Stygobromus foliatus Holsinger

Holsinger, John R., Ansell, Lynnette Meador & Shafer, Justin, 2011, Four new species of the subterranean amphipod genus Stygobromus (Amphipoda: Crangonyctidae) from shallow groundwater habitats on the Coastal Plain and eastern margin of the Piedmont in Maryland and Virginia, USA, Zootaxa 2972, pp. 1-21 : 14-19

Stygobromus foliatus Holsinger , sp. n.

Figures 11 , 12 , 13 , 14

Material examined. Maryland: Holotype male (8.0 mm), 2 paratype females (7.0 mm), spring on Hancock Rd., 373 m northeast of junction of Hancock and Adams roads, Naniemoy TNC preserve, Charles County, collected by Jim McCann, 6 February 2000; 1 female paratype, Poplar Hill Creek spring, 23 m S of Rt. 244, St. Marys County, collected by Katherine McCarthy and Pete Bendel, 11 April 2003; 1 female paratype, Poplar Hill Creek No. 2, 60 m south from Creek and Beauvue Rd. (Rt. 244), St. Marys County, collected by Pete Bendel, 29 April 1999. Virginia: 2 female paratypes, seep pool, 3.5 km north of Rappahannock Academy, Pettigrew Wildlife Management area, 0.5 km SE of Skinkers Corner, Caroline County, collected by S. M. Roble and K. L. Derge, 19 Oct. 2000; 3 female paratypes, seep 1 and pond of Mount Creek, Fort A.P. Hill, collected by A. C. Chazal and A.V. Evans, 24 March 2008; 4 juveniles and 1 female paratypes, seeps in Pittigrew Wildlife Management area, Skinkers Corner, 26 March 2008; seep flowing into backwater, Dragon Run drainage, Rt. 615 at Exol Swamp, King and Queen County, 2 female paratypes, collected by A. C. Chazal and A. Belden, 10 June 2003; 2 female (1 ovigerous) and 2 male paratypes, seep in Voorhees Nature Preserve, 8 km SW of Oak Grove, Westmoreland County, collected by Christopher Hobson, 11 April 2000. The holotype is deposited in the National Museum of Natural History, Smithsonian Institution ( USNM 1154087); paratypes are in the research collection of J. R. Holsinger.

Diagnosis. A r elatively large, shallow groundwater species clearly distinguished from all other known species of Stygobromus by possession of 2 pairs of large, leaf-like sternal gills (or processes) on pereonites 6 and 7. Additionally:gnathopod propodi relatively large; bases of pereopods 5, 6, and 7 moderately expanded; uropod 1 relatively heavily spinose and peduncular process absent from male uropod 1; posterior margins of pleonal plates 1 and 3 and ventral margins of plates 2 and 3 bearing numerous spines and/or setae; and broad brood plates of female bearing numerous long setae. Largest males, 8.0 mm; largest females, 8.0 mm.

Description. Female paratype (8.0 mm). Antenna 1 ( Fig. 11 a): 50% length of body, approximately 50% longer than antenna 2, primary flagellum with 20 to 21 articles, last 3 very small and bearing aesthetacs; accessory flagellum equal in length to first 2 flagellar articles. Antenna 2 ( Fig. 11 b) flagellum with 8 articles. Mandibles ( Figs. 11 c,d) generally similar but differing in a few small ways: spine row of right with 4 short spines and 2 tiny setae, that of left with 4 short spines; lacinia mobilis of right with 5 small teeth, that of left with 4 bluntly rounded teeth; incisors with 4 bluntly rounded teeth; molars prominent, each bearing single seta but differing in several ways as shown (e. g. right with damaged surface, left turned in opposite direction); palps similar, segment 2 with 8 to 10 setae on inner margins, segment 3 with 6 D setae, 4 long E setae, and 1 B seta. Lower lip ( Fig. 11 e): inner lobes relatively well developed. Maxilla 1 ( Fig. 11 f): inner plate with 5 apical plumose setae; outer plate with 5 distally serrate spines apically; palp with 7 or 8 apical setae. Maxilla 2 ( Fig. 11 g): inner plate with 4 plumose setae in oblique rowand 6 spines apically; outer plate with 9 rather long apical setae. Maxilliped ( Fig. 11 h): inner plate with 3 short spines and 2 short setae apically; outer plate with inner row of 10 rather short setae; palp article 2 not greatly expanded, approximately 2X length of palp segment 1, inner margin with row of 15 or 16 setae increasing in length distally; palp article 3 shorter than 2 and bearing approximately 8 thin setae apically; nail of palp article 4 long and sharply pointed.

Gnathopod 1 ( Fig. 12 a): propodus smaller than propodus of gnathopod 2 ( Fig. 12 b); palm slightly convex, armed with unequal double row of 23 or 24 mostly small, distally notched spine teeth; defining angle rounded; posterior margin weakly convex, about 40% length of palm, with 4 sets of ventral setae. Posterior margin of carpus of gnathopod 1 bearing several sets of prominent setae and 2 rastellate setae on weakly lobed posterior margin. Coxa of gnathopod 1 suboval, with 4 setae on subventral margin. Gnathopod 2 ( Fig. 12 b): propodus about 40% longer than broad, palm straight or nearly so, little longer than posterior margin, armed with irregular double row of approximately 14 small, distally notched spine teeth; defining angle rounded, with 1 large spine on outside and 2 much smaller spines on inside and 4 long setae; posterior margin little shorter than palm, slightly convex, with approximately 10 long ventral setae; medial setae few in number, singly inserted and rather short. Ventral margin of carpus of gnathopod 2 short, weakly lobate; bearing several plain and 2 relatively larger astellate setae. Coxa of gnathopod 2 ventrally rounded, about as broad as deep, with 4 setae on ventral margin.

Pereopods 3 and 4 ( Figs. 13 a, b) subequal except for coxal plates. Coxal plate of pereopod 4 broader and deeper than that of pereopod 3, bearing 10 marginal setae; coxal plate of pereopd 3 with only 6 marginal setae. Pereopods 6 and 7 ( Figs. 13 c,d) subequal in length, approximately 50% length of body, 10 – 15% longer than pereopod 5 ( Fig. 13 e). Posterior margins of pereopods 5 and 7 weakly convex, that of pereopod 6 nearly straight; distoposterior lobes little stronger on 5 and 7 than on 6. Dactyls of pereopods 6 and 7 approximately 30% length of corresponding propodi; that of pereopod 5 proportionately little longer.

Coxal gills ( Figs. 12 b, 13a,b) present on gnathopod 2 and pereopods 3 – 7 approximately equal in size except smaller on pereopod 7. Large, paired, leaf-like, lateral sternal gills ( Figs. 14 a) present on pereonites 5 and 7; 1 pair tiny narrow sternal gills on pleonite 1. Brood plates ( Fig. 14 b) relatively large, heavily setose, narrowing distally.

Pleonal plates ( Figs. 14 c): posterior margins variable in shape and number of short setae; that of plate 1 weakly convex with 5 setae; that of 2 relatively straight and with 3 setae; that of 3 convex and with 5 setae. Ventral margins (or submargins) also variable: plate 1 with 5 weak spines; plate 2 with 6 weak spines in row above margin; and plate 3 with 2 spines above margin. Pleopods typical for genus. Urosomites free, not fused. Uropod 1 ( Fig. 14 d): inner ramus subequal in length to outer ramus, approximately 65% as long as peduncle, armed with 10 spines (2 on apex much larger than others); outer ramus with 11 spines (1 on apex much larger); peduncle with 18 spines (most on outer margin). Uropod 2 ( Fig. 14 e) only about 50% length of uropod 1: inner ramus subequal in length to outer ramus, armed with 8 spines; outer ramus with 6 spines; peduncle broader and about 30% longer than rami, armed with 5 spines. Uropod 3 ( Fig.14 f): ramus only 30 – 35% length of peduncle, armed with 2 rather long apical spines. Telson ( Fig. 14 g) subquadrate, approximately 25% longer than broad, apical margin with very shallow gap and armed on either side with 6 spines (one-half of each set with 6 long and 6 short spines).

Male: Closely similar to the female excepting absence of brood plates and possession of slightly larger gnathopod propodi. Male also lacks a distal peduncular process on uropod 1, which is usually present in species of Stygobromus (see Holsinger 1978).

Type locality. Spring in Naniemoy Nature Conservancy (TNC) Preserve on Hancock Road, 373 m northeast of junction with Adams roads, Charles County, Maryland. In addition to the holotype male and 2 fragmented female paratypes the sample from this locality contained a specimen of the stygobiotic amphipod Stygobromus indentatus (Holsinger) and a juvenile specimen of the amphipod Crangonyx sp. The latter has small eyes and does not appear to be troglomorphic.

Etymology. The epithet foliatus is from the Latin meaning leaf, leafy or leaved, and is so named for the unique “leaf shaped” lateral sternal gills or processes of this species.

Distribution and ecology. To date this species is recorded from three headwater spring-like seeps in southwestern Maryland and five similar habitats on the opposite side of the Potomac River in east-central Virginia ( Fig. 1 ). These seeps typically serve as groundwater feeders for small surface creeks and/or swamps. The sample from the type-locality in Charles County also contained a single male specimen of the stygobiotic amphipod Stygobromus indentatus ( Holsinger1978) and a juvenile, eyed specimen of another amphipod probably Crangonyx stagnicolous Zhang & Holsinger (2003) . The sample from Popular Hill Creek spring in St. Marys County also contained 3 specimens of Stygobromus indentatus . The sample from the seep in Voorhees Nature Preserve, Westmoreland Co., VA contained a single, eyed specimen of Crangonyx and 3 troglomorphic asellid isopods Caecidotea jeffersoni ( Lewis 2009) . All specimens of S. foliatus examined to date have been collected from a variety of seeps and small springs fed by subterranean groundwater emanating from unconsolidated Coastal Plain sediments in southern Maryland and southeastern Virginia. These habitats are developed on either side of the Potomac River and occur in unconsolidated Quaternary-Tertiary sediments consisting primarily of sand, silt, clay and gravels. Presumably, the relative coarse, saturated sediments provide an ideal interstitial habitatfor both subterranean amphipods and other small, aquatic invertebrates such as isopods, flatworms, and snails.

Figures

FIGURE 1. Distribution of four new species of Stygobromus from the Coastal Plain and eastern Piedmont in Maryland and Virginia, United States. The restricted distributions of S. caecilius, S. felleri and S. paxillus. are indicated by single symbols. Stygobromus foliatus is more widely distributed and recorded from multiple locations on opposite sides of the Potomac River in Charles and S. Mary’s counties in Maryland, and Caroline, Westmoreland and King & Queen counties in Virginia. Several closely spaced localities in Caroline County are indicated by a single symbol. The curved line through MD and VA marks the approximate boundary between the Coastal Plain to the east and the Piedmont to the west.

FIGURE 11. Stygobromus foliatus, sp. n. Seep in Pettigrew Wildlife Management Area, 0.5 km SE of Skinkers Corner, Caroline Co., Virginia. Paratype female (8.0 mm): (a, b) antennae 1, 2 (aesthetacs on antenna 2 enlarged as indicated); (c) right mandible in part (palp not drawn, similar to left mandible); (d), left mandible; (e) lower lip; (f) maxilla 1; (g) maxilla 2; (h) maxilliped. All mouthparts drawn to same scale; antennae to smaller scale.

FIGURE 12. Stygobromus foliatus, sp. n. Seep in Pettigrew Wildlife Management Area, 0.5 km SE of Skinkers Corner, Caroline Co .. Virginia. Paratype female (8.0 mm): (a) gnathopod 1 (palmar region and rastellate setae greatly enlarged; (b) gnathopod 2 (propodus and rastellate setae greatly enlarged). Basic gnathopods drawn to same scale. Enlargements: gnathopod 1 (4 X); gnathopod 2 (2 X).

FIGURE 13. Stygobromus foliatus, sp. n. Seep in Pettigrew Wildlife Management Area, 0.5 km SE of Skinkers Corner, Caroline Co .. Virginia. Paratype female (8.0 mm): (a, b, c, d, e), pereopods 3, 4 (in part), 5, 6, 7 (in part). Note: pereopods 6 and 7 subequal except coxal plates differ as shown. Coxal gills shown on pereopods 3 and 4. All pereopods and basic gnathopods to same scale.

FIGURE 14. Stygobromus foliatus, sp. n. Seep in Pettigrew Wildlife Management Area, 0.5 km SE of Skinkers Corner, Caroline Co .. Virginia. Paratype female (8.0 mm): (a) leaf-like sternal gills (processes); (b) brood plate; (c) pleonal plates (left to right) 1, 2, 3; (d) uropod 1; (e) uropod 2; (f) uropod 3; (g) telson. Brood plates, sternal gills, uropods 1 and 2 drawn to same scale; uropod 3 and telson to larger scale. Pleonal plates greatly reduced and drawn to much smaller scale than other structures.