Two new species of Notodasus Fauchald, 1972 (Annelida: Capitellidae) from the Central Indo-Pacific region

Notodasus Fauchald, 1972 is a small genus of the polychaete family Capitellidae, including 10 described species worldwide. The genus is unusual in the Central Indo-Pacific, and there is no taxonomic record of Notodasus in this region. In this study, two new species of Notodasus are described and illustrated, namely Notodasus celebensis sp. nov. and N. chinensis sp. nov. The former species, collected from the mixed-species seagrass beds in the Indonesian island of Sulawesi, is mainly characterized by the longitudinally striated epithelium on thoracic segments and the completely separated notopodial lobes. The latter species, obtained from coastal waters off southern China, differs from its congeners with the following characters: tessellated epithelium present on anterior thorax as well as on the dorsum of chaetigers 11 and 12, notopodial lobes fused and chaetal fascicles almost touching each other on anterior abdomen, and branchial pores evident from anterior abdomen. Comparisons are made with closely related species in this paper, and a revised key is provided to all described Notodasus species. The descriptions of the two new species represent the first record of Notodasus in this region and expand the geographical distribution of the genus.


INTRODUCTION
Polychaetes, known as an ecologically important taxon of benthic macrofauna, are frequently found and numerically dominated in marine surveys. They exhibit high taxonomic diversity with over 11,000 valid species worldwide (Pamungkas et al., 2019). Of all known polychaete families, Capitellidae Grube, 1862 is a family with approximately 200 described species, represented by 43 genera (Magalhães & Blake, 2017;García-Garza, De León-González & Tovar-Hernández, 2019). This family is usually associated with organically enriched and disturbed sediments (Magalhães & Blake, 2017), and as such, some species can be used as environmental indicators (Pearson & Rosenberg, 1978;Reish, 1980;Warren, 1991). Although extensive taxonomic studies on capitellid polychaetes have been carried out, correct identification of capitellid species is fairly challenging due to their two new species also allows us to better understand the geographical distribution of the genus. A revised key to all Notodasus species is also provided in this paper.

MATERIALS AND METHODS
The Notodasus specimens were collected from the southern coasts of China ( Fig. 2A) during 2017-2018 and from mix-species seagrass beds of northern Sulawesi Island, Indonesia (Fig. 2B) in May 2014, respectively (for more detail, see Table 1). Indonesian specimens examined in this study were collected with permission of the Ministry of Research and Technology of the Republic of Indonesia (permit no. 135/SIP/FRP/SM/V/ 2014). In Indonesia, a PVC corer (10 cm in inner diameter) was used to collect sediment samples which were later washed through a 0.5 mm sieve in the field. In China, the Notodasus specimens were collected by means of a grab sampler (surface area 0.05 m 2 ), and then sieved through a 0.5 mm sieve on board. All retained specimens were fixed with 7% diluted formalin in seawater. In the lab, Notodasus specimens were transferred to 70% ethanol.
Light microscope images were obtained by means of a Leica M205A stereomicroscope equipped with Leica DFC 550 digital camera. The structure of hooded hooks was observed under a light microscope using oil immersion (Axio Imager Z2; ZEISS, Oberkochen, Germany). A scanning electron microscopy (SEM) analysis was conducted to observe the ultrastructure of abdominal hooks. In brief, the specimens were placed in an ultrasonic chamber with distilled water for 60 s to remove the hoods of the abdominal hooks. The treated specimens were dehydrated and then dried in a drying oven at 60 C for 5 min. Finally, specimens were mounted on a stub and coated with gold. SEM observations were performed using ZEISS SUPRA 55 SAPPHIRE at Xiamen University, China. The MGSP was used to identify the distribution of glandular areas, following the protocol of Warren, Hutchings & Doyle (1994). Morphological terminology and the characters used for classification follow those of Warren, Hutchings & Doyle (1994).
Type material of several Notodasus species were reviewed from the Natural History Museum of Los Angeles County-Allan Hancock Foundation (LACM-AHF) and the Colección Poliquetologica de la Universidad Autonoma de Nuevo León (UANL). The type material of the two new species described herein are deposited in the Third Institute of Oceanography, Ministry of Natural Resources, Xiamen, China.

Nomenclatural acts
The electronic version of this article in portable document format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank Life Science Identifiers (LSIDs) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org: pub:6342781B-D33C-4FF8-85BD-37D185FC2403. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.
Branchiae not known, as all examined specimens incomplete. Pygidium not known.
Variations. The holotype is a larger individual than paratypes. Meanwhile, the longitudinally striated epithelium is more evident in the holotype.
Methyl green staining (Figs. 4A-4D and 5G-5H). Thorax uniformly stained light green except by the presence of a medium green transverse band on peristomium. Methyl green stain on first two abdominal chaetigers slightly darker than on thorax. Abdominal chaetigers 3-13 with medium green stain on dorsolateral areas between noto-and neuropodial lobes and ventral areas around neuropodia, a longitudinal mid-ventral band stained with medium green, and light green stain on parapodial tori and lateral organs. Abdominal chaetiger 14 and following ones stained with a light green completely, and mid-ventral band faded. Etymology. The specific name is derived from the type locality, Chinese waters.
Branchiae digitiform in holotype, may be retractile, only observed on some superior neuropodial lobes of abdominal segments (Fig. 6F), arising from a small pore just above neuropodial fascicles. Pygidium not seen.
Variations. All specimens are incomplete, without posterior abdomen. Tessellated epithelium on chaetigers 11 and 12 are more evident in larger specimens. Fused notopodial lobes located on a raised coalesced lobe on abdominal chaetigers 2-4 in larger specimens, while on abdominal chaetigers 2-3 in smaller specimens.
Methyl green staining pattern (Figs. 8A-8C and 9A-9B). Thorax and abdominal segments completely stained light green except that dark green stain from chaetiger 6 to prechaetal area of chaetiger 7, and moderate green stain from chaetiger 11 to prechaetal area of chaetiger 12.

DISCUSSION
On N. celebensis sp. nov.
Among all 10 known Notodasus species worldwide, Notodasus celebensis sp. nov. (Fig. 5G) is most similar to N. magnus (Fig. 5A) and N. harrisae (Fig. 5C) from the Gulf of California, and N. fauchaldi (Fig. 5E) from the Andaman Sea by having longitudinally striated epithelium on thoracic segments, whereas the rest members of the genus bear thoracic segments with tessellated epithelium. However, N. celebensis sp. nov. differs from these three closely related species, based on other morphological characters (Table 2). N. celebensis sp. nov. is distinguished from N. magnus in that: (1) N. celebensis sp. nov. bears rounded prostomium with digitate palpode compared with conical prostomium with short palpode in N. magnus; (2) striated epithelium is present on more thoracic segments in N. magnus than in N. celebensis sp. nov.; (3) abdominal notopodial lobes are completely separated along the abdomen in N. celebensis sp. nov., while in N. magnus, they are fused with a median constriction in anterior abdomen; (4) N. celebensis sp. nov. has abdominal hooks with four rows of small teeth above main fang instead of three rows as in N. magnus. N. celebensis sp. nov. also differs from N. harrisae in that: (1) N. celebensis sp. nov. bears prostomium with digitate palpode and without eyespots compared with prostomium with short palpode and eyespots in N. harrisae; (2) striated epithelium are present on anterior 8 chaetigers of N. celebensis sp. nov. but on the entire thorax of N. harrisae; (3) abdominal notopodial lobes are completely separated along the abdomen in N. celebensis sp. nov. whereas they are fused in the anterior abdomen of N. harrisae; (4) abdominal hooks of N. celebensis sp. nov. have four rows of small teeth above main fang instead of three rows as in N. harrisae. Furthermore, N. celebensis sp. nov. differs from N. fauchaldi in that: (1) N. celebensis sp. nov. bears prostomium with digitate palpode and without eyespots compared with prostomium with short palpode and eyespots in N. fauchaldi; (2) abdominal notopodial lobes are completely separated along the abdomen in N. celebensis sp. nov. whereas they are fused in the anterior abdomen of N. fauchaldi; (3) in anterior abdomen, lateral organs are situated in a pit in N. celebensis sp. nov. but protruded above surface in N. fauchaldi.
The inhabiting environment is also different: N. celebensis sp. nov. was found in the shallow nearshore seagrass beds characterized by fine sand; N. magnus was collected from soft sediments mixed with sand, mud, and pebbles at depths of 29-35 m; N. harrisae was found to inhabit intertidal and shallow fine sand; and N. fauchaldi was recorded in a variety of sediments at depths of 21-55 m.
As for methyl green staining, the most relevant characteristic of N. celebensis is that it has a dark transverse band on peristomium and medium green stain on dorsolateral areas of abdominal chaetigers 3-13 (Figs. 5G-5H), which is distinct from the other three Notodasus species. Based on the original descriptions of type species, N. magnus has darker prechaetal and postchaetal transverse band on abdominal chaetigers 3-5 (Fig. 5B); N. harrisae has two dark dorsolateral longitudinal bands on abdominal chaetigers 3-22 (Fig. 5D); and N. fauchaldi has dark green stain on dorsum of abdominal chaetigers except for notopodial lobes and lateral organs (Fig. 5F). Moreover, the latter three species have uniform light green stain on anterior thorax, without a dark band on peristomium.
Quite a few monographs and papers dealing with Indonesian polychaetes have been published (Caullery, 1915(Caullery, , 1944Horst, 1903Horst, , 1910Horst, , 1912Horst, , 1915Horst, , 1916aHorst, , 1916bHorst, , 1917Horst, , 1924Pettibone, 1970Pettibone, , 1971AI-Hakim & Glasby, 2004;Pamungkas, 2015Pamungkas, , 2017. In these publications, seven capitellid genera were taxonomically recorded in Indonesian waters, namely Capitella, Dasybranchus, Mediomastus, Notomastus, Polymastigos, Promastobranchus, and Scyphoproctus. N. celebensis sp. nov., which is newly described from Sulawesi Island, represents the report of Notodasus in Indonesian waters for the first time. The number of capitellid genera in this area rises to eight genera. Notodasus chinensis sp. nov. mostly resembles N. oraria (Fig. 9C) from the waters off California, USA and N. dexterae (Fig. 9D) from the Pacific coast of Panama. These three species share the tessellated epithelium on thoracic segments, the fused notopodial lobes in anterior abdomen, and the mid-ventrally separated neuropodial lobes along abdomen. However, N. celebensis sp. nov. bears tessellated epithelium on the dorsum of chaetigers 11 and 12 and branchial pores commencing from abdominal chaetiger 2, which are not found in other species in the genus. In addition to the above morphological characters exclusive to N. chinensis sp. nov., N. chinensis sp. nov. can be distinguished from N. oraria in that: (1) eyespots are present in N. chinensis sp. nov while absent in N. oraria; (2) thoracic segments have tessellated epithelium on anterior 5 chaetigers in N. chinensis sp. nov. while on chaetigers 1-8 of N. oraria. N. chinensis sp. nov. also differs from N. dexterae in that: (1) lateral organs are situated in a pit in anterior abdomen in N. chinensis sp. nov. while protruded above surface in N. dexterae; (2) abdominal hooks have four rows of small teeth above main fang in N. chinensis sp. nov. instead of five rows of small teeth as in N. dexterae. For more details, see Table 3.
In terms of inhabiting environment, N. chinensis sp. nov. is described from shallow subtidal mud or muddy sand (7-12 m), N. dexterae inhabits intertidal sand, and N. oraria is mainly found in muddy sediment at depths of 1-180 m.
Furthermore, N. chinensis sp. nov. has a distinct MGSP: dark green stain on chaetigers 7-8, medium green stain on chaetigers 11-12, and light green stain on the remaining segments (Figs. 9A and 9B). According to the original descriptions of type materials, N. oraria has medium green stain from the postchaetal part of chaetiger 6 to prechaetal part of chaetiger 10, and dark green stain on chaetigers 11 and 12 (Fig. 9C); N. dexterae has medium green stain on chaetigers 9-13, and two dark dorsolateral bands from the third abdominal chaetiger (Fig. 9E).

On the generic definition of Notodasus
The genus Notodasus was originally erected by Fauchald (1972), distinct from other capitellid genera mainly by having only capillaries on all 11 thoracic chaetigers as well as on the first two abdominal ones. Since then, eight Notodasus species had been added to the genus, all in agreement with the generic definition. The genus Dodecaseta was initially established by McCammon & Stull (1978), then its definition was expanded by Green (2002) as having first one or two abdominal chaetigers with capillaries instead of first abdominal chaetiger as in the original definition. Of the three known Dodecaseta species, D. eibyejacobseni completely matched the generic diagnosis of Notodasus. The remaining two species, D. oraria and D. fauchaldi, agree well with the generic definition of Notodasus except that they bear abdominal capillaries only on first abdominal chaetiger instead of on first two abdominal chaetigers as in Notodasus. García-Garza, De León-González & Harris (2017) believed that the above morphological difference might be due to the fact that these two Dodecaseta species were described based on immature specimens. It is well known that the replacement of hooded hooks by capillaries occurs in some capitellid genera during ontogeny (Ewing, 1982;Blake, 2000), and the number of chaetigers with capillaries will change until the adult condition is reached. Based on the high morphological similarity, García-Garza, De León-González & Harris (2017) considered Dodecaseta as a junior synonym of Notodasus, without any technical change in the generic definition.
In addition to the presence of capillaries on all 11 thoracic chaetigers and first two abdominal chaetigers, Notodasus bears other distinctive morphological characters: partially developed neuropodia and protruded lateral organs on the last one or two chaetigers with capillaries (Green, 2002). In this study, one of the newly described species, N. celebensis sp. nov., completely matches the generic diagnosis of Notodasus. However, the other species, N. chinensis sp. nov., agrees with the generic diagnosis of Notodasus in most morphological characters except that all examined specimens are characterized by the presence of capillaries on first abdominal chaetiger and absence on the following abdominal segments, irrespective of body size. Given that the specimens of N. chinensis sp. nov. were collected in different seasons (October 2017, January 2018, and April 2018), we

CONCLUSIONS
Located between the Pacific Ocean and the Indian Ocean, the Central Indo-Pacific region is an important marine biodiversity hotspot with especially rich marine life. In this region, Notodasus is a poorly known group, and prior to this study, there was no taxonomic report of Notodasus species. The description of new Notodasus species from Sulawesi Island and southern China indicates that there is higher diversity within the genus than expected, and this contributes to better understand its diversity worldwide. Besides, this study provides more data about the ecological aspects of Notodasus. However, future efforts should be devoted to the taxonomy of polychaete fauna in this region due to relatively scant information on this group.