A new species of Eusirus from Jeju Island, Korea (Crustacea, Amphipoda, Eusiridae)

Abstract A new eusirid amphipod, Eusirus bulbodigitus sp. n., from Jeju Island, Korea is described with a detailed description and illustrations. Eusirus bulbodigitus sp. n. shows common features with the five known eusirid amphipods Eusirus abyssi Stephensen, 1944, Eusirus columbianus Bousfield & Hendrycks, 1995, Eusirus hirayamae Bousfield & Hendrycks, 1995, Eusirus laticarpus Chevreux, 1906, and Eusirus parvus Pirlot, 1934, such as the mandibular palp article 3 bearing a group of setae laterally. However, this new species is differentiated by the combination of the following characteristics: the eyes are poorly developed, the propodus on pereopod 4 is slightly shorter, the inner margin of dactylus on pereopod 4 is swollen, the length of pereopods 5–7 is moderate, the urosomite 1 has a dorsal protrusion distally, and the telson is shallowly cleft. This is the first record of the genus Eusirus Krøyer, 1845 from Korean waters.

Here, we report a new eusirid species, Eusirus bulbodigitus sp. n. from Jeju Island, Korea, belonging to the family Eusiridae with a detailed description and illustrations. This is the first record of the genus Eusirus from Korean waters.
Upper lip ( Fig. 1H) globular, apex convex and weakly produced, covered with marginal and submarginal fine setae.
Lower lip (Fig. 1I), inner lobe weak, covered with marginal and submarginal fine setae; outer lobe subovoid, round distally, covered with fine setae on apex and medial margin; mandibular process short.
Maxilla 2 (Fig. 1L), inner plate ovoid, broader, larger than outer plate, lined with 17 marginal setae from apex to distal half of medial margin, with 4 plumose setae subdistally and 6 facial setae on medial margin; outer plate with 18 setae on apical margin.
Left mandible ( Fig. 2A) with bi-dentate incisor and 6-dentate lacinia mobilis; accessory setal row composed of 2 simple and 4 dentate setae; palp 3-articulate, palp article 1 shortest, palp article 2 slightly curved, with 1 row of 11 setae on surface obliquely, palp article 3 1.2 times as long as palp article 2, gradually slender distally, covered with several rows of minute setae on distal surface, with 1 group of 4 serrate setae on lateral margin proximally and longest seta among them reaching apex of palp article 3, lined with simple and serrate setae on medial margin, apex oblique and with 5 long serrate setae. Right mandible ( Fig. 2B) with bi-dentate incisor, lacinia mobilis not split and with 1 row of small dentations apically; accessory setal row composed of 3 dentate setae; molar triturative, columnar; palp similar to that of left mandible.
Gnathopod 1 (Fig. 2C, D) stout, strongly subchelate, "eusiroidean" in form; coxa subtriangular, slightly expanded anteroventrally, with 13 submarginal setae on ventral margin irregularly, with 4 small notches bearing 1 minute seta and more expanded backwards posterodistally, with 9 setae on medial surface; basis steady in width, anterior margin lateral border shallowly lobate distally bearing 1 minute seta, with 6 short setae on distal half, medial border weakly lobate unevenly bearing 4 groups of elongate setae on distal 2/3, posterior margin lined with 19 submarginal 4 setae, with 1 group of 3 setae distally; ischium largely lobate, anterior margin lateral border expanded dis- tally, with 3 minute setae, medial border lobate, with 3 minute setae distally, posterior margin with 7 setae; merus subrectangular, as long as ischium, forming groove and broadly lobate on anterior margin, posterior margin lined with serrate setae, with 12 serrate setae on medial surface posterodistally; carpus elongate and slender, 0.8 times as long as basis in length of anterior margin, anterior margin lined with regularly spaced robust and simple setae, carpal lobe narrow, covered with elongate serrate setae mediodistally, posterior margin lateral border with acute protrusion distally, medial border broadly lobate; propodus wider than long, broad, very deep, shorter than that of carpus in length of anterior margin, with 1 plumose seta and 2 simple setae at anterodistal corner, proximal 2/3 of posterior margin fitting well to distal groove of carpus and with setae distally, with 1 group of 10 defining setae, palm long, convex, lined with numerous crooked setae submarginally, with 1 pair of plumose setae distally; dactylus falcate, stout, long, fitting palm, lined with minute setae along inner margin.
Gnathopod 2 (Fig. 3A, B) similar to gnathopod 1, stout, strongly subchelate, also "eusiroidean" in form; coxa rectangular, with 2 small notches bearing 1 minute seta at posterodistal corner, with 1 robust seta on posterior margin, with 8 setae on medial surface, ventral margin convex, with 10 submarginal setae; basis steady in width, anterior margin lateral border shallowly lobate distally bearing 1 minute seta and with 18 setae, medial border with 1 long and 12 short setae, with 1 pair of elongate setae at anterodistal corner, posterior margin lined with 12 setae on proximal 1/3, remainder just with 6 minute setae, with 1 group of 3 setae at posterodistal corner; ischium largely lobate, expanded distally, anterior margin with 5 minute setae on lateral and medial borders, respectively, posterior margin just with 2 setae, with 1 pair of setae at posterodistal corner; merus subrectangular, slightly longer than ischium, forming groove anteriorly, posterior margin lined with short setae, covered with 7 long serrate setae on posterodistal surface, posterodistal corner weakly produced, with 2 short setae; carpus elongate and slender, 0.7 times as long as basis, anterior margin lined with robust and simple setae regularly, carpal lobe narrow, covered with elongate serrate setae medially, posterior margin lateral border with acute protrusion distally, medial border broadly lobate, with 2 minute setae marginally, with 1 robust seta on medial surface; propodus wider than long, broad, very deep, shorter than that of carpus in length of anterior margin, anterior margin with 2 setae, with 1 plumose and 3 simple setae at anterodistal corner, proximal 2/3 of posterior margin fitting well to distal groove of carpus, with 1 group of 11 defining setae, palm long, convex, lined with numerous crooked setae submarginally, with 1 pair of plumose setae proximally; dactylus falcate, stout, long, fitting palm, lined with minute setae along inner margin.
Pereopod 3 (Fig. 4A, B) slender, length ratio of merus:carpus:propdus 1.0:0.7:0.9; coxa rectangular, convex, with 8 submarginal setae on ventral margin, expanded backwards, with 3 small notches bearing 1 minute seta at posteroventral corner, with 1 robust seta on posterior margin, with 23 setae on medial surface; basis linear and elongate, 1.4 times as long as coxa, anterior margin a little lobate distally, lined with 18 single and 2 pairs of setae on distal 2/3, with 1 pair of setae at anterodistal corner, posterior margin with 5 setae, with 1 long seta at posterodistal corner; ischium lobate  distally, with 3 minute setae on lateral and medial borders of anterior margin, respectively, with 1 pair of unequal setae at posterodistal corner; merus 0.6 times as long as basis, anterior margin with 7 minute setae, anterodistal corner weakly produced, with 1 pair of unequal setae, posterior margin with 6 pairs of short setae, with 1 group of 3 setae at posterodistal corner; carpus slightly dilated distally, 0.5 times as long as merus, anterior margin with 4 minute setae marginally and 1 seta distally, posterior margin with 5 pairs of unequal setae marginally and 1 pair of setae distally, distal margin obliquely truncated posteriorly, with 2 setae on lateral and medial borders, respectively; propodus, 0.9 times as long as merus, anterior margin with 8 minute setae, with 1 pair of minute setae at anterodistal corner, posterior margin with 11 groups of setae, with 1 pair of locking setae and 1 seta at posterodistal corner; dactylus elongate, 0.3 times as long as propodus, curved distally, with 5 rows of small teeth and 1 subdistal protrusion on distal half of posterior margin.
Pereopod 5 (Fig. 5A-C), coxa bilobate subequally, anterior lobe slightly larger than posterior lobe, both expanded posteroventrally, posterior lobe with 2 subacute teeth posteroventrally; basis ovoid, convex anteriorly, anterior margin with 4 long setae proximally and lined with 15 setae, with 1 small acute protrusion and paired robust and minute setae at anterodistal corner, posterior margin lateral border moderately expanded and with 22 strong serrations bearing 1 minute seta, medial border with 2 setae distally and with 3 setae at angulate distal corner; ischium short, anterior margin with 1 seta, with 1 small acute protrusion and paired robust and minute setae at anterodistal corner, posterior margin lateral border largely lobate, its apex acute and slightly lurched distally, medial border also lobate but not produced distally; merus 0.7 times as long as basis, anterior margin lined with 3 single setae and 4 groups of setae, with 1 group of 4 setae at anterodistal corner, posterior margin slightly expanded, with 5 setae irregularly, with 1 pair of unequal setae at produced posterodistal corner; carpus as long as merus, anterior margin with 5 groups of setae, anterodistal corner obliquely truncated, with 1 group of 8 setae, posterior margin with several setae and 1 group of 6 setae distally; propodus linear and elongate, 1.8 times as long as carpus, anterior margin with 11 single and 3 paired setae, with 1 pair of locking setae and 1 lateral seta distally, posterior margin setose irregularly, with 3 setae at posterodistal corner; dactylus also elongate, 0.3 times as long as propodus, curved distally, with 7 rows of small teeth and 1 subdistal protrusion on distal 2/3 of anterior margin, with 3 minute setae on posterior margin.
Pereopod 6 ( Fig. 5D-H) longer than pereopod 5; coxa bilobate, anterior lobe smaller, posterior lobe more dilated posteroventrally, with 2 small notches bearing 1 minute seta; basis ovoid, anterior margin with 15 setae proximally and lined with 12 setae, with 1 small acute protrusion, 1 robust and 1 simple seta at anterodistal corner, posterior margin lateral border moderately expanded, with 21 strong serrations bear- ing 1 minute seta, medial border with 10 setae on distal half; ischium short, anterior margin with 1 seta, with 1 seta and 1 small acute protrusion at anterodistal corner, posterior margin lateral border largely lobate, its apex acute and slightly lurched distally, medial border also lobate but not produced distally; merus as long as basis, anterior margin slightly concave, armed with setae of various combination, posterior margin broadly expanded, with 5 setae, posterodistal corner produced, with 1 group of 5 unequal setae; carpus 0.9 times as long as merus, anterior margin with 2 single, 1 paired short setae and 3 groups of setae, anterodistal corner obliquely truncated, with 1 group of 4 robust setae and 1 medial, 2 lateral setae, posterior margin with 2 setae, with 1 group of 4 setae at posterodistal corner; propodus linear and elongate, 1.8 times as long as carpus, anterior margin with 10 single and 7 paired setae, with 1 pair of locking setae and 1 lateral seta at anterodistal corner, posterior margin densely setose irregularly, with 1 seta at posterodistal corner; dactylus also elongate, 0.2 times as long as propodus, curved distally, with 6 rows of small teeth and 1 subdistal protrusion on distal 2/3 of anterior margin, with 2 minute notches on posterior margin.
Remarks. Eusirus bulbodigitus sp. n. shares the characteristic of a mandibular palp bearing a group of setae laterally on the 3 rd article with five known species: E. abyssi Stephensen, 1944;E. columbianus Bousfield & Hendrycks, 1995;E. hirayamae Bousfield & Hendrycks, 1995;E. laticarpus Chevreux, 1906;and E. parvus Pirlot, 1934(Chevreux 1906, Pirlot 1934, Stephensen 1944, Bousfield and Hendrycks 1995. However, E. abyssi is readily discriminated from E. bulbodigitus sp. n. by the absence of eyes, the massive carpal lobes of gnathopods 1 and 2, the elongate articles of pereopod 3, and the presence of dorsal teeth on urosomite 1 (Stephensen 1944). Both Eusirus columbianus and E. laticarpus can be differentiated from E. bulbodigitus sp. n. by their developed eyes and the protruding apical margin of each lobe on telson (Chevreux 1906, Bousfield andHendrycks 1995). In addition E. columbianus represents additional differences as follows: (1) the accessory flagellum of E. columbianus is longer than that of E. bulbodigitus sp. n.; (2) the inner plate of maxilla 2 in E. columbianus is smaller than that of E. bulbodigitus sp. n.; (3) among the lateral setae on mandibular palp article 3, the longest seta is not reaching at distal end of article 3 in E. columbianus (vs. reaching in E. bulbodigitus sp. n.); (4) the merus, carpus and propodus of pereopod 4 are slightly longer than those of pereopod 3 in E. columbianus (vs. slightly shorter in E. bulbodigitus sp. n.); (5) the posterior margin of pleonal epimeron 3 is serrate entirely in E. columbianus (vs. partially serrate on distal half in E. bulbodigitus sp. n.); and (6) the lateral margin of outer ramus on uropod 3 is lined with single setae in E. columbianus (vs. with paired setae in E. bulbodigitus sp. n.) (Bousfield and Hendrycks 1995). Eusirus bulbodigitus sp. n. is very similar to E. parvus. However, it can be distinguishable from the latter in following characters combined: (1) the eyes are poorly developed in E. bulbodigitus sp. n. (vs. well-developed in E. parvus); (2) the mandibular palp of E. bulbodigitus sp. n. is slender than that of E. parvus; (3) the longest seta of mandibular palp article 3 is reaching to the end of article in E. bulbodigitus sp. n. (vs. not reaching in E. parvus); (4) the articles of pereopods in E. parvus are slightly longer than those of E. bulbodigitus sp. n.; and (5) the inter-ramal process of uropod 1 is absent in E. bulbodigitus sp. n. (vs. present in E. parvus) (Pirlot 1934). Eusirus bulbodigitus sp. n. from Korean waters also closely resembles E. hirayamae from Japanese waters. However, it can be clearly distinguished from the latter with the combination of the following characteristic features: (1) the lateral border of carpus on ganthopod 2 is acutely produced posterodistally (vs. not produced and rounded in E. hirayamae); (2) both lengths of the carpus and propodus on pereopod 4 are slightly reduced (merus:carpus:propodus = 1.0:0.6:0.7) compared to those of pereopod 3 (merus:carpus:propodus = 1.0:0.7:0.9), but they are not reduced in pereopod 4 of E. hirayamae (merus:carpus:propodus = 1.0:0.7:1.0 in both pereopods 3 and 4); (3) on pereopod 4, the proximal three-quarters of posterior margin of the dactylus is swollen in E. bulbodigitus sp. n. (vs. moderate in E. hirayamae); (4) the dactylus of pereopods 3 and 5-7 is more elongate and slender in E. bulbodigitus sp. n. (vs. shorter and thicker in E. hirayamae); (5) the posterior margin of dactylus on pereopods 3-7 has rows of small teeth in E. bulbodigitus sp. n. (vs. no teeth in E. hirayamae), (6) the setations of articles on pereopods 3-7 in E. bulbodigitus sp. n. are weaker than those in E. hirayamae; (7) the lateral surfaces of pleonal epimera 1-3 are not covered with setae anteroventrally in E. bulbodigitus sp. n. (vs. densely covered with setae in E. hirayamae); and (8), the posterior margin of pleonal epimeron 3 is serrated in the distal half (vs. serrations occur along the whole posterior margin in E. hirayamae) (Bousfield and Hendrycks 1995).