Stygobromus obrutus Holsinger 1978

Stygobromus obrutus

MATERIAL EXAMINED.—VIRGINIA. Pittsylvania Co.: shallow well in woods, Mt. .Cross Rd., 1.6 km northwest of U.S. Rt. 48, W of Danville, holotype (USNM 168837), 63 12 , and 9 juv. paratypes (USNM), L. Hubricht, 4 Dec 1948.

DIAGNOSIS.—A small groundwater species distinguished by small size at sexual maturity, reduced setation of mandibular palp, relatively short antennae, short, simple lateral sternal gills, lightly spined peduncle of uropod 1, and short telson which lacks an apical notch and has relatively short spines. Largest male, 2.5 mm; largest females, 3.6 mm.

FEMALE.—Antenna 1, 40–43 percent length of body, about 45 percent longer than antenna 2; primary flagellum with 10–12 segments. Antenna 2, flagellum with 3 segments. Mandibles subequal; spine row with 3 spines; segment 2 of palp without setae, segment 3 with 4 short setae on inner margin, 4 long setae on apex. Maxilla 1: inner plate with 5 apical, plumose setae; palp with 4 stiff, apical setae. Maxilla 2, inner plate with oblique row of 6 or 7 plumose setae on inner margin. Maxilliped: apex of inner plate with 2 bladelike spines, 1 plumose spine and 2 naked setae, inner margin with 1 plumose seta or spine distally; outer plate with few naked setae on inner margin and apex and 1 apical, plumose seta. Lower lip with vestigial inner lobes.

Propod of gnathopod 1 a little larger than 2nd propod; palm armed with double row of 4 or 5 spine teeth; defining angle with 2 long spine teeth on outside, 1 shorter one inside; medial setae few in number, singly inserted. Dactyl nail of gnathopod 1 rather long. Coxal plate of gnathopod 1 longer than broad, margin with 2 setae. Gnathopod propod 2: palm with double row of 4 spine teeth; defining angle with 1 long spine tooth on outside, 1 shorter one on inside; posterior margin with 2 sets setae; medial setae few in number, singly inserted. Dactyl nail of gnathopod 2 rather long. Coxal plates of gnathopod 2 and pereopod 3 about as broad as long, margins with 3 setae each. Coxal plate of pereopod 4 broader than long, about 35 percent length of basis, margin with 3 setae. Pereopod 6 subequal in length to pereopod 7, 45–50 percent length of body, about 30 percent longer than pereopod 5. Pereopods 5–7: bases about as broad proximally as distally, posterior margins convex, distoposterior corners distinct and broadly rounded; dactyls of 6 and 7 about 30 percent length of corresponding propods, that of 5 about 37 percent. Three median sternal gills on pereonites 2–4; 2 pairs short, simple lateral sternal gills on pereonites 6 and 7; sternal gill absent from pleonite 1. Brood plates sublinear, not expanded distally.

Pleonal plates: posterior margins nearly straight, with 1 seta each; posterior corners small, narrowly rounded; ventral margins of plates 2 and 3 with 1 spine each. Uronites completely or incompletely fused. Uropod 1: inner ramus subequal in length to outer ramus, about 70 percent length of peduncle, armed with 5–7 spines; outer ramus with 5–7 spines; peduncle with 5 spines. Uropod 2: inner ramus about 35 percent longer than outer ramus, a little shorter than peduncle, armed with 6 spines; outer ramus with 5 spines; peduncle with 3 spines. Uropod 3: ramus about 1/3 length of peduncle, armed with 2 apical spines. Telson comparatively short, about as broad as long, gently tapering distally; apical margin entire, armed with 12 relatively short spines.

MALE.—Differing from female slightly as follows: Gnathopod propods proportionately a little smaller. Posterior margin of gnathopod propod 2 with 1 set setae. Peduncular process of uropod 1 small, apex trilobed.

TYPE-LOCALITY.—A shallow well in woods west of Danville, Pittsylvania Co., Virginia. This well is either in early Paleozoic or Precambrian granite gneiss or Triassic sandstone. The locality data given on the collection label were not specific enough to pinpoint the location precisely.

DISTRIBUTION AND ECOLOGY.—This species is known only from its type-locality, which is situated in the Roanoke River drainage basin and lies some 75 airline miles southeast of the range of any other species in the emarginatus group. In contrast to other species in this group, all of which occur west of the Blue Ridge Mountains in the Applachian Valley and Plateau provinces, S. obrutus is found in the Piedmont Province in south-central Virginia.

Five of the females in the type series had setose brood plates and varied in length from 2.7 to 3.6 mm.

ETYMOLOGY.—The epithet obrutus is from the Latin, meaning “buried,” “covered,” or “hidden.”