Sigambra sundarbanensis sp. nov. (Annelida, Pilargidae) from the Indian sector of Sundarbans Estuarine System, with remarks on parapodial glands

Abstract. A new pilargid species, Sigambra sundarbanensis sp. nov., is described from the rivers Matla and Thakuran, in the central Indian sector of the Sundarbans Estuarine System. This species is characterized by several characters such as the starting position of the notopodial hooks, the length of the median antenna and the variation in number of the neuropodial chaetae. These characters distinguish the new species from its congeners. Some parapodial glands have been found in individuals of this species. The new species closely resembles Sigambra parva (Day, 1963). Additionally, an updated key of genus Sigambra is provided, along with a table indicating their morphological variations and a global map showing their type localities.


Introduction
Pilargids are uncommon nereidiform marine annelids; most are free-living, and many are motile burrowers. They are ubiquitous in sediments at various depths in estuarine or oceanic realms, most preferably in coarse to mixed substratum (Jumars et al. 2015). They are considered carnivores and transported to the laboratory and later sorted and preserved in 70% ethanol. For better observation of the parapodial structures, and to get a clear visualization of the starting position of the notopodial hooks, the marginal papillae and the oocytes, specimens were mounted in a 1:1 solution of glycerin and 70% ethanol. Methyl green staining is often used to examine internal structures such as parapodial glands, as most glands contain phospholipids and the stain is more intensely fi xed upon them. Our specimens were fi rst immersed in an oversaturated methyl green solution in 70% ethanol (as samples were preserved in the same concentration of ethanol) for about 1 min; then, they were briefl y set on tissue paper and further rinsed in clean ethanol. Samples were scanned and photographed through stereo (Olympus SZX7) and compound (Nikon ECLIPSE Ci) microscopes. For environmental parameters (temperature, pH, salinity, sediment texture and organic content), bottom waters and sediment samples were collected on-board and analyzed using standard protocols (El Wakeel & Riley 1957;Buchanan 1984;Grasshoff et al. 1999). The holotype and all paratypes were deposited at the Zoology Museum, Department of Life Sciences, Presidency University, Kolkata, India (PUZ). Class Polychaeta Grube, 1850Order Phyllodocida Dales, 1962Suborder Nereidiformia Glasby, 1993 Family Pilargidae de Saint-Joseph, 1899 Subfamily Pilarginae de Saint-Joseph, 1899 Genus Sigambra Müller, 1858 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.

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 fi ner 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 T8) 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.

Fig. 2.
Type localities of the accepted species of Sigambra Müller, 1858, names of the species are denoted with the serial numbers from Table 3. PYGIDIUM. Laterally expanded with 2 ventral cirri, as long as 3-4 median chaetigers (Fig. 3G).

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 fi rst 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

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 than 30%, 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).

Key to species of Sigambra Müller, 1858
(   (Paterson & Glover, 2000) * As per Salazar-Vallejo et al. (2019), this is not completely identifi ed and based upon some specimens which have not been described yet, but are distinct and deserve to be described.

Discussion
In general, there are two patterns regarding positional variation of notopodial hooks in Sigambra. It either starts from a specifi c chaetiger (1-3 chaetiger variation) or shows variation. The position of the notopodial hook in S. sundarbanensis sp. nov. follows the fi rst pattern, whereas most of its congeners show variations. The general variability in the fi rst appearance of the hooks and the number of hookless chaetigers in pre-pygidial segments can limit its taxonomic position (Licher & Westheide 1997).
Based on literature, Sigambra sundarbanensis sp. nov. is the fi rst reported species of this genus with parapodial glandular structures. According to the methyl green stained images, these glands resemble the chromophile glands found in the neuropodial pinnae among the members of Tomopteridae (Grube, 1850) (e.g., Tomopteris helgolandica Greeff, 1879) which are pelagic in nature and involved in light production (Gouveneaux et al. 2017). For the taxonomic classifi cation of tomopterids, Fauvel (1923) mentioned these glands and their affi nity towards nuclear dyes like haematoxylin. Later on, a detailed histochemical examination on these light-producing parapodial glands by Gouveneaux (2016) confi rmed their similar staining property. However, no such parapodial structures have so far been reported for any species of Sigambra in particular or pilargids in general. Even though the histochemistry of this gland is unknown, methyl green staining images (Fig. 4B-F) depict a visual resemblance with the image in Gouveneaux et al. (2017: fi g. 2f). Moreover, the fi rst appearance of these glands from the anterior parapodia along with their gradual enlargement can be similar to what is shown in specimens of Enapteris euchaeta Chun, 1888 or Tomopteris elegans Chun, 1888. Their chromophile glands are very conspicuous and situated inferiorly from segment 4, although in Tomopteris apsteini Rosa, 1908 they are visible from the third pair of parapodia, progressively larger, becoming enormous globes hanging under ventral rami (Støp-Bowitz 1948;Böggemann 2009). However, without having a detailed knowledge on its histochemistry and function, the observed structure in our study should not be further named; rather, it would be more appropriate referring them as parapodial glands.
Sigambra sundarbanensis sp. nov. is described from a mangrove dominated estuary of the Indian Sundarbans. The species has been described with morphological features along with the environmental factors in different seasons from the rivers Matla and Thakuran, Sundarbans. This report contributes to the polychaete checklist of Indian waters. It also includes the fi rst documentation of some parapodial European Journal of Taxonomy 744: 49-66 (2021) glands in Sigambra. Moreover, it also delivers a global map of the type localities of all the accepted species of Sigambra.