Stygobromus bakeri Gibson & Hutchins, 2021

Gibson 1, Randy, Hutchins 2, Benjamin T., Krejca 3, Jean K., Diaz 4, Peter H. & Sprouse 3, Peter S., 2021, Stygobromus bakeri, a new species of groundwater amphipod (Amphipoda, Crangonyctidae) associated with the Trinity and Edwards aquifers of central Texas, USA, Subterranean Biology 38, pp. 19-45 : 19

Stygobromus bakeri Gibson & Hutchins sp. nov. Fig. 4

Type locality.

USA, Texas, Comal County, John Knox Ranch, John Knox Spring (29.9642, 98.1956).

Material examined.

All sites are in Texas, USA. Holotype female (4.6 mm) - Comal County, John Knox Ranch, John Knox Spring: collected by Randy Gibson (RG), 6 May 2010 , dissected slide mounts Texas State University Aquifer Biodiversity Collection. Allotype male (3.1 mm) - Comal County, John Knox Ranch, John Knox Spring: collected by RG, 16 April 2010 . Paratypes - John Knox Spring: juvenile (1.7 mm) collected by RG, 6 January 2010; 2 males (3.5 and 3.6 mm) and juvenile (2.4 mm) collected by RG, 6 April 2010; 2 males (2.5 and 3.4 mm) collected by RG, 2 May 2010; juvenile (2.4 mm) collected by RG, 6 May 2010; damaged female (4.5 mm) collected by RG, 10 May 2010; 2 males (3.1 mm and 4.1 mm) collected by RG, 13 May 2010; male (3.5 mm) and juvenile (2.3 mm) collected by RG, 21 May 2010. - Hays County, Jacob's Well (30.0345, -98.1261): male (4.2 mm) collected by Jean Krejca (JK), 21 October 2009 ; female (4.5 mm) collected by JK, 6 May 2010; female (3.5 mm) collected (≈ 43 m depth) by JK and James Brown, 4 August 2010; brooding female (4.4 mm), male (4.1 mm) and 2 juveniles collected by JK 31 May 2011; 2 males (4.4 and 2.9 mm) collected by JK, 16 June 2013; male (3.2 mm) collected by Matt Turner, 8 September 2013.

Additional material examined.

- Travis County, Mormon Spring no. 3 (30.3132, -97.7747): 1 damaged female (4.4 mm) collected by P. Sprouse, 30 January 2017 (Zara # 9450, TMM # 91,955). - Bell County , Middle Robertson Spring (Robertson Springs Complex) (30.9444, -97.5410): female (4.2 mm) collected by Andy Gluesenkamp, 12 Dec 2009; juvenile (1.7 mm) collected by RG and Pete Diaz (PD), 30 July 2015; male (2.7 mm) collected by PD, 26 April 2017. Sam Bass Spring (Robertson Springs Complex) (30.9444, -97.5410): male (4.5 mm) collected (drift net) by PD and RG, 1 Mar 2016. Upper Ludwigia Spring (Robertson Springs Complex) (30.9444, -97.5411): male (3.1 mm), collected (drift net) by PD, 20 January - 6 June 2017. Creek Spring (Robertson Springs Complex) (30.9445, -97.5414): male (3.0 mm) collected by PD, 12 April 2018; 2 males (4.1 and 3.2 mm) collected by PD, 27 November 2018. Big Boiling Spring (Downtown Springs Complex) (30.9437, -97.5366): male (3.2 mm) collected by RG and PD, 25 September 2015. Anderson Spring (Downtown Springs Complex (30.9441, -97.5346): damaged specimen (2.9 mm) collected (drift net) by PD, 1 November 2016 - 20 January 2017. Whole paratypes are deposited in the National Museum of Natural History, Smithsonian Institution ( USNM accession # 1638682-1638690) and Texas State University Aquifer Biodiversity Collection (ABC accession # 0021059-002171) .

Diagnosis.

Small stygobitic species similar to most members of the hubbsi group with mature females larger than males, gnathopod 2 larger than gnathopod 1, posterior margin of gnathopod 1 typically shorter than palm, and telson as long as broad with shallow notch in apical margin. Distinguished from other hubbsi group species by gnathopods 1 and 2 with 2 rows of 3 singly inserted setae on the inner palm, and combination of following characters: gnathopod 1 with row of 4 to 5 setae posterior to the defining angle (compare to S. quatsinensis Holsinger & Shaw, 1986 with 2 or 3 postmarginal setae, 3 hubbsi group species have 1 or 2 postmarginal setae and all others are lacking); pereopods 6 and 7 with broadly expanded bases and distinct distoposterior lobes (similar to S. salturus Wang & Holsinger, 2001); and uropod 3 with single, slightly distal peduncular seta (similar to S. lanensis Wang & Holsinger, 2001).

Description.

Female (4.6 mm). Antenna 1 (Fig. 5 , A1): 39% length of body, ca. 1.7 × longer than antenna 2; primary flagellum with 10 segments, aesthetacs on segment 3-10; accessory flagellum 2 segmented reaching up to flagellar segment 2. Antenna 2 (Fig. 5 , A2): flagellum with 3 segments. Mandibles (Fig. 5 , lMdb, rMdb): left mandible incisor 5-dentate, lacinia mobilis 5-dentate, with 5 robust, plumose accessory setae, molar protruding and non-triturative with tuft of setae on anterior margin, blade-like teeth and single long rastellate seta. Right mandible incisor 5-dentate, lacinia mobilis 3-dentate, 1 serrate, 1 forked apically, and last with row of denticles and long setae at base; accessory setae row with 2 robust serrate setae and tufts of long setae basally; molar and palp similar to left mandible. Palp 3-segmented, segment 2 and 3 subequal; segment 2 with 2 alpha and 1 beta setae, fringe of small hair-like setae lining inner margin and fringe of pubescence lining outer distal margin; segment 3 with 1 B seta, 5 E setae, 6 C setae, small hair-like D setae, A setae absent, and fringe of pubescence lining outer distal margin. Lower lip (Fig. 5 , Li): covered in fine hairs with inner lobes poorly developed. Maxilla 1 (Fig. 5 , Mx1) inner plate with 4 plumose setae; outer plate with 7 comb-spines (1 with inner medial margin lined with setae, 2 serrate, and 4 gaffate); palp 2-segmented, with 3-4 setae (2 apical, 1-2 subapical) and ca. 10 small hair like setae. Maxilla 2 (Fig. 5 , Mx2) inner plate with 13 setae, 7 longer plumose setae along inner margin and 6 shorter apical setae; outer plate with 9 setae (mostly finely serrate, and 1 with apical minute setule) and an additional shorter seta on outer margin. Maxilliped (Fig. 5 , Mp) inner plate with 3 apical spine-teeth, 2 plumose submarginal setae, and 2 longer setae arising at base; outer plate with 8-9 long simple setae and 1 outer apical seta with apical marginal setules; palp 4 segmented, second segment about 2 × longer and wider than third, segment 1 with seta on inner margin, segment 2 with 6-8 setae on inner margin, 2 of these finely serrate, apical seta longer than rest, segment 3 with 8-11 setae, a few finely serrate and many forked apically, a densely pubescent patch located on dorsal face from midline to outer margin of segment, segment 4 with 2 setae apically at base of nail, one seta on outer margin medially, nail subequal in length to segment 4.

Gnathopod 1 (Fig. 6 , G1): propodus shorter than that of gnathopod 2; coxal plate longer than broad, with 1 ventral and 2 anterior marginal setae; basis with 5 long setae inserted along posterior margin; ischium with 2 setae and scale patch along posterior margin; merus with 9 posterodistal setae and patch of pubescence; carpus 36% length of propodus, with 9 setae; propodus 1.5 × longer than widest point with 3 superior and 3 inferior medial setae, palm slightly convex with 10 inner and 11 outer bifurcate robust setae, 2 of these posterior to defining angle, defining angle rounded and sub angular, margin posterior to defining angle ca. half the length of the palm with 4-5 setae and scaly patch becoming finer at defining angle and granular along inner margin of dactylus depression; dactylus with 1 seta on outer margin and 3 on distal half of inner margin; dactylus nail ca. 21% length of dactylus.

Gnathopod 2 (Fig. 6 , G2): propodus 1.3 × longer than that of gnathopod 1; coxal plate quadrate, with 1 ventral and 2 anterior marginal setae; basis with 1 long seta on anterior margin, shorter seta on posterodistal margin; ischium with 1 seta and scale patch on posterior surface; merus with 2 posterodistal setae, scale patch on posterior surface; carpus 25% length of propodus, with 3 distal setae, 7 setae and scale patch on posterior surface; propodus ca. 2 × longer than widest point with 3 superior and 3 inferior medial setae, palm straight to slightly convex with 11 inner and 13 outer bifurcate robust setae, 2 of these posterior to defining angle and 3 sets of paired setae, 2 of these posterior to defining angle, defining angle rounded, margin posterior to defining angle ca. half the length of the palm with scaly patch on poster margin becoming finer at defining angle and granular along inner margin of dactylus depression; dactylus with 1 seta on outer margin; dactylus nail ca. 15% length of dactylus.

Pereopod 3 (Fig. 7 , P3): subequal in length to pereopod 4; coxal plate subquadrate with 2 apical setae; merus 1.2 × longer than carpus; propodus 1.4 × longer than carpus, 2 setae on outer distal margin; dactylus ca. 30% length of propodus, apical sensory seta on outer margin. Pereopod 4 (Fig. 7 , P4): coxal plate deep, quadrate with 3 ventral marginal setae, reaching more than 50% length of basis; merus subequal in length to carpus; propodus 1.3 × longer than carpus, 2 setae on outer distal margin; dactylus ca. 30% length of propodus, apical sensory seta on outer margin. Pereopod 5 (Fig. 7 , P5): coxal plate bi-lobate with single seta on anteroventral margin; basis broad with posterior margin slightly convex; merus 75% length of carpus; propodus subequal to carpus, 4 setae on outer distal margin, 1 of these longer than dactylus; dactylus ca. 30% length of propodus, apical sensory seta on outer margin. Pereopod 6 (Fig. 7 , P6): subequal in length to pereopod 7 and 25% longer than pereopod 5; coxal plate bi-lobate with single seta on posteroventral margin; basis broad with posterior margin convex; merus 90% length of carpus; propodus 1.2 × longer than carpus, 1 seta on outer distal margin; dactylus ca. 30% length of propodus, apical sensory seta on outer margin. Pereopod 7 (Fig. 7 , P7): coxal plate ovate with single seta on ventral margin; basis broad with posterior margin convex with well developed distoposterior lobe; merus 85% length of carpus; propodus 1.2 × longer than carpus, 1 long and 2 shorter setae on outer distal margin; dactylus ca. 30% length of propodus, apical sensory seta on outer margin.

Coxal gills on somites 2-7, stalked and subovate, smallest on pereopod 7 (Fig. 7 , P3-P7). Brood plates present on somites 2-5 (Fig. 6 , G2; Fig. 7 , P3-P5) bearing long distal setae (not illustrated). Sternal gills absent but small processes sometimes visible on sternites between pereopods 3-7.

Pleonal plates (Epimera) (Fig. 7 , Plp): posterior margins weakly convex with single setule; distoposterior corners weakly rounded; ventral margin weakly convex in plate 1 and 2 and nearly straight in plate 3, with single marginal seta on plate 2 and 2 setae on plate 3. Pleopods biramous, rami subequal, outer rami 7 segmented, inner rami 8 segmented, bearing many plumose setae, typical for genus (Fig. 6 , Pl1). Uropod 1 (Fig. 6 , U1): peduncle with 9-10 robust setae; inner ramus ca. 70% length of peduncle, slightly longer than outer ramus, armed with 3 inner and 5 apical robust setae; outer ramus with 1 inner and 6 apical robust setae. Uropod 2 (Fig. 6 , U2): peduncle with 4 robust setae; inner ramus subequal to peduncle, ca. 30% longer than outer ramus, armed with 5 inner and 4 apical robust setae; outer ramus with 2 inner and 4 apical robust setae. Uropod 3 (Fig. 6 , U3): peduncle with single slightly distal seta; 2ramus ca. 51-55% length of peduncle, with 2 apical setae. Telson (Fig. 6 , Tl): quadrate (subequal in length and width) with apical v-shaped notch with depth ca. 20% length of telson, 8 to10 apical setae and bearing tufts comprised of 2 sensory setae on each lateral side.

Male (3.1 mm). Closely resembles female but typically smaller, with fewer setae on appendages, and differing in the following characters: Antenna 1 proportionally longer, up to 46% length of body; primary flagellum with 9 segments. Uropod 2 inner and outer rami subequal in length.

Etymology.

Epithet Stygobromus bakeri honors David Baker, a citizen scientist, conservationist and visionary who has worked for decades to bring together scientists, politicians, funders, regulators and stakeholders from near and far to conserve the watershed of Jacob’s Well. David lived immediately over the underwater cave for some time, working hard to make his own property environmentally friendly and to educate those who visited. He formed the Wimberley Valley Watershed Association, a non-profit with the mission to protect Jacob’s Well and Cypress Creek.

Distribution and ecology.

To date, this small species has been collected in 2 karst springs in southwestern Hays and northwestern Comal counties, Texas, a single karst spring 50 km northeast, on the shoreline of the Colorado River (usually emerging beneath a boat dock in Lake Austin) in Austin, Travis County, Texas and 2 karst spring complexes 74 km further northeast in Salado, Bell County, Texas (Fig. 1 ). All 4 sites are hydrogeologically distinct.

Jacob’s Well (Fig. 2A-C ) is a natural spring and cave located in the bed of Cypress Creek with enterable passage typically 1-2 m in diameter, and mapped using cave SCUBA to about 42 m deep and 1.6 km in length. Water issues out of the cave from Glen Rose Limestone (Middle Trinity Aquifer) and flows down Cypress Creek: a tributary of the Blanco River ( Gary et al. 2019). Stygobromus bakeri sp. nov. was only sampled successfully by hand, with divers using basters, dipnets, and centrifuge vials. Typically, the species was seen in the water column or on the floor, which consisted of silt, breakdown and cobble. Drift nets, bottle traps and cotton lures were not effective at capturing this species at Jacob’s Well. It was seen only in a deeper portion of the cave between 70 m and 215 m penetration. At 70 m penetration, the cave passage descends from -30 m to -42 m through a vertical slot called "The Knife Edge", beyond which, S. bakeri sp. nov. was collected. But beyond 215 m, the passage ascends back to 30 m and the species was not observed in that section. The habitat was not obviously different in the areas where the species was observed, with the exception of the entrance area (within 70 m of the surface) that has both surface influence and passage constrictions with greater flow, leading to smooth, scoured rocks and less silt.

John Knox Spring (Fig. 2D, E ) emerges from Lower Glenn Rose Limestone (Middle Trinity Aquifer) through a 0.3 m vertical crevice-like orifice 1 meter below the water surface and along the bank of a limestone grotto pool forming the headwaters of Carper’s Creek which discharges into the Blanco River. Interstitial surface insect fauna collected from this spring along with Stygobromus russelli (the most widespread Stygobromus species in central Texas) suggests that some portion of the flow might arise from hyporheic or vadose origin. Details of this habitat are described in Külköylüoğlu and Gibson (2018).

Robertson Springs (Fig. 3A-D ) and Downtown Spring Complex (Fig. 3E, F ) are 0.4 km apart, located on opposite sides of Interstate Highway 35 in the city of Salado. Both spring complexes comprise several spring orifices issuing from Edwards and Comanche Peak limestones forming spring runs that empty into Salado Creek. A diverse interstitial groundwater fauna (Table 1 ) including relatively widespread species has been recorded from these springs. Stygobromus bakeri sp. nov. was initially sampled by drift netting at Middle Robertson Springs ( Norris et al. 2012). Robertson Springs has numerous spring openings (over 30 sites mapped during high flows) with variable discharge and substrate issuing from both banks and from under the 300 m long spring run. Stygobromus bakeri sp. nov. was collected at 4 spring sites throughout the run at different spring zones within the Robertson Springs complex ( Diaz et al. 2016; Diaz and Warren 2019). The upper springs cease flowing seasonally and often produce troglofauna (terrestrial cave fauna) when flows resume after adequate rain events, while the lower springs are perennial and occasionally produce troglofauna. The Downtown Spring Complex is more discrete than Robertson Springs, with each of the 7 spring sites issuing from a single origin. Most of the springflow issues from Big Boiling Spring and flows down a short 5 meter spring run that empties into Salado Creek. Anderson Spring is 350 m downstream of Big Boiling Spring, has much less flow, issues from a fissure underneath a spring run (250 m downstream of the headwater Critchfield Spring), and flows 21 m before joining Salado Creek ( Norris et al. 2012). Hydrological studies of this area in the Northern Segment of the Edwards Aquifer showed the Downtown Spring Complex to be part of an integrated fracture system with groundwater flow velocities recorded at 1.8 m/sec and confirmed mixing of groundwater and Salado Creek surface water at Big Boiling Spring ( Wong and Yelderman 2017).

Mormon Spring # 3 (Fig. 2F, G ), which normally lies beneath Lake Austin on the Colorado River, issues from Fredericksburg Group consolidation (limestone, dolomite, chert, and marl). Lake Austin was constructed in 1940, replacing the damaged Austin Dam, which may have also inundated the spring. From time to time, the Lower Colorado River Authority lowers the level of Lake Austin to allow for dock maintenance. During the last such lowering of 3 m in January 2017, multiple spring complexes were visible over a stretch of about 100 m along the east shore of the lake. North to south, Mormon Springs numbers 1 and 2 emerged from sediment, without visible bedrock apertures. Mormon Spring # 3 consists of multiple bedrock outlets in a cutbank excavated in bedrock below a boat dock. The spring appears associated with the Trinity Aquifer but is located near the Edwards Aquifer boundary and could potentially be a gravity spring from local recharge intersected by the Colorado River. Alternatively, this spring could represent flow from the north, from the Balcones Escarpment. No dye tracing has been conducted at this spring to characterize its springshed.

Stygobromus bakeri sp. nov. co-occurs with S. russelli (all sites), S. bifurcatus ( Jacob’s Well, Salado Springs, and Mormon Springs), and a large undescribed species of Stygobromus in the Stygobromus flagellatus group ( Jacob’s Well). The presence of 4 Stygobromus species at Jacob’s Well is notable, and presumably all 4 species occupy different ecological niches. Due to its unusually small size and robust body, S. bakeri sp. nov. is able to inhabit both deep cave and shallow spring habitats. It may also occupy interstitial habitats such as the hyporheic zone of surface streams, although it was not collected in hyporheic samples from Carper’s Creek, near John Knox Spring. The small adult size and general stout teardrop body shape is reminiscent of Seborgia relicta Holsinger, 1980 and S. hershleri Holsinger, 1992 found in groundwater habitats (deep aquifer caves and wells, springs, and hyporheos) throughout the Edwards Plateau and Balcones Escarpment. Seborgia has not been found associated with the Glen Rose formation of the Trinity Aquifer nor with the northernmost section of the Edwards Aquifer extending north of the Colorado River in Austin to Salado where Stygobromus bakeri sp. nov. occurs (Fig. 1 ). The furthest northern record for Seborgia relicta is from Cold Spring issuing from the Edwards Aquifer on the southern bank of the Colorado River (Ladybird Johnson Lake) only 4 km downstream of Mormon Spring #3. Groundwater fauna records for Stygobromus bakeri sp. nov. sites (Table 1 ) include interstitial and relatively widespread, ecological generalist stygobionts and crenobionts, yet widespread cavernicolous stygobionts such as Cirolanides spp. isopods are absent at these sites ( Holsinger 1967; Lewis 2000; Krejca 2009; Gibson et al. 2018; Hutchins 2018; Devitt et al. 2019; Schwartz et al. 2019; Alvear et al. 2020). Many of the springs have been thoroughly sampled and regularly monitored for multiple years, so the stygofauna is better characterized relative to many other Texas groundwater sites.

Figures

Figure 1. Stygobromus distribution in Texas. Red square expanded to view central Texas species. Individual undescribed species represented by S. sp. and S. spp. (Stygobromus flagellatus) refers to several undescribed site endemic species in the Stygobromus flagellatus group (Hutchins 2018).

Figure 2. Comal, Hays, and Travis counties, Texas spring sites for Stygobromus bakeri sp. nov. Jacob's Well A cave entrance B second constriction point (30 m water depth), Stygobromus bakeri sp. nov. were found deeper than this point C drift net at first constriction point (23 m water depth). John Knox Ranch D Blue Hole, perennial headwaters of Carper's Creek E John Knox Spring issuing within Blue Hole. Mormon Spring 3 F Spring exposed under boat dock during drought G drift net on spring site. Photograph credit: Jean Krejca (A-C); courtesy of John Knox Ranch (D); Randy Gibson (E); Peter Sprouse (F, G).

Figure 3. Salado, Bell County, Texas spring sites for Stygobromus bakeri sp. nov. Robertson Spring Complex A Beetle Spring B Middle Spring C Ludwigia Spring D Creek Spring upwelling below water surface. Downtown Spring Complex E Big Boiling Spring F Anderson Spring. Photographs by Pete Diaz.

Figure 4. Stygobromus bakeri sp. nov. collected live from Jacob's Well. Photograph by Jean Krejca.

Figure 5. Stygobromus bakeri sp. nov., Holotype ♀ (4.6 mm), John Knox Spring, Comal Co., Texas A 1 antenna 1 A 2 antenna 2 Li lower lip Mp maxilliped Mx 1 maxilla 1 Mx 2 maxilla 2. Paratype ♂ (3.1 mm), John Knox Spring, Comal Co., TX. lMdb left mandible and mandibular palp rMdb right mandible. Scale bars: 0.05 mm (Li, Mp, Mx 1, Mx 2, lMdb, rMdb); 0.1 mm (A 1, A 2).

Figure 6. Stygobromus bakeri sp. nov., Holotype ♀ (4.6 mm), John Knox Spring, Comal Co., Texas G 1, G 2 gnathopods (G 1 palmer margin enlarged 2 x) 1 - 2 U 1 - U 3 uropods 1 - 3 Pl 1 pleopod 1 Tl telson. All body parts drawn to same scale.

Figure 7. Stygobromus bakeri sp. nov., Holotype ♀ (4.6 mm), John Knox Spring, Comal Co., Texas. P 3 - P 7 pereopods 3 - 7 Plp pleonal plates (epimera).