Stygobromus tritus Holsinger 1974

Stygobromus tritus

MATERIAL EXAMINED.—MONTANA. Ravalli Co.: well at Victor Crossing, holotype female (USNM 142818) and 14 paratypes (USNM 142819), 19 Oct. 1952 (collector unknown); 11 paratypes (USNM 142820), Ocie Hessling, 11 Oct. 1952.

DIAGNOSIS.—A rather small subterranean species distinguished by the palms of the gnathopodal propods, which are slightly concave and have short spine rows; comparatively narrow propod of gnathopod 2; slender bases of pereopods 5–7 without distoposterior lobes; rather long marginal spines on the inner ramus of uropod 2; short telson (broader than long); and proportionately short ramus of uropod 3. Sexes generally similar. Largest females, 5.0 mm; largest males, 4.5 mm.

DESCRIPTION.—Antenna 1, 60 percent as long as body, 50 percent longer than antenna 2, with up to 14 segments in primary flagellum. Antenna 2, flagellum with 6 segments. Mandibles similar to those of S. hubbsi. Maxilla 1: inner plate with 6 apical, plumose setae; palp with 4 short spines and 2 setae apically. Maxilla 2, inner plate with oblique row of 7 or 8 plumose setae on inner margin. Maxilliped: inner plate with 3 serrate spines, 3 plumose spines and 2 stiff setae apically; outer plate with 1 small spine and several stiff setae apically to subapically on inner margin. Lower lip, inner lobes indistinct, vestigial or absent.

Gnathopod 1: Propod palm not oblique, rather straight, armed with double row of 5 spine teeth; posterior angle defined by 3 long spine teeth on outside, 4 shorter spine teeth on inside; posterior margin comparatively long, without setae; medial setae few in number, singly inserted; dactyl nail and accessory spine relatively long. Coxal plate of gnathopod 1 longer than broad, with 1 marginal seta. Gnathopod 2: propod larger than 1st propod, nearly twice as long as broad; palm oblique, slightly concave, armed with double row of 6 spine teeth; posterior angle poorly defined, with 1 long spine tooth and 1 long seta on outside and 1 short spine tooth on inside; posterior margin rather long, uneven, with 3 sets of long setae; medial setae rather prominent, singly, doubly, and triply inserted; dactyl nail rather long. Coxal plate of gnathopod 2 rather shallow, with 2 marginal setae. Coxal plates of pereopods 3 and 4 broader than long, with 4 marginal setae each. Pereopod 6 slightly longer than pereopod 7, 50 to 55 percent as long as body, about 20 percent longer than pereopod 5. Bases of pereopods 5–7 narrow, posterior margins with 6 or 7 setae, distoposterior lobes indistinct. Dactyls of pereopods 6 and 7 comparatively short, less than one-third length of corresponding propods. Coxal gills on pereopods 2–6. Brood plates of mature females small and narrow.

Pleonal Plates: Posterior margins convex, with 1 or 2 setae each; posterior corners tiny, rounded; ventral margins of plates 2 and 3 with 1 or 2 spines each. Uropod 1: inner ramus a little longer than outer ramus, shorter than peduncle, armed with 10 or 11 spines; outer ramus with 8 or 9 spines; peduncle with 9 or 10 spines. Uropod 2: inner ramus a little longer than outer ramus, about equal in length to peduncle, armed with 12 spines (some rather long); outer ramus with 7 spines; peduncle with 5 spines. Uropod 3: ramus comparatively short, only about 25 percent as long as peduncle, with 3 apical spines. Telson a little broader than long, apical margin with distinct notch, apical lobes with 5 or 6 spines each.

TYPE-LOCALITY.—A well at Victor Crossing, Ravalli County, Montana. The well is reported to be 20 feet deep and is located in or near the town of Victor, although a precise location was not given by the collector. This well is presumably excavated in Quaternary deposits which, according to geological maps, cover the floor of the valley formed by the Bitterroot River.

DISTRIBUTION AND ECOLOGY.—This species is known only from its type-locality, where it was collected at two different times in October 1952. The first sample, taken on 11 October, contained 11 specimens of S. tritus, 5 specimens of S. montanensis (described herein) and 2 specimens of S. obscurus (also described herein). The second sample (19 October) contained only 15 specimens of S. tritus. Females outnumbered males almost 3 to 1 in the two samples. Although several females had setose brood plates, none were ovigerous and nothing is known about the reproductive capacity of this species.

ETYMOLOGY.—The name tritus is from the Greek tritos, meaning “third.”