Benjamin T. Hutchins

Groundwater-obligate taxa (stygobionts) exhibit variable degrees of habitat specialization, ranging from those restricted to single micro-habitats to generalists occurring in a broad range of habitat types. The karstic Edwards Aquifer is the most biodiverse and well-studied groundwater habitat in the continental United States, but some Edwards Aquifer-associated species also occupy the hyporheic zone of rivers downstream from karst. The goals of this research were to 1) assess the potential of the off-karst hyporheic zone as habitat for ‘karst’ species and 2) to determine how stygobiont richness, diversity, and abundance related to distance from karst in the hyporheic zone of a spring-dominated river system. We enumerated stygobionts and quantified environmental characteristics for 55 samples from 11 sites in the San Marcos River between 3 and 140 river kilometers downstream from the rivers’ spring source. Linear mixed effects models and variance partitioning were used to investigate relationships between stygobiont assemblage metrics and environmental parameters (distance from karst, temperature, dissolved oxygen, and hydraulic conductivity). Stygobiont richness and diversity were significantly affected by distance from karst. Stygobiont richness and abundance were significantly affected by hydraulic conductivity. In richness and diversity models, predictive power was low, and additional environmental variables were significant in all models. This study demonstrates that stygobiont communities in the hyporheic zone can be structured by both local environmental conditions, via habitat filtering, and to a lesser extent, by spatial effects (i.e., proximity to source populations in adjacent aquifers). Furthermore, habitat specificity is poorly characterized for stygobiont taxa, inhibiting effective conservation planning and implementation.

Invertebrate communities in the hyporheic zone are structured by metacommunity processes including environmental filtering and spatial effects (e.g., dispersal). We investigated how these processes differentially influence two functional groups: groundwater-obligate invertebrates (stygobionts) and benthic insects. We predicted that environmental filtering would dominate for insects, while spatial effects would prevail for stygobionts. We collected 76 samples from 28 hyporheic sites along 109 km of the Rio Grande, Texas, USA, enumerating 54,508 individuals across 80 taxa. Concordance analysis and RDA revealed two communities: one dominated by insects, and another by crustaceans and soft-bodied organisms, including all stygobionts. In linear models, insect richness and abundance were influenced by temperature, alkalinity, oxygen, and pH. Stygobionts were associated with higher nitrate, higher temperature, and lower specific conductance (indicating groundwater discharge in the study area). Stygobiont presence, richness, and abundance increased near springs. Stygobiont communities exhibited spatial autocorrelation while insect communities did not. Insect responses to environmental variables suggest species sorting effects, which prevail for taxa with sufficient dispersal ability. Stygobiont responses to spatial variables suggest limited dispersal from source populations in adjacent aquifers. Dispersal ability may limit stygobionts’ ability to recolonize habitats after hydrologic disturbances, especially when connection with adjacent aquifers is lost.

In desert environments, unique communities depend on groundwater from springs, including a diverse radiation of small (Pseudotryonia Hershler, 2001, five nominal Tryonia W. Stimpson, 1865 (Cochliopidae) and seven Pyrgulopsis Call & Pilsbry, 1886 (Hydrobiidae). Four of these are classified as endangered under the US Endangered Species Act. Surveys for springsnails were conducted at 128 sites, including 13 sites that were previously reported localities, and 115 previously unsampled spring sites were also searched for new springsnail populations. Sequences of the DNA barcoding region were used to establish a database of known sequences from the named species and confirm identifications of new populations encountered. We report eight new springsnail populations, including new records for T. metcalfi Hershler, Liu & Landye, 2011, T. cheatumi (Pilsbry, 1935), P. ignota Hershler, Liu & Lang, 2010, P. metcalfi (D.W. Taylor, 1987), and P. texana (Pilsbry, 1935). We were not able to recollect Juturnia brunei (D.W. Taylor, 1987), T. oasiensis Hershler, Liu & Landye, 2011, or P. davisi (D.W. Taylor, 1987). The DNA barcoding gap for Tryonia ranged from 1.56–4.47% and for Pyrgulopsis from 0.68–1.68%.

Recent hyporheic, crenal and phreatic groundwater collections suggest a rich oligochaete fauna associated with the Edwards and Edwards-Trinity Aquifers in central Texas, including many species in the family Lumbriculidae. Based on limited invertebrate collections from earlier sampling programs, four sites were selected for more intensive oligochaete sampling. The lumbriculid Eclipidrilus palustris (Smith, 1900) is widespread in cleaner, cooler waters of the southeastern USA, and was common in crenal habitats. However, several lumbriculid species appear to be new and endemic, four of which are described here. Two of these can be assigned to the genus Eremidrilus Fend & Rodriguez, 2003 based on semiprosoporous male ducts with male pores in X, spermathecae in XI, and a filiform proboscis. Eremidrilus samacos n. sp. has typical morphology for the genus; E. parvitheca n. sp. (provisionally assigned to the genus) has duct-like spermathecae that join the gut, and unusual dorsal glands in posterior segments. Two morphotypes are assigned to E. parvitheca, based on spermathecal location in XI (typical) or XII (variant). A third species (Trichodrilus comalensis n. sp.) lacks a proboscis and has unusually elongated ventral chaetae. A problematic fourth species (Pararhynchelmis texana n. sp.) has semiprosoporous atria in X and duct-like spermathecae joining the gut in VIII. As it lacks apomorphies previously proposed for Rhynchelmis Hoffmeister, 1843 and Pseudorhynchelmis Hrabě, 1982, it is provisionally assigned to Pararhynchelmis Fend & Lenat, 2010.

Two new populations of Pyrgulopsis were found in springs in the Chihuahuan desert region of Texas. Pyrgulopsis Call & Pilsbry, 1886 are a diverse group of snails occupying freshwater springs, usually narrow-range endemics limited to a single spring. Their small ranges confer substantial conservation concern as the drying or disturbance of a single spring can lead to their extinction. We use mitochondrial COI and nuclear LSU sequences, shell morphometrics, and anatomical features to distinguish a new species of Pyrgulopsis endemic to the Big Bend region of Texas.

Pyrgulopsis Call & Pilsbry, 1886 is a genus of small (<5 mm) spring snails, usually endemic to single freshwater springs. Two new populations of Pyrgulopsis found in very small, isolated springs and spring runs in the mainstem Rio Grande watershed of western Texas are distinguished from congeners. Mitochondrial and nuclear sequences, morphometrics, and morphological characteristics support Pyrgulopsis rubra sp. nov. and Pyrgulopsis harrymilleri sp. nov. as distinct from other known Pyrgulopsis species, including the geographically proximate P. metcalfi.

Between 2015 and 2017, 58 ostracod taxa were collected at 30 locations in 111 samples from the hyporheic zone of surface streams in Texas, U.S.A. Six new species (Bradleycypris foresteri n. sp., Bradleystrandesia macula n. sp., Pseudocandona lordi n. sp., Indocandona rusti n. sp., Stenocypris sancari n. sp., Cyclocypris dalyana n. sp.) are described based on carapace and soft body parts. Additionally, 17 species and two genera (Vestalenula and Indocandona) are presented as new records for Texas, and Indocandona rusti n. sp. is the first record of the genus outside India. Taxonomic positions of the new species are discussed along with ecological information. One-hundred and six nonmarine ostracod species are now reported from Texas. However, ongoing studies suggest that actual species richness is much higher.

The Edwards Aquifer and related Edwards-Trinity Aquifer of Central Texas, USA, is a global hotspot of stygobiont biodiversity. We summarize 125 years of biological investigation at the San Marcos Artesian Well (SMAW), the best studied and most biodiverse groundwater site (55 stygobiont taxa: 39 described and 16 undescribed) within the Edwards Aquifer Groundwater Ecosystem. Cluster analysis and redundancy analysis (RDA) incorporating temporally derived, distance-based Moran’s Eigenvector Mapping (dbMem) illustrate temporal dynamics in community composition in 85 high-frequency samples from the SMAW. Although hydraulic variability related to precipitation and discharge partially explained changes in community composition at the SMAW, a large amount of temporal autocorrelation between samples remains unexplained. We summarize potential mechanisms by which hydraulic changes can affect community structure in deep, phreatic karst aquifers. We also compile information on 12 other Edwards and Edwards-Trinity Aquifer sites with 10 or more documented stygobionts and used distance-based RDA to assess the relative influences of distance and site type on three measures of β-diversity. Distance between sites was the most important predictor of total dissimilarity and replacement, although site type was also important. Species richness difference was not predicted by either distance or site type.

Three new species of Parabathynellidae (Crustacea: Bathynellacea) are described using morphological and molecular data, and additional populations identified as Texanobathynella cf. bowmani are placed within a molecular phylogeny for the group. The finding of Montanabathynella pecosensis sp. nov. entails a southern range expansion of over 1500 km for the genus, previously thought to be restricted to Montana, USA. Montanabathynella pecosensis sp. nov. displays a unique combination of morphological features, including eight main teeth on the labrum; four and six teeth on the pars incisiva and pars molaris of the mandible, respectively; eight teeth on the distal endite of MxI; epipod present on ThII and III; four teeth on the first segment of female ThVIII; and an unpronounced anal operculum. The other two new species described here belong to the genus Texanobathynella Delamare Deboutteville et al., 1975, previously consisting of only two species. Both species, Texanobathynella aaronswinki sp. nov and T. coloradoensis sp. nov., display combinations of morphological features distinct from the other two previously known species of the genus. Mitochondrial (COI) and nuclear (18S) markers support the validity of the new species and enable us to place them, along with T. cf. bowmani, within a larger phylogenetic framework of the Parabathynellidae.