New record of an estuarine polychaete, Neanthesglandicincta (Annelida, Nereididae) on the eastern coast of Peninsular Malaysia

Abstract An estuarine species of Nereididae (Annelida), Neanthesglandicincta (Southern, 1921) has been newly recorded on the eastern coast of Peninsular Malaysia located in the South China Sea based on 23 specimens collected from three estuaries (Tumpat, Kelantan Delta, Kelantan; Setiu Lagoon, Terengganu; Kuala Ibai, Terengganu). The morphological characteristics of the Malaysian specimens agree well with those of the previous original description and the redescription of N.glandicincta based on Indian, Myanmar and Singapore specimens. The number of paragnaths in all groups on the proboscis of our Malaysian specimens is within the range of the intraspecific variation of N.glandicincta as shown in the previous descriptions. An identification key to species of the Neanthesglandicincta species complex, which includes two morphologically similar species, is provided.


Introduction
Estuaries are ecologically important habitats which serve as critical transition zones between freshwater and marine habitats (Levin et al. 2001). In general, nereidid polychaetes often occur as major components of the macrobenthic fauna in estuaries and play important roles in the nutrient cycling of an estuary ecosystem (Sato 2017). However, taxonomic knowledge of nereidid fauna in tropical Asia seems to be insufficient although the area has the greatest diversity of coastal species in the world (Tittensor et al. 2010). In Malaysia, only six nereidid species belonging to three genera (Namalycastis rhodochorde, N. cf. abiuma, Namalycastis sp., Dendronereides sp., Perinereis cf. nuntia and P. aibuhitensis) have been recorded with a published taxonomic account (Idris et al. 2012, Ibrahim et al. 2017, while approximately 700 species belonging to 45 genera have been recorded worldwide (Santos et al. 2005, Read and. In Asian tropical estuaries, two nereidid species, Neanthes glandicincta (Southern, 1921) and Composetia burmensis (Monro, 1937), have been most commonly reported (Lee and Glasby 2015). Lee and Glasby (2015) demonstrated that C. burmensis is a junior synonym of N. glandicincta, and newly found a cryptic species from eastern Singapore, which is morphologically very similar to but distinct from N. glandicincta, and described as Neanthes wilsonchani.
During our survey of the nereidid fauna in estuaries located on the eastern coast of Peninsular Malaysia, we found N. glandicincta, which commonly occurs in all of the three sites surveyed, without any occurrence of N. wilsonchani in spite of geographical proximity between our sampling sites and the type locality of N. wilsonchani. In the present paper, we describe N. glandicincta as a new record from the eastern coast of Peninsular Malaysia.

Materials and methods
Field sampling for nereidid specimens was carried out at three estuaries located on the eastern coast of Peninsular Malaysia ( Specimens were collected from sediment samples dug out from intertidal bottoms using shovels, fixed in 80% ethanol, and transferred to fresh 80% ethanol for preservation. The salinity of the surface or interstitial water (water kept in a hole dug in the sediment surface at low tide) was measured using a SCT meter (Quanta Multi-Parameter Probe).
For preserved specimens, the anterior maximum body width, excluding parapodia (BW) was measured at a scale of 1-mm units; for complete specimens, its body length from the anterior end of the prostomium to the posterior end of the pygidium, excluding the anal cirri (BL) was measured, and the number of total chaetigers was also counted. The paragnaths in each group on the proboscis were counted under a stereoscopic microscope both the right and left sides of Groups II and IV were counted but only the larger count was reported. Photographs of the specimens were taken using a digital camera (AM4023X Dino Eye) attached to stereoscopic (Olympus SZX7) and compound (Leica DM300) microscopes. Drawings were prepared using a camera lucida attached to the microscopes. A map drawing was prepared using the ArcGIS 10.3 software.
The usage of terminology of paragnath groups on the proboscis, and parapodial and chaetal morphology is according to Bakken and Wilson (2005).
Specimens were deposited in the South China Sea Repository and Reference Center of Universiti Malaysia Terengganu (UMT) and the National Museum of Nature and Science, Tsukuba, Japan (NSMT).
Remarks. Neanthes is a large genus, considered to be polyphyletic (Bakken and Wilson 2005;Glasby et al. 2011). The generic diagnosis is modified here from Sato (2013) to allow for the occasional absence of paragnaths on the oral ring of the proboscis of the Neanthes glandicincta species complex (see below).

Neanthes glandicincta species complex
Diagnosis. Conical paragnaths present in all of groups I, II, III, and IV on maxillary ring of proboscis. Only few minute rudimentary paragnaths or none present in groups VI and VII-VIII on oral ring of proboscis; paragnaths absent in group V; single round papilla usually present in group VI, with single minute paragnath, or no paragnaths, seated on tip of papilla. Uniramous parapodia of first two chaetigers without notoacicula. In following biramous parapodia, notopodia, consisting of dorsal cirrus and three ligules/lobe (dorsal ligule, prechaetal lobe and ventral ligule) throughout. Neuropodia, consisting of four ligules/lobes (superior lobe, inferior lobe, postchaetal lobe, ventral ligule) and ventral cirrus present in anterior and middle body; superior lobe absent in posterior body. Upper neurochaetae includes homogomph spinigers with long blades and heterogomph spinigers with short blades throughout; some or most of heterogomph spinigers replaced by heterogomph falcigers in middle body. Lower neurochaetae include heterogomph spinigers with long blades and heterogomph spinigers with short blades throughout; some or most of heterogomph spinigers with short blades replaced by heterogomph falcigers in middle body. Heterogomph falcigers first appear around chaetiger 20. Conspicuous dark glandular patches present in notopodial dorsal ligules.
Remarks. Two species, Neanthes glandicincta (Southern, 1921) and N. wilsonchani (Lee & Glasby, 2015), are included in this species complex at present. The two species are distinguishable only by the numbers of paragnaths (Lee and Glasby 2015; see the key below).
Description. Largest complete specimen 70 mm BL, 2.0 mm BW, with 132 chaetigers. Colour in preserved specimens is whitish cream with brownish pigmentation on prostomium, anterior part of palps, and dorsum of anterior chaetigers. Sub-pentagonal prostomium with a pair of smooth tapered antennae situated at anterior end (Figs 3,  4A). A pair of palps with massive palpophores and short conical palpostyles. Two pairs of eyes arranged trapezoidally (anterior pair with space wider than that of posterior pair); anterior pair reniform and slightly larger; posterior pair round and smaller. Midlongitudinal white slit present on dorsal anterior surface of prostomium. Peristomium with four pairs of tentacular cirri of unequal length; posterodorsal tentacular cirri longest, reaching back to chaetigers 6-12 (Figs 3, 4A).
Proboscis with a pair of light brown jaws, each with approx. ten teeth. Typical conical paragnaths present on maxillary ring (Figs 3, 4A); number of paragnaths on each group are as follows (Table 1): group I: 3-13, scattered and unequal; group II: 13-20 in two arched rows, marked large paragnaths with sharply tapering and curved tip present in middle position; group III: 39-58, in three or four rows of transversely elongated bands; group IV: 11-17 in a triangular patch with markedly large paragnaths present in posterior position. Oral ring sometimes expanded into a trapezoidal shape at full-everted proboscis, with only a few or no rudimentary paragnaths; number of paragnaths on each group are as follows (Table 1): group V: none; group VI: 0 or 1 minute paragnath seated on tip of small round papilla, sometimes only papilla present; groups VII-VIII: 0 or 1 minute paragnath present. Total number of paragnaths 94-137.
Neuropodia consisting of superior lobe, inferior (acicular) lobe, postchaetal lobe, ventral ligule and ventral cirrus present in anterior and middle body, but lack superior lobe in posterior body (Fig. 4C, D); postchaetal lobe present throughout; all ligules/ lobes are conical with tapering tip throughout. Ventral cirrus is slender with tapering tip. Glandular patches present along ventral edge of neuropodial ligule/lobes. Notochaetae all homogomph spinigers having long blades with finely serrated edges (Fig. 4E). Upper neurochaetae include homogomph spinigers with long blades (posteriorly) and heterogomph spinigers with short blades (anteriorly, Fig. 4F) throughout; some or most of heterogomph spinigers are replaced by heterogomph falcigers with slender blades (Fig. 4G) in middle body. Lower neurochaetae include heterogomph spinigers with long blades (posteriorly) and heterogomph spinigers with short blades (anteriorly) throughout; some or most of heterogomph spinigers with short blades are replaced by heterogomph falcigers in middle body. Heterogomph falcigers first appear around chaetiger 20 in both upper and lower neurochaetae.
Reproduction. The coelom of a female specimen collected from Tumpat, Kelantan Delta on 13 August 2009 (BW 1.7 mm) was filled with oocytes (probably immature eggs) 100-140 µm in diameter.
Habitat. Intertidal sandy or muddy flats in estuaries. Salinity in habitats highly varied; the salinity of surface water at Muara Kuala Setiu in Setiu Lagoon varied in a range from 22.4 to 28.3 psu (Nicholas 2018), while the salinity of interstitial water at the other sites, Setiu Lagoon, Kuala Ibai, and Tumpat, was in the range from 3.0 (Site 103 in Setiu Lagoon) to 16.5 psu (Tumpat).
Geographical distribution. India, Myanmar, western Singapore, the eastern coast of Peninsular Malaysia. Based on synonymy with C. burmensis, and Southern (1921), Lee and Glasby (2015) and the present study.

Discussion and conclusion
In the present study, an estuarine nereidid species, Neanthes glandicincta (Southern, 1921) is newly recorded at the eastern coast of Peninsular Malaysia in the South China Sea. The morphological characteristics of the present specimens which were collected from three estuaries in Malaysia well agreed with those of N. glandicincta originally described by Southern (1921) based on Indian specimens, and also those redescribed by Lee and Glasby (2015) based on Indian, Myanmar and Singapore specimens. The number of paragnaths in all groups on proboscis of our Malaysian specimens was within the range of the variation of N. glandicincta shown in the original description and redescription (Table 1). Therefore, our specimens can be clearly identified as N. glandicincta.  (2) (1) (6-6, 2) (16-18, 2) (42-42, 1) (12-12, 2) (0-0, 2) (0-0, 2) (0-0, 2) ( (1) (1) (1) (1) Near Calcutta (original description by Southern Note: 1) Corresponding to site numbers in Fig. 1.; 2) Range and (number of available data); 3) Data from complete specimens; 4) Mean ± SD (range, and number of available data); 5) Larger value at a left or right side; 6) All total with numbers from both sides of groups II, IV, and VI; 7) Calculated based on the individual data shown in Table 3 in Lee and Glasby (2015); 8) A part of syntypes of Ceratonereis burmensis Monro, 1937;9) One of probable syntypes of Nereis (Nereis) glandicincta Southern, 1921. According to Atlas of Living Australia (2017), this species was previously recorded from Blue Lagoon which is situated near Port Dickson on the western coast of Peninsular Malaysia, based on specimen(s) deposited in the Northern Territory Museum and Art Gallery, Darwin, Australia (NTM W19065) (Chris Glasby, pers. comm.), though its taxonomic description has not yet been published. This record should be re-examined to clarify whether N. wilsonchani or other morphologically similar species are included or not. Lee and Glasby (2015) showed that Ceratonereis burmensis Monro, 1937 (type locality: Maungmagan in Myanmar and off Bombay in India) is a junior synonym of Neanthes glandicincta (Southern, 1921), and that N. glandicincta was distributed in western Singapore, whereas the closely similar species, N. wilsonchani Lee & Glasby, 2015 was distributed in eastern Singapore. In the present study on three estuaries in eastern Malaysia, we could not find N. wilsonchani in spite of geographical proximity between our sampling sites and the type locality of N. wilsonchani (eastern Singapore). The record of C. burmensis from Jeram, Selangor, western peninsular Malaysia by Polgar et al. (2015), is likely to represent one of these two species, but this needs to be verified. Lee and Glasby (2015) described the epitokous metamorphosis of both N. glandicincta and N. wilsonchani based on sexually mature males and females collected from Singapore in a period from December to April. However, we were not able to collect any epitokous specimens of N. glandicincta probably because our sampling period was limited to August. Previous records of "N. glandicincta" and "Ceratonereis burmensis" on the coasts of South China Sea (Khlebovich 1963, Wu 1967, Wu et al. 1985 and Australia (Rullier 1965) should be re-examined because some morphologically similar but distinct species (N. wilsonchani or other cryptic species) may have been confused with N. glandicincta or C. burmensis. Lee and Glasby (2015) and Sato (2017) suggested that another cryptic species similar to N. glandicincta and N. wilsonchani may be distributed on the coasts of South China Sea and East China Sea.