Microlysias triangulus, Heo & Hendrycks & Kim, 2020

Heo, Jun-Haeng, Hendrycks, Ed A. & Kim, Young-Hyo, 2020, Two new species of the genus Microlysias (Crustacea, Amphipoda, Tryphosidae) from Korean Waters, Zootaxa 4759 (4), pp. 543-556 : 550-555

Microlysias triangulus n. sp.

(Korean Name: Se-mo-deo-deum-i-gin-pal-yeop-sae-u, new)

( Figs. 7–10 )

Type material. Holotype, adult male, 11.2 mm, cat no. NIBRIV0000865028, Nokdo Island , Nokdo-ri , Ocheonmyeon, Boryeong-si, Chungcheongnam-do, Korea, 36° 16′ 07″N, 126° 15′ 59″E, 21 November 2001, collected by Y. Eun. Paratypes, 1 adult female, 7.0 mm, cat no. NIBRIV0000865030; 1 immature male, 10.1 mm, cat no. NIBRIV0000865029 (dissected), data same as holotype ; 1 ♂, CMNC 2020-0002 .

Etymology. The species name is from the Latin triangulus (triangle), referring to the expanded, triangular shaped peduncular article 5 of male antenna 2.

Description. Holotype adult male.

Body ( Fig. 7A ) dorsally smooth; head ( Fig. 8A ), lateral cephalic lobe produced, subacute apically; eyes ovoid, black, medium; epimeron 1 ( Fig. 8B ) posterior margin smooth, rounded posteroventrally; epimeron 2 ( Fig. 8B ) subquadrate posteroventrally; epimeron 3 ( Fig. 8B ) subquadrate, posterior margin slightly convex, all epimera with pubescence ventrally and without serration posteriorly; urosomite 1 ( Fig. 8B ) with a weak, dorsal depression and low, rounded carina, which slightly overhangs urosomite 2.

Antenna 1 ( Fig. 8C ) stout, short, length 0.33 x antenna 2; peduncular article 1 longer than peduncular articles 2–3 combined, with a row of 7 penicillate setae dorsally; peduncular articles 2–3 short subequal in length; length ratio of peduncular articles 1–3=1.00: 0.22: 0.21; flagellum 9-articulate, subequal in length to peduncle, with stout callynophore, calceoli absent; accessory flagellum 4-articulate, article 1 elongate, much longer than the other articles combined.

Antenna 2 ( Fig. 8D ) elongate; peduncular article 5 characteristic in form, subtriangular, length=width, broadly gibbous ventrally, length 1.79 x article 4; flagellum 74-articulate, article 1 elongate, length 0.68 x peduncular article 5, calceoli absent.

Right mandible ( Fig. 8E ), incisor smooth, with 1 tiny sharp tooth on the lower side; lacinia mobilis absent; accessory setal row with 6 spines; molar elongate and forms a setose ridge; palp 3-articulate, attached slightly proximal to molar; article 1 unarmed, elongated, length 0.61 x article 3; article 2 longest, with a row of 10 A2-setae; article 3 weakly falcate, length 0.91 x article 2, with 1 A3-seta, 12 D3-setae, and 3 E3-setae.

Left mandible ( Fig. 8F ) similar to left except lacinia mobilis present as a simple peg-like spine, and accessory setal row with 8 spinules.

Maxilla 1 ( Fig. 8G ), inner plate slender, with 2 pectinate setae apically and setules on inner margin, lacking apical spine; outer plate with 11 dentate spine-teeth; palp narrowly rectangular, biarticulate, curved, proximal article short, distal article with 5 blunt spines and 1 simple seta apically.

Maxilla 2 ( Fig. 8H ), inner plate slender, with 1 pappose and 9 simple setae apically, medial margin with pubescence; outer plate slender, length 1.23 x inner one, with pectinate and simple setae apically, both margins covered with pubescence.

Maxilliped ( Fig. 8I ), inner plate subrectangular, with 2 pectinate setae medially, apical margin with bifid cutting edge and 2 simple setae; outer plate elongate semielliptical, reaching distal end of article 2 of palp, with a row of 4 spinules medially; palp 4-articulate, articles 1–3 with simple setae medially and laterodistally; article 4 falcate, shorter than article 3.

Gnathopod 1 ( Fig. 8J ) subchelate; coxa large, widening distally, roundly produced anteroventrally, width 0.66 x length; basis with simple setae along anterior margin; merus, and carpus subequal in length with pubescence ventrally, carpus with short lobe ventrally; propodus subrectangular, narrowing distally, length 2.50 x width, slightly concave ventrally, length 1.41 x carpus; palm short, serrulate, gently convex, defined by 2 spines posterodistally; dactylus falcate, with an accessory tooth, fitting palm.

Gnathopod 2 ( Fig. 9A ), coxa subrectangular, width 0.41 x length; basis slender, elongate, with 1 simple seta posterodistally; ischium elongate, with 5 simple setae on posterior margin, subequal in length to carpus; merus length 0.73 x ischium, with patch of setules posteriorly and unequal simple setae posterodistally; carpus gradually widening distally, with 7 simple setae distally, half of ventrodistal surface covered with pubescence; propodus subovate and narrow, length 2.43 x width, minutely chelate, with 4 rows of pectinate setae medially, length 0.46 x carpus; dactylus acute, fitting palm.

Pereopod 3 ( Fig. 9B ), coxa subrectangular, unarmed; basis narrowing proximally, with 6 simple setae anteriorly and 3 simple setae posterodistally; merus length 1.82 x carpus, produced anterodistally, with 14 simple setae posteriorly; propodus subrectangular, slender, length 1.35 x carpus, with a row of 5 spines posteriorly; dactylus falcate, length 0.30 x propodus.

Pereopod 4 ( Fig. 9C ) similar to pereopod 3 except coxa much broader than that of pereopod 3, posterior margin excavate, posterodistal lobe produced, truncated, posteroventral corner rounded.

Pereopod 5 ( Fig. 9D ), coxa large, rounded quadrate, weakly bilobate, length 1.16 x width; basis broadly subcircular, width 1.07 x length, posterior margin weakly serrate, posteroventral lobe broadly rounded; with a row of spines and 12 simple setae along anterior margin; merus expanded posteriorly, anterior margin with 6 simple setae, posterior margin with 4 simple setae; propodus rectangular, length 1.46 x carpus, anterior margin with a row of 4 paired spines; dactylus falcate, length 0.42 x propodus.

Pereopod 6 ( Fig. 9E ), coxa subequal to coxa 7, weakly bilobate, posterior lobe produced posteroventrally; basis subovate, with a row of spines on distal half of anterior margin, posterior margin weakly serrate; merus expanded posteriorly, anterior margin with 6 long simple setae and 5 spines, posterior margin with 3 spines; propodus rectangular, length 1.25 x carpus, anterior margin with 4 paired spines; dactylus falcate, length 0.35 x propodus.

Pereopod 7 ( Fig. 9F ) similar to pereopod 6, but coxa not lobate; basis broader than that of pereopod 6, posterior margin convex and weakly serrate; merus narrow, not expanded.

Uropod 1 ( Fig. 10A ), peduncle subrectangular, length 1.67 x outer ramus, with row of spines on dorsolateral and dorsomedial margins; rami lanceolate, outer ramus slightly longer than inner one, dorsolateral margin with 4 spines; inner ramus with 3 spines on both dorsolateral and medial margins.

Uropod 2 ( Fig. 10B ), peduncle longer than outer ramus, with 3 spines dorsolaterally; outer ramus slightly longer than inner one, with 4 dorsolateral spines; inner ramus with 3 dorsolateral and 2 dorsomedial spines.

Uropod 3 ( Fig. 10C ), peduncle short, much shorter than outer ramus, with 5 spines distally; outer ramus biarticulate, length 1.17 x inner ramus, proximal article with 11 long plumose setae along inner margin, outer margin with 6 spines; distal article short, length 0.17 x proximal one; inner ramus broad, not reaching base of distal article of outer ramus, outer margin with 15 long plumose setae, inner margin with 2 spines and 3 plumose setae.

Telson ( Fig. 10D ) much longer than broad, length 1.62 x width, deeply cleft, 74% of its length, each lobe with a pair of penicillate setae and 3 spines dorsolaterally, 1 spine and 1 penicillate seta apically.

Paratype, immature male. 10.1 mm.

Antenna 1 ( Fig. 10E, F ) similar to that of adult male; peduncular article 1 with 2 penicillate setae distally; flagellum 9-articulate, calceoli absent.

Antenna 2 ( Fig. 10E, G ) short; similar to that of adult male, but peduncular article 5 less expanded ventrally; flagellum 54-articulate, compressed, much shorter than that of adult male, calceoli absent.

Paratype, adult female. 7.0 mm.

Head ( Fig. 10H ) similar to that of adult male; cephalic lobes slightly narrower and rounded.

Antenna 1 ( Fig. 10I ) similar to that of adult male; peduncular article 1 with 4 penicillate setae ventrally; flagellum 8-articulate; callynophore present, calceoli absent.

Antenna 2 ( Fig. 10J ) short, subequal in length to antenna 1; peduncular articles 4–5 subrectangular, narrow, without ventral expansion; flagellum 6-articulate, much shorter than that of adult male.

Uropod 3 ( Fig. 10K ) stout, inner ramus narrower than male and with less plumose setae than male.

Depth range. 20–30 m

Remarks. The relative position of the palp attached to the mandibular body has traditionally been an important character in higher level systematics in lysianassids. However, the form of the palp is as important as the relative position. Elongation of the first article of the palp is an unusual character in lysianassids. Barnard & Karaman (1991) mentioned that the mandibular palp of the genus Microlysias is attached strongly proximal to the molar, but this diagnosis was based on only a single species, M. xenokeras . However, Lowry and Kilgallen (2014), showed that M. xenokeras and M. soela have slightly differing positions of the mandibular palp, but they are all proximal to the molar. The character of the mandible in the genus Microlysias should be emended as follows: mandibular palp is attached slightly to strongly proximal to the molar.

Microlysias triangulus sp. nov. is similar to M. xenokeras Stebbing, 1918 , M. soela Lowry & Kilgallen, 2014 and M. rectangulatus sp. nov. (this paper) in having a broadened male peduncular article 5 of antenna 2. However, M. triangulus is distinguished from its congeners as follows: 1) antenna 2, peduncular article 5 characteristic in form, subtriangular (vs. subquadrate in M. xenokeras , M. soela and M. rectangulatus ); 2) gnathopod 1, propodus elongate and slender, length 2.50 x width (vs. stouter in M. xenokeras , 2.10 x width, M. soela , 1.73 x width and M. rectangulatus , 2 x width). As well, from M. xenokeras , it differs in the much stouter gnathopod 2 propodus and narrower pereopod 5 basis. From M. soela it also differs in the much smaller eyes, lack of maxilla 1 inner plate apical spine projection and slender maxilla 2. From M. rectangulatus sp. nov. (this paper) it also differs in the narrower propodus of gnathopods 1–2, much shorter article 1 of the mandibular palp and smooth posterior margins of epimeron plates 2–3. Microlysias triangulus is also the largest species, with males reaching 11.2 mm vs 7.6 mm in M. rectangulatus and 6 mm in M. xenokeras and M. soela .

The type species, M. xenokeras was collected from ascidians ( Stebbing, 1918). Lowry & Kilgallen (2014) mention that crinoids were in one of the collections of M. soela , but did not specifically mention any direct association. We have no information from our collections whether or not M. triangulus were associated with any other invertebrates when collected. If Microlysias species are commonly found to be associated with sessile invertebrates, we can expect many other species to be eventually described in the future.

We have no direct evidence of the feeding type of M. triangulus , but as M. rectangulatus is a carnivorous/scavenging species, it may be inferred that M. triangulus may also have a similar feeding habit.

Distribution. Korea (Nokdo-island, Boryeong-si).

Figures

FIGURE 7. Microlysias triangulus sp. nov., A, paratype, adult male, 11.6 mm, habitus; B, paratype, adult female, 7.0 mm, habitus. Scale bars=1.0 mm.

FIGURE 8. Microlysias triangulus sp. nov., holotype, adult male, 11.2 mm, cat no. NIBRIV0000865028: A, head, lateral; B, epimeral plates and urosomites 1–3; C, antenna 1; D, antenna 2; E, right mandible; F, left mandible; G, maxilla 1; H, maxilla 2; I, maxilliped; J, gnathopod 1. Scale bars=1.0 mm (A, B), 0.4 mm (C, D, J), 0.1 mm (E–I).

FIGURE 9. Microlysias triangulus sp. nov., holotype, adult male, 11.2 mm, cat no. NIBRIV0000865028: A, gnathopod 2; B, pereopod 3; C, pereopod 4; D, pereopod 5; E, pereopod 6; F, pereopod 7. Scale bars=0.2 mm.

FIGURE 10. Microlysias triangulus sp. nov., holotype, adult male, 11.2 mm, cat no. NIBRIV0000865028: A, uropod 1; B, uropod 2; C, uropod 3; D, telson; paratype, immature male, 10.1 mm, cat no. NIBRIV0000865029: E, head, lateral; F, antenna 1; G, antenna 2; paratype, adult female, 7.0 mm, cat no. NIBRIV0000865030: H, head, lateral; I, antenna 1; J, antenna 2; K, uropod 3. Scale bars=0.5 mm (H), 0.4 mm (E–G), 0.2 mm (A–D, I–K).