Gammarus caecigenus Hou & Li

Hou, Zhonge, Zhao, Shuangyan & Li, Shuqiang, 2018, Seven new freshwater species of Gammarus from southern China (Crustacea, Amphipoda, Gammaridae), ZooKeys 749, pp. 1-79 : 29-33

Gammarus caecigenus Hou & Li sp. n. Figs 39, 40, 41, 42, 43, 44

Material examined.

Holotype: male (IZCAS-I-A1587-1), 10.0 mm, Xingwen County (105.12°E, 28.19°N), altitude 840 m, Yibin City, Sichuan Province, China, April 25, 2014, collected by Yucheng Lin, Huifeng Zhao, Yunchun Li, Jianglang Wu and Fengyuan Li. Paratype: female (IZCAS-I-A1587-2), 5.9 mm, same data as holotype.

Etymology.

The epithet derives from the Latin word " caecigenus ", referring to the eyes absent; adjective.

Diagnosis.

Eyes absent; antenna II with long setae, calceoli absent; merus and carpus of pereopod III with clusters of long setae on posterior margins; armature of urosomites degenerated, urosomite I with setae on dorsal margin, urosomite II with two groups of spines accompanied by setae; uropod I peduncle without basefacial spine; uropods I–II with more marginal spines; inner ramus of uropod III reaching 0.9 times the length of outer ramus, terminal article of outer ramus vestigial.

Description of holotype male

(IZCAS-I-A1587-1), 10.0 mm.

Head (Fig. 39A): eyes absent, inferior antennal sinus deep.

Antenna I (Fig. 39B, C): peduncle articles I–III in length ratio 1.0: 0.8: 0.4, with distal setae; flagellum with 28 articles, most with aesthetascs; accessory flagellum with four articles; both primary and accessory flagella with short distal setae.

Antenna II (Fig. 39D): peduncle articles III–V in length ratio 1.0: 2.1: 2.4, articles IV and V of peduncle with long lateral and medial setae; flagellum with ten articles and one tiny distal article, with setae along dorsal and ventral margins; calceoli absent.

Upper lip (Fig. 39E): ventral margin rounded, bearing short minute setae.

Mandible (Fig. 39G, H): left mandible incisor with five teeth; lacinia mobilis with four teeth; spine row with seven pairs of plumose setae; articles I–III of palp in length ratio 1.0: 3.1: 2.6, second article with 12 marginal setae, article III with five A-setae, two clusters of B-setae, a row of D-setae, and five E-setae apically; incisor of right mandible with four teeth, lacinia mobilis bifurcate, with small teeth.

Lower lip (Fig. 39F): inner lobes lacking, outer lobes covered with thin setae.

Maxilla I (Fig. 39I, J): asymmetrical, left inner plate with 18 plumose setae and five fine setae on medial margin; outer plate with 11 robust serrated apical spines; second article of left palp with two simple setae and eight slender spines apically; second article of right palp with five stout spines, one stiff seta and one slender spine.

Maxilla II (Fig. 39K): inner plate with three simple setae and 14 plumose facial setae in an oblique row; inner and outer plates with long setae apically.

Maxilliped (Fig. 39L, M): inner plate with three stout apical spines, two subapical spines, and 25 plumose setae; outer plate bearing four simple setae, a row of 16 blade spines and five plumose setae apically; article IV of left palp missing, right palp hooked, with two setae at hinge of unguis.

Pereon.Gnathopod I (Fig. 40A, B): coxal plate bearing one seta on anterior and posterior margins each; basis with long setae on anterior and posterior margins; merus bearing setae on posterodistal corner; carpus 1.6 times as long as wide, 0.8 times as long as propodus, bearing four clusters of setae along ventral margin and two clusters of setae on dorsal margin; propodus oval, palm with one medial spine and 13 spines on posterior margin and surface; dactylus with one seta on outer margin.

Gnathopod II (Fig. 40C, D): coxal plate bearing two setae and one seta on anterior and posterior margins, respectively; basis with setae on anterior and posterior margins; merus bearing setae on posterodistal corner; carpus 2.2 times as long as wide, 0.8 times as long as propodus, bearing eight clusters of setae along ventral margin and two clusters of setae on dorsal margin; propodus subrectangular, palm margin with one medial spine and seven spines on posterodistal corner; dactylus with one seta on outer margin.

Pereopod III (Fig. 41A, B): coxal plate bearing two setae and one seta on anterior and posterior margins, respectively; basis elongated, with setae along anterior and posterior margins; merus with two short setae and five clusters of long straight setae on posterior margin and two groups of spines accompanied by setae on anterior margin, anterodistal corner with one spine accompanied by setae; carpus with three groups of spines accompanied by long setae on posterior margin, anterodistal corner with one spine accompanied by setae and posterodistal corner with two spines accompanied by setae; propodus with two spines accompanied by setae on posterior margin and two spines on posterodistal corner; dactylus with one plumose seta on anterior margin, and two setae at hinge of unguis.

Pereopod IV (Fig. 41C, D): coxal plate concave, bearing two setae on anterior margin and five setae on posterior margin; basis with setae along anterior and posterior margins; merus with a single seta and four clusters of straight setae on posterior margin and one spine accompanied by two setae on anterior margin, anterodistal corner with two spines accompanied by setae; carpus and propodus with spines accompanied by setae on posterior margins; dactylus with one plumose seta on anterior margin, and two setae at hinge of unguis.

Pereopod V (Fig. 41E, F): coxal plate bearing one seta and two setae on anterior and posterior margins, respectively; basis expanded, with two setae and five spines accompanied by setae on anterior margin, anterodistal corner with two spines accompanied by setae, posterior margin with a row of 13 setae; merus with one spine accompanied by clusters of short setae on anterior margin and one spine accompanied by two setae on posterior margin, anterodistal corner with one spine accompanied by setae and posterodistal corner with two spines accompanied by setae; carpus and propodus with three to four groups of spines accompanied by fine setae on anterior margins; dactylus with one plumose seta on posterior margin, and two setae at hinge of unguis.

Pereopod VI (Fig. 41G, H): coxal plate bearing two setae on posterior margin; basis elongated, with two long setae and three spines accompanied by setae on anterior margin, anterodistal corner with three spines accompanied by one seta, posterior margin with a row of 13 setae; merus with three spines accompanied by setae on anterior margin and two spines accompanied by two setae on posterior margin, anterodistal and posterodistal corners with four and three spines accompanied by setae respectively; carpus with groups of spines accompanied by setae on anterior and posterior margins; propodus with groups of spines accompanied by fine setae on anterior margin; dactylus with one plumose seta on posterior margin, and two setae at hinge of unguis.

Pereopod VII (Fig. 41I, J): coxal plate with five setae on posterior margin; basis with one seta and four spines accompanied by setae on anterior margin, anterodistal corner with one spine accompanied by setae, posterior margin with a row of 15 setae; merus with three spines accompanied by setae on anterior margin and two spines accompanied by two setae on posterior margin, anterodistal and posterodistal corners with five and three spines accompanied by setae, respectively; carpus with groups of spines accompanied by setae on anterior and posterior margins, respectively; propodus with groups of spines accompanied by fine setae on anterior margin and three clusters of long setae on posterior margin; dactylus with one plumose seta on posterior margin, and two setae at hinge of unguis.

Coxal gills: coxal gill of gnathopod II and gill of pereopod V a little shorter than bases; gill of pereopod III approx. as long as basis; gill of pereopod IV a little longer than basis; gill of pereopod VI approx. half of the basis; gill of pereopod VII smallest, less than half of the basis.

Pleon.Epimeral plates (Fig. 40 E–G): plate I ventrally rounded, bearing five long setae on anteroventral margin and four setae on posterior margin; plate II with four spines on ventral margin and seven setae on posterior margin, posterodistal corner blunt; plate III with five spines on ventral margin and seven setae on posterior margin, posterodistal corner blunt.

Pleopods I–III (Fig. 42 A–C): similar, peduncle with two retinacula accompanied by one to three setae; outer ramus approx. as long as inner ramus, both rami fringed with plumose setae.

Urosome.Urosomites (Fig. 40H): urosomite I with two clusters of dorsal setae; urosomite II with one spine accompanied by five setae on each side; urosomite III with one spine accompanied by three setae on each side.

Uropods I–III (Fig. 42 D–F): uropod I peduncle longer than rami, without basofacial spine, with four and seven spines on inner and outer margins, respectively, inner and outer distal corners with one spine and two spines respectively; inner ramus with three spines on inner margin and two spines on outer margin; outer ramus with two spines and five spines on inner and outer margins, respectively; both rami with five terminal spines. Uropod II short, peduncle a little longer than rami, bearing two spines on inner margin, four spines on outer margin and one distal spine on each corner; inner ramus with three spines on inner and outer margins each; outer ramus with one spine on inner margin and four spines on outer margin; both rami with five terminal spines. Uropod III peduncle with two setae on surface and seven distal spines; inner ramus 2.0 times as long as peduncle, reaching 0.9 times the length of outer ramus, with two spines accompanied by simple and plumose setae on inner margin, four plumose setae accompanied by simple setae on outer margin and two spines accompanied by long setae distally; proximal article of outer ramus with three groups of spines accompanied by simple setae on outer margin, simple and plumose setae on inner margin, and four distal spines, terminal article vestigial.

Telson (Fig. 41K): deeply cleft, 1.1 times as long as wide, each lobe with four setae on surface; left lobe with three distal spines accompanied by two simple setae and right lobe with two distal spines accompanied by three simple setae.

Description of paratype female

(IZCAS-I-A1587-2). 5.9 mm.

Pereon.Gnathopod I (Fig. 43A, B): coxal plate bearing one seta on anterior and posterior margins each; basis with long setae on anterior and posterior margins; propodus oval, palm with four spines on posterior margin, bearing long setae along anterior and posterior margins; dactylus with one seta on outer margin.

Gnathopod II (Fig. 43C, D): coxal plate bearing one seta on anterior and posterior margins each; basis with setae on anterior and posterior margins; propodus subrectangular, palm margin with three spines on posterodistal corner, bearing long setae along anterior and posterior margins; dactylus with one seta on outer margin.

Pereopods III and IV (Fig. 44E, F): merus and carpus with fewer setae on posterior margins than those of male.

Pereopods V–VII (Fig. 44 G–I): similar to those of male.

Oostegite (Fig. 44 A–D): oostegite of gnathopod II broad, with marginal setae, oostegites of pereopods III and IV elongated, oostegite of pereopod V smallest.

Urosome.Uropods I–III (Figs 42H, I; 44J): uropod I peduncle longer than rami, without basofacial spine, with three and five spines on inner and outer margins, respectively, inner distal corner with one spine and outer distal corner with three spines; inner ramus with two spines on inner and outer margins each; outer ramus with three spines on inner margin; both rami with five terminal spines. Uropod II short, peduncle a little longer than rami, bearing three spines on outer margin, each corner with one distal spine; inner ramus with two spines on inner and outer margins each; outer ramus with one spine on inner margin and three spines on outer margin; both rami with five terminal spines. Uropod III peduncle with two setae on surface and six distal spines accompanied by setae; inner ramus 1.1 times as long as peduncle, reaching 0.7 times the length of outer ramus, with one spine accompanied by three simple setae and two plumose setae on inner margin; proximal article of outer ramus with two pairs of spines accompanied by simple setae on outer margin and three plumose setae accompanied by three simple setae on inner margin, terminal article much shorter than distal setae; adjacent spine absent.

Telson (Fig. 42J): cleft, 1.1 times as long as wide, right lobe with one spine on surface, each lobe with two distal spines accompanied by two setae.

Variability.

Outer ramus of uropod III without terminal article or much shorter than adjacent spines.

Habitat.

This species was collected from a cave, where a pool with an area of one square meter was formed by dripping water from stalactites.

Remarks.

The new species of Gammarus caecigenus Hou & Li, sp. n. is most similar to G. hirtellus Hou, Li & Li, 2013 in eyes absent; epimeral plate III posterodistal corner subacute; and uropod III inner ramus 0.9 times the length of outer ramus, terminal article vestigial. Gammarus caecigenus Hou & Li, sp. n. differs from G. hirtellus Hou, Li & Li, 2013 ( G. hirtellus in parentheses) in antenna II calceoli absent (present); pereopod III with long straight setae on posterior margin (with long curled setae); pereopods V–VII with groups of spines on anterior margins, but with few setae (with spines accompanied by long setae); uropods I and II with more spines along peduncle and both rami (with one spine on each side of inner and outer rami); and urosomites I and II with two groups of spines and setae (with four groups of spines and setae).

The comparison between these seven species is presented in the following key.