The new amphipod was known only from the urban seep where its type specimens were collected, making the site both the basis for its description and the principal limit on what is known about its ecology and geographic range. Stygobromus is a Holarctic genus of obligate subterranean amphipods associated with groundwater habitats. The source study noted that the genus contains about 137 described species and several undescribed forms, with its greatest diversity in the Nearctic.

The Piedmont and Atlantic Coastal Plain of Maryland, Virginia and the District of Columbia are especially rich in these organisms. Thirteen species had previously been described from that region, and seven had been documented in hypotelminorheic habitats and associated seepage springs in and near the Washington metropolitan area. The description of S. anacostensis brings the regional total of described species to 14 and the number documented from hypotelminorheic habitats in and near metropolitan Washington to eight.

The type locality is a small hypotelminorheic seep at the extreme end of Shepherd Parkway, a unit of National Capital East managed by the National Park Service. Hypotelminorheic habitats are shallow subterranean environments characterized by small flows of water in slight depressions lined with decaying leaves. Seepage springs drain small areas, often less than a hectare, and the associated habitat reaches only a few metres in depth.

The Washington-area census in which the specimens were collected had identified more than 150 seeps. At the Anacostia Park site, groundwater emerges along a two-metre-high rock face approximately five metres from Malcolm X Avenue, within a narrow strip of parkland bordered by the road. The site was discovered after a park ranger noticed extensive ice on the adjacent sidewalk, indicating that water was flowing from the seep even under cold conditions.

Most S. anacostensis individuals were found in the water flowing over moss-covered rocks, while a few occurred in decaying leaves at the base of the rock face. The seep retains water through most of the year. The new species co-occurs with the groundwater isopod Conasellus, also identified in the study as Caecidotea kenki, but it is not known to share the seep with another Stygobromus species.

Little is currently known about its ecology or life history. The authors noted that S. tenuis potomacus occurs at another seep about one kilometre away in Anacostia Park, although the two sites lie in different HUC10 drainages and the authors did not establish that the two species co-occur. Recognition of the new species depended on a suite of small anatomical characters examined under microscopes.

The male holotype measured 5.9 millimetres from the rostrum to the base of the telson, while the female allotype measured 5.3 millimetres. Another diagnostic trait was the presence of rastellate, or brush-like, setae on the posterodistal margin of the second gnathopod’s carpus. The researchers also documented differences in the antennae, mandibles, mouthparts, walking legs, gills, pleopods, uropods and telson, including the numbers and arrangement of specialized setae.

The diagnosis separates S. anacostensis from four close members of the S. tenuis group. Its second antenna is equal to or shorter than its first, distinguishing it from S. tenuis tenuis. It has only two C-setae and up to eight E-setae on the mandibular palp, whereas S. tenuis potomacus has more E-setae.

The weakly convex posterior margins of the bases of walking legs five through seven and the distally tapering telson distinguish it from S. allegheniensis. Less spinose first and second uropods and fewer robust setae at the telson apex separate it from S. hayi. The serrate gnathopod edge was clearest after digestion and staining, but it was also visible in undigested and juvenile specimens, indicating that it was not an artefact of the laboratory treatment.

For the genetic analysis, the researchers amplified 2,738 base pairs across two mitochondrial loci—cytochrome oxidase subunit 1, or co1, and 16S ribosomal DNA—and three nuclear loci—histone H3, 18S ribosomal DNA and 28S ribosomal DNA. Maximum-likelihood and Bayesian phylogenetic analyses produced similar trees. Analyses of mitochondrial data, whether alone or combined with nuclear data, placed the Anacostia population apart from other sampled members of the S. tenuis group with high bootstrap support.

S. tenuis potomacus was its closest sampled relative. Average uncorrected mitochondrial co1 distance was 6.5 percent between S. anacostensis and populations at Caledon State Park and 12.6 percent between S. anacostensis and the nearest sampled S. tenuis potomacus population in Anacostia Park, separated by about 10 kilometres. The nuclear loci showed less overall variation but still contained two fixed differences at h3 and one at 18s between the new species and that nearby population.

Three molecular species-delimitation methods were applied to the mitochondrial dataset. Automatic Barcode Gap Discovery identified nine molecular operational taxonomic units, and Poisson Tree Processes returned the same divisions. Multi-rate Poisson Tree Processes identified seven units, but all specimens assigned to S. anacostensis again formed one group.

These methods also divided several sampled S. tenuis potomacus populations into separate candidate units, consistent with the finding that those populations did not form a single monophyletic group in the phylogenetic analyses. The agreement among the delimitation approaches supported the recognition of the Anacostia population as distinct, although the analysis also indicates that additional evolutionary lineages may remain to be investigated within the broader species group. Because the species is known from only one small seep, its extent of occurrence and area of occupancy were calculated as extremely small.

The researchers assigned it a NatureServe rank of Critically Imperiled, or G1, and assessed it as Critically Endangered under criterion B1 of the International Union for Conservation of Nature Red List because of its extremely restricted range. Its location on National Park Service land provides some protection, but the affected strip is narrow and the authors identified human intrusion, disturbance, habitat degradation, pollution, road salt and changes to drainage near the sidewalk as major threats. The conservation assessment therefore concerns a species known from one urban locality; it does not establish trends in population size, and no abundance estimate was reported.

The discovery adds another example of the genetic diversity found in groundwater organisms within the Washington region. Earlier phylogenetic work had found up to 13.7 percent uncorrected mitochondrial co1 divergence among populations of the S. tenuis group in Virginia and Washington, D.C., while a later assessment cited 9.2 percent divergence between S. tenuis potomacus populations only 7.2 kilometres apart in a straight line. The new species adds a morphologically and genetically distinct member of that assemblage.

Its description also illustrates the potential for additional species to be missed where morphologically similar groundwater populations are separated by substantial mitochondrial sequence divergence. The authors cautioned that the serrate gnathopod feature may occur in other Stygobromus species but had not previously been documented, making re-examination of that structure across the genus necessary to determine its full diagnostic value.

Subject of Research: Morphological, phylogenetic and conservation description of a new Stygobromus amphipod species from a Washington, D.C., seepage spring

Article Title: A new species of Stygobromus Cope, 1872 ( Amphipoda , Crangonyctidae ) from a hypotelminorheic seepage spring in Washington, D.C., USA

Article References: Niemiller, M. L., Cannizzaro, A. G., Sawicki, T. R., & Culver, D. C. (2024). A new species of Stygobromus Cope, 1872 (Amphipoda, Crangonyctidae) from a hypotelminorheic seepage spring in Washington, D.C., USA. Subterranean Biology, 48, 117-146. https://doi.org/10.3897/subtbiol.48.112984

Image Credits: AI Generated

DOI: 10.3897/subtbiol.48.112984

Keywords: Stygobromus anacostensis, groundwater amphipod, new species, Washington DC, seepage spring, subterranean biodiversity, cryptic diversity, molecular phylogeny, species delimitation, urban conservation, species, Stygobromus