A new species of Strongylacron (Copepoda, Harpacticoida, Cletodidae) from intertidal mudflats in the Korean peninsula

Abstract A new species, Strongylacron glabrum sp. n. is described from intertidal mudflats in the Korean peninsula. The new species is assigned to the monospecific genus Strongylacron Gee & Huys, 1996 in accordance with the generic morphological features of the rostrum, antennary exopod, and thoracic legs. However, Strongylacron glabrum sp. n. is clearly distinguished from the type species, Strongylacron buchholtzi (Boeck, 1873), by the naked anterior margin of rostrum, the elongate exopod and endopodal lobe of female P5 approximately 3.5 and 2.7 times as long as width, respectively, and the presence of 8–10 rod-like projections on prosomites. The genus Strongylacron is first recorded from Korean waters by the present study.


Introduction
The harpacticoid copepod genus Strongylacron Gee & Huys, 1996 belonging to the family Cletodidae T. Scott, 1904 was established by Gee and Huys (1996) as a part of an effort to resolve the relationship among members of Enhydrosoma Boeck, 1873. The latter genus was recognized as a heterogeneous group by Fiers (1987) and Mielke (1990). After the studies of Lang (1936Lang ( , 1948, this genus was distinguished from Cletodes Brady, 1972 by subtle morphological characteristics such as the form and setation of the distal segment on legs 3 and 4 (Gee 1994(Gee , 2001Gee and Huys 1996). In the revision of Enhydrosoma, Gee (1994) first proposed the buchholtzi-species group representing differences from others in the rostrum, antenna, maxillule and male P3 endopod. In addition, he suggested that this group should be removed from Enhydrosoma. Thereafter, the buchholtzi-species group was revised and the following three genera were established: Schizacron Gee & Huys, 1996, Spinapecruris Gee, 2001, and Strongylacron Gee & Huys, 1996(Gee 1994, 2001Gee and Huys 1996). Strongylacron is a monospecific genus containing St. buchholtzi (Boeck, 1873) (Walter and Boxshall 2016). The type species has been chiefly reported from European waters and occurs usually around estuary sediments containing high organic content (Boeck 1873;Sars 1909;Wells 1963;Gee and Huys 1996).
While studying harpacticoid copepods from Korean waters as a part of the 'Survey of indigenous biological resources of Korea', a new harpacticoid copepod belonging to the genus Strongylacron was discovered and reported here as Strongylacron glabrum sp. n. along with detailed description and illustrations.

Materials and methods
Sampling was performed with a sieve of 212 µm mesh from intertidal mudflats on the south-western coasts of Korea. In ebb tides, surface sediments (< 5 cm sediments depth) were obtained by using large spoons. Samples remaining on sieve were fixed initially with 5% formaldehyde-seawater solution. Harpacticoid specimens were preserved with 99.9% ethanol after sorting in the laboratory. They were dissected by using tungsten needles under stereo microscope (Discovery, V8; Carl Zeiss, Germany) and then mounted on polyvinyl lactophenol or lactophenol. The observations and drawings were performed by light microscope (ECLIPSE 80i; Nikon, Japan) equipped with a drawing tube. Several specimens were examined in a scanning electron microscope (SEM). They were cleaned by an ultrasonic machine, prefixed by 4% glutaraldehyde, postfixed by 2% OsO 4 , dehydrated through graded ethanol solutions, air-dried, and coated with gold. The dried materials were observed under SEM (VEGA 3 LM; Tescan, Czech Republic), with an accelerating voltage of 20 kV and working distances between 17.90-20.50 mm. Descriptions and line drawings examined under 400-1,000× magnifications were made based on the paratypes. All materials examined were deposited in Chosun University and the National Institute of Biological Resources (NIBR), Korea.
Diagnosis. Habitus semi-cylindrical, approximately 900 µm; each prosomite with 8 or 10 rod-like projections. Rostrum fused to cephalothorax basally, slightly recurved dorsally; anterior margin rounded and naked. Genital field with vestigial P6 represented by seta. Caudal rami approximately 2.5 times (female) and 3.6 times (male) as long as width; tube pore on outer margin inserted proximally; caudal seta VI shorter than seta IV. Antennary endopod with stout spine-like seta at distal corner. Mandibular gnathobase with 3 bicuspid teeth, without seta; basis with 2 short and 1 long setae. Female P5 exopod 3.5 times as long as width; innermost seta on exopod shorter than exopod in length; endopodal lobe approximately 2.7 times as long as width. Male P5 endopodal lobe, outer most seta on endopodal lobe reaching half of middle seta on exopod.
Prosome ( Caudal rami (Figs 6D, 9E, F) cylindrical, tapering posteriorly, as long as anal somite in length, with 2 tube pores and 7 caudal setae: lateral seta I along with seta II inserted in proximal fifth of ramus; seta III half of caudal ramus in length; seta IV small, fused to well-developed seta V at its base; seta V 3.0 times as long as caudal ramus; seta VI shortest, located at inner distal corner; seta VII located in dorsal surface proximally, articulated basally.
Distribution. The south-western coasts of South Korea. Etymology. The epithet of the specific name, glabrum, is derived from the Latin adjective glaber, meaning 'hairless' or 'bare'. This name refers to the naked anterior rostral margin of the new species. Remarks. Gee and Huys (1996) redefined the taxonomic status of four Enhydrosoma species, E. buchholtzi (Boeck, 1873), E. barnishi Wells, 1967, E. bifurcarostratum Shen & Tai, 1965, and E. vervoorti Fiers, 1987, belonging to the buchholtzi-species group (see Gee 1994), and they established two genera, Schizacron Huys, 1996 andStrongylacron Gee &Huys, 1996. These genera share a distinctive U-shaped female P5, which is known as a unique structure of the family Cletodidae T. Scott, 1904, but they are typically divided in terms of the structure of rostrum (Gee and Huys 1996). Schizacron is characterized by having a recurved dorsally and markedly bifid anterior rostral margin, while Strongylacron's rostrum is non-recurved and has a broadly rounded anterior margin (Gee and Huys 1996). Additionally, they recognized the presence of a row of fine setules on the anterior rostral margin as a significant generic characteristic of Strongylacron. The genus Strongylacron was erected based on only one species, St. buchholtzi, with a restricted distribution in the north Atlantic Ocean (northwestern Europe and Canada) (Boeck 1873;Sars 1909;Willey 1929;Wells 1963;Gee and Huys 1996).
Members of the family Cletodidae are known as mud-burrowers from shallow and sublittoral marine habitats (Por 1986;Boxshall and Halsey 2004;Kim et al. 2014;Song et al. 2014). Strongylacron species were also reported from muddy bottoms. Strongylacron buchholtzi was known from intertidal and sublittoral (depth of 20 m) habitats on the Atlantic Ocean (northwestern Europe, Canada) (Boeck 1873;Sars 1909;Willey 1929;Wells 1963;Gee and Huys 1996). Strongylacron glabrum sp. n. was found from intertidal mudflats on the south-western coasts of South Korea. The genus Strongylacron herein is first recorded from the Pacific Ocean.