Stygobromus gradyi Holsinger 1974

Stygobromus gradyi

MATERIAL EXAMINED.—CALIFORNIA. Tuolumne Co.: Crystal Palace Cave, holotype female (USNM 142780) and 6 paratypes (JRH), Mark Grady, 15 Aug. 1968.

DIAGNOSIS.—A medium-sized cavernicolous species closely related to S. grahami but differing from that species by being smaller at sexual maturity; lacking inner lobes on the lower lip; proportionately smaller gnathopodal propod 2 of female with shorter and slightly convex palm; fewer spines on ventral margins of pleonal plates 2 and 3; and distally serrate peduncular process of male uropod 1. Largest females, 7.0 mm; largest males, 4.5 mm.

FEMALE.—Antenna 1: 50 percent as long as body, about 55 percent longer than antenna 2; primary flagellum with 14 to 17 segments. Antenna 2: peduncular segments similar to S. grahami; flagellum with 6 or 7 segments. Mandibles similar to S. grahami except for few less spines in spine row. Maxilla 1: inner plate with 12 or 13 plumose setae apically; palp with 5 spines and 3 or 4 setae apically and subapically. Maxilla 2: inner plate with oblique row of 14 plumose setae on inner margin. Maxilliped like that of S. grahami. Lower lip without inner lobes.

Gnathopodal Propod 1: Palm not very oblique, slightly concave, armed with double row of 5 spine teeth plus 2 additional spine teeth on outside; posterior angle with 3 long spine teeth on outside, 6 forked spine teeth on inside; posterior margin without setae; medial setae few in number, mostly singly inserted; dactyl nail long. Coxal plate of gnathopod 1 rather deep, longer than broad, with 5 marginal setae. Gnathopodal propod 2 a little longer than 1st propod; palm oblique, slightly convex, armed with double row of 5 spine teeth plus 2 additional spine teeth on outside; posterior angle with 1 long spine tooth and 6 setae on outside, 2 shorter spine teeth on inside; posterior margin with 1 set of 3 setae; medial setae singly inserted; dactyl nail rather long. Coxal plates of gnathopod 2 and pereopod 3 deep, nearly twice as long as broad, with 7 marginal setae on plate 2 and 9 on plate 3. Coxal plate of pereopod 4 deep, expanded distally (as in S. grahami), with 10 marginal setae. Pereopod 6 a little longer than pereopod 7, 45 to 50 percent as long as body, 20 to 25 percent longer than pereopod 5. Bases of pereopods 5–7 broader proximally than distally; posterior margins slightly convex; distoposterior lobes distinct, rounded. Dactyls of pereopods 6 and 7 about one-third the length of corresponding propods. Coxal gills present on pereopods 2–6. Brood plates of mature females small and narrow.

Pleonal Plates: Posterior margins of 2 and 3 slightly produced distally, that of 1 nearly straight; posterior corners small, distinct, rounded; ventral margin of plate 2 with 4 spines, that of plate 3 with 3 spines. Uropod 1: inner ramus a little longer than outer ramus, about two-thirds the length of peduncle, armed with 8 spines; outer ramus with 6 or 7 spines; peduncle with 6 spines. Uropod 2: inner ramus longer than outer ramus, equal in length to peduncle, armed with 7 or 8 spines; outer ramus with 5 or 6 spines; peduncle with 3 spines. Uropod 3: ramus about 40 percent as long as peduncle, with 4 or 5 apical spines. Telson slightly longer than broad; apical margin with shallow notch; apical lobes with 5 or 6 spines per lobe.

MALE.—Differing from the female slightly in the structure of the gnathopodal propods and uropod 1 as follows: Gnathopodal propod 1: palm nearly straight, armed with double row of 3 small and 1 large spine teeth; posterior angle with 3 spine teeth on outside, 5 forked spine teeth on inside. Gnathopodal propod 2: palm not as oblique, nearly straight, armed with unequal double row of 4 small spine teeth plus 2 large spine teeth on outside; posterior angle with 1 spine tooth and 5 setae on outside, 2 spine teeth on inside. Peduncular process of uropod 1 with fine serrations on upper margin toward distal end.

TYPE-LOCALITY.—Crystal Palace Cave, about 10 miles east of Angels Camp, Tuolumne County, California. This cave is just north of the South Fork of the Stanislaus River and was described in detail by Halliday (1962). It is developed in steeply dipping marble or limestone of the Calaveras formation and contains two small streams. Crystal Palace Cave lies about 12 miles southeast of the cluster of caves in central Calaveras County which contain S. grahami.

DISTRIBUTION AND ECOLOGY.—This species is known only from five females and two males collected in August 1968 from intermittent stream pools in the upper section of Crystal Palace Cave. Two of the females (measuring 6.0 to 7.0 mm) had setose brood plates and were sexually mature.

ETYMOLOGY.—This species is named in honor of its collector, Mark Grady.

Stygobromus gradyi, commonly called Grady's cave amphipod, is a troglomorphic species of amphipod in family Crangonyctidae. It is endemic to California in the United States.