Stygobromus franzi Holsinger 1978

Stygobromus franzi

Stygonectes species.—Holsinger, 1969a:32 [species B].—Franz and Slifer, 1972:26.

MATERIAL EXAMINED.—MARYLAND. Garrett Co.: Crabtree Cave, holotype (USNM 168821) and 1 paratype (USNM), A. and B. Norden, 31 Dec 1972; additional paratypes (in JRH) collected by L. R. Franz as follows: 2 on 2 Jan 1967, 1 in 1969 (?), and 1 on 16 May 1974; Sand Cave. 2 , 1 paratypes (JRH), L. R. Franz, 1 Mar 1970; Washington Co.: Round Top Mine No. 2, 13 paratypes (USNM), A. and B. Norden, 4 Mar 1973 and 13 and 1 paratypes (JRH), 26 Oct 1974.

DIAGNOSIS.—A relatively small cavernicolous species allied morphologically with S. emarginatus but distinguished from that species by smaller size at sexual maturity, and having fewer flagellar segments in antennae, fewer spine teeth on palms of gnathopod propods, fewer sets of setae on posterior margin of gnathopod propod 2, proportionately shorter bases of pereopods 5–7, more spinose uropods 1 and 2, and proportionately longer outer spines on apex of telson. Largest males, 3.5 mm; largest female, 6.9 mm.

FEMALE.—Antenna 1, 50–55 percent length of body, about 50 percent longer than antenna 2; primary flagellum with 14–18 segments. Antenna 2, flagellum with 4 or 5 segments. Mandibles subequal; spine row with 5 spines; segment 2 of palp with 8 long setae, segment 3 with 2 long setae on outer margin, row of shorter setae on inner margin, 4 long setae on apex. Maxilla 1: inner plate with 5 or 6 apical, plumose setae; palp with 2 stiff setae and 2 slender spines apically. Maxilla 2, inner plate with oblique row of 6 plumose setae on inner margin. Maxilliped: inner plate apically with 2 bladelike spines, 2 pulmose spines and 2 naked setae; outer plate with row of naked setae on inner margin and apex and 2 apical, plumose setae. Inner lobes of lower lip vestigial.

Propod of gnathopod 1 subsequal in size to 2nd propod; palm with double row of 7 spine teeth; defining angle with 3 spine teeth on outside, 2 shorter ones on inside; medial setae mostly singly inserted. Dactyl nail of gnathopod 1 rather long. Coxal plate of gnathopod 1 longer than broad, with 4 marginal setae. Gnathopod propod 2: palm with double row of 6 spine teeth; defining angle with 1 long spine tooth on outside, 3 shorter ones on inside; inferior medial setae singly inserted, superior medial setae variable as shown. Dactyl nail of gnathopod 2 rather long. Coxal plates of gnathopod 2 and pereopod 3 a little longer than broad, margins with 5 setae each. Coxal plate of pereopod 4 rather shallow, broader than long, margin with 7 setae. Pereopods 6 and 7 subequal in length, 45–50 percent length of body, 20 percent longer than pereopod 5. Pereopods 5–7: bases comparatively short, not much broader proximally than distally, posterior margins nearly straight, distoposterior lobes poorly developed; dactyls 27–31 percent length of corresponding propod. Three long median sternal gills on pereonites 2–4; 2 pairs rather long, bifurcate lateral sternal gills on pereonites 6–7; 1 pair sternal gills on pleonite 1. Brood plates rather narrow but some what expanded distally.

Pleonal plates: posterior margins slightly convex, with 1 seta each; posterior corners small, subacute; ventral margins of plates 2 and 3 with up to 3 spines each. Uronites completely fused. Uropod 1: inner ramus equal in length to outer ramus, 60–65 percent length of peduncle, armed with 13 spines; outer ramus with 12 spines; peduncle with 19 spines. Uropod 2: inner ramus a little longer than outer ramus, a little shorter than peduncle, armed with 12 spines; outer ramus with 10 spines; peduncle with 5–7 spines. Uropod 3: ramus ¼ to 1/3 length of peduncle, apex with 3 or 4 (occasionally 2) spines; peduncle sometimes with spinule. Telson a little longer than broad; apical margin entire, armed with 12–16 (usually 12) spines of which several are distinctly longer than the others and up to 75–100 percent length of telson.

MALE.—Differing from female as follows: Gnathopod propods with fewer palmar spines and medial setae. Sternal gills absent from pleonite 1. Ventral margins of pleonal plates 2 and 3 with 1 or 2 fewer spines. Peduncular process of uropod 1 triangular in shape, upper margin serrate toward distal end.

VARIATION.—As shown in Figure 9, the shape of the telson varies both between and within populations. The proportionate length of, and the number of apical spines on, the ramus of uropod 3 is also somewhat variable as noted in the description.

TYPE-LOCALITY.—Crabtree Cave, located 27 km east-northeast of Oakland in Garrett County, is the largest known cave in Maryland, contains 1280 meters of passage and is developed in the Greenbrier limestone (Franz and Slifer, 1971:57–60). Type-specimens have been collected from a small stream and, according to Franz and Slifer (1971:26), also from drip pools.

DISTRIBUTION AND ECOLOGY.—To date this species has been collected from three localities in western Maryland—two caves in Garrett County and one cave-like, limestone mine in Washington County. The range, which is disjunct, covers a linear distance of approximately 105 km and lies within the Potomac River drainage basin. Although the range of S. franzi comes to within about 24 km of that of S. emarginatus, these two species have never been found in the same cave and their ranges in western Maryland lie within separate drainage systems. Stygobromus franzi; does, however, occur sympatrically with two other species in the genus—a small, undescribed form in Crabtree Cave (see “Stygobromus sp.,” elsewhere in this paper) and S. allegheniensis in the Round Top mine complex in Washington County.

Stygobromus franzi inhabits both small cave streams and pools. It was found in a tiny stream in Round Top Mine No. 2, in small, seep- or spring-fed pools in Sand Cave and presumably in both a small stream and drip pools in the type-locality. Ovigerous females have not been found to date, but females (measuring 4.2–6.9 mm) with setose brood plates have been taken during fall, winter, and spring.