Stygobromus mackenziei Holsinger 1974

Stygobromus mackenziei

MATERIAL EXAMINED.—CALIFORNIA. Santa Cruz Co.: Empire Cave, holotype female (USNM 142791) and 5 paratypes (JRH), David McKenzie, 28 May 1967.

DIAGNOSIS.—A small cavernicolous species, only distantly related to the cave species of the Mother Lode region of California and distinguished by having 4 apical spines on the inner plate of the maxilliped, lightly spined palms of the gnathopodal propods (especially the 2nd), relatively large, deep coxal plates of the gnathopods and pereopods 3 and 4, proportionately broad bases of pereopods 5–7, lightly spined peduncles of uropods 1 and 2, and 8 apical spines on the telson. Largest females, 4.7 mm; largest male, 4.0 mm.

FEMALE.—Antenna 1, about 45 percent as long as body, about 30 percent longer than antenna 2; primary flagellum with 10 to 12 segments. Antenna 2, flagellum with 5 or 6 segments. Mandibles subequal; spine row with 5 or 6 spines; palpal segment 2 with 6 setae toward distal end; palpal segment 3 comparatively short, about two-thirds the length of segment 2, with row of short setae on inner margin and 4 long setae apically. Maxilla 1: inner plate with 8 or 9 plumose setae apically; outer plate with 5 slender spines and 3 setae apically to subapically. Maxilla 2: inner plate with oblique row of 9 plumose setae on inner margin. Maxilliped: apex of inner plate with 4 serrate spines and 4 or 5 stiff setae, inner margin with 2 or 3 plumose setae; outer plate with 1 bladelike spine and a number of stiff setae apically. Lower lip without inner lobes.

Gnathopodal Propod 1: Palm not very oblique, slightly convex, armed with double row of 4 or 5 small spine teeth; posterior angle with 2 large spine teeth and 1 long setae on outside, 2 shorter spine teeth on inside; posterior margin without setae; medial setae few in number, singly inserted. Coxal plate of gnathopod 1 nearly twice as long as broad, with 2 marginal setae. Gnathopodal propod 2 about one-fourth longer than 1st propod; palm oblique, slightly concave, armed with double row of 5 spine teeth plus 2 additional spine teeth on outside toward posterior angle; posterior angle with 1 long spine tooth and several long setae on outside, 2 short spine teeth on inside; posterior margin with 2 sets of setae distally; medial setae mostly singly inserted; dactyl nail rather large. Coxal plates of gnathopod 2 and pereopod 3 longer than broad, with 4 marginal setae each; coxal plate of pereopod 4 about as broad as long, with 5 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 convex, distoposterior lobes small, bluntly rounded. Dactyls of pereopods 6 and 7 about one-third the length of corresponding propods. Coxal gills on pereopods 2–6. Sternal blisters on pereonites 2–7. Broad plates of mature females small and narrow.

Pleonal Plates: Posterior margins nearly straight to slightly convex; posterior corners broadly rounded; ventral margin of plate 2 with 2 spines, that of plate 3 with 4 spines. Uropod 1: inner ramus slightly longer than outer ramus, about two-thirds the length of peduncle, armed with 8 spines; outer ramus with 7 spines; peduncle with 6 or 7 spines. Uropod 2: inner ramus about one-fourth longer than outer ramus, about four-fifths the length of peduncle, armed with 7 spines; outer ramus with 6 spines; peduncle with 3 spines. Uropod 3: ramus about one-half the length of peduncle, with 3 apical spines. Telson longer than broad; apical margin with small notch; apical lobes with 4 spines each.

MALE.—Generally similar to female with the following exceptions: Uropod 1: inner ramus a little longer than outer ramus, 60 to 65 percent as long as peduncle, armed with 6 spines; outer ramus with 7 spines; peduncle with 5 spines and small distal process. Uropod 2: inner ramus nearly twice the length of outer ramus, about equal in length to peduncle, armed with 5 or 6 spines; outer ramus with 4 spines; peduncle with 1 or 2 spines. Uropod 3: ramus with 2 or 3 spines. Telson only slightly longer than broad.

TYPE-LOCALITY.—Empire Cave, Santa Cruz County, California. The type-locality is a small cave located in or near the city of Santa Cruz and, according to Halliday (1962), is developed in metamorphosed limestone of unknown geologic age. Although the cave is subject to intermittent flooding, it does not contain a permanent stream but contains gours which are often filled with water (Halliday, 1962).

DISTRIBUTION AND ECOLOGY.—This species is known only from its type-locality. The type-series consisted of one male and five females. One of the females, measuring 3.6 mm, was ovigerous and contained three eggs in the brood pouch. The remaining females ranged in size from 4.0 to 4.7 mm and most had setose brood plates.

ETYMOLOGY.—This species is named in honor of its collector, David McKenzie.

Stygobromus mackenziei, commonly called Mackenzie's cave amphipod, is a troglomorphic species of amphipod in family Crangonyctidae. It is endemic to California, where it occurs only in Empire Cave. This cave is part of the Porter Caves, in the grounds of University of California, Santa Cruz.