Stygobromus borealis Holsinger 1978

Stygobromus borealis

Stygobromus sp.—Bell, 1971:41, fig. 81.

MATERIAL EXAMINED.—VERMONT. Rutland Co.: Morris Cave, holotype (USNM 168800), 17 and 6 paratypes (JRH), S. B. Peck, 20 May 1968; additional paratypes from Morris Cave as follows: 6 (USNM). S. B. Peck, 23 Apr 1967; 25 , 2 (USNM). S. B. Peck and W. A. Shear, 18 Oct 1968; 6 (JRH), R. M. Norton, 14 Feb 1970; 2 (JRH), G. Bilyard, 1970 (?). NEW YORK. Rensselaer Co.: springhouse, 1.6 km S of Berlin, 1 paratype (JRH), S. B. Peck, 11 May 1969.

DIAGNOSIS.—A relatively small groundwater and cavernicolous species distinguished by absence of setae on palp segment 2 of mandible, reduced number of setae on inner plates of maxillae 1 and 2, unnotched spine teeth of gnathopod propods, shallow coxal plates of pereopods 3 and 4, narrow bases of pereopods 5–7, and narrow, l-spined ramus of uropod 3. Largest males, 3.0 mm; largest females, 4.0 mm.

FEMALE.—Antenna 1, 50–60 percent length of body, 45–50 percent longer than antenna 2; primary flagellum with 9–12 segments. Antenna 2, flagellum with 3 segments. Mandibles subequal; spine row with 2 or 3 spines; segment 2 of palp without setae, segment 3 with 1 long seta on outer margin, row of moderately short setae on distal half of inner margin, 4 or 5 long setae on apex. Maxilla 1: inner plate with 2 apical, plumose setae; palp with 3 stiff setae and 1 slender spine (or stiff seta?) apically. Maxilla 2, inner plate with oblique row of 2 plumose setae on inner margin. Maxilliped: inner plate with 2 bladelike spines, 1 plumose spine and 1 naked seta apically, 1 naked and 1 plumose seta subapically; outer plate with naked setae on inner margin and apex, and 1 plumose seta apically. Inner lobes of lower lip small to vestigial.

Propod of gnathopod 1 subequal in size to 2nd propod but broader proximally; palm armed with double row of 3 unnotched spine teeth; defining angle with 1 long spine tooth on outside, 2 shorter ones on inside; posterior margin about 2/3 length of palm, with 2 thick setae; inferior medial setae rather long, singly inserted, superior medial setae reduced in number, singly inserted. Dactyl nail of gnathopod 1 moderately long. Segment 5 of gnathopod 1 without rastellate setae. Coxal plate of gnathopod 1 longer than broad, margin with 2 setae. Gnathopod propod 2: palm rather short, straight, armed with double row of 2 unnotched spine teeth; defining angle with 1 long spine tooth on outside, 2 shorter ones on inside; posterior margin longer than palm, with 3 sets setae; medial setae singly inserted. Dactyl nail of gnathopod 2 moderately long. Segment 5 of gnathopod 2 without rastellate setae. Coxal plates of gnathopod 2 and pereopod 3 shallow, about as broad as long, margins with 2 long setae each. Coxal plate of perepod 4 shallow, broader than long, reaching only about 30 percent length of basis, margin with 3 setae. Pereopod 7 slightly longer than pereopod 6, 45–50 percent length of body, about 25 percent longer than pereopod 5. Pereopods 5–7: bases narrow, distoposterior lobes poorly developed; dactyls, 39–42 percent length of corresponding propods. Coxal gills absent from pereopod 7. Three median sternal gills on pereonites 2–4; 2 pairs simple, moderately long lateral sternal gills on pereonites 6 and 7; sternal gills absent from pleonite 1. Brood plates narrow, not expanded distally.

Pleonal plates: Posterior margins convex, with 1 seta each; posterior corners small to nearly indistinct; ventral margins without spines. Uronites sometimes partly fused, sometimes completely fused. Uropod 1: inner ramus a little longer than outer ramus, about 60 percent length of peduncle, armed with 7 spines; outer ramus with 7 spines; peduncle with 11 spines. Uropod 2: inner ramus longer than outer ramus, a little shorter than peduncle, armed with 7 spines; outer ramus with 5 spines; peduncle with 3 spines. Uropod 3: ramus narrow, about 30 percent length of peduncle, armed with 1 rather large spine. Telson longer than broad, gently tapering distally; apical margin convex, sinuate, armed with 9 or 10 spines.

MALE.—Differing from female in structure of uropods 1 and 2 as follows: Uropod 1: inner ramus with 7 spines; outer ramus with 6 spines; peduncle with 10 spines; peduncular process subtriangular, apex serrate. Inner ramus of uropod 2 with 8–9 spines.

TYPE-LOCALITY.—Morris Cave, located just south of Danby in Rutland Co., Vermont, is a small cave in a restricted band of marble with a deep pool of water in the rear. The pool, which is inhabited by S. borealis, has a silt and/or sand bottom and slopes down at an angle of approximately 45 degrees to a depth probably exceeding 9 meters (S. B. Peck, in litt.). According to Peck, the pool is probably in contact with phreatic water in the marble bedrock.

DISTRIBUTION AND ECOLOGY.—This species is recorded from two localities, Morris Cave in Vermont and a spring in Rensselaer Co., New York, both of which are located in the Taconic Mountains (New England Province) and lie about 81 km apart. The Vermont locality is in the Lake Champlain drainage basin, whereas the New York locality is in the Hudson River drainage. Collections are available from winter, spring, and fall, and females (2.8–4.0 mm in length) with setose brood plates have been observed during all three seasons.