Stygobromus dicksoni Holsinger 1978

Stygobromus dicksoni

Stygobromus sp.—Holsinger, 1969a:32.—Holsinger and Peck, 1971:30.

MATERIAL EXAMINED.—ALABAMA. Jackson Co.: Sauta Cave, holotype (USNM 168809), 12 and 3 paratypes (JRH), J. R. Holsinger, S. B. Peck, and R. M. Norton, 17 Mar 1964; Horseskull Cave, 4 paratypes (JRH), S. B. Peck and A. Fiske, 5 Aug 1967; Indian Rocks Cave, 6 paratypes (JRH), S. B. Peck and A. Fiske, 7 Aug 1967; Out Cave, 5 paratypes (USNM), S. B. Peck and A. Fiske, 13 Aug 1967; Sheldons Cave, 1 paratype (JRH), S. B. Peck, 25 Jan 1967; Madison Co.: Varnedoe Cave, 1 paratype (JRH), E. Steenburn, 17 Dec 1972; Marshall Co.: Cathedral Caverns (all collections by S. B. Peck), 1 paratype (USNM), 3 Aug 1965, 1 paratype (JRH), 12 Aug 1965, 3 , 1 paratypes (USNM), 17 Aug 1965, 1 , 5 juv. paratypes (JRH), 1 Sept. 1965. GEORGIA. Chatooga Co.: Blowing Spring Cave, 1 , 2 paratypes (JRH), J. R. Holsinger, S. B. Peck, R. A. Baroody, and A. Fiske, 11 Jun 1967; Dade Co.: Byers Cave, 15 , 2 paratypes (JRH), J. R. Holsinger, S. B. Peck, and A. Fiske, 18 Jun 1967; Howards Waterfall Cave, 3 paratypes (JRH), J. R. Holsinger, S. B. Peck, and R. M. Norton 15 Mar 1964; Rustys Cave, 1 paratype (JRH), A. Iles, 23 Sep 1967; Walker Co.: Pettijohn Cave, 1 paratype (JRH), J. R. Holsinger, S. B. Peck, R. A. Baroody, and A. Fiske, 10 Jun 1967.

DIAGNOSIS.—A relatively small to medium-sized cavernicolous species, very closely allied with S. mackini, but distinguished from that species by reaching sexual maturity at a smaller size, lacking distal spines on peduncle of uropod 3, proportionately shorter ramus of uropod 3, and longer peduncular process of uropod 1 of the male which is about 60 percent length of outer ramus. Largest male, 3.8 mm; largest female, 6.7 mm.

FEMALE.—Antenna 1, 38–42 percent length of body, 30–40 percent longer than antenna 2; primary flagellum with 14 or 15 segments. Antenna 2, flagellum with 5 or 6 segments. Mandibles subequal; spine row with 4 spines; segment 2 of palp with row of 5 setae (3 long, 2 short) on inner margin, segment 3 with 1 or 2 long setae on outer margin, 1 long seta and row of short setae on inner margin, and 3 long setae on apex. Maxilla 1: inner plate with 5 or 6 apical, plumose setae; palp with 2 or 3 slender spines and 4 or 5 thick setae apically. Maxilla 2, inner plate with oblique row of 6 plumose setae on inner margin. Maxilliped: inner plate with 3 bladelike spines, 1 plumose spine, and 1 naked seta apically, and 1 plumose spine (or coarse seta?) and 1 naked seta subapically; outer plate with row of naked setae on inner margin and apex and usually 1 plumose seta apically. Inner lobes of lower lip small.

Propod of gnathopod 1 subequal in size to 2nd propod; palm armed with double row of 7 or 8 spine teeth; defining angle with 3 or 4 long spine teeth on outside, 2 or 3 shorter ones on inside; posterior margin with 3 setae; medial setae singly inserted. Dactyl nail of gnathopod 1 rather long. Coxal plate of gnathopod 1 longer than broad, margin with 3 setae. Gnathopod propod 2: palm with double row of 7 or 8 spine teeth; defining angle with 3 long spine teeth on outside, 2 or 3 shorter ones on inside; posterior margin with 3 sets setae; medial setae singly inserted. Dactyl nail of gnathopod 2 rather long. Coxal plates of gnathopod 2 and pereopod 3 longer than broad, margins with 4 or 5 setae each. Coxal plate of pereopod 4 relatively broad and deep, nearly as broad as long, reaching about 55 percent length of basis; margin with 8 setae. Pereopod 7 slightly longer than pereopod 6, 40–45 percent length of body, 15–30 percent longer than pereopod 5. Bases of pereopods 5–7 about like those of S. mackini; dactyls of pereopods 6 and 7, 30–35 percent length of corresponding propods, that of pereopod 5, 35–40 percent length of corresponding propod. Coxal gill present or absent on pereopod 7. Two median sternal gills usually present on pereonites 2 and 3; 2 pairs simple, lateral sternal gills on pereonites 6 and 7; sternal gills absent from pleonite 1. Brood plates somewhat expanded distally (similar to those of S. mackini).

Pleonal plates: posterior margins slightly sinuate, plates 1 and 2 with 5 or 6 setae each, plate 3 with 1 or 2 setae; posterior corners of plates 2 and 3 small and rounded, that of plate 1 indistinct; ventral margin of plate 2 with 1 spine, that of plate 3 with 2 spines. Uronites free. Uropod 1: inner ramus subequal in length to outer ramus, about 70 percent length of peduncle, armed with 10–12 spines; outer ramus with 9–11 spines; peduncle with 12–13 spines. Uropod 2: inner ramus longer than outer ramus, subequal in length to peduncle, armed with 9–11 spines; outer ramus with 6 or 7 spines; peduncle with 4 or 5 spines. Uropod 3: ramus about 25 percent length of peduncle, armed with 2 or 3 apical spines. Telson a little longer than broad; apical margin incised about 25 percent the distance to base, with 12–16 spines.

MALE.—Differing from female as follows: Flagellum of antenna 1 with 10 or 11 segments. Gnathopod propods with few less spines and setae. Coxal gill absent from pereopod 7. Uropod 1: inner ramus with 8–11 spines; outer ramus with 7–9 spines; peduncle with 7–9 spines; peduncular process elongate and narrowing distally, reaching about 60 percent length of outer ramus, weakly serrate distally. Uropod 2: inner ramus with 7–9 spines; outer ramus with 7 spines; peduncle with 3 or 4 spines. Apical margin of telson armed with 12 spines.

VARIATION.—The specimens from caves in Chatooga and Walker counties, Georgia, lacked median sternal gills, but these structures were present in all other populations examined. The presence or absence of a coxal gill on pereopod 7 of the female was also variable throughout the range—specimens from Byers Cave in Dade Co., Georgia, and Cathedral Caverns in Marshall Co., Alabama, lacked this structure. Three out of four females from Horseskull Cave in Jackson Co., Alabama, also lacked a coxal gill on pereopod 7, whereas the other female in the sample possessed a coxal gill on only one of the two seventh pereopods. All other females examined possessed a coxal gill on both seventh pereopods.

TYPE-LOCALITY.—Sauta Cave, located 11.3 km southwest of Scottsboro in Jackson Co., Alabama, is a large, stream-passage cave developed in the Bangor limestone of Mississippian age. The type series was collected from mud-bottom, rimstone pools in the “Great Lakes” section of the cave. The pools contained pieces of rotting wood and are located on an upper level of the cave well above the stream.

DISTRIBUTION AND ECOLOGY.—The range of this species is contained within the Cumberland Plateau and occupies a part of the Tennessee River drainage basin in northeastern Alabama and northwestern Georgia and a part of the Coosa River drainage (upper Alabama River basin) in northwestern Georgia. The range, which covers a linear distance of approximately 108 km from east to west, extends from southeastern Madison County and northern Marshall County northeast through Jackson County in Alabama and then south-southeast from Dade County through western Walker County to northwestern Chatooga County in Georgia. The northernmost record for this species (Horseskull Cave in Jackson County) is situated about 150 km southwest of the southernmost record for S. mackini (Berry Cave, Roane Co., Tennessee).

All collections made to date have been from drip and seep pools in caves developed in Mississippian limestones. Although collections are available from all seasons of the year, ovigerous females have not been found. However, females (size range, 3.8–6.7 mm) with setose brood plates have been collected during spring, summer, and fall, and the September collection from Cathedral Caverns contained five newly hatched young.

ETYMOLOGY.—It is a pleasure to name this species in honor of Gary W. Dickson, who has materially assisted with the collection and study of cavernicolous amphipods.