Stygobromus morrisoni (Holsinger 1967)

Stygobromus morrisoni (Holsinger)

Stygonectes morrisoni Holsinger, 1967:28–31, fig. 4 [type-locality: Witheros Cave, Bath Co., Virginia]; 1969a:32–34; 1972:59.—Holsinger, Baroody, and Culver, 1976:24.

Stygobromus morrisoni (Holsinger).—Karaman, 1974:114.—Holsinger, 1977:262.

MATERIAL EXAMINED.—VIRGINIA. Bath Co.: Witheros Cave, 7 , 7 (Holsinger, 1967:28). WEST VIRGINIA. Hardy Co.: Dyers Cave, 1 , J. R. Holsinger, 29 Aug 1966; Pendleton Co.: Kenny Simmons Cave, 1 , J. R. Holsinger, 25 Aug 1966.

DIAGNOSIS AND DESCRIPTION.—Corresponding to the diagnosis and description by Holsinger (1967: 28–31) with the following additions and modifications: Largest male, 6.0 mm; largest female, 8.0 mm. Antenna 1, 50–55 percent length of body; primary flagellum with 15–20 segments. Spine row of mandible with 6 or 7 spines. Maxilla 1: inner plate with 7 or 8 apical, plumose setae; palp with 3 or 4 small spines and 4–6 setae apically. Maxilla 2, inner plate with oblique row of 7–9 plumose setae on inner margin. Maxilliped: inner plate with 2 bladelike spines, 1 plumose spine and 2 or 3 naked setae on apex, few plumose setae or spines on inner margin; outer plate with naked setae on inner margin and apex and 1 apical, plumose seta. Defining angle of gnathopod propod 1 with 3–5 spines on outside and 4–7 on inside. Coxal plate of gnathopod 1 with 4–6 marginal setae. Posterior margin of gnathopod propod 2 with 3 or 4 sets setae. Coxal plate of gnathopod 2 with 5–9 marginal setae; coxal plate of pereopod 3 with 7–11 marginal setae; coxal plate of pereopod 4 with 10–12 marginal setae. Dactyls of pereopods 5–7, 30–40 percent length of corresponding propods. Lateral sternal gills bifurcate. Brood plates of sexually mature females rather narrow but somewhat expanded distally. Uronites completely fused. Uropod 1: inner ramus with 11–13 spines; outer ramus with 10–13 spines; peduncle with 11–19 spines. Uropod 2: inner ramus with 8–11 spines; outer ramus with 7–9 spines; peduncle with 4–6 spines. Uropod 3: ramus with 3–5 apical spines; peduncle with 1 or 2 distolateral spines. Telson with 6–8 apical spines per lobe in female, typically 4 per lobe in male.

VARIATION.—Several structures varied among the population samples as follows: The single female (8.0 mm) from Kenny Simmons Cave had more spines on the defining angle of the 1st gnathopod propod than females from Witheros (6.0–6.7 mm) or Dyers (5.0 mm) caves. Females from West Virginia had 4 sets of setae on posterior margins of 2nd gnathopod propods as opposed to only 3 sets in the Virginia specimens. The proportionate lengths of pereopod dactyls were less in the two West Virginia samples (30–34 percent in contrast to 40 percent for the Virginia material). Sternal gills were present on the 1st pleonite in females from Witheros and Kenny Simmons caves but absent from the 1st pleonite of the Dyers Cave female. The peduncle of uropod 1 had 16–19 spines in females from West Virginia but only about 11 in females from Virginia. The number of apical spines on the ramus of uropod 3 varied from 3 to 5 and the number of spines on the peduncle from 1 to 2; however, no geographic pattern was discernible for the variation noted in this structure.

DISTRIBUTION AND ECOLOGY.—The range of this species, which is very disjunct, extends from Bath Co., Virginia, north-northeast to Hardy Co., West Virginia, and covers a linear distance of approximately 161 km. Witheros Cave is situated in the upper James River drainage, whereas Kenny Simmons and Dyers caves are in the upper Potomac River drainage. All three caves are developed in Silurian-Devonian limestone, but these strata are not continuous between them. The geographic variation in the several structures noted above is not surprising in view of the long, discontinuous distribution of this species. Whether the northernmost population in Dyers Cave actually represents a distinct subspecies as I have previously implied (Holsinger, 1969a:46; Holsinger, Baroody, and Culver, 1976:24) must await the collection of additional material. Undoubtedly, additional populations of this species remain to be discovered in the large disjunctions between the presently known localities.

Stygobromus morrisoni inhabits a small, gravel-bottom stream in Witheros Cave, where it occurs syntopically with S. mundus (Holsinger, 1967:31). In Kenny Simmons Cave a single female of this species was collected along with two specimens of S. emarginatus from an old, partly submerged wooden boat in a mud-bottom lake at the rear of the cave. In Dyers Cave a single female was taken from a gravel-and-silt bottom, residual stream pool, which was also occupied by several specimens of S. allegheniensis. To date, ovigerous females have been found only in Witheros Cave (Holsinger, 1967:31).

Stygobromus morrisoni, commonly called Morrison's cave amphipod, is a troglomorphic species of amphipod in family Crangonyctidae. It is native to Virginia and West Virginia, in the United States.