Ampelisca pygmaea Shellenberg, 1938

Kim, Kyung-Won, Choi, Jae-Hong, Shin, So-Yeon & Kim, June Kim and Young-Hyo, 2024, Amphipod (Crustacea: Malacostraca) fauna of the continental shelf region in the Southern Sea of Korea, Journal of Species Research 13 (2), pp. 159-177 : 161-163

4. Ampelisca pygmaea Shellenberg, 1938

꼬Ṙü경ăNjẹ (ṳạ)* ( Figs. 3, 4 , 11D )

Ampelisca pygmaea Shellenberg, 1938: 10 , Fig. 4 ; Ledoyer, 1978: 211, Fig. 3; Ren, 2006: 121, Fig. 40.

Material examined. 2♀♀, Site 4, 17 August 2022.

Description. Female (MABIK CR00254533): Body ( Figs. 3A, 11D ) smooth, 5.6 mm long: Head ( Fig. 3A) subtriangular; rostrum short; two paired eyes are composed of corneal lens, one pair located behind rostrum, the other located to ventral margin of cephalic lobe.

Right mandible ( Fig. 3B), incisor with 6 blunt teeth; lacinia mobilis with 5 tiny blunt teeth; molar truncate, accessory setal row of 7 setae between lacinia mobilis and molar; palp well developed, triarticulate, mandibular palp article 2 characteristically swollen.

Gnathopod 1 ( Fig. 3C), coxa expended ventrally, with plumose setae ventrally, ventral margin rounded with a notch posteroventrally; basis narrowing proximally, anterior margin with short marginal setae, proximal half with long simple setae, posterior margin with 8 simple long setae and row of marginal robust setae; carpus elongate, width 0.36 times length, twice as propodus, with several setae on each margin; propodus subovate, palm subchelate; dactylus broad, falcate.

Gnathopod 2 ( Fig. 3D) similar to gnathopod 1, but coxa elongate and narrowed distally; carpus and propodus more elongated than those of gnathopod 1; dactylus falcate; length ratio of articles 2 - 7 = 1.00: 0.19: 0.31: 0.73: 0.30: 0.15.

Pereopod 3 ( Fig. 3E), coxa slender, subrectangular, similar to that of gnathopod 2; basis with marginal setae anteriorly, 4 long simple setae posteriorly; merus elongate, posterodistal corner expanded distally with plumose setae; carpus subrectangular, posterior margin with simple setae, 2 plumose setae posteromedially; propodus subovate, with 6 robust setae posteromarginally, dactylus elongate; length ratio of articles 2 - 7 = 1.00: 0.17: 0.65: 0.29: 0.28: 0.37.

Pereopod 4 ( Fig. 3F) subsimilar to pereopod 3, but more setose; coxa subquadrate, posterior margin with posteroproximal excavation, ventral margin setose; basis distal half of posterior margin setose, with 4 robust setae distally; merus both margins with plumose setae, posterodistal quarter with robust setae; carpus posterior margin with 2 long robust setae, 4 plumose setae posteromedially; propodus with 5 robust setae posteromarginally; dactylus falcate, slightly longer than propodus; length ratio of articles 2 - 7 = 1.00: 0.20: 0.81: 0.29: 0.24: 0.27.

Pereopod 5 ( Fig. 4A ), coxa bilobate; basis roundly expanded anteriorly and posteroproximally, anterior margin with marginal robust setae and 5 plumose setae; carpus slightly expanded posterodistally with 4 robust setae, anterodistal corner with 1 robust seta; propodus club shaped; length ratio of articles 2 - 6 = 1.00: 0.18: 0.33: 0.48: 0.41.

Pereopod 6 ( Fig. 4B ) similar to pereopod 5, but coxa much smaller, basis subcircular, expanded both marginally; length ratio of articles 2 - 6 = 1.00: 0.17: 0.26: 0.50: 0.38.

A B C D E F

Pereopod 7 ( Fig. 4C ), basis expanded posteroventrally, posterodistal lobe not reaching distal end of ischium, with marginal plumose setae; ischium subovate, with 2 robust setae anteriorly; dactylus foliaceous; length ratio of articles 2 - 7 = 1.00: 0.25: 0.11: 0.12: 0.26: 0.27.

Uropod 1 ( Fig. 4D ), peduncle rectangular, slightly longer than rami, with 4 dorsolateral, 7 dorsomedial, and 8 ventrolateral robust setae; rami slightly shorter than peduncle, outer ramus slender, subequal to inner one.

Uropod 2 ( Fig. 4E ) short, peduncle rectangular, 1.1 times as long as inner ramus; both rami dorsomedially serrulate.

Uropod 3 ( Fig. 4F ), peduncle much shorter than rami; both rami subequal in length, outer ramus slender, medial margin serrate, with 3 marginal setae; inner ramus distal half with plumose setae.

Telson ( Fig. 4G ) subovate, narrowing distally, about 0.6 times as long as wide, deeply cleft, each lobe with dorsal and apical setae.

Distribution. Korea (south coast), China, Russia, Indonesia, Vietnam.

Remarks. In the genus Ampelisca , degree of projection of rostrum, shape of the cephalic lobe, and the variation of number and location of lens-shaped eyes are different depending on the species. Ampelisca pygmaea belongs to a group with two lens-shaped eyes, the second pair located on the ventral margin of the cephalic lobe. Among the species of genus Ampelisca , A. pygmaea is morphologically distinguished from other Ampelisca species by having following three characteristics: (1) mandibular palp, article 2 broadly expended (vs. generally thin except for A. ceratophoculata , A. hainanensis , etc.); (2) pereopods 3, 4, meri expended posterodistally (vs. generally unexpanded or weak to that extent of A. pygmaea ); and (3) uropod 3, rami shape (slender out- er ramus; leaf-shaped inner ramus with plumose setae) ( Schellenberg, 1938; Ren, 2006). As Ren noted, individuals found in the South China Sea differ from those in other regions by having shorter antennae and concave posteroventral margin of pereopod 6 basis. Our Korean specimen is 5.6 mm long, which is larger than the individuals reported from the Bismarck-Archipelago by Shellenberg (1938) and the Indian Ocean by Ledoyer (1978), but in good agreement with the original description by Schellenberg (1938).

Figures

Fig. 4. Ampelisca pygmaea Shellenberg, 1938, adult female.A, pereopod 5; B, pereopod 6; C, pereopod 7; D, uropod 1; E, uropod 2; F, uropod 3; G, telson. Scale bars:A-G=0.2 mm.

Fig. 11. A, Ampelisca alatopedunculata; B, Ampelisca cyclops; C, Ampelisca naikaiensis; D, Ampelisca pygmaea; E, Byblis japonicus; F, Byblis longiflagelis; G, Ampithoe lacertosa; H, Kamehatylus japonicus.