Comparative development and ocular histology between epigean and subterranean salamanders (Eurycea) from central Texas

The salamander clade Eurycea from the karst regions of central Texas provides an ideal platform for comparing divergent nervous and sensory systems since some species exhibit extreme phenotypes thought to be associated with inhabiting a subterranean environment, including highly reduced eyes, while others retain an ancestral ocular phenotype appropriate for life above ground. We describe ocular morphology, comparing three salamander species representing two phenotypes—the surface-dwelling Barton Springs salamander (E. sosorum) and San Marcos salamander (E. nana) and the obligate subterranean Texas blind salamander (E. rathbuni) - in terms of structure and size of their eyes. Eyes were examined using confocal microscopy and measurements were made using ImageJ. Statistical analysis of data was carried out using R. We also provide a developmental series and track eye development and immunolocalization of Pax6 in E. sosorum and E. rathbuni. Adult histology of the surface-dwelling San Marcos salamander (E. nana) shows similarities to E. sosorum. The eyes of adults of the epigean species E. nana and E. sosorum appear fully developed with all the histological features of a fully functional eye. In contrast, the eyes of E. rathbuni adults have fewer layers, lack lenses and other features associated with vision as has been reported previously. However, in early developmental stages eye morphology did not differ significantly between E. rathbuni and E. sosorum. Parallel development is observed between the two phenotypes in terms of morphology; however, Pax6 labeling seems to decrease in the latter stages of development in E.rathbuni. We test for immunolabeling of the visual pigment proteins opsin and rhodopsin and observe immunolocalization around photoreceptor disks in E. nana and E. sosorum, but not in the subterranean E. rathbuni. Our results from examining developing salamanders suggest a combination of underdevelopment and degeneration contribute to the reduced eyes of adult E. rathbuni.

Subterranean movement inferred by temporary emigration in Barton Springs salamanders (Eurycea sosorum)

Movement behavior is an important aspect of animal ecology but is challenging to study in species that are unobservable for some portion of their lives, such as those inhabiting subterranean environments. Using four years of robust-design capture-recapture data, we examined the probability of movement into subterranean habitat by a population of endangered Barton Springs salamanders (Eurycea sosorum), a species that inhabits both surface and subterranean groundwater habitats. We tested the effects of environmental variables and body size on survival and temporary emigration, using the latter as a measure of subterranean habitat use. Based on 2,046 observations of 1,578 individuals, we found that temporary emigration was higher for larger salamanders, 79% of which temporarily emigrated into subterranean habitat between primary sampling intervals, on average. Body size was a better predictor of temporary emigration and survival compared to environmental covariates, although coefficients from lower ranked models suggested turbidity and dissolved oxygen may influence salamander movement between the surface and subsurface. Surface population dynamics are partly driven by movement below ground and therefore surface abundance estimates represent a fraction of the superpopulation. As such, while surface habitat management remains an important conservation strategy for this species, periodic declines in apparent surface abundance do not necessarily indicate declines of the superpopulation associated with the spring habitat.

Phenotypic Variation in Endangered Texas Salamanders: Application of Model-Based Clustering for Identifying Species and Hybrids

The endangered Barton Springs and Austin blind salamanders (Eurycea sosorum and E. waterlooensis, respectively) are micro-endemics to the Barton Springs segment of the Edwards Aquifer and its contributing zone in Central Texas. Although vertically segregated within the aquifer system, both species are known from the same spring outflows and occasionally hybridize. We used geometric morphometrics and model-based clustering applied to a large sample of standardized salamander photographs to evaluate the potential for objective phenotypic assignment to either species, as well as putative hybrids. In addition to characterizing variation in head shape, our analyses inferred sets of clusters corresponding to ontogenetic series in both species but did not infer any distinct hybrid clusters. Eurycea sosorum and E. waterlooensis have distinctive head size to trunk length allometries, which contributed to the effective clustering of species, even at small body sizes. We also observed subtle, but significant, microgeographic variation in E. sosorum, suggesting the possibility of population substructuring, phenotypic plasticity, or undetected hybridization.

Mitigation of Impacts to Groundwater Quality from Highway Runoff in a Karst Terrain

There is a concern that urban development and the construction and operation of highways in particular represent a substantial threat to groundwater quality in areas underlain by karst geology. The objective of this paper is to determine whether common stormwater treatment practices are effective at preventing adverse impacts. The Barton Springs portion of the Edwards Aquifer, a karst system in Central Texas, presents a unique opportunity to address this issue, in that stormwater treatment requirements have been in place over a period of 20 years, when a substantial amount of development, including new highways, has been undertaken. This paper analyzes water quality data from Barton Springs over this period to identify constituents that exhibit changes in concentration and identifies potential causes of these changes. This analysis indicates that the constituents for which water quality has exhibited degradation appear to be due mainly to increases in pumping for water supply and onsite disposal of wastewater. No changes in constituent concentrations were observed for those most closely associated with highway operation, including total suspended solids, heavy metals, pesticides, and bacteria. It was found that sand filters produce an effluent quality equal to or better than runoff from undeveloped land. Consequently, it can be concluded that sand filtration, or an equivalent treatment, is an effective strategy for preventing groundwater degradation from highway operation in karst terrains.

New occurrence records for Eurycea sosorum Chippindale, Price & Hillis, 1993 (Caudata, Plethodontidae) in Travis and Hays counties, Texas, USA

We present 7 new occurrence records for the Barton Springs Salamander (Eurycea sosorum Chippindale, Price & Hillis, 1993) from Hays and Travis counties, Texas, USA, including the first for this species from the Trinity Aquifer. Eurycea sosorum is listed as endangered under the Endangered Species Act of 1973 due to ongoing threats from urbanization and aquifer overdraft throughout its narrow range. Although this species is more widely distributed than when it was first described in 1993, its range is still exceptionally small, restricted to portions of only two watersheds (Onion and Barton creeks) in one of the fastest-growing metropolitan areas in the United States (Austin, Texas). Under any ecologically-relevant criterion that is based on the best available scientific evidence, this species remains in danger of extinction throughout its range.