that was an ovigerous female with BW of 1.0 mm, whereas it was present in a few parapodia in chaetigers 5–10 in holotype and some large paratypes of C . tokashikiensis with a BW of 1.5 mm or more ( Sato 2020). In the present study, the presence and absence of the notoacicular process was regarded as an intraspecific variation depending on the body size.

Figures

Fig. 8. Composetia tokashikiensis Sato, 2020, from Songkhla Lagoon, Thailand (PMBC 29970). A, dorsal views of the anterior body with the everted proboscis; B, ventral view of the everted proboscis; C, posterior view of right parapodium 1; D, posterior view of right parapodium 5; E, anterior view of right parapodium 5; F, posterior view of right parapodium 20. G, posterior view of right parapodium 46. H, homogomph spiniger from upper neurochaetae in parapodium 1; I, heterogomph spiniger with short blade from lower neurochaetae (lower position) in parapodium 5; J, heterogomph falciger from upper neurochaetae in parapodium 1. Abbreviations: g, glandular patch; i, neuropodial inferior lobe; ne, neuroacicula; no, notoacicula; p, neuropodial postchaetal lobe. Scale bars: 1mm (A, B); 0.1mm (C–G); 0.05mm (H–J).

Fig. 1. Distributions of two species of Composetia. *Mitochondrial COI gene was analyzed.

Fig. 6. Geographical variation in the number of paragnaths on areas I (A), II (B), III (C), IV (D) and all areas (E) in proboscis of two species of Composetia collected from 10 localities in the middle and southern Ryukyu Islands, southern Japan. Numbers of paragnaths on the areas II and IV were represented by the larger number in either right or left sides. Average and SD bar are shown in each locality. The number on each datum indicates sample size. The localities are arranged from north to south. Difference of mean numbers between the sympatric populations of C. kumensis and C. tokashikiensis was not significant (NS, P>0.05) or significant (*P<0.05; **P<0.001; ***P<0.0001) by the Mann-Whitney U-test. a, Amami-oshima (site numbers in Fig. 1 and Table 1: 1–5); b, Kakeromajima (6, 7); c, Kikai-jima (8); d, Tokuno-shima (9); e, Okinawa-jima (10–15); f, Tokashiki-jima (16), g1, northern Kume-jima (17–20); g2, southern Kume-jima (21); h, Ishigaki-jima (22–27); i, Yonaguni-jima (28).

Fig. 7. Geographical variation in the number of anterior chaetigers of two species of Composetia collected from 10 localities in the middle and southern Ryukyu Islands, southern Japan. Average and SD bar are shown in each locality. The number on each datum indicates sample size. The localities are arranged from north to south. Abbreviations of localities are the same as in Fig. 6. *Difference of mean numbers between the sympatric populations of C. kumensis and C. tokashikiensis was significant (Mann–Whitney Utest, P=0.0016).