Sigambra sundarbanensis Bhowmik & Ghoshal & Salazar-Vallejo & Mandal 2021, sp. nov.

Sigambra sundarbanensis sp. nov.

urn:lsid:zoobank.org:act: D315C406-6F83-413C-BFCA-E00A8D83070C

Figs 2–5; Table 2

Diagnosis

A species of Sigambra with median antenna reaching up to chaetigers 3–4, 2–3 times as long as lateral antennae; tentacular segment 3–4 times as wide as long. Pharynx with 14 prismatic projected lobes. Dorsal cirri larger than ventral ones, largest in chaetiger 1. Ventral cirri absent in chaetiger 2. Notopodial hooks start in chaetiger 8, accompanied by notoacicula; neuropodia with various types of capillary chaetae. Parapodial spaces with glandular, tubular structures.

Etymology

The type locality (river Thakuran) is a tidal estuarine river of the Sundarbans Estuarine System. The epithet of this new species refers to the entire estuarine system, i.e., Indian Sundarbans.

Type material

Holotype INDIA • complete spec.; river Thakuran, stn T8; 21°39′3.73″ N, 88°30′25.17″ E; depth 26 m; Aug. 2019; Moumita Bhowmik and Sumit Mandal leg.; in sediment; PUZ 501.

Paratypes INDIA • 4 complete specs; river Thakuran, stn T6; 21°45′35.90″ N, 88°29′8.53″ E; depth 10 m; Aug. 2019; Moumita Bhowmik and Sumit Mandal leg.; in sediment; PUZ 502 to PUZ 505 • 3 complete specs; river Thakuran, stn T8; 21°39′3.73″ N, 88°30′25.17″ E; depth 26 m; Aug. 2019; Moumita Bhowmik and Sumit Mandal leg.; in sediment; PUZ 506 to PUZ 508 • 6 complete specs; river Thakuran, stn T8; 21°39′3.73″ N, 88°30′25.17″ E; depth 26 m; Dec. 2019; Moumita Bhowmik and Sumit Mandal leg.; in sediment; PUZ 514 to PUZ 519 • 2 incomplete specs; river Matla, stn M5; 21°45′18.20″ N, 88°38′25.20″ E; depth 11 m; Jan. 2019; Moumita Bhowmik and Sumit Mandal leg.; in sediment; PUZ 490 to PUZ 491.

Sampling site and type locality

Various environmental factors that characterize the sampling sites are in Table 1. Bottom water salinity ranged from 17.0 in August to 23.42 in January 2019. Sediment temperature was found to be at its maximum in August 2019. Organic enrichment in sediment was moderate, ranging from 0.78 to 1.78%. In terms of granulometry, the study sites are mostly silty with comparatively finer and coarser particles that vary seasonally. The lowest proportion of clay was represented in the soil texture during the monsoon (0.15–0.35%). The sediment texture of the type locality was characterized by a high silt percentage and a lower sand percentage that further decreased in the post-monsoon season (Dec. 2019). Bottom water salinity level varied from 17 to 21 (Table 1). Morphological and morphometric data are in Table 2 and the comparison of the new species with all other accepted species of Sigambra is in Table 3.

The holotype of Sigambra sundarbanensis sp. nov. was collected from the river Thakuran (station T 8) and paratypes were collected from both the rivers Thakuran and Matla in January 2019, August 2019 and December 2019. A morphometric analysis was performed for all the collected specimens. Moreover, a global map (Fig. 2) has been presented for all the accepted species of Sigambra based on their type locations.

Description

Holotype (PUZ 501)

MEASUREMENTS. Complete, 5.63 mm long, 0.32 mm wide at chaetiger 8–9 (average width 0.28 mm), 64 chaetigers (Fig. 3A).

BODY. Obconic, sub cylindrical along anterior end, depressed thereafter.

PROSTOMIUM. Blunt, bilobed, three times as wide as long. Palps biarticulated directed ventrally; palpophores large, palpostyles small. Pharynx exposed with 14 prismatic marginal papillae, tips distinct

(Fig. 3C). Antennae cirriform, lateral antennae subdistally located, smaller than median one (Fig. 3B). Median antenna 2.3 times as long as laterals, reaching up to chaetiger 4.

TENTACLES. Tentacular segment 3–4 times as wide as long; two pairs of tentacular cirri, dorsal tentacular cirri slightly larger than ventral ones.

CIRRI. Parapodial cirri triangular, tapered, foliose, longer than wide. Dorsal cirri longer than ventral cirri throughout, largest in chaetiger 1, reaching up to chaetiger 5 (Fig. 3D). Chaetiger 2 with smallest dorsal cirri, without ventral cirri. Parapodia with reduced notopodia and well developed neuropodia.

NOTOPODIA. Include distally curved dorsal hooks from chaetiger 8 (Fig. 3D), head of hook not exposed outside body wall to chaetiger 22, fully exposed from chaetiger 23, continued along body (Fig. 3E) up to last 2 pre-pygidial chaetigers (Fig. 3G). From chaetiger 8 onwards, hooks accompanied with acicula (Fig. 5A–B). Neurochaetae include 2–4 short wide pectinate chaetae with variable number of spinulose or serrated capillaries (Figs 3F, 5A).

GLANDS. Parapodial glands starting from chaetiger 5, developed gradually up to chaetiger 60. Each gland with 2–6 large tubular cells, varying in shape and size (Fig. 4B, 5D). These tubular structures converge ventrally from wide base of coelomic ramus. Tubular structures rudimentary (L: 19 µm, W: 11 µm) or fully developed (L: 50 µm, W: 8 µm); inner features unknown.

PYGIDIUM. Laterally expanded with 2 ventral cirri, as long as 3–4 median chaetigers (Fig. 3G).

OOCYTES. Not seen.

Paratypes

A total of 13 complete and 2 incomplete paratypes show a minor characteristic variation. They were 2.18–8.91 mm long (5.09 ± 2.29 mm), 0.08–0.41 mm wide (0.15 ± 0.08 mm); median antennae were 0.2–0.57 mm long (0.36 ± 0.11 mm) reaching up to chaetigers 3–4. Oocytes (Figs 4A, 5C) 12–36 µm in diameter (23.33 ± 6.90 μm). Glandular structures in parapodial spaces have been found in most paratypes, they were 14–74 µm long (43.88 ± 17.69 µm) (Table 2). Large tubular glandular cells in chaetigers 47–49 of paratype PUZ 506 are shown in Fig. 4C–D. In other parapodia (chaetigers 12 and 13), tubular cells invade into coelomic space (Fig. 4E–F).

Remarks

Following the redescription of S. parva by Moreira & Parapar (2002), it can be stated that S. sundarbanensis sp. nov. resembles S. parva Day, 1963. They have similar characteristics, such as median antenna longer than lateral ones, reaching chaetigers 3–4, and pharynx with 14 marginal papillae. However, they differ in several features, the most notable ones being the starting point of the dorsal hooks and the absence of capillary chaetae in the notopodia. In S. sundarbanensis sp. nov., the first appearance of dorsal hooks from chaetiger 8 remains constant in all 16 specimens, irrespective of specimen size. The hooks are accompanied by a single acicula, and the last two chaetigers are hookless. The notopodia are devoid of any capillary chaetae, neuropodia with 2–4 short pectinate chaetae with a variable number of spinulose or serrated capillaries, and the relative size of the median antenna is 2.3 times as long as the lateral ones. In comparison with S. parva, the median antenna is 1.5 times as long as the lateral ones, the notopodial hook starts from chaetigers 4–5 and is accompanied by single capillary chaetae in the posterior parapodial segments, neuropodia with 1–2 pectinate chaetae, but the number of hookless chaetigers is not mentioned in the literature (Day 1963; Moreira & Parapar 2002).

Distribution

Sigambra sundarbanensis sp. nov. is only known from the rivers Matla and Thakuran of the Indian Sundarbans.

Ecology

All specimens of this new species were found in mangrove habitats with silty sand sediments, in depths of 11 to 26 m. Mature specimens, with developed oocytes, were recorded in August and December 2019 from Thakuran River. Among all the abiotic factors, salinity plays a pivotal role in ecology and

distribution of species across the globe, as this acts as a physiological barrier for both stenohaline and euryhaline species. Sigambra parva was recorded from Cape Province, South Africa (Day 1963) and the Mediterranean coast of Spain (Moreira & Parapar 2002), where the water salinity remains higher than30%, whereas the localities of S. sundarbanensis sp. nov. had a salinity of 17–23.42%. Additionally, S. parva had a comparatively higher range of depth variation from 2 to 97 meters (Day 1963; Moreira & Parapar 2002).