Gammarus alius, Sidorov, Dmitry A., 2012

Sidorov, Dmitry A., 2012, Two new species of freshwater amphipods (Crustacea: Gammaridae) from Central Asia, with comments on the unusual upper lip morphology, Zootaxa 3317, pp. 1-24 : 12-22

Gammarus alius sp. nov.

Figures 45–89

Gammarus pulex ( Linnaeus, 1758) : Martynov, 1930, p. 62; Chatyr-Kul Lake.

Diagnosis. Gammarus alius sp. nov. belongs to the pulex species group. Body stout but soft ( Fig. 45 ), dorsal surface of body segments smooth. Inferior antennal sinus deep, rounded ( Fig. 46 ). Antenna 1 20% longer than antenna 2. Antenna 2 with flagellum flattened dorso-ventrally ( Figs 81–83 ), bearing calceoli. Coxal plate of gnathopod 1 ( Fig. 63 ) sub-linear. Pereopods 3–7 elongate. Dorsal surface of urosomites 1–3 ( Fig. 50 ) armed with robust notched setae on the following manner: 1 (2-2-2), 2 (3-2-3), 3 (2-0-2). Pleopods 1–3 ( Figs 84, 85 ) with rami flattened dorsoventrally. Uropod 3 with lanceolate rami bearing plumose setae. Body length 15.0–16.0 mm (males).

Material examined. Holotype: male, 16.0 mm, X31194 /Cr-1368- FESU, Kyrgyzstan, south part of Chatyr-Kul Lake (40º 58.3 N; 75 º 32.0 E), 20 Aug 2006, 3555 m above sea level, coll. E.I. Shornikov, V.M. Yakymov. Paratypes: 16/2sd-IBSS, 2 males (2 × 15.0 mm), with same data as holotype.

Description. Male (16.0 mm, holotype). Head. Antenna 1 ( Fig. 47 ): 40% length of body, about 20% longer than antenna 2; peduncular articles 1–3 have a length ratio of 1:0.60:0.4; primary flagellum with 28 articles bearing very short setae, aesthetascs absent; accessory flagellum 3–articulate, terminal article small, bearing 3 simple setae. Antenna 2 ( Fig. 48 ): gland cone acute without setae; peduncular article 4 as long as article 5, both articles setose with short and long stiff setae on lateral and ventral margins correspondingly; flagellum with 15 articles, flattened dorso-ventrally ( Figs 81–83 ), each flagellar article bearing long stiff bifurcate setae on inner face, calceoli present. Mandibles subequal: left mandible ( Fig. 51 ) with incisor 5–dentate, lacinia mobilis 5–dentate, setal row with 11 serrate trifurcate setae, triturative molar large with long plumose seta; palp article 2 ( Fig. 53 ) 0.25 × longer than article 3 with 10 simple setae, article 3 bearing 3 A-setae, two groups of 3 B-setae, 32 D-setae and 5 E-setae; incisor of right mandible ( Fig. 52 ) 4–dentate, lacinia mobilis bifurcate, setal row with 11 serrate trifurcate setae. Maxilla 1, left ( Fig. 55 ): inner plate with 21 plumose setae, outer plate with 10 pectinate robust setae; left palp feeble, palp articles 1–2 have a length ratio of 0.27:1, article 2 bearing 8 short strong plumose and 5 simple stiff setae on apex; right palp massive ( Fig. 57 ), articles 1–2 have a length ratio of 0.30:1, article 2 bearing 4 strong peg setae separated by cuticular hillock from 1 strong and 2 stiff setae on apex. Maxilla 2 ( Fig.54 ): inner plate with oblique row of 34–36 small plumose setae on inner margin; outer plate almost twice broader than inner plate with 5 stiff serrate and 10 slender setae on apex. Upper lip ( Figs 58 , 86–89 ): subrhomboid, with two acute lower lobes and minute setae at apex. Lower lip ( Fig. 59 ): inner lobes indistinct. Maxilliped ( Fig. 60 ): peduncle trapeziform without setae on inner face, 1 stiff setae on disto-lateral corner; inner plate ( Fig. 62 ) with 3 simple strong peg setae accompanied with simple and plumose setae on apex, 11 plumose setae on inner margin; outer plate ( Fig. 61 ) with a row of 15 medial robust naked setae and with parallel row of 29 stiff setae, 3 robust plumose and 2 long pappose setae on apex; palp four articulate, article 2 weak with 8 serrate setae at outer margin.

Pereon. Gnathopod 1 ( Fig. 63 ): coxal plate deep, sub-linear, ventral margin with 3 short setae on anterior and posterior corners; basis stout with long simple setae on anterior and posterior margins; carpus 0.75 × as long as propodus; propodus of gnathopod 1 ( Fig. 64 ) as long as propodus of gnathopod 2; propodus sub-triangular, ovate, palm straight with cutting margin developed and armed with 1 distally notched robust setae on medial face, 2 long distally notched robust setae near defining angle; posterior margin shorter than palm bearing six groups of small robust setae; dactylus with 1 seta on outer face. Gnathopod 2 ( Fig. 65 ) larger than gnathopod 1; coxal plate deep ventral margin narrowed with 3 short setae on anterior and posterior corners; basis stout with long simple setae on anterior and posterior margins; carpus 0.85 × as long as propodus; propodus sub-rectangular ( Fig. 66 ), palm slightly concave with cutting margin developed and armed with 1 distally notched robust setae on medial face, two pairs of distally notched robust setae near defining angle three of them long; posterior margin 1.5 × longer than palm with 7 sets of setae; dactylus similar to that of gnathopod 1. Pereopod 3 ( Fig. 67 ): shorter than pereopods 6 and 7, sub-equal in length to pereopod 5; coxal plate rather deep with 3 short setae on ventral margin; basis sub-linear with sets of long setae on anterior and posterior margins; merus densely setose with long sparse uncurved setae on posterior margin; carpus setose with long sparse setae accompanied by 6 short robust setae on posterior margin; propodus setose with 5 sets of long setae accompanied by 6 short robust setae; dactylus about 35% length of corresponding propodus, bearing 1 simple seta on outer margin and 2 setae in base of nail. Pereopod 4 ( Fig. 68 ): subsimilar to pereopod 3 but distinctly shorter; coxal plate sub-quadrate bearing 7 short setae along ventral margin.

Pereopods 5–7 ( Figs 75–77 ): slender, sub-similar but pereopod 6 longest; pereopod 7 0.95 × as long as pereopod 6; coxal plates 5–7 bilobate, posterior lobes indistinct and armed with 3–8 short setae on posterior margin; bases with slightly serrate posterior margins, basis 5 with disto-posterior lobe acute, bases 5 and 6 elongate and tapered downward, disto-posterior margins straight or convex; meri densely setose with minute setules; carpi 6 and 7 elongate, 1.8 × as long as corresponding meri; posterior margins of propodi 5 and 6 bearing 4 sets of short to long setae; dactyli about 25–32% length of corresponding propodi, bearing 1 simple seta on outer margin and 2 setae in base of nail. Coxal gills 2–7 ( Figs 65 , 77 ) stalked and saccular; genital papillae on ventral surface of pereonite 7.

Pleon. Epimera 1–3 ( Fig. 78 ): epimeron 1 with 16 long and 1 robust notched setae on anteroventral corner, posterior corner obtuse, recessed, posterior margin with 3 setae; epimeron 2 posterior corner acute, tapered, posterior margin sinuate, bearing 7 robust and 2 stiff setae in two rows on sub-ventral margin and 4 setae on posterior margin; epimeron 3 disto-posterior corner weakly produced, bearing 5 notched and 2 stiff setae on sub-ventral margin, posterior margin unarmed. Pleopods 1–3 ( Figs 79, 80 , 69 ): subequal, peduncle with 2 retinacula each accompanied by 1–4 simple setae on anterior corner; 2 or 3 bifurcate setae on outer margin of first article of inner ramus; small process enclosed with plumose setae ( Fig. 79 ) on inner face of first articles of outer rami; rami flattened dorso-ventrally ( Figs 84, 85 ), sub-equal in length and fringed with plumose setae. Urosome ( Figs 45 , 50 ): urosomites 1–3 dorsally prominent with 2 or 3 groups of robust notched and sparse fine setae; lateral groups of setae on urosomites 1–2 located angularly. Uropod 1 ( Fig. 70 ): slender, rami tapered; peduncle with 1 small basofacial seta, outer margin with 4 and inner margin with 1 setae; inner ramus 1.1 × as long as outer ramus, 65% length of peduncle; inner ramus with 1 seta on outer margin; outer ramus with 2 setae on inner margin; both rami with 4 distal simple setae, two of them long. Uropod 2 ( Fig. 71 ): slender, rami tapered; peduncle 0.6 × longer than outer ramus, with 3 setae on inner margin, outer margin with 1 seta; outer ramus 0.75 × as long as inner ramus, both margins unarmed; inner ramus with 2 setae on outer margin; both rami with 3 distal setae but outer ramus with 2 subdistal setae additionally. Uropod 3 ( Figs 72, 73 ): biramous, rami lanceolate; peduncle with 1 seta on lateral surface and two groups of robust setae at distal margin; inner ramus long, 0.45 × longer than peduncle and about 0.80 × as long as outer ramus, inner and outer margins with plumose setae; proximal article of outer ramus with 3 robust setae accompanied with long plumose setae on outer margin, inner margin densely setose with 12 sets of long plumose setae, terminal article twice longer than 3 adjacent notched setae. Telson ( Fig. 74 ): longer than uropod 3 peduncle; slightly longer than broad, completely cleft; each lobe with 3 apical notched setae accompanied by thin setae.

Variability. The number of setae on the inner plate of maxilla 1 varies between 20 and 21. Infrequently, twin (inosculate) setae occur ( Fig. 56 ) in the same location, which may be considered to be a malformation. Article 2 of the right palp maxilla 1 bears 4 or 5 strong peg setae.

Sexual dimorphism. No females were found.

Taxonomic comments. Gammarus alius sp. nov. possesses several unique features that distinguish it from known species of the pulex -group and that contradict the genus diagnosis based on the type-species by Pinkster (1970) (see Karaman & Pinkster 1977, pp. 8–13). These features include the following: outer plate of maxilla 1 with broad pectinate robust setae similar to a scraper but not similar to a comb; oblique row of inner plates of maxilla 2 with a bushy row of thin, small, plumose setae; setal row of mandibles with trifurcate setae; lower lip with vestigial inner lobes; and epistome of upper lip with hooks (see discussion below).

Gammarus alius sp. nov. shares some similarities with G. lacustris (see Barnard & Dai 1988), such as a deep rounded inferior antennal sinus, a relatively short antenna 1, and slender dactyli of pereopods 3–7 besides G. l a c u s - tris is also similar to G. hanhi Safronov, 2006 , and G. zagrosensis Zamanpoore et al., 2009 . However, the new species differs from G. lacustris based on the sub-rectangular shape of gnathopod 2 propodus, elongated pereopods, urosomites that are dorsal with elevation, and the pattern setation of antenna 2 which are different but bear some resemblance to G. fossarum due to the stiff, long bifurcate setae on the inner face of the flagellum.

The following features (the number of characteristics of G. alius sp. nov. are in parentheses) are common to G. alius sp. nov. and G. niglangensis Hou & Li, 2003 , from Lugu Lake, Yunnan, China: inner plate of maxilla 1 with 17 (21) setae, maxilla 2 with 25 (34–36) setae in oblique row, molar large, epimeron 2 with two rows of setae on the subventral margin, and lateral groups of robust setae on urosomites 1–2 located angularly (this feature is also common to several other gammarid species). Both species differ significantly in the shape and armament of gnathopod 1–2 propodi, pereopods 3–7 and uropods 1–2; however, the rami of uropod 3 of G. a l i u s sp. nov. are rather lanceolate and most closely resembles those of G. odettae Mateus & Mateus, 1990 .

Type locality. Central Tien-Shan, Chatyr-Kul Lake (40º 58.3 N; 75º 32.0 E).

Ecology. Species dwells among clumps of hydrophytic plants in Chatyr-Kul Lake on the depth 50 cm and predominates in quantity.

Etymology. Species epithet alius (lat.) adjective means “another”.

Discussion. It is important to clarify the terminology relative to the morphology of antenna 2 flagellum. The widespread term “swollen” used to indicate the modified articles of the antenna 2 flagellum is misleading because it does not reflect that the flagellum is different in form depending on how it is viewed. In side view the flagellum of antenna 2 is apparently slender (non-swollen) for most species of the genus Gammarus ( Fig. 83 ) but if viewed ventrally (or dorsally) the flagellum specifically appears flattened ( Figs 81, 82 ). The flattened dorso-ventrally viewed flagellum creates an impression that all articles are swollen, but viewed laterally antenna 2 appears as a slender flagellum (non-swollen). So it is necessary to indicate the view (dorsal, ventral or lateral) when describing the condition of antenna 2 flagellum.

The dorso-ventrally flattened antenna 2 flagellum is a well known significant feature that is considered to be one of the most important for taxonomy of G. pulex ( Linnaeus, 1758) , besides former the same feature is certainly common to the several species are known from Balkans, Asia Minor and Persia: G. komareki Schäferna, 1922 , G. agrarius G. Karaman, 1973 , G. arduus G. Karaman, 1975 , G. uludagi G. Karaman, 1975 , G. f r a t e r Karaman & Pinkster, 1977, G. pretzmanni Mateus & Mateus, 1990 , G. o d e t t a e Mateus & Mateus, 1990 and G. topkarai Özbek & Balik, 2009 . All of them generally dwells in the running freshwaters.

In addition, the flattened rami of pleopods 1–3 correspond to most gammarid species to a varying degree, probably due to the increase in the “paddle surface” of the appendices that are used for swimming. The pleopods of G. alius sp. nov. have a bulge (exopodal clasping hook) that has been noted in several taxa of amphipods ( Boudrias 2002, Sidorov 2010). However, almost the same structure with a barely noticeable bulge were identified by myself in some other species of the genus Gammarus , including those for G. montaniformis sp. nov. As noted by Grinzov (2010), it is likely that the bulges function to synchronise the movement of both branches.

Due to the remarkable morphology of the upper lip that bears “hooks” on the epistome, it is still difficult to precisely determine the taxonomic position of this species, as similar structures have not been observed for the genus. Similar structures are indicated in the literature for Gammarellus homari ( Fabricius, 1779) (see Schellenberg 1942, p. 3) and Elasmopus rapax Costa, 1853 (see Bellan-Santini 1999). I investigated several species of gammarids stored in my collection, including G. pulex , G. lacustris Sars, 1863 , G. varsoviensis Jażdżewski, 1975 , G. fossarum Koch, 1836 , G. suifunensis Martynov, 1925 , and G. koreanus Ueno, 1940 , and I am sure that these species have common adjacent sclerites near the epistome but lack hooks. The epistomal hooks of G. a l i u s sp. nov. are integrated but not articulated, and these structures are clearly evident and form a unified structure (see Figs 86–88 ). There is little doubt that this feature is not an artefact due the dissection because the hooks occasionally break off with careless preparation ( Fig. 89 ). Therefore, further research using new methods is required.

Several characters indicate a possible relation between G. a l i u s sp. nov. and Issykogammarus hamatus Chevreux, 1908 , provisionally placed in the family Acanthogammaridae . Features common to these taxa, include the inner plates of both maxilla 1 and 2 with numerous plumose setae and uropod 3 with lanceolate rami fringed with plumose setae. Issykogammarus hamatus possesses the sharp shape of coxal plates and only 6–7 setal-teeth on the outer plate of maxilla 1. The presence of these features along with the lack of a complete morphology of the upper lip in Chevreux’s description makes a clear comparison of both species difficult. Barnard & Barnard (1983) included Issykogammarus in a separate group, “Baikalian Escapee”, although the authors did not explain how Issykogammarus got from Lake Baikal to Issyk-Kul. Takhteev (2000, p. 17) correctly pointed out the inaccuracy of this conclusion and the absence of clear evidence for placing this genus in Acanthogammaridae .

Notably, the gammarid fauna of Central Asia is quite diverse in its composition and origin. Undoubtedly, the endemic Issykogammarus inhabiting the depths of Lake Issyk-Kul is an ancient element of the gammarid fauna, and probably, the origin and age of the genus is associated with the history of high altitude lakes of the central Tien- Shan. It will be interesting in the future to investigate alpine lakes in the region to clarify their relationship with the fauna of Issyk-Kul.

Figures

FIGURE 45. Gammarus alius, sp. nov., from left side, male, 15.0 mm, paratype.

FIGURES 46 – 53. Gammarus alius, sp. nov., male, 16.0 mm, holotype: (46) head; (47) antenna 1; (48) antenna 2, lateral view; (49) antenna 2, lateral view, inner face, in part; (50) urosome; (51) mandible, left; (52) mandible, right; (53) mandibular palp. Scale bars 0.2 mm, if not indicated otherwise.

FIGURES 54 – 62. Gammarus alius, sp. nov., male, 16.0 mm, holotype: (54) maxilla 2; (55) maxilla 1, left; (56) inosculate setae of maxilla 1 inner plate; (57) palp of maxilla 1, right; (58) upper lip; (59) lower lip; (60) maxilliped; (61) outer plate of maxilliped; (62) inner plate of maxilliped. Scale bars 0.2 mm.

FIGURES 63 – 66. Gammarus alius, sp. nov., male, 16.0 mm, holotype: (63) gnathopod 1; (64) propodus of gnathopod 1; (65) gnathopod 2; (66) propodus of gnathopod 2. Scale bars 0.2 mm.

FIGURES 67 – 74. Gammarus alius, sp. nov., male, 16.0 mm, holotype: (67) pereopod 3; (68) pereopod 4; (69) pleopod 3; (70) uropod 1; (71) uropod 2; (72) uropod 3; (73) terminal article of uropod 3; (74) telson. Scale bars 0.2 mm.

FIGURES 75 – 77. Gammarus alius, sp. nov., male, 16.0 mm, holotype: (75) pereopod 5; (76) pereopod 6; (77) pereopod 7. Scale bars 0.2 mm.

FIGURES 78 – 80. Gammarus alius, sp. nov., male, 16.0 mm, holotype: (78) epimera 1 – 3; (79) pleopod 1; (80) pleopod 2. Scale bars 0.2 mm.

FIGURES 81 – 85. Gammarus alius, sp. nov., male, 16.0 mm, holotype: (81) flagellum of antenna 2, ventral view, in part; (82) the same enlarged; (83) flagellum of antenna 2, lateral view, in part; (84) pleopod 1, ventral view, in part; (85) pleopod 1, lateral view, in part. Scale bars 0.2 mm.

FIGURES 86 – 89. Different conditions of upper lip of Gammarus alius, sp. nov., male, 15.0 mm: (86) dorsal view (with visible adjoining sclerite); male, 16.0 mm, holotype: (87) dorsal view; (88) ventral view; male, 15.0 mm: (89) hook damaged owing to careless preparation, dorsal view.