Two new species of Leptopsyllus from Korea (Copepoda, Harpacticoida, Paramesochridae)

Abstract Two new species of Leptopsyllus are described from the subtidal zone of Korea. Both species were assigned to the subgenus Leptopsyllus (Leptopsyllus) T. Scott, 1894, based on following three characters: two-segmented rami of P1, absence of endopod on P2 and P3, and presence of one-segmented endopod of P4. L. (L.) pundius sp. n. is most closely related to L. (L.) punctatus Mielke, 1894, however clearly distinguishable from it based on mandibular exopod with two setae, shape of P6, and caudal seta III ornamented with spinules in the new species. L. (L.) koreanus sp. n. is clearly distinguishable from its congeners by the second segment of P1 endopod armed with one element, male baseoendopod of P5 with one seta, and one segmented endopod of mandibular palp. The world distribution and updated key to the species of the genus Leptopsyllus are provided.


Introduction
Although the marine biodiversity of Korea is very high according to a recent estimation (Costello et al. 2010), many small interstitial organisms, including harpacticoid copepods, still remain unknown. Harpacticoid copepods play an important role in the benthic food web, and are an important source of biodiversity in Korea. Thus far, 88 harpacticoid species (58 genera and 23 families) including planktonic, free-living benthic, and invertebrate-associated species have been reported from Korean waters (Song et al. 2012).
Paramesochrid harpacticoids, with their reduced appendages and vermiform body shapes, successfully inhabit subtidal and intertidal sandy bottoms (Boxshall and Halsey 2004). Many free-living genera of the family Paramesochridae have adapted to living on various sandy sediments irrespective of depth and salinity (Plum and George 2009). For example, Emertonia clausi Pointer & Veit-Köhler, 2013 was collected from the deep sea (Pointner et al. 2013) while Remanea naksanensis Back, Lee & Huys, 2011 was collected from brackish water (Back et al. 2011). So far, nine species belonging to four genera in the family Paramesochridae have been discovered in Korea (Back and Lee 2014).
A survey of harpacticoid copepods from subtidal zones in Korea resulted in the discovery of two new species belonging to Leptopsyllus (Leptopsyllus). Here these two species are described and an updated key to species of the genus is provided.

Materials and methods
Specimens were collected from sediments in the subtidal zone near Pung Island off the west coast of Korea (Leptopsyllus (Leptopsyllus) pundius sp. n.), and Maemul Island (Leptopsyllus (Leptopsyllus) koreanus sp. n.) off the south coast of Korea. Sediments were collected using a grab (surface area 0.1 m 2 ) and fixed with 5% buffered formalin. Copepods were extracted from the sediment samples using the Ludox method (Burgess 2001) and preserved in 70% ethanol. Dissected specimens were mounted on several slides separately using lactophenol as a mounting medium. Slides were sealed with transparent nail varnish. Observations were made using a microscope (Olympus BX51) equipped with differential interference contrast and a drawing tube.
The descriptive terminology of Huys et al. (1996) was adopted. Abbreviations used in the descriptions are:
Specimens were deposited in the National Marine Biodiversity Institute of Korea (MABIK). Scale bars in figures are in μm.
Genital field located mid-ventrally halfway the length of the genital doublesomite; copulatory pore located near posterior border of genital field and covered by small process (Fig. 2D); P6 represented by transverse plate with 1 bare seta (Fig.  2D); penultimate somite with well-developed pseudoperculum; anal operculum not developed.
P6 (Fig. 5C) symmetrical, with 2 bare setae arising from small protrusion on inner part of P6, and 1 outer longest bare seta.
Etymology. The specific name refers to the type locality of the new species in Korea.

General status and zoogeography of the genus Leptopsyllus T. Scott, 1894
Since the genus Leptopsyllus was proposed by Scott T (1894) based on the reduction of legs, several species have been described in Leptopsyllus. Though many species were originally assigned to the genus Leptopsyllus, some of them were moved to new genera according to new classifications based on leg characteristics (reduced or absent). Boxshall and Halsey (2004) proposed the number of species in each genus and the key to genera based on Kunz (1981), Huys (1987), and Cottarelli and Forniz (1994). As a result, the genus Leptopsyllus currently consists of 13 valid species including the two new species described in this study. However, the complete descriptions of mouthparts are lacking for many species, because of the small body size of these organisms. In addition, the abundant of species in Leptopsyllus is usually low. Unfortunately, we founded only one female of L. (L.) pundius sp. n. and one male of L. (L.) koreanus sp. n. during the study. Until now, many species belonging to genus Leptopsyllus have been found mainly in Europe in Atlantic Ocean (Fig. 9). Most species are distributed along the coast including islands intertidal zone, but three species, Leptopsyllus (Paraleptopsyllus) arcticus (Lang, 1936), L. (L.) elongates Drzycimski, 1967, and L. (L.) abyssalis Becker, Noodt & Schriever, 1979, were collected from deep sea (deeper than 200 m). Belonging to the family Paramesochridae, species of Leptopsyllus are well known for living in sandy bottom, however, some species were found in muddy sediment (Lang, 1948;Drzycimski, 1967;Plum and George, 2009). In this study, L. (L.) pundius sp. n. is found in the sandy sediment, while L. (L.) koreanus was collected from muddy sand sediment. In conclusion, species belonging to the genus Leptopsyllus are thought to inhabit a variety of sediments and depths.

Relationships between two new species and their congeners
The two new species are placed in the genus Leptopsyllus T. Scott, 1894 based on the absence of an endopod on P2 and P3, P4 endopod armed with one apical seta, and presence of two setae/spines on the distal segments of the exopods on P2-P4. The two new species are placed in subgenus Leptopsyllus on account of the absence of P3 endopod because the discrepancy in the diagnostic characters between the subgenera Leptopsyllus T. Scott, 1894 , andParaleptopsyllus Lang, 1944 is presence or not of P3 endopod.   (Table 1). However, L. (L.) pundius sp. n. can be differentiated from the congener by (1) the one-segmented mandibular exopod armed with two setae; whereas L. (L.) punctatus has four setae on the exopod of the mandible (Table 1), (2) P6 with one element, and (3) caudal seta III is stout, decorated with spinules. In addition, the structure of the rostrum with a pair of small sensillae of the new species is also unique character in the genus Leptopsyllus. Several species, (Caligopsyllus primus Kunz, 1975, Diarthrodella ergeneae Sonmez, Karaytug & Sak, 2015, Emertonia clausi Pointer & Veit-Köhler, 2013 Mielke, 1984. The caudal ramus in the genus Leptopsyllus is conical and its distal margin is bluntly pointed; however, caudal seta formula varies among congeners. Leptopsyllus (L.) koreanus sp. n. clearly has caudal seta I, although this seta is obscure in other congeners. Both new species have a tri-articulated seta VII, while L. (L.) punctatus and L. (L.) platyspinosus have a bi-articulated seta VII arising from a chitinous outgrowth on the dorsal surface. However, the caudal rami and setae of other species in Leptopsyllus have not been described in detail in other previous studies.
Four distinctive groups within the genus Leptopsyllus can be recognized based on the shape of the female P5 (Table 1) n. are cannot be assigned to any of the four groups because they have been described based on the male specimen. More studies will be necessary to prove the relationship among four groups including female and male P5.

Key to the species of the genus Leptopsyllus
The latest key proposed by Bodin & Jackson (1987) includes nine species of Leptopsyllus, and does not include the two species L. (P.) arcticus and L. (L.) abyssalis. Because only the males of some species have been described, the extent of sexual dimorphism in mouthparts or P1-P4 was not known. An updated key is developed on the basis of selected characteristics from the original description that identifies species within the genus Leptopsyllus.