Stygobromus mackini Hubricht 1943

Stygobromus mackini Hubricht

Stygobromus mackini Hubricht, 1943:695–697, pl. 3 [type-locality: Sikes Cave, 7.2 km N of Lebanon, Russell Co., Virginia].—Pennak, 1953:443, fig. 276G.—Barnard, 1958:74.—Hubricht, 1959:878.—Nicholas, 1960:128–129.—Barr, 1961:32—Holsinger, 1963:29; 1964:62.—Poulson and White, 1969:973.—Holsinger, 1969a:30–32.—Holsinger and Peck, 1971:30.—Holsinger, 1972:71.—Culver, 1973:103.—Culver, Holsinger, and Baroody, 1973:691.—Karaman, 1974:114.—Holsinger, Baroody, and Culver, 1976:23.—Estes and Holsinger, 1976:487.—Holsinger, 1977:262.

MATERIAL EXAMINED.—TENNESSEE. Anderson Co.: Offutts Cave, 9 , 2 , J. A. Payne, 11 Jul 1965; Grainger Co.: Indian Cave, 1 , (USNM), L. Hubricht, 8 Aug 1939; Hancock Co.: Cantwell Valley Cave, 1 , J. R. Holsinger and C. W. Maus, 28 Oct 1966; Hawkins Co.: Sensabaugh Saltpetre Cave, 4 , 2 , J. R. Holsinger, 15 Apr 1967; Roane Co.: Berry Cave, 1 , R. M. Norton, 15 Jul 1964; Union Co.: Oaks Cave, 3 , R. M. Norton, 17 Jul 1965; Ridenour Pit Cave, 1 , R. M. Norton, 2 Jul 1967. VIRGINIA. Bland Co.: Hamilton Cave, 2 , (USNM), T. C. Barr, Jr., 6 Jun 1958, and 1 , 2 , 1 juv., J. R. Holsinger 10 May 1965; Repass Saltpetre Cave 11 , 10 (5 specimens in Museo Civico Di Storia Naturale, Verona), J. R. Holsinger, 10 May 1965; Giles Co.: Ballards Cave, 2 , J. R. Holsinger and L. M. Ferguson, 29 Jun 1974; spring at Bear Cliff near Mt. Lake, 3 , 5 (USNM), H. H. Hobbs, Jr., 25 Jun 1947; Starnes Cave, 1 , J. R. Holsinger and S. W. Hetrick, 19 May 1974; Tawneys Cave, 1 , J. R. Holsinger and E. Bauer, 20 Aug 1965; Russell Co.: Banners Corner Cave, 1 , 1 , D. C. Culver and P. J. Starr, 18 Aug 1967; Bundys Cave No. 2, 1 , 1 juv., J. R. Holsinger, 9 May 1965; Burns Cave, 1 , J. R. Holsinger, 13 Apr 1968; Grays Cave, 2 , J. R. Holsinger, 6 Apr 1968; Jessie Cave, 3 , 1 , J. R. Holsinger, 9 May 1965; Johnson Cave, 4 , 1 juv., J. R. Holsinger, S. S. Taylor, and J. M. Beck, III, 11 May 1968; Munsey Cave, several specimens (D. C. Culver coll.), J. R. Holsinger and D. C. Culver, 29 Aug 1971; Porgie Bundys Cave, 4 , 1 , J. R. Holsinger and D. C. Culver, 16 Aug 1972; Sikes Cave, 5 , 1 cotypes (syntypes) (USNM 79324) and 3 , 2 cotypes (USNM), L. Hubricht, 26 Aug 1939; Smiths Drop Cave, 2 , 1 , 1 juv., J. R. Holsinger and D. C. Culver, 29 Jul 1974; Scott Co.: Blair-Collins Cave, 5 , J. R. Holsinger and S. S. Taylor, 6 Nov 1966; Blowing Hole Cave, 1 , J. R. Holsinger, 22 Apr 1962; Deep Spring Cave, 1 , J. R. Holsinger, S. S. Taylor and J. M. Beck, III, 11 May 1968; Flannery Cave, 6 , 2 , J. R. Holsinger and S. B. Peck, 1 Aug 1964; Grigsby Cave, 7 , J. R. Holsinger and C Rippey, 18 Jul 1965; Hill Cave, 3 , J. R. Holsinger and J. Tichenor, 3 Aug 1967; Jack Cave, 1 , 4 , J. R. Holsinger, 10 Jun 1966; Jackson Cave, 1 , 1 , J. R. Holsinger, 13 Nov 1966; Kerns Smoke-Hole Cave, 2 , 2 , J. R. Holsinger, 19 Aug 1969; McDavids Cave, 1 , J. R. Holsinger and D. C. Culver, 3 Aug 1974; McNew Cave, 1 , 3 , J. R. Holsinger, 18 Jul 1976; Moccasin Valley Cave, 10 , 2 , J. R. Holsinger, 25 Aug 1969; Natural Tunnel Cavern, 1 , J. R. Holsinger and J. A. Estes, 22 Nov 1976; Pond Cave, 7 , 1 , J. R. Holsinger and S. S. Taylor, 5 Nov 1966; Spurlock Cave, 1 , J. R. Holsinger, 25 Nov 1966; Taylor Cave No. 1, 1 , J. R. Holsinger, 6 May 1967; Winding Stair Cave, 9 , 1 , 1 juv., J. R. Holsinger and G. Powers, 12 Aug 1969; Wolfe Cave, 1 , J. R. Holsinger, G. and A. Powers, 14 Aug 1969; Smyth Co.: Buchanan Saltpetre Cave, 2 , 1 , J. R. Holsinger, 13 Apr 1963, and 2 , 2 , G. Marland, Jul 1964; McMullin Cave, 1 juv. (?), J. R. Holsinger and P. M. Hauer, 17 Jul 1969; Tilson Saltpetre Cave, 1 1 , J. R. Holsinger, and D. C. Culver, 15 Aug 1972; Tazewell Co.: Cauliflower Cave, 1 , J. R. Holsinger, S. W. Hetrick, and J. Tichenor, 29 Apr 1972; Chimney Rock Cave, 5 , 13 (USNM), L. Hubricht, 25 Aug 1939; Crocketts Cave, 1 , J. R. Holsinger and G. W. Dickson, 10 Apr 1974; Fallen Rock Cave, 1 , J. R. Holsinger, 9 Nov 1968, and 8 , J. R. Holsinger, G. D. Corbett, G. W. Dickson, and S. W. Hetrick, 13 Oct 1973; Glenwood Church Cave, 4 , 1 , J. R. Holsinger, R. M. Norton, and R. Schultetus, 28 May 1966; Hugh Young Cave, 3 , J. R. Holsinger, R. M. Norton, and R. Schultetus, 28 May 1966, and 2 , J. R. Holsinger and D. C. Culver, 30 Apr 1972; Lost Mill Cave No. 1, 1 , J. R. Holsinger, 28 May 1966; Steeles Cave, 4 , J. R. Holsinger and D. C. Culver, 15 Aug 1972; Ward Cove Cave, 8 , 2 , J. R. Holsinger, R. M. Norton, and R. Schultetus, 29 May 1966; Washington Co.: covered spring 1.6 km NW of Court House in Abingdon, 3 , R. M. Norton, 30 Jul 1964; Singleton Cave, 1 J. R. Holsinger and C. B. Moore, 9 Dec 1967; Wise Co.: Wildcat Saltpetre Cave, 1 , J. R. Holsinger, G. W. Dickson, and T. C. Kane, 27 Nov 1975. WEST VIRGINIA. Mercer Co.: Beacon Cave, 1 , J. R. Holsinger, 12 May 1966; Big Springs Cave, 5 , 3 , J. R. Holsinger, D. C. Culver, and P. J. Starr, 20 Aug 1967; Panther Cave, 1 , G. Park, 29 Oct 1972, and 2 , 27 Dec 1972; Thompson School Cave, 1 , J. R. Holsinger, 12 May 1966; Monroe Co.: Cross Road Cave, 1 , J. R. Holsinger, 1 Sep 1967; Greenville Saltpetre Cave, 1 , J. R. Holsinger, D. C. Culver, and R. A. Baroody, 5 Apr 1971.

DIAGNOSIS.—A medium-sized cavernicolous species distinguished by the gnathopod propods which are subequal in size, the 1st gnathopod propod which has a long, oblique palm and short posterior margin with few setae, the peduncle of uropod 3 which has 1–6 rather conspicuous distal spines, and the distinct apical notch (incision) of the telson. Largest males, 7.5 mm; largest female, 10.0 mm.

FEMALE.—Antenna 1, 38–65 percent length of body, 28–64 percent longer than antenna 2; primary flagellum with 14–23 segments. Antenna 2, flagellum with 5–7 segments. Mandibles subequal; spine row with 5 spines; segment 2 of palp with row of rather long setae on inner margin, segment 3 with 3 long setae on outer margin, proximally long and distally short setae on inner margin, and 3 or 4 long setae on apex. Maxilla 1: inner plate with 6–9 apical, plumose setae; palp with 5–8 setae and 2 or 3 slender spines apically and subapically. Maxilla 2, inner plate with oblique row of 6–10 plumose setae on inner margin. Maxilliped: inner plate with 3 or 4 bladelike spines, 1 or 2 plumose spines and 2 naked setae apically, and 3 plumose spines (or coarse setae?) on inner margin distally; outer plate with numerous naked setae on inner margin and apex and 1 or 2 lightly plumose setae apically. Inner lobes of lower lip vestigial.

Propod of gnathopod 1 subequal in size to 2nd propod; palm armed with double row of 7–12 spine teeth; defining angle with 2–4 (usually 3 or 4) long spine teeth on outside, 2–4 shorter ones on inside; posterior margin with 3–8 setae; medial setae mostly singly inserted. Dactyl nail of gnathopod 1 moderately long. Coxal plate of gnathopod 1 longer than broad, margin with 4–6 setae. Gnathopod propod 2: palm with double row of 7–12 spine teeth; defining angle with 2 or 3 spine teeth of unequal length on outside, 2–4 short spine teeth on inside; posterior margin with 3–5 sets setae; medial setae singly inserted. Dactyl nail of gnathopod 2 moderately long. Coxal plate of gnathopod 2 longer than broad, margin with 5–9 (usually 7) setae. Coxal plate of pereopod 3 longer than broad, margin with 5–11 (usually 9–11) setae. Coxal plate of pereopod 4 relatively broad and deep, about as broad as long, reaching approximately 55–60 percent length of basis; margin with 6–17 (usually 10–14) setae. Pereopod 6 little longer than pereopod 7, 40–60 percent length of body, 10–30 percent longer than pereopod 5. Bases of pereopods 5–7: posterior margin of 7 usually more convex than in 5 and 6; distoposterior lobes distinct, broadly rounded; anterior and posterior margins with variable number of spines and setae, respectively. Dactyls of pereopods 6 and 7, 35–37 percent length of corresponding propods; dactyl of pereopod 5 about 30 percent length of corresponding propod. Coxal gill present on pereopod 7. Three median sternal gills on pereonites 2–4; 2 pairs simple lateral sternal gills on pereonites 6 and 7; sternal gills absent from pleonite 1. Brood plates somewhat expanded distally.

Pleonal plates: posterior margin of plate 1 convex, with 3–6 setae, that of plate 2 nearly straight, with 5 or 6 setae, that of plate 3 slightly sinuate, with 4–6 setae; posterior corners small, bluntly rounded; ventral margin of plate 2 with 3–5 spines, that of plate 3 with 4–6 spines. Uronites free. Uropod 1: inner ramus subequal in length to outer ramus, about 60–65 percent length of peduncle, armed with 9–15 spines; outer ramus with 8–15 spines; peduncle with 10–27 (usually 13–24) spines. Uropod 2: inner ramus a little longer than outer ramus, a little shorter than peduncle, armed with 8–12 spines; outer ramus with 7–13 spines; peduncle with 4–10 (usually 5–7) spines. Uropod 3: peduncle with 1–6 (usually 1–3) distal spines; ramus 35–50 percent length of peduncle, with 2–5 (usually 2–4) apical spines. Telson longer than broad, gently tapered distally; apical margin incised 20–45 (usually 20–33) percent the distance to base, armed with 10–16 (usually 11–15) spines.

MALE.—Differing only slightly from female as follows: Coxal gill sometimes absent from pereopod 7. Peduncular process of uropod 1 about 25 percent length of outer ramus, narrowing distally, apex usually weakly serrate.

VARIATION.—As indicated in the description there is considerable morphological variation in meristic characters (i.e., numbers of setae and spines) in this species. Variation was also noted in the length and number of flagellar segments of the antennae and in the depth of incision of the telson. In view of the wide range of this species and its occurrence in several different drainage basins and in many geologically isolated valleys, this variation is not surprising. With the exception of a few populations possibly isolated geographically from the main part of the range, much of the meristic variation does not appear to have a geographic pattern. In some instances, variation in meristic characters was noted among specimens in the same population, especially between males and females. In other instances, meristic characters appeared to vary between populations and can be attributed in part to the different sizes at which animals in different populations reach sexual maturity (e.g., females from 4.5 to 10.0 mm and males from about 4.5 to 7.5 mm). Larger specimens of the same species frequently have more spines and setae on a given structure than smaller ones (see also Holsinger, 1967:149). With one exception, all specimens examined had from 1 to 3 distal spines on the peduncle of uropod 3. A sexually mature female (7.2 mm long with setose brood plates) from Tilson Saltpetre Cave in Smyth Co., Virginia, differed in having 5 and 6 spines, respectively, on the peduncles of uropod 3 and a few more spines on uropods 1 and 2 and the telson. Because of the limited sample size from this cave, it is difficult to assess the significance of this variation. However, Tilson Saltpetre Cave does not appear to be any more isolated geographically (or geologically) than many other caves that contain populations of S. mackini.

The variation in the depth of the incision of the telson, like the meristic characters, did not correspond to a geographic pattern. The depth of the notch of the telson varied between 20 and 33 percent in most populations examined critically, and in only two populations from different parts of the range did it exceed 33 percent.

The variation in only one character examined—the ratio of the length of antenna 1 to body length—appeared to have a geographic correlation. As indicated by the data in Table 1, populations from caves in Mercer Co., West Virginia, which are isolated in a valley just north of East River Mountain, differ from populations elsewhere in the range by having longer first antennae. The ratio of the length of antenna 1 to body length in most populations examined outside of Mercer County generally fell between 0.40 and 0.55, and the samples analyzed in Table 1 from Repass Saltpetre Cave (New River drainage), Moccasin Valley Cave (Holston River drainage), and Offutts Cave (Clinch River drainage) appear typical in this respect. Although Moccasin Valley Cave vs. Offuts Cave (p > .10) (not significant) the ratios obtained from the Mercer County samples varied to some extent (between 0.47 and 0.65), the mean, as indicated by the t-tests in Table 1, was significantly higher than in the three other populations compared.

DISTRIBUTION AND ECOLOGY.—The range of this species, the second largest of the cave species treated in this paper, extends from Roane Co., Tennessee, in the Tennessee River drainage basin north-northeastward to Monroe Co., West Virginia, in the New River drainage basin and covers a linear distance of 403 km. Populations appear to be more common in caves near the middle of the range in limestone valleys drained by the Clinch River (upper Tennessee River drainage) in southwestern Virginia. In Russell, Scott, and Tazewell counties, Virginia, more than 80 percent of the caves examined with superficially suitable habitats contained populations of S. mackini. Elsewhere, populations are found in limestone valleys drained by the Holston River (upper Tennessee River drainage) and the New River (upper Kanawha River drainage), and a single population is recorded from the Powell River Valley (upper Tennessee River drainage) in Wise Co., Virginia. With the exception of two spring localities, all other populations have been sampled from caves, many of which are developed in Ordovician limestones.

Stygobromus mackini is primarily an inhabitant of small, mud-bottom drip and seep pools, although a few populations have been found in small streams with gravel or mud substrates. Out of 62 samples, 49 have come from pools, 11 from streams and 2 from surface springs (Table 2). As is the case with many other species of the genus, females are far more abundant in collections than males. Of the 244 specimens examined during this study, 184, or 75 percent, were females. Sexually mature females (with setose brood plates), ranging in size from 4.5 to 8.7 mm, have been recorded from all seasons of the year. Ovigerous females have been found during the months of May, July, and August. Juveniles have been found in samples from the same months. The clutch size of seven ovigerous females, ranging in size from 4.5 to 8.0 mm (X = 6.80, SD = 1.24, C.V. = 0.18) is summarized as follows: N = 7, range = 1–12 eggs or embryos, X = 6.43, SD = 3.78, C.V. = 0.59. Newly hatched young are approximately 1.7 mm in length.

REMARKS—Although I indicated earlier (Holsinger, 1969a:44–45) that S. mackini occurred in Craig Co., Virginia (upper James River drainage), a re-examination of the material from this county (a single specimen from New Castle Murder Hole Cave) has led me to conclude that, while this specimen is closely allied with S. mackini, it represents a distinct species but one that cannot be described pending collection of additional specimens (see “Stygobromus spp.”).

Hubricht (1943:697) designated Sikes Cave (7.2 km north of Lebanon, Russell Co., Virginia) as the type-locality for this species. I have made several attempts to locate this cave but have so far failed. In the general vicinity of Hubricht's Sikes Cave, however, I was able to find a small, dry cave in a limestone bluff just south of the Clinch River which the owner referred to as “Sykes Cave” (Holsinger, 1975a:259). This cave contained no water and, according to L. Hubricht (in litt.), was not the same cave which he designated as the type-locality.