Description of two new species of Paraonidae (Annelida) from the Gulf of Thailand, Western Pacific

Abstract Two new species of Aricidea Webster, 1879 (Paraonidae), Aricidea (Acmira) anusakdiisp. nov. and Aricidea (Aricidea) thammapinanaesp. nov. were collected from 10–26.5 m depth, in soft bottoms with mud mixed with sand and shells at Songkhla Sea, the Gulf of Thailand between 2011–2018. Aricidea (Acmira) anusakdiisp. nov. is clearly distinguished from other species of the subgenus Acmira by having a rounded bilobed prostomium divided by a slight notch on the anterior margin; red pigments on the subdistal to the tip of each branchia (new character); two prebranchial chaetigers; 48–68 pairs of branchiae; and modified neurochaetae as strong curved spines with blunt shafts surrounded by pubescence from chaetigers 19–44. On the other hand, Aricidea (Aricidea) thammapinanaesp. nov. can be separated from other members of the subgenus Aricidea by the presence of a biarticulated median antenna; distinctive notopodial lobes as broad triangular with short distal protuberances on chaetiger 3, 4–8 pairs of branchiae; and modified neurochaetae as bidentate neurochaetae with a long pubescent subterminal arista on the concave side. All data have been archived and are freely available from the Dryad Digital Repository (https://doi.org/10.5061/dryad.hqbzkh1cn).


Introduction
Polychaetes in the seas around Thailand are poorly known, especially those belonging to Paraonidae, a family of small burrowing polychaetes usually found in soft sediments (Rouse and Pleijel 2001). Until now, the only study of Thai paraonid species was published by Lovell (2002), who reported 19 taxa of paraonids from the Andaman Sea around Phuket Island, of which three species were newly described. The present study is the result of a monitoring program carried out between 2011 and 2018 entitled "Status of Coastal and Marine Resources and Ecosystem in Songkhla's Sea and Monitoring projects of Petroleum Production Area in Songkhla Sea". The family Paraonidae was one of the most species-rich families in the study area with over 20 undescribed taxa. The genus Aricidea is the most diverse in the Paraonidae, with more than 75 known species (Blake 2019). Strelzov (1973) divided the genus into four subgenera: Aricidea sensu stricto, Aedicira Hartman, 1957, Allia Strelzov, 1973(= Strelzovia Aguirrezabalaga, 2012, and Acesta Strelzov, 1973(= Acmira Hartley, 1981, which were separated based on the nature of the modified neurochaetae. Hartley (1981) pointed out that the chaetal differences are unclear to justify the generic status for the four subgenera (Blake 2019), and these subgenera have largely been accepted by taxonomists (Blake 1996(Blake , 2019Lovell 2002;Arriaga-Hernández et al. 2013, among others). During the identification process, we observed several specimens from the genus Aricidea Webster, 1879 that had a combination of taxonomic characters not found in the previously described species.
Another subgenus, Aricidea (Aricidea), is characterized by the presence of cirriform prostomial antennae, usually articulated, and modified neurochaetae either pseudocompound or hooked with subterminal spines on the concave side (Blake 1996). Fifteen species have been described from various localities around the world (Table  1), but in the seas around Thailand only Aricidea (Aricidea) fragilis Webster, 1879, A. (Aricidea) multiantennata Lovell, 2002 and A. (Aricidea) thailandica Lovell, 2002 have been reported. The specimens examined in the present study were characterized by the shape of the median antenna, the structure of the modified chaetae, and particularly by the shape of the third notopodial lobe. The latter was broadly triangular with a short round distal protuberance; this feature suggested that these paraonids could belong to an undescribed species.
The aim of this study is to examine in detail the morphological characteristics of these specimens using scanning electron microscope (SEM) images and light microscope photographs to verify differences from previously described species, and to confirm them as new species or not. Comparative tables of the diagnostic features of the new species and of those observed in closely similar taxa are included.

Materials and methods
Specimens were collected between 2011 and 2018 in the southern Gulf of Thailand (7°14'21"-7°49'21"N, 100°24'42"-100°49'00"E) ( Fig. 1), with a Van Veen grab (0.1 m 2 ) at depths ranging from 10 to 26.5 m. The collected samples were sieved with 2.0 mm, 1.0 mm and 0.5 mm mesh screens in the field. Later, water and sediment from the sieved grab samples were passed through a 300 µm filter bag. Specimens retained by both separation methods were separately fixed with a 4% formaldehyde in seawater solution.
In the laboratory, samples were washed with fresh water and transferred to 70% ethanol. The polychaetes were sorted into taxonomic groups using a stereomicroscope and those belonging to the proposed new species were examined under dissection and compound light microscopes. SEM images were produced with a JEOL JSM-5800LV microscope and Apreo-Field Emission Scanning Electron Microscope (FESEM) at the Office of Scientific Instrument and Testing (OSIT), Prince of Songkla University, Hatyai, Thailand. Light microscope photographs were taken with a Leica digital camera in the OSIT, Prince of Songkla University and Olympus SZX16 with DP74 digital camera at MEM. The morphological measurements of the holotype are indicated in the taxonomic description. Information on character variability found in the paratypes is included in parentheses. The confirmation of the taxonomic status of the new species was based on the excellent revision and compilation of the diagnostic characteristics of all recognized species of the subgenera Aricidea (Acmira) and Aricidea (Aricidea) by Blake (1996), Lovell (2002) and (1 spec.), Sta. S12-3, 9 Feb. 2012; AM W.52895 (1 spec.), Sta. S10-9, 6 May 2018.
Reproduction. Holotype and paratypes of A. (Acmira) anusakdii sp. nov. collected in March, May, June, and August had eggs in their branchial chaetigers. Eggs were also found in October in non-type material.
Etymology. The species was named in honor of, and to remember, Mr Anusakdi Plathong, Sakanan's deceased father.
Habitat. At 10-26.5 m depth, in mud mixed with sand and shells substrates. Distribution. Songkhla Sea, Gulf of Thailand, Western Pacific. Remarks. Currently, the subgenus Aricidea (Acmira) Hartley, 1981 is represented by 20 species, including the new species described in the present study. The species that make up this subgenus can be separated by the features of modified neurochaetae (teeth, hood, distal arista, and pubescence), the length and shape of the median antenna, the number of prebranchial chaetigers and the number of branchiae (Arriaga-Hernández et al. 2013). Previously, only one species, A. (Acmira) simonae Laubier & Ramos, 1974, had been described with two prebranchial chaetigers. However, this taxon, originally described from Marseille, France, and common in Mediterranean and Black Sea is entirely different from the new species collected in Thailand. Aricidea (Acmira) simonae has smooth neuropodial spines, without pubescence, a very short antenna on the insertion area, bears only 20-32 pairs of branchiae and lacks neuropodial lobes. Aricidea (Acmira) anusakdii sp. nov. has curved spines with blunt shafts surrounded by pubescence, an antenna that reaches the posterior margin of the prostomium, has neuropodial lobes and bears a significantly higher number of branchial pairs (48-68 pairs).
Although the modified spines in A. (Acmira) mirifica and A. (Acmira) finitima have no hood and do not bear distal or subdistal pubescence, in the first species the spines sometimes bear a short distal arista and in the second they almost always bear arista. Nonetheless, both these species can also be separated from the new species because they have three prebranchial chaetigers, their antennae are longer (reaching chaetiger 1-3 or 6), and they bear fewer branchiae, 12 and 14-27 pairs, respectively.
It is important to note that previously, the presence of lobes and notches on the anterior margin of the prostomium had only been reported in two species: A. (Acmira) simonae, which has three lobes in ventral view (Laubier and Ramos   1973) and other differences, smooth neuropodial spines, a very short antenna on the insertion area, bears only 20-32 pairs of branchiae and lacks neuropodial lobes, with the new species has been previously argued; and Aricidea (Acmira) trilobata Imajima, 1973, distributed on the continental shelves of Japan and California (Blake, 1996), which also bears three lobes on the anterior edge of the prostomium and the branchiae start from chaetiger 4. However, unlike the new species, this last species also bears three lobes on the anterior edge of the prostomium, the branchiae start from chaetiger 4, the median antenna extending to chaetiger 2 and only bears 18 to 20 branchial pairs. Description. Holotype complete with approximately 50 chaetigers (posterior region coiled, difficult to count segments), 5.47 mm long, 0.3 mm wide (Fig. 5A-C); two complete paratypes with 29 and 45 chaetigers, others incomplete with 21 to 32 chaetigers, 1.8-4.5 mm long and 0.01-0.23 mm wide. Body small, new preserved specimens reddish-orange in prebranchial and branchial regions (Fig. 5A); dorsal ciliary bands present on the prebranchial and branchial chaetigers. Prostomium conical, distally rounded, longer than wide; one pair of small black or brown eyes present; two pairs of long ciliary bands, one pair located above nuchal grooves and other at lateral margins of prostomium. Median antenna biarticulated, basal portion clavate, distal portion triangular, ciliated on distal end; basal portion of median antenna about two times longer than distal portion, extending to chaetiger 1 (Figs 6B, D, 9A). Nuchal   organs as pair of oblique, deep slits. Posterior buccal lip with six longitudinal folds, extending to chaetiger 1, with one pair of ciliary patches above the buccal region (Figs 6C, 9B). Numerous small filaments along body, and thin papillae present on the body (Fig. 6F) and notopodial pores (Fig. 7F).
Reproduction. Holotype and paratypes of Aricidea (Aricidea) thammapinanae sp. nov. collected in March, August, and September had eggs in the coelomic cavities of postbranchial chaetigers.
Etymology. The species epithet thammapinanae, is after the family name of Ms Vorramaz Thammapinan. This species is named in honor of her initiation, coordination, and assistance to the research project in Songkhla Sea.
Habitat. At 20-24 m depth, mud with sand and shells. Distribution. Songkhla Sea, Gulf of Thailand, Western Pacific.
Remarks. This is a small species of the subgenus Aricidea (Aricidea) having a maximum length of 5.47 mm (holotype) and with only 4-8 pairs of branchiae. The presence of eggs (Fig. 7C) in individuals collected during several sampling months implies that the small size of this new taxon is a specific characteristic. The presence of bidentate chaetae is unusual in species belonging to the subgenus Aricidea (Aricidea). Until now, 15 species have been described in this subgenus but only Aricidea (Aricidea) capensis Day, 1961 from South Africa (Day 1961) has bidentate modified chaetae (Table 1). However, the species presents clearly different characteristics from those observed in A. (Aricidea) thammapinanae sp. nov., since the bidentate modified chaetae of A. (Aricidea) capensis Day, 1961 are smooth, without pubescence along the shaft or on the subterminal spine. Besides the antenna, extending to chaetiger 2, is faintly annulated, eyes are lacking, 14 branchial pairs are present, and all prebranchial notopodial lobes are small and slender (Table 1). In contrast, the proposed new species has bidentate modified neurochaetae with pubescence on the distal shaft and along the subterminal spine, a biarticulated antenna that extends to chaetiger 1, and a pair of eyes. Only 4-8 branchial pairs are present, and on chaetiger 3, distinctive broad triangular notopodial lobes with short distal protuberances.
due Ratchakorn Sirijarukul and Monrach Intarasiri for assisting with photographs. Thanks also to Mrs Apinya Sukolra, Ms Benjaporn Nooklay, and all staff at the SEM section at the Office of Scientific Instrument and Testing (OSIT), Prince of Songkla University. Special thanks to Mr Pacharadon Plathong for drawing the illustrations. Finally, special thanks to Larry Lovell for checking and commenting on the Aricidea (Aricidea) species list. We are grateful to editor, Dr. Christopher J. Glasby and reviewer, Dr. Melih Ertan Çinar for provided comments and suggestions that helped us to improve the quality of the manuscript.