Stygobromus tahoensis Holsinger 1974

Stygobromus tahoensis

Stygobromus hubbsi Shoemaker.—Frantz and Cordone, 1966: 7 (in part).

MATERIAL EXAMINED.—All material from Lake Tahoe at the indicated locations and depths. CALIFORNIA. Placer Co.: between Skunk Harbor and Tahoe City (1534–1623 ft), holotype female (USNM 142804) and 96 paratypes (USNM 142805), T. C. Frantz et al., 14 Dec. 1962; approximately 450 additional paratypes (USNM 142806–142817) as follows—Placer Co.: 2 collections off McKinney Bay (820–840 ft and 1255 ft), A. J. Cordone and T. C. Frantz, 24 Sept. 1962; Tahoma (455–460 ft), Frantz and Curran, 21 Dec. 1962; El Dorado Co.: Emerald Bay (116–198 ft), Frantz, Curran, and Saake, 11 July 1962; South Tahoe shelf (250–425 ft), Frantz, Curran, and Saake, 20 Aug. 1962. NEVADA. Washoe Co.: 2 collections from Crystal Bay (410–444 ft), Frantz, Curran, and Saake, 13 Aug. and 20 Dec. 1962; Douglas Co.: dredge from bottom in Cave Rock area (245–264 ft), T. C. Frantz, 22 May 1962; 2 collections at Logan Shoals (618–630 ft and 1165 ft), Frantz and Cordone, 16 Aug. 1962; 2 collections at Cave Rock (215–501 ft and 245 ft), Frantz, Curran, and Saake, 17 July and 27 Oct. 1962.

DIAGNOSIS.—A medium-sized, deep-lake dwelling species, apparently related to S. sierrensis and S. sheldoni, but differing from these species in the female which has proportionately more narrow gnathopodal propods with longer posterior margins and more spines on the peduncle of uropod 1; in the male by the short outer ramus of uropod 2 and the proportionately smaller telson with only 6 apical spines; and in both sexes by having fewer ventral spines on pleonal plates 2 and 3 and 2 setae each on the posterior margins of these plates. Largest females, 6.5 mm; largest males, 5.0 mm.

FEMALE.—Antenna 1, about 45 percent as long as body, about 50 percent longer than antenna 2; primary flagellum with 12 segments. Antenna 2, flagellum with 5 segments. Mandibles subequal; spine row with 6 spines; palpal segment 2 with 7 long setae on inner margin; palpal segment 3 with 4 short setae on inner margin, 5 long setae apically. Maxilla 1: inner plate with 6 plumose setae apically; palp with 3 spines and 3 setae apically. Maxilla 2: inner plate with oblique row of 7 plumose setae on inner margin. Maxilliped: inner plate with 3 bladelike spines, 2 serrate spines and 4 setae apically; outer plate with 1 bladelike spine and several setae apically. Lower lip with small inner lobes.

Gnathopodal Propod 1: Palm about as long as posterior margin, armed with double row of 4 spine teeth, plus 2 additional spine teeth on outside; posterior angle with 3 spine teeth and 1 long setae on outside; 6 spine teeth on inside; posterior margin without setae; medial setae mostly singly inserted; dactyl nail rather long. Coxal plate of gnathopod 1 longer than broad, with 3 marginal setae. Gnathopodal propod 2 a little longer than 1st propod; palm oblique, slightly convex, armed with double row of 7 or 8 spine teeth; posterior angle rounded, with 1 long spine tooth and 5 setae on outside, 2 shorter spine teeth on inside; posterior margin with 1 set of 3 long setae; medial setae mostly singly inserted; dactyl nail rather long. Coxal plates of gnathopod 2 and pereopod 3 longer than broad, with 3 or 4 marginal setae each; coxal plate of pereopod 4 about as broad as long, with 5 marginal setae. Pereopod 6 a little longer than pereopod 7, about 45 percent as long as body, about 20 percent longer than pereopod 5. Bases of pereopods 5–7 rather narrow, not much broader proximally than distally; distoposterior lobes poorly developed, broadly rounded. Dactyls of pereopods 6 and 7 relatively long, about 40 percent as long as corresponding propods. Coxal gills on pereopods 2–6. Brood plates of mature females small and narrow.

Pleonal Plates: Posterior margins of plates 1 and 3 convex, that of plate 2 nearly straight; margins of plates 2 and 3 with 2 setae each; posterior corners distinct, bluntly rounded; ventral margins of plates 2 and 3 with 4 spines each. Uropod 1: inner ramus a little longer than outer ramus, about two-thirds the length of peduncle, armed with 11 spines; outer ramus with 8 spines; peduncle with 11 or 12 spines. Uropod 2: inner ramus about one-third longer than outer ramus, equal in length to peduncle, armed with 10 spines; outer ramus with 7 spines; peduncle with 4 spines. Uropod 3: ramus about one-third the length of peduncle, with 3 or 4 apical spines. Telson about as long as broad; apical margin with tiny notch; apical lobes with 5 spines each.

MALE.—Differing from the female as follows: Antenna 1, 35 to 40 percent as long as body, primary flagellum with 8 segments. Antenna 2, flagellum with 5 segments. Gnathopodal propod 1 with few less spines at defining angle. Gnathopodal propod 2 proportionately stouter, only about one-fourth longer than broad; palm slightly convex; posterior angle better defined; posterior margin with row of 6 setae just below posterior angle. Distoposterior lobes of bases of pereopods 5–7 better developed, not as broadly rounded. Uropod 1 with fewer spines and large, distally acute peduncular process. Uropod 2: inner ramus rather broad and proportionately long, 50 percent longer than outer ramus and about 25 percent longer than peduncle, armed with 6 spines; outer ramus with 4 spines; peduncle with 2 spines. Telson about one-fourth longer than broad; apical margin nearly entire or with very shallow emargination; apical lobes with 3 spines each.

VARIATION.—Smaller ovigerous females (about 4.5 to 5.0 mm) have a few less spines on the palm of gnathopodal propod 2 and 1 or 2 fewer setae on the posterior margins of the bases of pereopods 5–7 than larger ovigerous females (about 5.5 to 6.5 mm).

TYPE-LOCALITY.—Lake Tahoe between Skunk Harbor and Tahoe City (depth = 1534 to 1623 ft), Placer County, California.

DISTRIBUTION AND ECOLOGY.—This species is known only from Lake Tahoe, where it was extensively sampled in the early 1960s by T. C. Frantz and A. J. Cordone during a limnological survey of the lake. Two reports by these workers (Frantz and Cordone, 1966, 1967) usefully summarized the animal and plant life of the lake and included a detailed map showing the collecting sites. The records for this species (and for S. lacicolus, described later) are the first reported occurrences for blind, white amphipods from a deep, oligotrophic lake in the United States, although species of similar subterranean facies have been reported previously from deep lakes in Europe (e.g., Niphargus foreli) and Siberia (Stygobromus pusillus). In addition to the two species of Stygobromus, a blind, white planarian (Dendrocoelopsis hymanae) was also collected on this survey and was described by Kawakatsu (1968).

Approximately 550 specimens of S. tahoensis were represented in a series of 13 samples taken from the lake periodically from May to December 1962. The majority of specimens were taken at depths ranging from 200 to 1623 feet; only one sample (July) was obtained at less than 200 feet. The talitrid amphipod Hyalella azteca (Saussure) was also sampled but was almost exclusively restricted to depths less than 200 feet. Two of the 13 samples also contained Stygobromus lacicolus (see below) and three additional samples contained only the latter species. The substrate preference of Stygobromus in Lake Tahoe is apparently not living plants, since, as shown by Frantz and Cordone (1967), plants were most concentrated at depths of 200 to 350 feet, beyond which only small quantities were found at 400 feet and none were taken at 500 feet or deeper. The greatest concentrations of Stygobromus were found at depths of 600 to 900 feet and again at 1500 to 1623 feet (Frantz and Cordone, unpublished data).

Out of the approximately 550 specimens studied, 51 were ovigerous females ranging in size from 3.8 to 6.5 mm ( = 4.8 mm). The number of eggs and/or embryos per female ranged from 1 to 5 ( = 2.25). Ovigerous females were found in samples taken during May, July, August, September, October, and December. Males and juveniles were also found in most of these samples, but females were always the most abundant.

COMMENTS.—On the basis of a mistaken determination, Frantz and Cordone (1966) listed the Stygobromus from Lake Tahoe as S. hubbsi. They also indicated the presence of two species of Hyalella—H. azteca and H. inermis. The latter species, however, is usually regarded as a synonym of the former.