Stygobromus phreaticus Holsinger 1978

Stygobromus phreaticus

MATERIAL EXAMINED.—VIRGINIA. Fairfax Co.: well at Vienna, holotype (USNM 168839), 11 , 3 , and 2 juv. paratypes (USNM), W. S. Abbott, 27 Dec 1921; Alexandria, well water, 12 paratypes (USNM), R. M. Lynch, Dec 1948.

DIAGNOSIS.—A unique, medium-sized groundwater species easily distinguished by the characters given for the phreaticus group and by heavily spinose uropods 1 and 2. Largest male, 6.8 mm; largest female, 7.0 mm.

MALE.—Antenna 1, 45–50 percent length of body, 40–45 percent longer than antenna 2; primary flagellum with 14–20 segments. Antenna 2, flagellum with 6 segments. Mandibles subequal; spine row with 2 or 3 spines; segment 2 of palp with row of setae of unequal length on inner margin, segment 3 with 2 long setae on outer margin, row of short setae on distal half of inner margin, and 4 long setae on apex. Maxilla 1: inner plate with 7–11 apical, plumose setae; setae; palp with 2 slender spines apically and 4 or 5 stiff setae subapically. Maxilla 2, inner plate with oblique row of 7–10 plumose setae on inner margin. Maxilliped: inner plate with 2 bladelike spines, 2 plumose spines and 1 naked seta apically, 1 naked and 2 plumose spines (or coarse setae?) subapically; outer plate with row of naked setae on inner margin and apex and 1 or 2 plumose setae apically. Inner lobes of lower lip vestigial.

Propod of gnathopod 1 about 2/3 size of 2nd propod; palm weakly convex, armed with double row of 9 or 10 spine teeth; defining angle poorly delimited, with 3 long spine teeth on outside, 4 shorter ones on inside; posterior margin with row of mostly singly inserted setae; medial setae singly inserted. Dactyl nail of gnathopod 1 moderately long. Coxal plate of gnathopod 1 longer than broad, margin with 3 setae. Gnathopod propod 2: palm rather long, nearly straight, armed with double row of 10 spine teeth; defining angle with 2 long spine teeth on outside, 2 shorter ones on inside; posterior margin with 4 sets setae; medial setae singly inserted. Dactyl nail of gnathopod 2 relatively short. Coxal plates of gnathopod 2 and pereopod 3 longer than broad, margins with 5 or 6 setae each. Coxal plate of pereopod 4 a little longer than broad, reaching 45–50 percent length of basis, margin with 9 setae. Pereopods 6 and 7 about equal in length, 40–45 percent length of body, 15–20 percent longer than pereopod 5. Pereopods 5–7; posterior margins of bases weakly convex, distoposterior lobes distinct, bluntly rounded; segments 5 and 6 rather heavily spinose on anterior margins; dactyls, 25–30 percent length of corresponding propods. Coxal gill absent from pereopod 7. Median and pleonite sternal gills absent; 2 pairs small, simple lateral sternal gills on pereonites 6 and 7.

Pleonal plates: posterior margins weakly convex, with 3 or 4 setae each; posterior corners small, distinct, usually subacute; ventral margin of plate 2 with 1–3 spines, that of plate 3 with 2 or 3 spines. Uronites free to partly fused but discernible suture between 1 and 2. Uropod 1: inner ramus a little longer than outer ramus, about 60 percent length of peduncle, armed with 25 spines; outer ramus with 17 spines; peduncle with 49–50 spines; peduncular process rather long, 30–35 percent length of outer ramus, slightly curved distally. Uropod 2: inner ramus longer and thicker than outer ramus, a little shorter than peduncle, armed with 20 spines; outer ramus with 15 spines; peduncle with 9 spines. Uropod 3: ramus variable, sometimes absent, sometimes vestigial, when present armed with 1, or rarely 2, apical spines; peduncle with 1 or 2 apical spines when ramus absent. Telson a little broader than long; apical margin with shallow notch, armed with 14–20 (usually 16–18) spines of unequal length.

FEMALE.—Differing only slightly from male as follows: Propod of gnathopod 2 proportionately a little smaller, with 2 or 3 more spine teeth. Segment 5 of gnathopod 2 without rastellate setae. Brood plates sublinear, only slightly expanded distally.

VARIATION.—The specimens from Alexandria differed slightly from those from Vienna by having 2 more setae on the inner plate of maxilla 1, 1 or 2 more setae on the inner margin of the inner plate of maxilla 2, proportionately smaller 3rd uropod, and 2 to 4 more apical spines on the telson. The most variable structure, however, both within and between populations and even on the same specimen in some cases, was uropod 3. In some specimens from both localities the ramus was vestigal, whereas in others it was absent. In one female from Vienna, one 3rd uropod lacked a ramus but had 2 apical spines on the peduncle, while the other 3rd uropod had a vestigial ramus with one apical spine. The variation of this structure is shown in Figure 57.

TYPE-LOCALITY.—A well at Vienna in Fairfax Co., Virginia. Because of the lack of additional information on the label in the collection, it is impossible to pinpoint the location of this well. In view of the early collection date (Dec. 1921), the well was probably of the shallow, hand-dug variety. In recent years, due to the rapid urbanization around Vienna, many of the old hand-dug wells in the area have been abandoned and either filled in or completely destroyed and it will probably be impossible to ever locate the original site of this collection.

DISTRIBUTION AND ECOLOGY.—This species is known from two collections made 27 years apart from single wells in Vienna and Alexandria, Virginia. The locality given on the label in the collection from Alexandria simply read “well-water, Alexandria, Virginia”; thus this well, like the one at Vienna, cannot be specifically pinpointed. Vienna and Alexandria are situated approximately 16 km apart and both are drained by tributaries of the nearby Potomac River. The city of Alexandria is underlain principally by Coastal Plain sediments of Cretaceous age, in contrast to Vienna and vicinity which is underlain by metamorphosed sedimentary rocks and Greenstone volcanics (lower Paleozoic or Precambrian in age) of the Piedmont Province.

Three females in the December collection from Alexandria had setose brood plates and ranged in length from 4.5 to 5.0 mm. None of the females from Vienna (December collection) had setose brood plates, although some were 6.0–7.0 mm in length.

ETYMOLOGY.—The epithet phreaticus is derived from the Greek word phreatos, meaning “well” or “reservoir.”

The araeus Group

DIAGNOSIS.—Adults, 3.5–7.0 mm in length. Mature male larger than mature female; sexual dimorphism pronounced. Inner lobes of lower lip small but well developed. Gnathopod 1: propod palm and posterior margin about equal in length, the latter with row of singly inserted setae; segment 5 with small rastellate setae. Propod of gnathopod 2 sexually dimorphic, about twice size of 1st propod in male, only a little larger in female; palm of male with deep excavation; posterior margin about equal in length to palm, with row of distally split setae. Segment 5 of gnathopod 2 with small rastellate setae. Pereopod 7 a little longer than pereopod 6, without coxal gill. Lateral sternal gills bifurcate. Pleonal plates: posterior margins with 2–5 setae each; posterior corners typically well developed but larger and more acute in female. Uronites fused. Uropod 1 sexually dimorphic; elongate and with long, slender spines on peduncle in male; not elongate and with short spines on peduncle in female. Uropod 3: ramus vestigial or absent; ramus or peduncle with 1 apical spine. Telson sexually dimorphic: that of male about 50 percent longer than broad, apical margin entire and slightly convex; that of female not much longer than broad, apical margin with tiny notch.