Stygobromus anacostensis Cannizzaro, Sawicki, & Niemiller, 2024

Niemiller, Matthew L., Cannizzaro, Andrew G., Sawicki, Thomas R. & Culver, David C., 2024, A new species of Stygobromus Cope, 1872 (Amphipoda, Crangonyctidae) from a hypotelminorheic seepage spring in Washington, D. C., USA, Subterranean Biology 48, pp. 117-146 : 117

Stygobromus anacostensis Cannizzaro, Sawicki, & Niemiller sp. nov.

Figs 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8

Type material.

Holotype: male 5.9 mm, from USA, Washington, District of Columbia, Anacostia Park, (38.83059°N, - 76.9995°W), deposited in the collection of the United States National Museum of Natural History, Smithsonian Institution, Washington, D.C (USNM 1606902); female allotype 5.3 mm (USNM 1606903). Holotype male and female allotype collected 18 October 2021 by Lizzy Sartain.

Paratypes: 1 male (USNM 1606904) and 2 females (USNM 1606905-1606906) collected on 18 October 2021 by Lizzy Sartain from type locality; 1 female collected on 20 September 2021 by Lizzy Sartain from the type locality (USNM 1606907).

Etymology.

The specific epithet anacostensis refers to its occurrence in Anacostia neighborhood in Washington, D.C., USA. It is part of the Anacostia River drainage.

Type locality.

USA. Washington, District of Columbia, hypotelminorheic seepage spring in a highly urbanized area that emerges from a small, 2-m high rockface ca. 5 m from Malcolm X Avenue SE in Shepherd Parkway (Figs 9 , 10 , 38.83059°N, - 76.9995°W). Shepherd Parkway is part of National Capital Parks East. Most individuals were collected in the water flowing over moss-covered rocks. A few individuals were also present in decaying leaves at the base, a more usual hypotelminorheic habitat ( Culver et al. 2006, 2012). The site is at the extreme tip of Shepherd Parkway, a unit of National Capital East (National Park Service). The width of park land is about 20 m and is bordered by Malcolm X Avenue. The site was discovered when a park ranger noticed extensive ice on the adjacent sidewalk resulting from flow from the seep.

Diagnosis.

Small stygomorphic species distinguished from other members of the Stygobromus tenuis group by size, largest male 5.9 mm, largest female 5.3 mm and as follows: S. tenuis tenuis - by antenna 2 subequal to or shorter than antenna 1; S. tenuis potomacus - only 2 C-setae on mandibular palp and up to 8 E-setae; S. allegheniensis - pereopods 5-7 basis posterior margin weakly convex, and telson tapering distally; S. hayi - by significantly less spinose uropods 1 and 2, and telson with significantly fewer apical robust setae. Further distinguished from all Stygobromus tenuis group species by gnathopods 1 and 2 with a serrate blade-like edge running the length of both palms, and by the posterodistal margin of gnathopod 2 carpus possessing rastellate seta(e).

Description.

Male: holotype, USNM 1606902 (Fig. 1A ); Size 5.9 mm.

Antennae. Antenna 1 (Fig. 2A ): 45% body length, 60% length of antenna 2 (in paratype (USNM 1606904); primary flagellum with 18 segments, aesthetascs on most segments, aesthetascs shorter than respective segments; accessory flagellum 2-segmented, reaching beyond first segment in length.

Antenna 2 (Fig. 2B ): damaged in holotype, description based on paratype (USNM 1606904); gland cone distinct; peduncle 80% length of flagellum, with weak plumose setules concentrated on postero and anterodistal margins of segments 4 and 5, peduncle segment 4 subequal in length to 5; flagellum 12-segmented, segment 5 with robust seta on anterodistal margin and segments 6 and 7 with robust seta placed along posterodistal margins, small calceoli-like structures apically on flagellar segments 5-12.

Mouthparts (Figs 2C, D , 3 ). Mandibles: left mandible (Fig. 2C ) incisor 5-dentate, lacinia mobilis 5-dentate, with 7 robust serrate and numerous plumose accessory setae; molar process reduced with simple seta; palp with 3 segments, second segment 85% length of third, with inner margin bearing 8 setae and sparse fine setae; segment 3 with 2 C-setae, 5 E-setae, 1 B-seta, and 8 plumose D-setae, lacking A-setae; face of article covered in numerous, fine pubescent setae.

Right Mandible (Fig. 2D ): incisor 4-dentate, lacinia mobilis bifurcate, both lobes with numerous protuberances; accessory setae row with 4 robust, serrate setae and numerous plumose setae; molar process reduced with simple seta. Palp with 3 articles, relative articles length and setation patterns as in left mandible.

Upper Lip (Fig. 3A ): rounded, apical margin of labrum with numerous fine setae. Lower Lip (Fig. 3B ): inner lobes distinct; outer margin of outer lobe sparsely covered in fine setae; inner margin of outer lobe heavily setose.

Maxilla 1 (Fig. 3C ): missing in holotype, description based on paratype (USNM 1606904); inner plate with 4 plumose marginal setae and numerous fine, pubescent setae covering entire plate; outer plate with 7 apical comb spines, pubescence covering inner margin, decreasing laterally and proximally; palp with 2 segments, distal segment covered in pubescence; subapical margin of distal article with 3 long setae, apical margin with 4 setae.

Maxilla 2 (Fig. 3D ): missing in holotype, description based on paratype (USNM 1606904); both inner and outer plates covered in pubescent setae; outer plate not as wide as inner plate, not narrowing distally, with numerous distal setae; inner plate narrowing slightly distally, with numerous apical setae and 3 large plumose facial setae.

Maxilliped (Fig. 3E ): inner plate shorter than outer plate, with 4 naked cuspidate setae 3 setae along apical margin, surface of plate covered in fine pubescence; outer plate armed with numerous setae covering inner and apical margins; palp second segment with numerous marginal setae, third article with numerous marginal/submarginal setae; dactyl with 2 outer setae and 2 inner setae.

Gnathopods. Gnathopod 1 (Fig. 4A ): coxal plate with 3 apical setae; basis with long setae inserted along anterior, and posterior margins, small patch of pubescence on posterodistal corner; ischium with 4 setae and pubescence along posterior margin; merus weakly pubescent along posterior surface, numerous distal setae, and robust seta along anterior margin; carpus approximately 50% length of propodus with robust seta along anterior margin and a group of setae on anterodistal margin, one of which is approximately 50% length of propodus, posterior margin with single group of plumose setae and 6 submarginal setae directed distally; propodus 1.3 × longer than broad, with 1 marginal anterior seta, 4 superior medial setae, with middle group of medial setae paired, 4 setae inserted at anterodistal corner, 6 inferior medial setae and numerous plumose posterior setae; palm oblique, concave, with serrate blade-like edge running the length, 7 outer and 7 inner bifid robust setae, 6 outer setae, and inner seta; inner margin of defining angle with 3 bifid robust setae, outer margin with 4 bifid robust setae; dactylus with outer seta and 7 short setae covering the entire inner margin and 3 setae placed along the inner margin at base of nail.

Gnathopod 2 (Fig. 4B ): coxal plate with 4 apical setae and facial seta; basis with long setae inserted along anterior, and posterior margins, small patch of pubescence on posterodistal corner; ischium with 3 setae and pubescence along posterior margin; merus with pubescence covering posterior surface and 4 posterodistal setae and robust seta along anterior margin; carpus approximately 75% length of propodus, with robust seta along anterior margin and two setae on anterodistal margin, one of which is approximately 33% length of propodus, posterior margin with 4 groups of plumose setae, distal-most bearing 3 rastellate setae, and 3 submarginal setae directed distally; propodus 1.3 × longer than broad, with marginal anterior seta, 5 superior medial setae, distal-most paired, 5 setae inserted at anterodistal corner, 5 inferior medial setae, proximal-most paired, and 8 groups of plumose setae along posterior margin; palm oblique, straight, with serrate blade-like edge running the length, 5 outer and 5 inner bifid robust setae, 5 outer setae, and 2 inner setae; inner margin of defining angle with 6 bifid robust setae, outer margin with 5 bifid robust setae; dactylus with outer seta and seta placed along the inner margin at base of nail.

Pereopods. Pereopod 3 (Fig. 5A ): coxal plate with 5 apical setae; merus 1.4 × longer than carpus, carpus approximately 85% of propodus in length; dactylus approximately 50% length of propodus, with plumose seta on posterior margin, 2 setae along anterior margin followed by thin seta on medial margin.

Pereopod 4 (Fig. 5B ): subequal to pereopod 3 in length; coxal plate armed with 4 anterior and 3 posterior apical setae; merus approximately 1.6 × longer than carpus; carpus approximately 60% length of propodus; dactylus approximately 40% length of propodus, setation as in pereopod 3.

Pereopod 5 (Fig. 5C ): coxal plate large, bilobate with distinct anterior and posterior lobes, posterior lobe with 4 robust setae on distal corner; basis posterior margin weakly convex with 9 shallow serrations, anterior margin with 6 split-tipped robust setae and 3 distal split-tipped setae; merus subequal in length to carpus; carpus subequal to propodus, dactylus approximately 50% length of propodus, setation as in pereopod 4.

Pereopod 6 (Fig. 5D ): coxal plate bilobate, with weakly produced anterior lobe, posterior lobe bearing 2 robust apical setae; basis posterior margin weakly convex with 8 serrations, anterior margin with 5 split-tipped robust setae, and 3 robust setae at anterodistal corner; merus approximately 1.2 × length of carpus; carpus approximately 90% of propodus in length, dactylus approximately 50% length of propodus, setation as in pereopod 5.

Pereopod 7 (Fig. 5E ): coxal plate small, subtriangular, with 4 posterior setae; basis posterior margin weakly convex with 10 serrations and straight distal corner, anterior margin with 8 split-tipped robust setae, and 2 robust setae at anterodistal corner; merus subequal in length to carpus; carpus approximately 80% length of propodus, dactylus approximately 40% length of propodus, setation as in pereopods 5, 6.

Gills (Fig. 5F ). coxal gills on somites 2-6, somites 6 and 7 with bifurcate sternal gills.

Pleon. Epimera (Fig. 6A ): first epimeron ventral margin with robust seta, distoposterior corner rounded, posterior margin with 2 setae. Second epimeron ventral margin with 3 robust setae, distoposterior corner rounded, posterior margin with 2 setae. Third epimeron ventral margin with 3 robust setae, distoposterior corner rounded, posterior margin with 2 setae.

Pleopods: pleopod 1 (Fig. 6B ) peduncle lacking setae, with 2 coupling hooks; outer, inner rami with 8 and 11 segments respectively, basal segment of outer ramus with clothes-pin setae. Pleopod 2 peduncle lacking setae, with 2 coupling hooks; outer, inner rami with 7, 11 segments respectively, basal segment of outer ramus with clothes-pin setae. Pleopod 3 outer, inner rami with 7, 7 segments respectively, basal segment of outer ramus with clothes-pin setae.

Urosome. Mostly bare, with sparse setae covering dorsal surface. Uropod 1 (Fig. 6C ): peduncle 1.4 × inner ramus in length, with 8 outer robust setae and inner robust seta(e), posteromedial margin with distinct protuberance approximately 20% of inner ramus in length, dorsal margin weakly serrate; outer ramus approximately 80% length of inner, with 2 inner and outer robust setae and 4 apical robust setae; inner ramus possessing 3 outer and two inner robust setae, and 5 apical robust setae.

Uropod 2 (Fig. 6D ): peduncle subequal in length to inner ramus, with 2 outer robust setae and inner robust seta; outer ramus approximately 88% length of inner ramus without robust setae along the inner and outer margins, and 4 apical robust setae; inner ramus with 2 outer and 2 inner robust setae, with 5 apical robust setae.

Uropod 3 (Fig. 6E ): small, shorter than telson, uniramous; peduncle 2 × length of ramus; ramus with 3 apical robust setae.

Telson (Fig. 6F ). Telson entire, elongated, 1.5 × longer than broad, weakly tapering distally; apex with 10 robust setae, and plumose seta, 2 plumose setae arise dorsolaterally from both outer margins.

Female: allotype USNM 1606903 (Fig. 1B ); Size 5.3 mm. Differing from male in several points, including, antennae; gnathopod shape and setation; uropods 1 and 2 shape and setation. Structures not described below are as in male.

Antennae. Antenna 1 (not illustrated, but see Fig. 1B ): 50% body length, 1.5 × longer than antenna 2; peduncle, flagellum lacking robust setae; primary flagellum with 16 segments. Antenna 2 (Fig. 7A ): gland cone distinct; peduncle 1.5 × longer than flagellum, with robust setae anteriorly, laterally on segments 3, 4, peduncle segment 4 subequal in length to segment 5; flagellum 7-segmented, without small calceoli-like structures apically on distal flagellar segments.

Gnathopods. Gnathopod 1 (Fig. 7B ): coxal plate with 3 apical and 2 facial setae; ischium with 2 setae and pubescence along posterior margin; carpus approximately 40% length of propodus with robust seta along anterior margin and a group of setae on anterodistal margin, one of which is approximately 50% length of propodus, posterior margin with single group of plumose setae and 4 submarginal setae directed distally; propodus 1.25 × longer than broad, with 1 marginal anterior seta, 3 superior medial setae, 3 setae inserted at anterodistal corner, 3 inferior medial setae and numerous plumose posterior setae; palm oblique, straight, with serrate blade-like edge running the length, 5 outer and 5 inner bifid robust setae, 4 outer setae, and inner seta; inner margin of defining angle with 3 bifid robust setae, outer margin with 4 bifid robust setae; dactylus with outer seta and 4 short setae covering the inner margin and 2 setae placed along the inner margin at base of nail.

Gnathopod 2 (Fig. 7C ): coxal plate with 6 apical setae and 2 facial setae; ischium with 2 setae and pubescence along posterior margin; merus with pubescence covering posterior surface and 4 posterodistal setae, without robust seta along anterior margin, and two robust setae along distal margin; carpus subequal in length to propodus, with robust seta along anterior margin and two setae on anterodistal margin, one of which is approximately 33% length of propodus, posterior margin with 3 groups of plumose setae, distal-most bearing rastellate seta, and 3 submarginal setae directed distally; propodus 1.1 × longer than broad, with marginal anterior seta, 3 superior medial setae, 4 setae inserted at anterodistal corner, 4 inferior medial setae, and 5 groups of plumose setae along posterior margin; palm oblique, straight, with serrate blade-like edge running the length, 3 outer and 3 inner bifid robust setae, 3 outer setae, and 1 inner seta; inner margin of defining angle with 4 bifid robust setae, outer margin with 4 bifid robust setae; dactylus with outer seta and 4 short setae covering the inner margin and seta placed along the inner margin at base of nail.

Gills and brood plates. Gills as in male with coxal gills on somites 2-6, somites 6 and 7 with bifurcate sternal gills (Fig. 8A illustrates somite 7). Brood plates early in development in allotype, present on somites 2-5.

Urosome. Uropod 1 (Fig. 8B ): peduncle 1.5 × length of inner ramus, with 6 outer robust setae and inner robust seta(e), posteromedial margin lacking protuberance; outer ramus approximately 90% length of inner, with 1 inner and outer robust seta, and 4 apical robust setae; inner ramus possessing 2 outer and inner robust seta(e), and 5 apical robust setae.

Uropod 2 (Fig. 8C ): peduncle subequal in length to inner ramus, with 2 outer robust setae and inner robust seta; outer ramus approximately 66% length of inner ramus with outer robust seta, and 4 apical robust setae, inner robust setae lacking.

Uropod 3 (Fig. 8D ): small, shorter than telson, uniramous; peduncle 1.5 × length of ramus; ramus with 4 apical robust setae.

Telson (Fig. 8E ). Telson entire, elongated, 1.5 × longer than broad, weakly tapering distally; apex with 9 robust setae, 2 plumose setae arise dorsolaterally from both outer margins.

Variation.

The new species was shown to vary slightly in several morphological characteristics, particularly between males and females (Table 3 ).

Molecular diagnosis.

Average uncorrected pairwise genetic distance at the mitochondrial co1 locus between S. anacostensis and the most closely related populations of S. t. potomacus sampled at Caledon State Park is 6.5%, with 32 fixed mutations separating the two taxa. Between S. anacostensis and the closest S. t. potomacus population (seepage spring near Malcolm X Ave in Anacostia Park; SP101), p-distance is 12.6%, with 67 fixed mutations. P-distance at the mitochondrial 16s locus between S. anacostensis and the populations of S. t. potomacus sampled at Caledon State Park is 2.3%, with eight fixed mutations. Nuclear loci exhibited low levels of variation among all S. tenuis species group taxa sampled; however, some diagnostic genetic variation was noted. Two fixed mutations in the h3 locus and one fixed mutation in the 18s locus exist between S. anacostensis and the closest S. t. potomacus population (SP101).

Geographical distribution.

The species is known to date only from the type locality in Shepherd Parkway, which is a 1200-acre national park located on the southern bank of the Anacostia River just upstream from where the river flows into the Potomac River (Fig. 9 ). Shepherd Parkway is part of National Capital Parks East (NACE).

Habitat and ecology.

Like other species of Stygobromus , S. anacostensis is a stygobiotic species occurring in groundwater habitats. All specimens have been collected from a seepage spring just off Malcolm X Avenue SE that marks the resurgence of hypotelminorheic groundwater at the surface (Fig. 10 ). Amphipods have been observed and collected from underneath moss-covered rocks, moss, and leaf litter on the small, 2-m high rockface as well as the small pool of the seepage spring. The seepage spring possesses water throughout most of the year Little is known regarding the ecology and life history currently. Stygobromus anacostensis co-occurs with the groundwater isopod Conasellus (= Caecidotea ) Stygobromus kenki (Bowman, 1967).

Conservation.

Stygobromus anacostensis is known only from the type locality. The NatureServe conservation rank calculated is Critically Imperiled (G1). Under IUCN Red List criteria, S. anacostensis was assessed as Critically Endangered (CR B1) because of an extremely small EOO and AOO (known from a single small seep) in an urban area. Major threats to the species include increased risk of human intrusion and disturbance, habitat degradation, and pollution. The type locality population is offered some protection by occurring on National Park Service land, but the area controlled by the NPS is very narrow, and the site is highly vulnerable to road salt as well as any attempt to “improve” the drainage in the vicinity of the sidewalk.

Genetic and phylogenetic analyses

We amplified in total 2,738 bp of five loci. Uncorrected mtDNA p-distance between S. anacostensis and populations of S. tenuis potomacus at Caledon State Park (SP104 and SP105) was 6.5% and 12.6% between S. anacostensis and the nearest S. t. potomacus population sampled in Anacostia Park (SP101). Average uncorrected nucDNA p-distance was substantially lower, averaging 0.001 between S. anacostensis and S. tenuis potomacus at Caledon State Park (SP104 and SP105), and 0.004 between S. anacostensis and the nearest S. t. potomacus population sampled in Anacostia Park (SP101).

The optimal substitution models for first, second, and third positions of co1 were TrNef+I ( Tamura and Nei 1993), F81 (Felsenstein 1981), and K81+I ( Kimura 1981), respectively. The optimal substitution model was HKY + I + G ( Hasegawa et al. 1985) for 16s, K80 ( Kimura 1980) for 18s, TIM2+I for 28s, and JC (Jukes and Cantor 1980) for h3. Phylogenetic tree topologies obtained for ML and Bayesian inference were highly similar. Phylogenetic trees reconstructed using both ML and Bayesian inference for the mtDNA (co1 + 16s; Fig. 11 ) and mtDNA+nucDNA datasets (Fig. 12A ) delimited individuals of Stygobromus anacostensis from the type locality as distinct from other populations of the S. tenuis species group sampled with high bootstrap support. Populations of S. tenuis potomacus did not form a monophyletic group for any dataset. Nuclear loci exhibited low levels of variation among all S. tenuis species group taxa sampled (Figs 12B , 13 ). Two fixed mutations in the h3 locus and one fixed mutation in the 18s locus exist between S. anacostensis and the closest S. t. potomacus population (SP101).

Species delimitation

For the mtDNA dataset (Fig. 11 ), the ABGD approach resulted in nine MOTUs, with convergence of initial and recursive partitions at prior intraspecific divergence (P) = 0.028, which remained stable until P = 0.0359. The PTP approach yielded the same MOTU delimitations. All S. anacostensis samples formed a MOTU, while several S. tenuis potomacus populations were delimited as distinct MOTUs. The mPTP approach estimated seven MOTUs, with highly similar designations to the ABGD delimitations. Stygobromus anacostensis individuals were grouped as a single MOTU, as were several S. tenuis potomacus populations. Stygobromus allegheniensis and S. hayi were grouped into a single MOTU.

Figures

Figure 1. Stygobromus anacostensis sp. nov., habitus: A holotype male, 5.9 mm (USNM 1606902) B Allotype female, 5.3 mm (USNM 1606903). Scale bar: 1 mm.

Figure 2. Stygobromus anacostensis sp. nov., Holotype male, 5.9 mm (USNM 1606902): A antenna 1 (single aesthetasc enlarged) C left mandible (palp omitted) D right mandible (lacinia mobilis enlarged). Paratype male, 5.7 mm (USNM 1606904): B antenna 2 (single calceolus enlarged). Scale bars: 0.5 mm (A, B); 0.25 mm (C, D).

Figure 3. Stygobromus anacostensis sp. nov., Paratype male, 5.7 mm (USNM 1606904): A upper lip D maxilla 2. Holotype male, 5.9 mm (USNM 1606902): B lower lip C maxilla 1 E maxilliped (distal margin of inner plate enlarged). Scale bars: 0.25 mm.

Figure 4. Stygobromus anacostensis sp. nov., Holotype male, 5.9 mm (USNM 1606902): A gnathopod 1 (palm and dactyl enlarged) B gnathopod 2 (rastellate seta, palm and dactyl enlarged). Scale bar: 0.5 mm.

Figure 5. Stygobromus anacostensis sp. nov., Holotype male, 5.9 mm (USNM 1606902): A pereopod 3 B pereopod 4 C pereopod 5 D pereopod 6 E pereopod 7 F bifurcate sternal gill located on somites 6 and 7. Scale bar: 0.5 mm.

Figure 6. Stygobromus anacostensis sp. nov., Holotype male, 5.9 mm (USNM 1606902): A epimera 1 - 3 B pleopod 1 (coupling spines and clothes pin seta enlarged) C uropod 1 (posteromedial protuberance enlarged) D uropod 2 E uropod 3 F telson. Scale bars: 0.25 mm.

Figure 7. Stygobromus anacostensis sp. nov., Allotype female, 5.3 mm (USNM 1606903): A antenna 2 B gnathopod 1 (palm and dactyl enlarged) C gnathopod 2 (rastellate seta, palm and dactyl enlarged). Scale bars: 0.25 mm (A); 0.5 mm (B, C).

Figure 8. Stygobromus anacostensis sp. nov., Allotype female, 5.3 mm (USNM 1606903): A coxa and basis of pereopod 7 showing placement of bifurcate sternal gill B uropod 1 C uropod 2 D uropod 3 E telson. Scale bars: 0.5 mm (A-C); 0.25 mm (D, E).

Figure 9. Distribution of Stygobromus anacostensis sp. nov. and other S. tenuis group species in the Washington D. C. area, USA

Figure 10. The type locality of S. anacostensis is a small hypotelminorheic seepage spring just off of Malcolm X Avenue, Shepherd Parkway, Washington, D. C., USA. Photograph by Jenna Keany.

Figure 11. Maximum-likelihood phylogeny and species delimitations of Stygobromus anacostensis and other S. tenuis species group taxa for the mtDNA dataset (co 1 + 16 s loci). Asterisk represents bootstrap node support greater than 90. Colored bars represented hypothesized MOTU groupings (i. e., species) based on corresponding delimitation analyses.

Figure 12. Maximum-likelihood phylogenies of of Stygobromus anacostensis and other S. tenuis species group taxa for the (A) mtDNA + nucDNA dataset (co 1 + 16 s + 18 s + 28 s + h 3 loci) and (B) nucDNA dataset (18 s + 28 s + h 3 loci). Asterisk represents bootstrap node support greater than 90.

Figure 13. Median joining networks for nuclear loci (18 s, 28 s, and h 3) generated in PopART v 1.7.