Namalycastis sp.

Ibrahim, Nur Fazne, Ibrahim, Yusof Shuaib, Kan, Kotaro & Sato, Masanori, 2024, Nereidid polychaetes (Annelida) inhabiting the inside of decaying fronds of the mangrove palm Nypa fruticans in a tropical estuary in Malaysia, with special reference to the life history of the dominant species, Namalycastis sp., Journal of Natural History 58 (45 - 48), pp. 2043-2074 : 2049-2057

Namalycastis sp.

( Figures 3–8 )

Namalycastis sp. : Ibrahim et al. 2017: 115–116, fig. 1b.

Material examined

Six stations in the estuarine lagoon of Setiu Wetlands, Terengganu, in the eastern coast of Peninsular Malaysia, September 2015 to September 2016 ( Table 1 ; Figure 2 ), coll. N.F. Ibrahim et al., 507 specimens (UMT-Ann 1701–2146, NSMT-Pol 113582, 113583). The following description is mainly based on one of them (UMT-Ann 1741 collected from Sta. 2 on 27 November 2015) .

Description

Complete female 4.1 mm BW, 180 mm BL, with 339 chaetigers ( Figure 4 (a)). Body stout with dorsum convex and venter flat, tapering gradually posteriorly. Dorsum brown anteriorly, whitish posteriorly under preservation; venter pale.

Prostomium trapezoidal, with shallow cleft in anterior margin, with narrow longitudinal groove ( Figures 4 (b) and 5(e,h,i)). Pair of antennae short, sub-conical, aligned at inner edge of palps, extending to one-quarter of palpophore ( Figures 4 (b) and 5(e,h,i)). Two pairs of eyes, black or purple, arranged obliquely or transversely, with posterior pair slightly smaller ( Figures 4 (b) and 5(i)). Jaws with single robust terminal tooth and around seven subterminal teeth, with gap separating terminal and subterminal teeth ( Figures 4 (b,d) and 5(h)).

Four pairs of tentacular cirri with cirrophores distinct. Anterodorsal and posterodorsal tentacular cirri sub-equal. Posterodorsal tentacular cirri reaching chaetiger 2 or 3 ( Figures 4 (b) and 5(e,h,i)).

Parapodia consist of triangular dorsal cirrus, neuroacicular ligule and ventral cirrus. Neuroacicular ligule distally bilobed in anterior chaetigers ( Figure 4 (e)); superior lobe papilliform; inferior lobe globular. Dorsal cirri becoming slender and flattened posteriorly ( Figures 4 (f–j) and 5(f,g)). Ventral cirri short, becoming slender posteriorly ( Figure 4 (f–j).

Notochaetae all sesquigomph spinigers ( Figure 6 (a)); more than 20 spinigers and 8 spinigers present in right parapodia of chaetigers 50 and 100, respectively.

Neurochaetae type A according to Glasby (1999). Upper neurochaetae (supra-acicular) consisting of sesquigomph spinigers posteriorly and heterogomph falcigers anteriorly. Sesquigomph spinigers with finely serrated blade ( Figures 6 (b) and 7(a) for NSMT-Pol 113582) present throughout (more than 20, and more than 10 spinigers present in right parapodium of chaetigers 50, and 100, respectively). Heterogomph falcigers with finely serrated blade ( Figure 6 (e,g)) present throughout; single nose-like projection sometimes presents at base of serrated edge ( Figure 6 (e)).

Lower neurochaetae (sub-acicular) consisting of heterogomph spinigers posteriorly and heterogomph falcigers anteriorly. Heterogomph spinigers with minutely serrated blade ( Figures 6 (c) and 7(b) for NSMT-Pol 113583) present throughout; additionally, a few heterogomph spinigers having blades with coarse serrations proximally ( Figures 6 (d) and 7(c) for NSMT-Pol 113583) present in posterior parapodia, first appearing at around chaetiger 60; heterogomph spinigers absent in posterior-most parapodia. Heterogomph falcigers with finely serrated blade ( Figure 6 (f,h,i)) present throughout. A few (usually 1 or 2) heterogomph falcigers having blades with coarse serrations proximally present at uppermost of sub-acicular position in posterior parapodia, first appearing at around chaetiger 140 ( Figures 6 (j), 7(f)). A few additional thin sesquigomph spinigers (epitokal chaetae sensu Glasby 1999) present in lower neurochaetae, first appearing at around chaetiger 40 ( Figure 7 (g) for NSMTPol 113582).

Pygidium with multi-incised rim, and anus dorsoterminal ( Figures 4 (c)) and 5(g)). Pair of anal cirri arising ventrolaterally, as long as pygidium length.

Mature eggs about 140µm in diameter, containing many yolk granules (lipid droplets) ( Figure 3 (a)).

Variation

Body size ranged from 1.0– 5.2 mm BW, with the largest complete specimen of 340 mm BL with about 400 chaetigers, and the smallest complete specimen of 20 mm BL with 30 chaetigers.

Colour of dorsum in live specimens was highly variable: pale red, dark red, or brown ( Figure 5 (a,b)). Colour of dorsum of preserved specimens also variable from whitish cream to dark brown throughout, or dark or light brown anteriorly and pale posteriorly, with consistently brown epidermal pigment on pygidium ( Figure 5 (c,d)).

Notochaetae (sesquigomph spinigers) were most abundant (up to 30) in chaetigers 10–50; sometimes few notochaetae occurring until around chaetiger 150. Notochaetae were absent throughout in 12 of 507 specimens examined.

Neuropodial supra-acicular sesquigomph spinigers ( Figure 7 (a)) were most abundant (up to 40) in chaetigers 10–50; larger specimens tended to have more spinigers ( Figure 8 ).

Neuropodial supra-acicular heterogomph falcigers were abundant (up to 10) in chaetigers 10–40, reducing in numbers in the posterior chaetigers (up to 2 at chaetiger 160).

Neuropodial sub-acicular heterogomph spinigers with finely serrated blade ( Figure 7 (b)) were present up to around 20. A few heterogomph spinigers with coarsely serrated blade ( Figure 7 (c)) were present in posterior parapodia, first appearing at various chaetigers from around chaetiger 20 to around chaetiger 100, depending on body size.

Neuropodial sub-acicular heterogomph falcigers present up to around 20, sometimes having a blade with only few small serrations proximally ( Figure 7 (d,e)). A few heterogomph falcigers with coarsely serrated blade ( Figures 6 (j) and 7(f)) were present at the uppermost of the sub-acicular position in posterior parapodia, first appearing at various chaetigers from around chaetiger 30 to around chaetiger 250, sometimes lacking throughout body.

A nose-like projection on the proximal edge of the blade of heterogomph falcigers with finely serrated blade was variable in size and shape ( Figure 7 (d,e,g)).

In some specimens of both males and females (1.8–4.4 mm BW, probably during sexual maturity), thin semi-transparent sesquigomph spinigers (epitokal chaetae, up to 10) exist in sub-neuroacicular fascicle ( Table 1 ; Figure 7 (g)).

Habitat

All specimens were found in the fibrous inside of the decaying fronds of the palm Nypa fruticans in estuarine mangroves, where the salinity of the ambient water seasonally fluctuates from 3.5 to 34 psu ( Table 2 ). They have never been found in mudflats around the palm trees. It is unknown where this species spawn and where its larvae and smaller juveniles live.

Feeding habit

Plant fibres and plant cells were found in the gut contents of an individual ( Figure 9 (a,b)), suggesting that this species may feed on the decaying tissue of the palm fronds. In another individual, however, nereidid parapodia with aciculae and chaetae were found in the gut contents ( Figure 9 (c,d)), though it is unclear whether the prey worm belongs to Namalycastis sp. or not. This species may be a facultative carnivore.

Remarks

Our specimens showed high variability in such characteristics as the colour of dorsum, the presence or absence of notochaetae, the number of neuropodial sesquigomph spinigers, and the presence or absence of neuropodial heterogomph falcigers with coarsely serrated blade. According to Glasby (1999), some of our specimens with more than 10 neuropodial sesquigomph spinigers are keying out to N. multiseta Glasby, 1999 , while the others with fewer than 10 neuropodial sesquigomph spinigers are keying out to the Namalycastis abiuma species group, which was established as an informal taxon including five nominal species or subspecies ( Lycastis meraukensis Horst, 1918 ; N. meraukensis zeylanica Silva, 1961 ; Lycastis nipae Pflugfelder, 1933 ; N. rigida Pillai, 1965 ; and Lycastis vivax , 1933) by Glasby (1999). However, our results suggest that the number of neuropodial sesquigomph spinigers may increase continuously according to the growth of each individual ( Figure 8 ). Therefore, we tentatively judged that our specimens belong to a single undetermined species ( Namalycastis sp. ) with high intraspecific variations.

The exact identity of Namalycastis sp. should be clarified based on the future taxonomic revision of the related nominal species ( N. multiseta and members of the Namalycastis abiuma species group). The present study revealed that a dense population of Namalycastis sp. is maintained within the decaying fronds of the Nypa trees, although this species has never been found in mudflats around the palm trees. The unique habitat preference of Namalycastis sp. may be a species-specific characteristic. Namalycastis nipae was described based on the specimens collected from the inside of the decaying leaf axes of Nypa fruticans in Medan-Belawan Strait, Indonesia ( Pflugfelder 1933). No other species of the Namalycastis abiuma species group has been recorded from such a unique habitat, except for the following example: specimens collected from the inside of the fibrous husks of Nypa palm nuts immersed in brackish water (salinity, 5.6%) in a lagoon in the Solomon Islands, the eastern distributional limit of Nypa fruticans , were identified as Namalycastis indica ( Southern, 1921) by Gibbs (1971); later, N. indica sensu Gibbs 1971 was also considered as a member of the Namalycastis abiuma species group by Glasby (1999).

Figures

Figure 2. Map showing seven sampling sites and distributions of three nereidid species along the lagoon of Setiu Wetlands, Terengganu, on the eastern coast of Peninsular Malaysia. M, Station M (Muara Kuala Setiu); 1, Station 1 (Beting Lintang); 2, Station 2 (in front of Pulau Tebing Tinggi); 3, Station 3 (in front of Pulau Che Hing); 4, Station 4 (in front of Pulau Gemia); 5, Station 5 (northern stream in front of Pulau Awang Jin); 6, Station 6 (southern stream in front of Pulau Awang Jin). Filled circles, Namalycastis sp.; open circle, N. rhodochorde; filled triangles, Perinereis aibuhitensis. Arrow indicates the single opening of Setiu Wetlands.

Figure 3. Coelomic cells in gametogenesis of Namalycastis sp. (a) Oocyte of a female individual collected from Station 6 on 27 November 2015. (b) Round cluster of spermatogonia or spermatocytes of a male individual collected from Station 4 on 28 September 2016. Scale bars: 0.05 mm.

Figure 4. Namalycastis sp., a specimen (UMT-Ann 1741; 4.1 mm BW, 180 mm BL) collected at Station 2 on 27 November 2015. (a) Whole body, dorsal view. (b) Anterior end, dorsal view. (c) Posterior end, dorsal view. (d) Pair of jaws, dorsal view. (e) Parapodium of chaetiger 10, posterior view. (f) Parapodium of chaetiger 65, posterior view. (g) Parapodium of chaetiger 93, posterior view. (h) Parapodium of chaetiger 111, posterior view. (i) Parapodium of chaetiger 142, posterior view. (j) Parapodium of chaetiger 166, posterior view. Scale bars: a = 5 mm; b, c = 1 mm; d–j = 1 mm.

Figure 5. Living (a, b) and preserved (c–i) specimens of Namalycastis sp. (a) Pale purple worm collected from Station 2 on 28 March 2016. (b) Greenish dark-brown worm collected from Station 5 on 28 September 2016. (c) Specimen with dark-brown pigmentation throughout, collected from Station 1 on 30 September 2015. (d) Whitish specimen, with anterior-most segments light brown, collected from Station 3 on 18 May 2016. (e–g) Light brown specimen (UMT-Ann 1712) collected at Station 2 on 30 September 2015, dorsal view: (e) Anterior end with everted proboscis; (f) Middle body; (g) Posterior end. (h) Anterior end of dark brown specimen, with everted proboscis (UMT-Ann 1707), collected from Station 1 on 30 September 2015, dorsal view. (i) Anterior end of whitish-cream specimen with brownish pigmentation in prostomium and a few anterior-most segments (UMT-Ann 1724), collected from Station 1 on 27 November 2015, dorsal view. Scale bars: c, d = 10 mm; e–i = 1 mm.

Figure 6. Namalycastis sp., UMT-Ann 1741 (4.1 mm BW, 180 mm BL) collected at Station 2 on 27 November 2015. (a) Sesquigomph spiniger in notochaetae, chaetiger 93. (b) Sesquigomph spiniger in upper neurochaetae, chaetiger 93. (c) Heterogomph spiniger with finely serrated blade in lower neurochaetae, chaetiger 142. (d) Heterogomph spiniger having blade with coarse serrations proximally, chaetiger 111. (e–h) Heterogomph falcigers with finely serrated blade in upper (e, g) and lower (f, h, i) neurochaetae in chaetigers 10 (e, f), 93 (g, h), and 142 (i). (j) Heterogomph falciger with coarsely serrated blade proximally in lower neurochaetae of chaetiger 142. Scale bar 0.1 mm.

Figure 7. Namalycastis sp., NSMT-Pol 113582 (4.2 mm BW, 200 mm BL) collected at Station 4 on 27 November 2015 (a, e, g); NSMT-Pol 113583 (2.5 mm BW, 250 mm BL) collected at Station 3 on 27 November 2015 (b–d); and UMT-Ann 1816 (2.6 mm BW, incomplete) collected from Station 2 on 27 November 2015 (f). (a) Sesquigomph spiniger in upper neurochaetae in posterior body. (b) Heterogomph spiniger with finely serrated blade in lower neurochaetae, chaetiger 5. (c) Heterogomph spinigers having blade with coarse serrations proximally in posterior body. (d) Heterogomph falciger with finely serrated blade, chaetiger 5. (e) Heterogomph falciger with finely serrated blade in posterior body. (f) Heterogomph falcigers having blade with coarse serrations proximally, chaetiger 200. (g) Heterogomph falcigers with finely serrated blade (f), and additional thin sesquigomph spinigers (epitokal chaetae) (ep) located at bottom position of lower neurochaetae in posterior body. Scale bars: a–e, g = 0.1 mm; f = 0.01 mm.

Figure 8. Percentage distribution of the number of sesquigomph spinigers in upper neurochaetae at chaetiger 35 in nine ranks of body size of Namalycastis sp. collected from Setiu Wetlands, Terengganu, Malaysia. A total of 229 specimens collected from six sites from September 2015 to September 2016 were examined. The number at the top of each bar is the number of individuals examined.

Figure 9. Gut contents of Namalycastis sp. (a, b) Plant cells (a) and plant fibres (b) in an individual collected from Station 2 on 27 July 2016. (c, d) Fragments of nereidid parapodia with aciculae (c) and chaetae (d) in another individual collected from Station 5 in 27 July 2016. Scale bars: a, c = 0.1 mm; b= 1 mm; d = 0.05 mm.