Four new species of isopods (Crustacea, Isopoda) from South Korea

Abstract Four new isopods, Amakusanthura intermediasp. nov., Apanthura laevipedatasp. nov., Idarcturus trispinosussp. nov., and Neastacilla paralongipectussp. nov., are reported from the sublittoral zones in Korean waters. Amakusanthura intermediasp. nov. differs from its congeners by the following features: the uropodal exopod is sinuous distally and with pointed apex; the maxillipedal endite is present and reaching to the distal end of fused palp articles I and II; and the propodal palm of pereopod I is stepped. Apanthura laevipedatasp. nov. can be distinguishable from its related species by the following characteristics: the eye is lacking; the propodal palm of pereopod I is not stepped; and the uropodal exopod is not sinuous. Idarcturus trispinosussp. nov. is diagnosed by the following features: the cephalon has three dorsal spines and a pair of lateral spines; pereonite IV has two pairs of small dorsolateral spines, four pairs of dorsal spines, and one posterior spine; and the pleotelson has three pairs of wings laterally. Neastacilla paralongipectussp. nov. can be distinguished by the following characteristics: the body is smooth and lacking dorsal spines or tubercles; pereonite IV is approximately 5.4× longer than pereonites II and III together; and the pleotelson has two pairs of lateral wings. In this paper, detailed descriptions and illustrations of the four species are presented. A key to the genera of the family Arcturidae and keys to the species of the four genera are also provided.


Introduction
The Anthuridae Leach, 1814, characterized by having an elongate cylindrical body and the exopod of uropod attaching to the protopod dorsally, is a large family of marine isopods that dominated in the tropical regions (Poore 2001a;Poore and Bruce 2012;Chew et al 2014). Anthurids usually live in sediment burrows as abundant and important components of the offshore soft-sediment marine environment, also commonly occurring in algal mats on hard substrates in both littoral and sublittoral habitats (Brusca and Iverson 1985). Among the 26 anthurid genera, Amakusanthura Nunomura, 1977 and Apanthura Stebbing, 1900 each contain 43 species and are the largest genera in the family (Boyko et al. 2008a, b). It is known that the Amakusanthura is more common in the tropical regions and the Apanthura is more abundant in the temperate regions (Poore 2001a). In the Far East of the temperate region, four Amakusanthura species have been reported only from Japan (Nunomura 2016) and six Apanthura species from Russia (the Kuril Islands), Japan, and Korea (Mezhov 1976;Wägele 1984;Nunomura 1993;Song and Min 2015).
In this study, we describe two new anthurids, Amakusanthura intermedia sp. nov. and Apanthura laevipedata sp. nov., and two new arcturids, Idarcturus trispinosus sp. nov. and Neastacilla paralongipectus sp. nov., from the sublittoral zones in Korean waters. We also provide a key to the genera of the family Arcturidae and keys to the species of these four genera. This is the first reports of Amakusanthura, Idarcturus, and Neastacilla from Korea.

Materials and methods
The materials of the present study were collected from 13 sampling stations of the sublittoral zones in Korean waters by SCUBA diving and using a Smith-McIntyre grab ( Fig. 1; Table 1). The collected materials were sorted using a sieve with a 1 mm mesh and immediately fixed in 94% ethyl alcohol. The observation and dissection of materials were conducted under a dissecting microscope (Nikon SMZ 1500) and a compound microscope (Olympus BX 50). Measurements and drawings of the specimens  were performed with the aid of a drawing tube. The drawings were scanned, inked, and arranged digitally using the methods described by Coleman (2003Coleman ( , 2009). The examined materials in this study were deposited at the National Institute of Biological Resource (NIBR) and Chosun University in Korea (CUK). Paratypes. 2 ♀♀, same data as for holotype; NIBRIV0000880420. Etymology. The specific name, intermedia, originates from the Latin word intermedius, meaning "that is between". This name refers to the length of maxillipedal endite comparing to the related species.
Description of holotype female. Body ( Fig. 2A, B) 5.4 mm, 13× longer than wide, smooth and slender. Cephalon square to globular, 0.8× as long as pereonite I; rostrum as long as anterolateral lobes; eye very small; color not pigmented, white. Pereonites rectangular; pereonites I-VI similar to each other in length; pereonite VII ~ 0.7× as long as pereonite VI; coxal plates of pereonites IV-VI visible dorsally. Pleonites 1.2× longer than pereonite VII; pleonites I-V separated by folds except dorsally between pleonites IV and V; pleonites IV and V visible partial sutures laterally; pleonite VI visible dorsally, with dorsal notch posteriorly.
Antennule (Fig. 2C) composed of three peduncular and three flagellar articles, sequentially slender distally in width. Peduncular article I rectangular, with one simple and a single penicillate seta distally; article II oblong, 0.6× as long as article I, with three simple and four penicillate setae distally; article III rectangular, 1.2× longer than article II, with three simple setae distally. Flagellar article I 0.4× as long as wide, with one simple and one penicillate seta; article II elongated, ~ 4× longer than article I, without seta; article III minute, square, with seven simple setae distally.
Antenna (Fig. 2D) consisting of five peduncular and four flagellar articles. Peduncular article I ~ 0.4× as long as article II, with two simple setae laterally; article II 1.8× longer than article I, with one short simple seta laterally and one simple seta distally; article III 0.7× as long as article II, with three simple setae distally; article IV square, subequal to article III, with six simple setae and one penicillate seta distally; article V elongate, 1.4× longer than article IV, with 13 simple setae on both lateral margin and one penicillate seta on distal end. Flagellar articles much shorter than peduncular article V, consecutively shortened; article I with four simple setae distally; article II with one simple seta; article III without setae; article IV with seven simple setae; articles III and IV minute.
Mandible (Fig. 2E), incisor with one prominent cusp and serrated margin; molar process bluntly rounded; palp article II 1.5× longer than article I, slightly stouter than other articles, with two simple setae distally; article III shortest in length, with four serrated setae and a row of spines laterally, one serrated seta distally.
Maxilla ( Fig. 2F) with five teeth distally and several fine setae laterally. Maxilliped (Fig. 2G), endite extending to proximal region of palp article III, with one short simple seta apically; palp articles I and II fused, with one simple seta distally; article III free, with two simple setae laterally and two simple setae distally; articles IV and V fused, oblique, smaller than other articles, with four simple setae laterally.
Pereopod I (Fig. 3A), basis continuously stouter distally, with three simple setae and three penicillate setae on superior margin; ischium slender than basis, rectangular, with one simple seta on inferodistal end; merus much shorter than ischium, wider than long, with one simple seta on superior and inferior distal end, respectively; carpus triangular, protruding inferodistally, with rough margin inferodistally and eight simple setae along with inferior margin; propodus stepped on palm, with twelve simple setae on inferior margin and three simple setae on distal end; dactylus with five simple setae distally, a row of spines and one simple seta laterally; unguis as long as dactylus, much longer than those of other pereopods, with small accessory unguis. Pereopods II and III (Fig. 3B, C), basis elongate, oval, with one simple seta on inferodistally; ischium slightly shorter than basis, with two simple setae on both lateral margins; merus ~ 0.5× as long as ischium, tapering proximally, with several simple setae on superior and inferior margins; carpus triangular, with several simple setae on inferodistal angle and fine setae along inferior margin; propodus elongate and oval, with simple setae on both lateral margins and one stout seta on inferodistal angle; dactylus with simple setae distally; unguis 0.5× as long as dactylus, with small accessory unguis. Pereopods IV-VI ( Fig. 3D-F), carpus more or less rectangular, with one stout seta on inferodistal angle (absent in pereopod V); propodus with several fine setae along both lateral margins and one stout seta on inferodistal angle. Pereopod VII (Fig. 3G), carpus and propodus with dentate margin baring simple setae along inferior margin.
Pleotelson (Fig. 4H) 2.2× longer than wide, concave proximally, convex medially, tapering distally, with two statocysts on dorsal surface proximally; apex with five simple setae; distal region with several simple setae on dorsal surface and lateral margins.
Distribution. Southern coast of Jeju-do in South Korea.
Key to known species of the genus Amakusanthura in the Far East Etymology. The specific name, laevipedata, originates from the combination of the Latin words laevis, meaning smooth and pedis, meaning foot. This name refers to having the propodus of pereopod that is smooth, not stepped.
Description of holotype female. Body (Fig. 5A, B) smooth and slender, 5.5 mm in length, 11× longer than wide. Cephalon square, 0.7× as long as pereonite I; rostrum extending as long as anterolateral lobes; eye absent. Pereonites oblong; pereonites I-III similar in length; pereonites IV-VI longer than previous three pereonites, subequal each other in length; pereonite VII ~ 0.7× as long as pereonite VI; coxal plates of pereonites V-VII visible dorsally. Pleonites I-V not separated by folds dorsally and laterally; pleonite VI with middorsal notch posteriorly.
Antennule ( Fig. 5C) consisting of three peduncular and three flagellar articles. Peduncular article I oblong, longer than wide, with three penicillate setae and one simple seta along lateral margin; article II square, ~ 0.5× as long as article I, with three penicillate setae on distal end; article III similar to article II in length, slender than article II, with one penicillate seta and three simple setae distally. Flagellar article I much shorter than other articles, 0.1× as long as peduncular article III; article II oblong to oval, tapering distally, 0.8× as long as peduncular article III; article III square to globular, 0.3× as long as article II, with two simple setae and two aesthetascs distally.
Antenna (Fig. 5D) longer than antennule, composed of five peduncular and three flagellar articles. Peduncular article I rectangular to globular, with one simple seta distally; article II 1.5× longer than article I; article III oblong, 0.4× as long as article II, with three simple setae distally; article IV similar to article III in shape, 0.7× as long as article III, with four simple setae distally; article V elongated oblong, 2.3× longer than article IV, with three penicillate and four simple setae on distal end. Flagellar article I elongated rectangular, ~ 0.5× as long as peduncular article V; articles II and III minute, with several simple setae on distal end.
Mandible (Fig. 5E), incisor with one cusp; molar process blunt; palp article I elongated oblong, with two simple setae distally; article II 1.4× longer than article I, slightly thicker than other articles; article III slander, slightly tapering distally, 0.4× as long as article II, with two short simple setae distally.
Maxilla (Fig. 5F) with six irregular teeth distally and fine setae laterally. Maxilliped (Fig. 5G), epipods globular; endite slightly exceeding distal end of fused palp articles I and II, tapering distally, with one simple seta distally; palp articles I and II fused, rectangular, with one simple seta distally and one simple seta laterally; article III free, almost square, with three simple setae on distal end; articles IV and V fused, triangular, much smaller than other palp articles, with five simple setae along inner margin. Pereopod I (Fig. 6A) basis as long as ischium, gradually tapering proximally; ischium rectangular, with one simple seta on inferodistal angle; merus 0.4× as long as ischium; superior margin of merus extending upwardly, with five simple setae on inferior margin, one short simple seta on superior margin, and one simple seta on mesial margin; carpus triangular, with five simple setae along inferior margin; propodus not stepped on palm, with two rows of simple setae along inferior margin and several fine setae on superior margin; dactylus oblique, with five simple setae; unguis slightly longer than dactylus; much longer than other pereopods, with accessory unguis distally. Pereopods II-VII (Fig. 6B-G) basis oblong to oval, with several simple setae and penicillate setae on superior margin and one simple seta on inferodistal angle; ischium subequal in length, gradually tapering proximally, with several simple setae along with inferior margin and none or one simple seta on superodistal angle; merus tapering proximally, with several simple setae on inferior margin and 1-3 simple setae on superior margin; carpus of pereopods II and III triangular, with several simple setae on inferior margin without stout seta on inferodistal angle; carpus of pereopods IV-VII subsquare to globular or trapezoidal, with one stout seta on inferodistal angle, several simple setae on both lateral margins, none or one penicillate seta on superior margin; propodus slightly elongate and oval, with several simple setae on both lateral margins and one stout seta on inferodistal angle; dactylus slender, with several simple setae distally, unguis, much shorter than dactylus, with minute accessory unguis on distal end.
Distribution. Southern coast of Jeju-do in South Korea.
Habitat. Sublittoral zone of sandy bottom. Remarks. Apanthura laevipedata sp. nov. differs from its congeners by the following features combined: (1) the integument is not pigmented; (2) the eyes are lacking; (3) pleonites I-V are not separated by folds dorsally and laterally; (4) the propodal palm of pereopod I is not stepped; and (5) the uropodal exopod is not sinuous.
Besides the new species, four species that have no eyes are known in the genus Apanthura Stebbing, 1900: A. insignifica Kensley, 1978;A. tyrrhenica Wägele, 1980;A. drosera Poore & Lew Ton, 1985;andA. fusei Nunomura, 1993 (Kensley 1978;Wägele 1980;Poore and Lew Ton 1985;Nunomura 1993). Among these species, Apanthura laevipedata sp. nov. is most similar to A. fusei in terms of dorsally fused pleonites I-V and not sinuous uropodal exopods. However, the new species is easily distinguished from the latter by not stepped propodal palm of pereopod I (vs. Figure 6. Apanthura laevipedata sp. nov., holotype, female A pereopod 1 B pereopod 2 C pereopod 3 D pereopod 4 e pereopod 5 F pereopod 6 G pereopod 7. stepped in the latter), upwardly extending carpus (vs. not extending in the latter), laterally invisible suture in pleonites I-V (vs. visible in the latter), and parallel lateral margins of the pleotelson (vs. proximally concave lateral margins in the latter) (Nunomura 1993). The new species also differs from A. drosera in having not stepped propodal palm of pereopod I (vs. stepped in the latter) (Poore and Lew Ton 1985), while can be distinguished from A. insignifica and A. tyrrhenica in that all pleonites are not separated by folds (vs. separated in the latter species) (Kensley 1978;Wägele 1980).  Remarks. Classification of the genera of Arcturidae had difficult because many genera were inadequately and ambiguously described (Kussakin 1972;King 2003b). To resolve this problem, Kussakin (1972) suggested using the structure of pereopods I-IV to diagnose genera and presented a key based on these features to eight Arctic and boreal genera. Since then Poore (2001b) transferred Antarcturus zur Strassen, 1903 and related genera Antarcturidae Poore, 2001. Parapleuroprion Kussakin, 1972 is now synonym of Arcturus Latreille, 1829 (Boyko et al. 2008d). Arcturella G. O. Sars, 1897 is synonym of Astacilla Cordiner, 1793 (Kensley et al. 2007;Rincón et al. 2018). Here we submit the key to all genera of Arcturidae.

Idarcturus trispinosus
Description of holotype female. Body (Fig. 8A, B) moderately flattened dorsoventrally, slightly geniculated between pereonites IV and V; length 4.5 mm, 4.5× longer than wide. Cephalon with three dorsal spines, one pair of lateral spines; anterior margin deeply concave, with small median process; anterolateral lobe expended anteriorly, concave distally; eye large, round, positioned laterally. Pereonites with dorsal spines; pereonite I with one pair of dorsal spines; pereonites II and III similar to each other in length, with one pair of dorsal spines, two pairs of lateral spines; dorsal spines of pereonite III bigger than dorsal spines of pereonite II; pereonite IV widest, ~ 1.5× longer than pereonites II and III together, with two pairs of small dorsolateral spines, four pairs of dorsal spines, one middorsal spine on posterior margin; posterior dorsal spines larger than other dorsal spines; anterolateral angle expanded laterally; oostegite IV with suture line posteriorly; pereonites V-VII similar to each other in length, with one pair of small dorsal spines, two or three tuberculate elevations. Pleon ~ 1.3× longer than pereonites V-VII, with two pairs of dorsal spines; anterior spines smaller than posterior spines; pleotelson gradually tapering posteriorly, with three pairs of wings laterally; proximal wing smaller than proceeding wings; apex round.
Antennule (Fig. 9A) exceeding peduncular article II of antenna, consisting of three peduncular articles and single-articled flagellum; peduncular article I globular, with one projection dorsally and two penicillate setae; article II cylindrical, with five penicillate setae; article III smaller than article II; flagellum with three aesthetascs on anterodistal end and three simple setae on distal end. Antenna (Fig. 9B-D) almost half of body length, slender, consisting of five peduncular articles and three flagellar articles; peduncular article I small; article II ×longer than article I; article III ×longer than article II, with two short simple setae distally; articles IV and V similar to each other, almost 3× longer than article III, with numerous minute simple setae along with lateral margin; article IV with one penicillate seta laterally; flagellar article I longer than flagellar articles II and III; articles II and III with one low of spines resembling saw-teeth on ventral margin; article III with one large claw apically.
Distribution. Southern coast of South Korea.
Etymology. The composite epithet of the specific name trispinosus is a combination of the Latin tres, meaning three, and spinosus, meaning thorny. This name refers to having three spines on the dorsal surface of the cephalon.
Remarks. Within the genus, Idarcturus trispinosus sp. nov. is distinguished from I. platysoma, the type species of the genus, by having dorsal spines in both sexes (lacking the dorsal spines in I. platysoma) and the pleotelson bearing lateral wings (lacking in the latter) (Barnard 1914).
Idarcturus trispinosus sp. nov. is similar to I. allelomorphus in having dorsal spines, but the former differs by having three dorsal spines on the cephalon (vs. two in the lat-ter) and six pairs of dorsal spines on pereonite IV (vs. one pair in the latter) (Menzies and Barnard 1959).
Idarcturus trispinosus sp. nov. most resembles I. hedgpethi in having anterolaterally expanded pereonite IV and dorsal spines. However, they could be distinguished by the anterolateral lobe of the cephalon (concave distally in the former vs. truncated in the latter) and the numbers of dorsal spines on the cephalon (three in the former vs. two in the latter), on pereonite IV (six pairs in the former vs. one pair in the latter), and on the pleon (two pairs in the former vs. one pair in the latter) (Menzies 1951).
Antennule (Fig. 13A) over second peduncular article of antenna, consisting of three peduncular articles and single-articled flagellum; peduncular article I square to globular, articles II and III cylindrical; article II slightly longer than article III; flagellum elongated oval, with five aesthetascs along with anterodistal region and three simple setae on distal end.
Antenna (Fig. 13B, C) slender, exceeding half of body length, composed of five peduncular articles and 3-articled flagellum; peduncular articles I and II subequal to each other; article IV longest; article V slightly shorter than article IV; articles IV and V with I-III pairs of simple setae; flagellar articles I and II with one row of spines resembling saw-teeth, article II with five simple setae on distal end.
with one penicillate seta on superior margin in pereopod VI; propodus with two or three penicillate setae on superior margin; dactylus bi-unguiculate, secondary unguis tiny.
Distribution. Southern coast of Jeju-do in South Korea.
Habitats. Sublittoral zone of sandy bottom. Etymology. The composite epithet of the specific name paralongipectus is a combination of the Greek prefix para-and the specific name of Neastacilla longipectus Nunomura, 2008, which means near longipectus, refering to the close resemblance to N. longipectus.
Remarks. Seven species of Neastacilla have pereonite IV at least 5× as long as pereonites II and III together in female as in the new species: N. algensis Hale, 1924, N. deducta Hale, 1925, N. kanowna King, 2003, N. lawadi King, 2003, N. longipectus Nunomura, 2008, N. monoseta (Guiler, 1949, and N. soelae King, 2003(Hale 1924, 1946King 2003b;Nunomura 2008). Among these species, Neastacilla paralongipectus sp. nov. is easily distinguishable from N. algensis, N. lawadi, and N. soelae in terms of the absence of dorsal tubercles on the cephalon in female (vs. presence in the latter species) (Hale 1924;King 2003b). Neastacilla paralongipectus sp. nov. is similar to N. deducta, N. kanowna, and N. monoseta in having a smooth body lacking dorsal elevations (King 2003b). However, Neastacilla paralongipectus sp. nov. differs from N. deducta, N. kanowna, and N. monoseta in that oostegite IV has suture posteriorly (vs. mesially in N. deducta and N. monoseta) and the pleotelson has rounded apex (vs. truncated in N. kanowna) (King 2003b). Neastacilla paralongipectus sp. nov. is easily distinguishable from N. longipectus in terms of the shape of the anterior margin of the cephalon (deeply concave in the former vs. slightly concave in the latter), the shape of the penes (elongated triangle in the former vs. rectangle in the later), and the structure of pereonite I and pleonite I (fused to cephalon and pleotelson, respectively, in the former vs. not fused in the latter) (Nunomura 2008).
Key to known species of the genus Neastacilla in the northwest Pacific