A new species of Leiochrides from the Korean subtidal waters with notes on the taxonomic status of the genus Pseudomastus (Annelida, Capitellidae)

Abstract Leiochrides yokjidoensis sp. n., collected from the sublittoral muddy bottom in southern Korea, is described as a new species. The taxonomic status of the monospecific genus Pseudomastus has been a subject of controversy for many years, as its characteristics overlap those given in recent generic definitions of Leiochrides. The results of a comprehensive review and comparison regarding the two genera, based on previous records showed minor differences. In this study, a detailed description of L. yokjidoensis sp. n. is given and a comparison with closely related species is tabulated and discussed.The taxonomic status of Pseudomastus is discussed and the genus placed in synonymy with Leiochrides.


Introduction
Polychaetes are an important component of the macrobenthic community and they play a crucial role in the functioning of benthic communities in the recycling and reworking of the benthic sediments, bioturbation, and in the burial of organic matter (Hutchings 1998). The Capitellidae Grube, 1862 are found in many types of sediments from the intertidal region to the deep sea and is a frequently dominant component of the benthic infaunal communities, especially in organically enriched sediments (Blake 2000). Owing to their accessibility and their importance in sedimentary environments, capitellids have been the subject of numerous ecological studies (Blake 2000). Despite their ecological importance, the taxonomy of Capitellidae has been largely ignored (Dean 2001). Since the first designation by Grube (1862), the family Capitellidae is currently composed of 48 genera, including 24 monospecific genera (Read 2017). The genera in this family have been defined by the number of segments with capillary chaetae or hooded hooks or mixtures of both (Blake 2000, Fauchald 1977). However, several previous records of Capitellidae include incorrect taxonomic information due to the ontogenetic variations in the chaetal arrangement and an indistinct demarcation of the peristomium or the transitional segment (Blake 2000, Green 2002, García-Garza and León-González 2011. Moreover, the high percentage of monospecific genera and the use of the insufficient characters in identification keys (i.e. based mostly on segmental distribution and chaetal composition) have steadily raised questions about taxonomic concepts of many Capitellidae (Ewing 1991, Blake 2000. The genus Leiochrides was established by Augener (1914) with the description of L. australis Augener, 1918 from southwest Australia. The additional seven described species in the genus are L. africanus Augener, 1918 from western Africa, L. pallidior (Chamberlin, 1918) and L. biceps Hartman, 1954 from California, L. hemipodus from Southern California, L. norvegicus Fauchald, 1972from western Norway, and L. branchiatus Hartman, 1976and L. andamanus Green, 2002 from the Indian Ocean. The genus Leiochrides was formerly defined as having 12 thoracic chaetigers with only capillaries, abdominal chaetigers with only hooks, an indistinct boundary between the thorax and the abdomen, and the absence of branchiae (Augener 1914). However, the generic definition has continuously been modified for several decades. Hartman (1963 described L. branchiatus and Leiochrides sp. from California and revealed that some species of Leiochrides had transitional chaetigers in the thorax as a unique characteristic that had not been noted in previous studies. The genus Leiochrides is represented by having 12 chaetigers with capillary chaetae and only one or two species have the transitional chaetigers with capillary notochaetae and neurohooks. On the other hand, Day (1967) and Fauchald (1977) did not accept the new character proposed by Hartman (1963 and followed the original generic definition of Augener (1914). Recently, Blake (2000) and Green (2002) reconfirmed the existence of the transitional thoracic chaetigers in Leiochrides species and emended the generic definition in the light of Hartman (1963. We follow the expanded generic definition of Green (2002) in this study.
The genus Pseudomastus established by Capaccioni-Azzati and Martin (1992) was defined by the 12 thoracic chaetigers, which comprised 11 thoracic chaetigers with capillaries and the last thoracic chaetiger with capillary notochaetae and neuropodial hooks. The chaetal arrangement of Pseudomastus coincides with the expanded generic definition of Leiochrides by Blake (2000) and Green (2002). The taxonomic relation-ship for these two genera, however, has never been precisely defined since the erection of Pseudomastus as a new monospecific genus.
This study provides a detailed description of a new species of Leiochrides from southern Korea and reveals its morphological distinctiveness through comprehensive comparisons with closely related species. Additionally, the taxonomic status of the genus Pseudomastus is discussed.

Materials and methods
Sediment samples were obtained from the sublittoral muddy-sand bottom of the southern coast of Korea with a 0.05 m 2 Van Veen grab followed by elutriation on a 0.35 mm sieve in a 30 L seawater container. The remaining organisms on the sieve were transferred to a 1 L collecting jar with a 7% MgCl 2 solution for anesthesia. The relaxed samples were initially fixed in a 10% formalin solution for an hour before they were preserved in 90% ethanol for subsequent analysis. For the identification of the morphological features, the samples were stained with Shirlastain A (SDLATLAS, Inc.) for three seconds and sorted under a zoom stereomicroscope (Nikon SMZ745T). Line drawings were conducted using a differential interference contrast microscope (Eclipse Ci-L, Nikon) and a digital pen display (Cintiq 22HD, Wacom). The MGSP of the examined specimens were described and photographed as described in Jeong et al. (2017). A Scanning Electron Microscopy (SEM) analysis was carried out to confirm the detailed morphological structure. The specimens were placed in an ultrasonic chamber with distilled water for 30-60 seconds to remove the hoods of the hooded hooks. The treated specimens were dehydrated through a series of increasing concentrations of ethanol ending with two changes of absolute ethanol, followed by critical point drying. The dehydrated specimens were coated with gold and then photographed using the Hitachi S-3000N.
The type materials were deposited in the collections of the Marine Biodiversity Institute of Korea (MABIK) in Seocheon, Korea.

Leiochrides yokjidoensis
Description. One complete specimen and eight incomplete specimens. Largest specimen 27 mm long, 0.34 mm wide for 159 chaetigers. Smallest specimens 7 mm long, 0.25 mm wide for 34 chaetigers. Body thread-like, cylindrical, widest in anterior thoracic chaetigers, tapering from abdomen to pygidium. Color in alcohol yellowish or reddish brown.
Transition between thoracic and abdominal region distinguished by change in position and type of chaetae and length of segments; abdominal segments longer and slightly narrower than thoracic segments, gradually smaller posteriorly; notopodial hooded hooks first present on chaetiger 13 (first abdominal chaetiger) ( Fig. 2A-B). Abdominal chaetigers with hooded hooks on posterior end of segment; thoracic chaetigers with capillary chaetae or neuropodial hooks in center of segment ( Fig. 2A-B).
Hooded hooks short, with main fang extending slightly beyond hoods; hood flared; shaft slightly enlarged like manubrium (Figs 2F, 3E). Hooks with three rows of small teeth above main fang; two in basal row, three in middle row, and 3-5 in superior rows (Figs 2E, 3F).
Etymology. The new species is named for its occurrence in Yokjido, Korea. Distribution. Leiochrides yokjidoensis sp. n. is distributed in the subtidal habitat (53 m) of the southern part of Korea.
Ecology. The surface sediment is mainly composed of sandy mud with fragmented shells. Mediomastus Hartman, 1944 andNotomastus M. Sars, 1851, also belonging to the Capitellidae, also occurred at the same location.
Remarks. Leiochrides yokjidoensis sp. n. is distinct in the morphological combination of 12 thoracic chaetigers, chaetigers 1-11 with only capillary chaetae, and the last thoracic chaetiger with the notopodial capillaries and neuropodial hooks. Among the genera of Capitellidae the presence of 12 thoracic chaetigers and neurohooks in last thoracic chaetiger are shared with Leiochrides, Pseudomastus, and Scyphoproctus Gravier, 1904. However, Scyphoproctus is clearly separated from the new species by the presence of the unique anal plaque, acicular spines in the posterior abdomen, and two achaetous segments in the anterior part of the thorax. In this study, the new species . Abbreviations: ac, anal cirri; br, branchia; cc, capillary chaeta; ch, chaetiger; hh, hooded hook; lo, lateral organ; mf, main fang; per, peristomium; pro, prostomium was placed under the genus Leiochrides, because the generic diagnosis of Pseudomastus mostly agreed with that of Leiochrides (see details in discussion section). Leiochrides yokjidoensis sp. n. closely resembles L. hemipodus and P. deltaicus in the presence of the neurohooks in the thorax, the uniramous first chaetiger, the two distinct basal teeth above the main fang, and the presence of the branchiae in the posterior abdomen (Table 1). In particular, L. yokjidoensis sp. n. and P. deltaicus correspond in the presence of multiple cirri on the pygidium (Table 1). However, the new species is discriminated from P. deltaicus by the following morphological characteristics: the absence of eyespots in the preserved materials, dorsal notochaetae without distal spinules, a proboscis without cilia on the tip of the papillae, the number of capillary chaetae (8-12 vs. 13-20) and neurohooks (8-10 vs. 11-12) per fascicle, the number of teeth above the main fang (8-10 vs. 7), the absence of neuropodial hooks in chaetiger 11, and the number of anal cirri (4 vs. 3). On the other hand, the holotype of L. hemipodus has identical chaetal arrangement to L. yokjidoensis sp. n., but it differs in the number of teeth above the main fang (3 vs. 8-10), the maximum number of branchiae per fascicle (12 vs. 5), and the species-specific MGSP (chaetigers 1-12 vs. 6-11, Table 1). In addition, L. hemipodus has been reported only in deep basin near California, but L. yokjidoensis sp. n. was found in shallow waters of southern Korea (Table 1). The paratype of L. hemipodus has a different chaetal arrangement (chaetigers 11-12 with neuropodial hooks) to the holotype of L. hemipodus, however these type specimens were almost identical in the remaining characteristics (Blake 2000, Green 2002 , Table 1). Generally, capitellids have developmental variation in the chaetal arrangement on the thorax, and the hooded hooks are replaced by capillaries with development (Fredette 1982, Blake 2009). Thus, the differences in chaetal arrangement between the type specimens of L. hemipodus may be associated with the developmental variation within a species. Blake (2000) had reported L. hemipodus from Santa Maria basin (603 m) and compared his specimens with the characteristics of the paratype of L. hemipodus Hartman, 1960. According to his description, L. hemipodus also differs from L. yokjidoensis sp. n. in the presence of a band of glands on chaetiger 6 and the shape of the branchiae (Table 1).

Discussion
Capaccioni-Azzati and Martin (1992) established the genus Pseudomastus to accommodate the only species, P. deltaicus characterized by 12 thoracic chaetigers, the first chaetiger with only notochaetae, chaetigers 1-10 with capillaries, the last two thoracic chaetigers with notopodial capillaries and neuropodial hooks, the posterior end of each notopodium with 2-4 digitate branchiae on chaetigers 205-210 to chaetigers 255-260, and the pygidium with three anal cirri. At this point, the genus Leiochrides was earlier assigned with morphological characteristics that coincided with Pseudomastus. However, Capaccioni-Azzati and Martin (1992) did not detect the close relationship in morphology between Leiochrides and their new genus, Pseudomastus. Hartman (1963 had proposed the presence of transitional chaetigers, with capillary notochaetae  1992) and neurohooks, as an important characteristic of Leiochrides but it was not accepted as a generic diagnosis by Fauchald (1977). Blake (2000) later added this characteristic to the generic definition, which was redefined as 12 chaetigers with capillary chaetae but sometimes one or two of the chaetigers having notopodial capillaries and neurohooks. According to this expanded generic definition, the characteristics of Pseudomastus also matched those of Leiochrides. In addition, the uniramous first chaetiger, the multiple branchiae in the posterior part of abdomen, the two distinct basal teeth above the main fang of the hooks, and the presence of multiple anal cirri are also identical features that are found in some members of Leiochrides. Although P. deltaicus is unique in having spinules on the distal edge of the dorsal capillary notochaetae and cilia on the papillae of the proboscis, these seem to be species level characteristics within the Capitellidae. Thus, we regard the monospecific Pseudomastus as a junior synonym of Leiochrides.
This study provides the detailed morphological features of L. yokjidoensis sp. n. including the presence/absence of the lateral organ and genital pore, the number of chaetae per fascicle, MGSP, and the number of anal cirri. Green (2002) used variations in the thoracic chaetal distribution and the details of the hooded hooks among six of the described Leiochrides species in an identification key to species. Nevertheless, the use of additional taxonomic information on the distribution of the lateral organs, nephridia or genital pores, MGSP, and characteristics of the pygidium are still insufficient to distinguish many of the previously described Leiochrides species, including L. hemipodus. Therefore, a precise examination of the morphological details and a re-examination of existing Leiochrides species is needed in the future and may prove useful in a clarification of the taxonomic relationship between Leiochrides species. Hooded hooks with 3 teeth above main fang in 2 rows; posterior abdomen with 3-12 branchiae per fascicle; pygidium without anal cirri ...L. hemipodus 9