Neanthes chilkaensis ( Southern, 1921 )

Villalobos-Guerrero, Tulio F., Kara, Jyothi & Idris, Izwandy, 2021, Redescriptions of Neanthes Kinberg, 1865 (Annelida: Errantia: Nereididae) species from worldwide regions, European Journal of Taxonomy 760 (1), pp. 101-135 : 104-109

Neanthes chilkaensis ( Southern, 1921)

Fig. 1

Nereis (Nereis) chilkaensis Southern, 1921: 52–54 , pl. 22, fig. 8a–r, text-fig. 4a–c [type locality: Chilika Lake, India].

Nereis chilkaensis – Fauvel 1932: 94–95; 1940: 258; 1953: 185–186, fig. 94a–c.

Neanthes chilkaensis – Hartman 1959: 250; 1974: 618. — Fauchald 1972: 409 (group IIB 1). — Wilson 1984: 225 (group IIB 1) (all previous in species list). — Sunder Raj & Sanjeeva Raj 1987: 92, pl. 3 fig. 7. — Nesemann & Sharma 2007: 106, 110, pl. 29 figs 1–2. — Villalobos-Guerrero & Idris 2021: 559 (table 1), 561 (table 2), 564 (table 3).

Nereis (Neanthes) chilkaensis – Misra et al. 1987: 72. — Nageswara Rao 1995: 324–325.

Type material

Lectotype (hereby designated)

INDIA • atokous; Orissa, Chilika Lake , “Maludaikuda” [now Maladeikuda ] Island; Chilka Lake Survey leg.; NHMUK 1938.5.7.27.

Comparative material

Syntypes of Neanthes indica brunnea ( Day, 1957)

MOZAMBIQUE • 2 specs; Inhambane, Morrumbene Estuary ; J.H. Day leg.; NHMUK 1961.16.23 • 1 atokous; same collection data as for preceding; SAM-A-20983 .

Description

COLOUR AND MEASUREMENTS. Lectotype atokous, complete, in good condition, 43 mm TL, 12.2 mm L15, 2 mm W15, with 74 chaetigers. Body colour brownish, with brown pigmentation in dorsum of prostomium and palps, and present throughout in dorsum and venter of segments ( Fig. 1A–B ).

PROSTOMIUM. Campanulate, 1.4 times longer than wide ( Fig. 1B ); anterior end broad, distally complete; anterolateral gap beside palpophore broad, twice as wide as antennal diameter; dorsal groove distinct, shallow, running mid-subdistally. Nuchal organs barely exposed, broad, 1.5 times as wide as diameter of posterior pair of eyes.

PALPOPHORES. Sub-conical, as wide as long ( Fig. 1B ), as long as three-fifths of entire prostomium; with distinct sub-distal transverse groove. Palpostyles ovoid, thick, with diameter as wide as one-third of palpophore.

ANTENNAE. Tapered, slender, long, extending forwards beyond tip of palpophore and posteriorly to halflength of prostomium; antennae well separated, with gap 1.2 times as wide as basal diameter of antennae.

EYES. Paired eyes blackish, arranged in a trapezoid form; gap between both pairs two-fifths as wide as diameter of posterior pair of eyes ( Fig. 1B ); anterior pair of eyes rounded, 1.5 times as wide as basal diameter of antennae, gap between both eyes 3 times as wide as diameter of eyes, with lens slightly distinct, dark, covering 20% of eye; posterior pair of eyes rounded, 1.2 times as wide as basal diameter of antennae, with lens barely distinct, dark, placed in middle of eye and covering 15% of it.

APODOUS ANTERIOR SEGMENT. Segment 2.5 times wider than long, 1.6 times as long as chaetiger 1, with flattening anterior margin, dorsum without marked transverse wrinkle.

TENTACULAR CIRRI. Slender, multi-articulated ( Fig. 1B ); postero-dorsal cirri extending posteriorly to chaetiger 5, twice as long as antero-dorsal cirri; antero-dorsal cirri extending posteriorly to chaetiger 2; postero-ventral cirri extended over opposite side of prostomium; antero-ventral cirri as long as posteroventral cirri and extending beyond 1.3 times length of palpophore; dorsal cirrophores cylindrical, ventral cirrophores ring-shaped, postero-dorsal cirrophores as long as antero-dorsal cirrophores, antero-ventral cirrophores twice as wide as postero-ventral cirrophores.

PHARYNX. Non-everted, previously dissected. Jaws not dissected, with 10 well-developed and blunt denticles; pulp cavity with two canals. Brownish paragnaths on maxillary and oral rings ( Fig. 1C ), except reddish on areas VII–VIII, consisting of conical and p-bars; merged paragnaths and plate-like basements absent. Area I: 8, irregular patch of uneven cones, distal ones smaller; areas IIa: 18 and IIb: 22, three slightly regular rows of uneven cones in eyebrow-shaped patch, cones in outer-most row larger ( Fig. 1C ); area III: 29, four irregular rows of uneven cones in sub-rectangular patch, without distinct laterally-isolated cones, distal cones smaller ( Fig. 1C ); areas IVa: 35 and IVb: 37, pear-shaped patch with proximal half consisting of four irregular rows and distal half with three slightly regular rows of uneven cones ( Fig. 1C ); area V: 4, two oblique rows of two small, worn, conical paragnaths, near but placed behind level of paragnaths on area VI ( Fig. 1C ); areas VIa: 5 and VIb: 7, one transverse row of distal, uneven p-bars becoming shorter outwards (two in VIa, four in VIb; Fig. 1C–D ) and one irregular patch of proximal, even cones (two in VIa and VIb; Fig. 1C–D ); areas VII–VIII: 51, two bands of p-bars ventrally well separated, with anterior and posterior bands consisting each of two transversely aligned rows, furrow row and ridge row with one p-bar on each region ( Fig. 1C ). Areas VI–V–VI ridge pattern, λ-shaped. Gap between area VI and areas VII–VIII narrow, as wide as palpostyle.

PAIRED OESOPHAGEAL CAECA. Present.

PARAPODIA. Without glandular, dorsal patches. Notopodia consisting of dorsal cirrus, dorsal ligule (distal and proximal), notopodial prechaetal lobe, and median ligule in biramous parapodia. Neuropodia consisting of neuroacicular ligule with inferior and postchaetal lobes, ventral ligule, and ventral cirrus; superior lobe absent throughout.

DORSAL CIRRI. Cirriform, long, extending markedly beyond distal region of dorsal ligule throughout ( Fig. 1E–I ); dorsal cirri 3.5–4 times as long as proximal region of dorsal ligule in anteriormost and anterior parapodia ( Fig. 1E–F ), 4–5 times as long as that in middle parapodia ( Fig. 1G–H ), 3 times as long as that in posterior ( Fig. 1I ) and posteriormost parapodia; attached basally to dorsal ligule in anteriormost parapodia ( Fig. 1E ), one-third in anterior parapodia ( Fig. 1F ), medially in middle parapodia ( Fig. 1G–H ), three-quarters in posterior ( Fig. 1I ) and posteriormost parapodia.

DORSAL LIGULE. Proximal region even towards posterior end; shorter than distal region of dorsal ligule in anteriormost and anterior parapodia ( Fig. 1E–F ), as long as that in following parapodia ( Fig. 1G–I ); glandular patch small, sub-oval, present on the base of dorsal cirri, more distinct in middle and posterior parapodia ( Fig. 1H–I ). Distal region well developed with similar length throughout ( Fig. 1E–I ); bluntly conical in anterior parapodia, conical in following parapodia, smaller than median ligule throughout; projecting distinctly beyond notoacicula throughout; one whitish, cylindrical, glandular patch throughout.

NOTOPODIAL PRECHAETAL LOBE. Present from parapodia 5 to parapodia 18 ( Fig. 1F ); bluntly conical, half as long as median ligule in anterior parapodia, then reducing and tapering gradually to notoacicular process that disappears in about parapodia 25.

MEDIAN LIGULE. Bluntly conical in anterior parapodia ( Fig. 1F ), conical in following parapodia ( Fig. 1G– I ), becoming slightly longer and narrower from middle parapodia towards posterior end.

NEUROACICULAR LIGULE. Smaller than ventral ligule in anteriormost parapodia ( Fig. 1E ), as long as in following parapodia ( Fig. 1F–I ), 1.5 times as wide as ventral ligule throughout.

NEUROPODIAL INFERIOR LOBE. Slightly developed and longer than neuroacicular ligule in first five parapodia, absent in following chaetigers.

NEUROPODIAL POSTCHAETAL LOBE. Present throughout; digitiform in anteriormost parapodia, bluntly conical in anterior parapodia, conical in following parapodia; slightly longer than neuroacicular ligule in anteriormost and posterior parapodia ( Fig. 1E, I ), as long as that in remaining parapodia.

VENTRAL LIGULE. Well developed throughout; bluntly conical, thick in anteriormost and anterior parapodia ( Fig. 1E–F ), conical and slender in following chaetigers; smaller than median ligule throughout, more distinct in posterior parapodia ( Fig. 1I ); extending beyond distal region of dorsal ligule in parapodia 1 and 2 ( Fig. 1E ).

VENTRAL CIRRI. Cirriform, slender; two-thirds as long as ventral ligule in anteriormost and anterior parapodia ( Fig. 1E–F ), one-third as long as that in following chaetigers ( Fig. 1G, I ).

ACICULAE. Black, with basal end uncoloured. Notoaciculae absent in first two chaetigers ( Fig. 1E ). Neuroaciculae extending beyond distal end of notoaciculae throughout, more distinct in anteriormost and anterior parapodia ( Fig. 1F ), with proximal half 1.2–1.4 times as wide as notoaciculae.

NOTOCHAETAE. All homogomph spinigers ( Fig. 1J ); 14–15 spinigers present in anterior parapodia, 9–10 spinigers in middle parapodia, 4 spinigers in posterior parapodia and 2 spinigers in posteriormost parapodia.

SUPRACICULAR NEUROCHAETAE. Mostly broken or with blades missing; consisting of homogomph spinigers ( Fig. 1K ) and heterogomph falcigers, both present throughout.

SUBACICULAR NEUROCHAETAE. Mostly broken or with blades missing; consisting of heterogomph spinigers ( Fig. 1L ) and heterogomph falcigers ( Fig. 1M ), both present throughout.

BLADES. Both homogomph ( Fig. 1J–K ) and heterogomph ( Fig. 1L ) spinigers with blades finely serrated towards toothed edge, evenly spaced, long (B/A ratio 8–17.5). Heterogomph falcigers with blades slender and of medium size (1.2–1.6), convex, terminal tooth blunt with inconspicuous tendon; serrations present in about two-fifths (0.4–0.45) of total blade length ( Fig. 1M ). Shaft of supracicular falcigers thicker than subacicular ones in posterior parapodia; camerated, with cavity divided sub-distally into two longitudinal partitions.

PYGIDIUM. Distinctly projected, bluntly conical, as long as wide, with anal cirri as long as last 9 chaetigers; cirrophores of anal cirri barely developed ( Fig. 1A ).

Remarks

Neanthes chilkaensis was not compared with other species in the original description. Among all the currently known species of Neanthes , N. chilkaensis resembles N. indica brunnea and N. mossambica ( Day, 1957) , both from Mozambique, and N. talehsapensis ( Fauvel, 1932) from the Songkhla Lake in Thailand. These species share a proximal region of dorsal ligule of similar size throughout the body (or slightly enlarged in posterior parapodia), presence of neuropodial postchaetal and notopodial prechaetal lobes at least in some anterior chaetigers, absence of neuropodial superior lobes, areas VII–VIII with two well-defined bands of more than 20 paragnaths, and area I with two or more paragnaths (see Villalobos- Guerrero & Idris 2021: table 2).

Nonetheless, N. chilkaensis is distinguishable from the species mentioned above according to the following diagnostic features: (I) the presence of p-bars on area VI, in contrast to its absence in those species; (II) the presence of solely p-bars on areas VII–VIII, in comparison to either conical only or both p-bars and conical ones in those species; (III) the multi-articulated tentacular cirri, in contrast to smooth ones in those species; (IV) four paragnaths on the area V of the pharynx, in comparison to none in those species; (V) a higher number of paragnaths on area III (29), in contrast to a lower number in N. indica brunnea (5–9) and N. talehsapensis (14); (VI) 5–7 paragnaths on area VI of the pharynx, in comparison to one in N. mossambica ; (VII) the presence of oesophageal caeca, in contrast to its absence in N. mossambica ; (VIII) aciculae mostly black, in contrast to pale coloured ones in N. talehsapensis ; (IX) the notopodial prechaetal lobe distinctly smaller than median ligule throughout, in comparison to a subequal in anterior chaetigers of N. talehsapensis ; (X) the dorsal cirri extending markedly beyond distal region of dorsal ligule, in contrast to those not extending beyond it in N. indica brunnea ; (XI) the presence of a neuropodial postchaetal lobe throughout, in comparison to it being present in anterior chaetigers only in N. indica brunnea ; (XII) the blade of heterogomph falcigers with blunt terminal tooth and inconspicuous tendon in middle and posterior parapodia, in contrast to hammer-headed terminal tooth and distinct tendon in N. indica brunnea ; (XIII) the antennae well separated from each other, in comparison to those closely together in N. indica brunnea ; and finally (XIV) the dorsum of segments with pigmentation bands, in contrast to it being present only in apodous segment in N. indica brunnea .

The species was described and illustrated in detail by Southern (1921) and placed within the genus Nereis using specimens in immature and reproductive stages collected from the southern half of the Chilika Lake, India. The original description is a combination of the available specimens’ morphology, which consisted of four epitokous (two females and males) specimens and seemingly four atokous specimens with different lengths and numbers of chaetigers: 92 mm, 81 chaet.; 65 mm, 84 chaet.; 56 mm, 66 chaet.; 44 mm, 81 chaet. However, the atokous specimen examined here does not match the size and number of chaetigers stated originally for those specimens. Southern (1921) used the specimen with 56 mm and 66 chaetigers to describe and illustrate the structures associated with the parapodia 1, 10, 50 and 60, which seems to correspond with one of two syntypes showing traces of desiccation deposited at the Zoological Survey of India (ZEV6281/7; http://www.zsicollections.in/search/ZSI0000004205) but not available for the present study. In general terms, the syntype examined matches the original description of N. chilkaensis . Hence, in order to fix the species definition (ICZN 1999, Art. 74.7.3), the specimen NHMUK 1938.5.7.27 is here selected as lectotype (ICZN 1999, Art. 74.1, 74.7.1) and described in detail with illustrations (ICZN 1999, Art. 74.7.2).

Moreover, some variations among the species deserve discussion. Southern (1921) described N. chilkaensis without paragnaths on area V, which are present in the lectotype in two oblique rows of four conical paragnaths near but placed behind the level of paragnaths of area VI. According to Southern (1921), the blades of subacicular heterogomph falcigers can wear distally giving two appearances: (I) long, slender, with many teeth; and (II) small, thick, with few teeth. In the syntype, only the first blade type was found possibly because most of the neurochaetae are broken or missing; however, judging Southern’s falciger illustrations, the presence of the second type of falciger, as occurs in other species of Neanthes ( Villalobos-Guerrero & Idris 2021; this study), is not disregarded.

The species was transferred to Neanthes by Hartman (1959), most likely founded on her proposal of distinguishing Neanthes from Nereis by the absence of homogomph falcigers (present in the latter genus; Hartman 1940, 1954). The species has been recognised in Neanthes until nowadays ( Fauchald 1972; Wilson 1984; Nesemann & Sharma 2007), sometimes incorrectly treated in the unaccepted subgenus Nereis (Neanthes) ( Misra et al. 1987; Nageswara Rao 1995). The species was briefly characterised by Fauvel (1932, 1953) from some specimens collected in the type locality and other sites in the east of India. Later, Misra et al. (1987) and Nageswara Rao (1995) did the same using specimens exclusively from the Chilika Lake, although the characterisations were very short and incomplete.

Neanthes chilkaensis has been recorded from several localities along the shores of India ( Parulekar 1971; Sunder Raj & Sanjeeva Raj 1987; Nesemann & Sharma 2007; Markande et al. 2014) and Sri Lanka ( de Silva 1965), which were mostly based on Fauvel’s (1932, 1940, 1953) studies from India. These records are considered valid, although re-assessment is needed, especially those from Sri Lanka and the south and west of India.

Distribution

East India: Chilika Lake (Ossira), Mandras, Pamban (Tamil Nadu) ( Fauvel 1932, 1953; Ganapati 1946; Misra et al. 1987; Nageswara Rao 1995); west India: Neendakara (Kerala) ( Fauvel 1940), Dabhol, Devgad, Malvan, Mumbai, Vengurla and Vijaydurg (Maharastra) ( Markande et al. 2014; Pati et al. 2015); Sri Lanka ( de Silva 1965).

Ecology

Common in muddy estuarine habitats ( Misra et al. 1987). In Chilika Lake, living in brackish water under stones, sand above and below high-water mark, amongst algae on the rocks, and in sponges Spongilla Lamarck, 1816 and Laxosuberites Topsent, 1896 (= Hymeniacidon Bowerbank, 1858 sensu van Soest 2020 ) ( Southern 1921). Host of the intestinal gregarine parasite Lecudina ganapatii Vivier, Ormieres & Tuzet, 1964 [as L. pellucida (von Kölliker, 1848) Mingazzini, 1891 ] ( Ganapati 1946).

Reproduction

According to Southern (1921), males and females undergo an epitokal metamorphosis but not as developed as in the heteronereis stage. Epitokous specimens were found in January, February and November. Males gyrate in a spiral course on water column to release the sperm.

Figures

Fig. 1. Neanthes chilkaensis (Southern, 1921). Lectotype (NHMUK 1938.5.7.27) from Chilika Lake, Orissa, India, atokous. A. Entire body in dorsal view (inset: approach of pygidium). B. Anterior end in dorsal view. C. Non-everted pharynx in ventral view. D. Dorsum of oral ring of non-everted pharynx in ventral view. E. Left parapodium from chaetiger 2 in posterior view. F. Left parapodium from chaetiger 13 in posterior view. G. Right parapodium from chaetiger 26 in anterior view. H. Right parapodium from chaetiger 38 in anterior view with ventral cirri contracted and hidden behind the neuroacicular ligule. I. Parapodium from chaetiger 52 in anterior view. J. Homogomph spiniger from notochaetae (chaetiger 13). K. Homogomph spiniger from supracicular neurochaetae (chaetiger 13). L. Heterogomph spiniger from subacicular neurochaetae (chaetiger 2). M. Heterogomph falciger from subacicular neurochaetae (chaetiger 13). Roman numerals refer to the areas and Arabic numbers to chaetigers. Scale bars: A = 3 mm; B = 1 mm; C = 0.5 mm; D–I = 0.2 mm; J–M = 20 µm.