Stygobromus paxillus 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 : 10-14

Stygobromus paxillus Holsinger , sp. n.

Figures 8 , 9 , 10

Material examined. Holotype female (3.0 mm) and 19 paratypes (all possibly females, approximately 3.03. 5 mm in length), from Prettyboy West Spring, 590 m southwest of Dam Breast, Baltimore Co., Maryland; 10 collected by Daniel Feller and Scott Smith, 26 March 2007 and 5 collected from same locality by Daniel Feller, 25 March 2008. The holotype is deposited in the National Museum of Natural History, Smithsonian Institution ( USNM 1154086); paratypes are in the research collection of J. R. Holsinger.

Diagnosis. Small, possibly parthenogenetic species, distinguished by robust gnathopod propodi bearing relatively few non-bifid, peg-like palmar margin spines; prominent cutting plates of right mandible, and heavily spinose uropods 1 and 2 and telson. Largest specimens (females), 3.0 – 3.5 mm; male unknown.

Description. Female (3.0 mm). Antenna 1 ( Fig. 8 a) 50% length of body, about 30 % longer than antenna 2, primary flagellum with 7 articles, none with aesthetacs; accessory flagellum slightly longer than first flagellar article. Antenna 2 ( Fig. 8 b): flagellum with 3 articles. Mandibles ( Figs. 8 c,d) subequal: spine row of right with 3 plumose spines, that of left lacking (?) spine row; lacinia mobilis of left 2 cutting plates, that of right larger and with 2 cutting plates; incisors of both prominent, with 7 or 8 teeth; molar of right cone-shaped and protruding, with short seta, that of left obsolete; segment 3 of right palp with 3 E and 1 D setae, segment 3 of left palp with 3 E and 5 D setae. Lower lip ( Fig. 8 e): inner lobes vestigial. Maxilla 1 ( Fig. 8 f): inner plate with 2 apical, plumose setae; outer with 5 spines (most serrate) apically; palp with 4 apical spines. Maxilla 2 ( Fig. 8 g): inner plate with 2 plumose spines near apex; palp with 4 spines on apex. Maxilliped ( Fig. 8 h): inner plate with 2 plumose and 3 naked spines apically; outer plate with row of approximately 6 or 7 setae on inner margin and 1 plumose spine on apex; article 2 of palp broad and nearly 2X length of segment 1, inner margin with row of 8 rather long setae; palp article 3 shorter than previous article, with several long setae apically; article 4 (dactyl) much shorter than article 3 and bearing long, sharply pointed nail.

Gnathopod 1 ( Fig. 9 a): propodus subtriangular and slightly larger than propodus of gnathopod 2, palm nearly straight, without serrations, approximately 3X length of posterior margin, bearing unequal double row of 8 prominent peg-like spines and 3 long setae; dactyl nail long, approximately 75% length of dactyl; posterior margin very short and lacking spines or setae; carpus approximately subtriangular, about as broad as deep, with 3 prominent ventral setae and thick seta on opposite margin; coxa suboval, short and longer than broad, with single, short marginal seta. Gnathopod 2 ( Fig. 9 b): propodus subequal in size to that of gnathopod 1, palm very slightly convex, without serrations, approximately equal in length to posterior margin, bearing 4 peg-like spines and 6 longish setae; nail of dactyl approximately 35% length of dactyl; posterior margin bearing 2 sets of 2 long setae each. Posterior margin of carpus of gnathopod 2 convex and bearing 7 or 8 mostly longish setae, none rastellate. Coxa of gnathopod 2 suboval and broadly rounded, bearing 3 ventral setae. Bases of both gnathopods with 4 to 6 long setae on posterior margins.

Pereopods 3 &4 ( Figs. 10 a,b) subequal, approximately equal in length to pereopod 5, coxa suboval, broader than deep, with 3 short setae on ventral margins; dactyls relatively long, approximately 50% length of corresponding propodi. Pereopods 5, 6 & 7 ( Figs. 10 c,d,e): 6 little longer than 7 and much longer than 5; posterior margins of bases nearly straight to weakly convex, distoposterior lobes of 5 and 6 broadly rounded, that of 7 not as prominent; dactyls approximately 65% length of corresponding propodi. Coxal gills ( Fig. 10 e) present on gnathopods 2 and pereopods 3 7. Medial sternal gills and 2 pairs lateral sternal gills ( Fig. 10 f) narrow, not bifurcate. Brood plates (attached to pereopods 3 & 4—see Figs. 10 a,b) elongate but not fully mature, lacking marginal setae.

Pleonal plates similar ( Fig. 10 g): ventral margins straight to slightly convex, lacking spines or setae; posterior margins slightly expanded and irregular, bearing 3 setae each. Pleopods typical for genus. Urosomites free, not fused. Uropod 1 ( Fig. 10 h): inner ramus equal in length to outer ramus, 3540 % length of penducle, armed with 9 spines of unequal length; outer ramus with 7 spines of unequal length; peduncle with 2 rows of 5 short spines each. Uropod 2 ( Fig. 10 i): inner ramus little longer than outer ramus and nearly as long as peduncle, armed with 11 long apical spines and 5 shorter marginal (dorsal); outer ramus with 3 long apical spines and 1 marginal (dorsal) spine; peduncle with 2 spines. Uropod 3 ( Fig. 10 j): ramus very short and less than 20% length of peduncle; armed with 4 relatively long dorsal spines. Telson ( Fig. 10 k): subquadrate, dorsal margin slightly convex and slightly longer than length, bearing 16 relatively long, stout spines.

Type locality. Prettyboy Dam West spring, 590 m southwest of Dam Breast, Baltimore County, Maryland ( Fig. 1 ).

Etymology. The epithet paxillus is from Latin meaning “peg” or “small stake” in recognition of the unusual peg or stalk-like palmar margin spines on the gnathopods that differ from those of most other species of Stygobromus .

Distribution and ecology. To date all known specimens of this rather unique species have been collected from Prettyboy Dam West spring. However, in addition to the specimens of S. paxillus , 5 specimens of another undescribed species of Stygobromus were collected here in 2008 and 2010. These samples contained 1 tiny (ca. 2.2 mm) female bearing 1 large egg and 3 juveniles (1.5 – 2.0 mm). Although these tiny specimens appear to differ from those of S. paxillus and for that matter from other species of Stygobromus , all but one are obviously immature and additional collecting effort is necessary to obtain more specimens (hopefully mature) for a taxonomic determination of this second species. The spring habitat of this species is apparently developed in sediments associated with Precambrian metamorphic rocks of the Piedmont.

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 8. Stygobromus paxillus, sp. n. Prettyboy Dam West Spring, Baltimore Co., Maryland. Paratype female (3.0 – 3.5 mm): (a, b), antennae 1, 2 (accessory flagellum of antennae 1 enlarged); (c) right mandible (spine row, lacinia mobilis and incisor enlarged); (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 9. Stygobromus paxillus, sp. n. Pretty Boy Dam West Spring, Baltimore Co., Maryland. Paratype female (3.0 – 3.5 mm): (a) gnathopod 1 (palmar spine enlarged); (b) gnathopod 2 (palm and palmar spines enlarged in part). Gnathopods drawn to same scale.

FIGURE 10. Stygobromus paxillus, sp. n. Pretty Boy Dam West Spring, Baltimore Co., Maryland. Paratype female (3.0 – 3.5 mm): (a, b, c, d, e) pereopods 3, 4, 5, 6, 7; (f) median sternal gills (processes); (g) pleonal plate; (h, i, j) uropods 1, 2, 3; (k) telson. All pereopods, pleonal plate, uropods 1 & 2 drawn to same scale; uropod 3 and telson to larger scale.