Stygobromus pusillus ( Martynov, 1930 )

Sidorov, Dmitry A., Holsinger, John R. & Takhteev, Vadim V., 2010, Two new species of the subterranean amphipod genus Stygobromus (Amphipoda: Crangonyctidae) from Siberia, with new data on Stygobromus pusillus (Martynov) and remarks on morphology and biogeographic relationships, Zootaxa 2478, pp. 41-58 : 44-46

Stygobromus pusillus ( Martynov, 1930)

Figures 2 – 7

Eucrangonyx pusillus Martynov, 1930: 106 –109, 125–126, figs 13–19 (original description), (Type locality: Teletskoye Lake, Siberia , U.S. S.R.).

Stygobromus pusillus (Martynov) . – Birstein 1940: 418; 1950: 114, 116. – Barnard 1958: 74. – Vandel 1965: 134. – Birstein 1969: 32. – Holsinger 1972: 65; 1974: 1. Karaman 1974: 117. – Holsinger 1977: 262. – Barnard & Barnard 1983: 59 –60, 441. – Holsinger 1986: 537, 547; 1987: 249–252, figs 1–2.

Stygobromus sp. – Holsinger 1978: 126, 127.

Material examined. Lectotype: female, 2.8 mm, 7/l57294-ZINRAS, Russia, Altay Republic, Teletskoye Lake, Biya River near Kokaikha, (date missing), collected by S.G. Lepneva. Paralectotypes: female, 3.0 mm, 13/88499-ZINRAS, same data as lectotype; female (3.0 mm), 2 males (2.8 mm, 9/57342-ZINRAS, Teletskoye Lake, near Yajilu, depth 145 m, 24 Aug 1934, collected by S.G. Lepneva. Additional material: 3 females (3.8 mm, 2.5 mm, 2.0 mm), 3 males (3.0 mm, 2.5 mm, 2.0 mm), 14/20sd-IBSS, Russia, Altay Republic, southern part of Teletskoye Lake opposite mouth of the Kokshi River (51°34΄185ʺ N; 87°40΄780ʺ E), depth 81–156 m, 1 Jul 1998, collected by L.V. Yanygina.

Diagnosis. Eyeless, unpigmented species of stygomorphic facies corresponding to the diagnosis by Holsinger (1987) with the following modifications and/or additions. Inner plate of maxilla 1 with 1 or 2 plumose apical setae. Molar of mandible bearing lightly plumose seta. Segment 4 of maxilliped palp approximately 60 % as broad as segment 3; dactyl (palp segment 4) prominent, bearing long claw. Propodus of female gnathopod 2 longer than that of gnathopod 1. Propodi of gnathopods 1 and 2 bearing 4 spines at defining angle but lacking spines elsewhere in palmar region. Paired median sternal gills on pereonites 2–4, 1 pair simple lateral sternal gills on pereonite 7. Largest female 3.8 mm; largest male 3.0 mm.

Description (in part). Female. Antenna 1 ( Fig. 7 ) 25 % length of body, about 50 % longer than antenna 2; peduncular segments 1–3 in length ratio 1: 0.75: 0.5; segment 1 of peduncle bearing 1 plumose seta distally; primary flagellum with 8–9 segments; most flagellar segments bearing aesthetascs accompanied by setae. Mandibles subequal: left mandible ( Fig. 5 ) with incisor 5-dentate, lacinia mobilis 5-dentate, spine row with 4 serrated spines, triturative molar with plumose seta; segment 3 of palp shorter than segment 2 with 1 A-setae, 2 D-setae and 4 E-setae. Maxilla 1 ( Fig. 6 ): inner plate with 2 apical, plumose setae; outer plate with 7 serrate spines (2 longer); palp segment 2 nearly twice length of segment 1, bearing 5 apical setae. Maxilliped ( Fig. 4 ): inner plate with 4 serrated spines and 2 naked setae apically or sub-apically; outer plate with row of 7 naked setae on rounded apex; palp segment 2 nearly twice the size of segment 1, inner margin bearing row of 9 or 10 rather long setae; nail of dactyl (palp segment 4) relatively long and sharply pointed.

Gnathopod 1 ( Fig. 2 ): propodus of gnathopod 1 as broad as but not as long as propodus of gnathopod 2; basis about as long as combined length of segments 3,4, and 5, with 1 long setae on posterior margin and with 2 long and 2 short setae on inner face; palmar margin slightly convex and equal in length to posterior margin, armed with stiff setae and 4 distally notched spine teeth in double row at defining angle; posterior margin slightly convex, with two setae; dactyl with 3 median seta on inner margin (1 near base of nail). Coxa of gnathopod 1 suboval and bearing 2 short setae on ventral margin. Gnathopod 2 ( Fig. 3 ): basis straight, with 2 long setae on posterior margin; palmar margin notched, slightly convex and short in length with posterior margin, armed with 1 long distally notched spine tooth on outside and 2 short distally notched spine teeth on inside on defining angle; posterior margin convex with two group of long setae; dactyl with 2 median seta on the inner face and 1 seta near base of nail. Coxa of gnathopod 2 subquadrate, with 2 short setae on ventral margin. Coxal plates of pereopods 5 and 6 bilobate, posterior lobes smaller than anterior lobes; coxal plate 7 broader than deep.

Coxal gills on pereopods 2–6 stalked and subovate. Paired median sternal gills on pereonites 2–4; 1 pair simple lateral sternal gills on pereonite 7. Oostegites (brood plates) on pereopods 2–5 sublinear, with long marginal setae. Pleonal plates 1–3: posterior margins nearly straight, with 1 seta each; posterior corners broadly rounded; ventral margin of plate 3 with 1 small spine. Urosomites free (separated by pronounced commissures); armed with sparse setae on dorsal face.

Type locality. Russia, Teletskoye Lake, the Biya River near Kokaikha, depth 25 m ( Fig. 1 ).

Remarks. The few morphological differences noted between the earlier descriptions of this species and the present one are apparently influenced by differences in the size and sex of the specimens examined. The first redescription of S. pusillus by Holsinger (1987) was based on two submature males that measured approximately 2.5 mm in length. In comparison with the larger female described herein, the palp of maxilla 1 of the male is more tapered; segment 3 of the maxilliped palp is narrower; and the palmar margin of gnathopod 1 is slightly concave (see Holsinger 1987, p. 251). A few other minor differences between the present redescription and the older one by Holsinger (1987) might also be noted but do not appear significant. Types were not designated in the original description of S. pusillus by A. V. Martynov (1930), and the sex and size of the specimens used in his original description were not mentioned. A type-locality was also not designated except that the material was said to have been collected from three different places in Teletskoye Lake ( Fig. 1 ). The material we examined from the Zoological Institute in St. Petersburg included one vial with two individuals, designated in Martynov’s original description as the first locality record from Teletskoye Lake near Kokaikha, station 210, probe 212, depth 25 m, silt, collected by S. Lepneva, determined by A. Martynov (date missing). From this syntype series we have designated a lectotype and specifically designated the source of the Biya River (opposite the settlement of original Kokaikha) in Teletskoye Lake as the typelocality.

Distribution and ecology. As previously noted, specimens of Stygobromus pusillus have been collected from several parts of Teletskoye Lake proper as well as from some of the surrounding drainage. In the northern part of the lake, specimens have been collected from the source of the Biya River opposite the settlement of original Kokaikha, in the lake proper opposite the settlement of Yajilu (mistakenly called Fajilu in Holsinger 1987) and near Korbu falls (Kurum creek), and elsewhere in the central part of the lake opposite the Kokshi River mouth. Stygobromus pusillus inhabits depths of 22 to 156 m where the substrate consists of grey silt. The esophagus of some of the collected specimens was filled with diatoms.

Figures

FIGURE 1. Distribution of Stygobromus in Central Asia: (1) S. pusillus (Martynov) in the Biya River and Teletskoye Lake; (2) S. mikhaili n. sp. in the Chuya River basin; (3) S. anastasiae n. sp. in the Angara River basin; (4) S. kazakhstanica Kulkina in the Chernaya River basin.

FIGURES 2 – 7. Stygobromus pusillus (Martynov), female, lectotype: (2) gnathopod 1; (3) gnathopod 2; (4) maxilliped; (5) left mandible; (6) maxilla 1; (7) antenna 1. Scale bars 0.1 mm.