Date of Award


Degree Name

Doctor of Philosophy


Biological Sciences


Elizabeth J. Walsh


Aquatic species living in ephemeral habitats without a continual influx of dissolved organic carbon (DOC) are at increased risk of the effects of ultraviolet radiation (UVR) as the climate changes. Climate change is predicted to cause changes in DOC, some habitats will experience extended periods of drought and reduced input from DOC sources, and other environmental features that increase UVR reaching aquatic habitats. UVR can cause damage to cellular structures and biomolecules such as DNA, proteins, and lipids. The pigmented bdelloid Philodina sp. (Philodina) found in rock pools in the Chihuahuan Desert naturally experiences high UVR, yet it appears to thrive. Here I: (1) Examine the relationship between DOC and pigmentation in bdelloids, (2) Determine whether red pigmentation provides protection from UVR, (3) Investigate the consequences of transgenerational UVB exposure, and how this affects offspring, and (4) quantify gene expression in response to UVB exposure highly pigmented and non-pigmented bdelloids using a combination of field and laboratory approaches. I observed that highly pigmented bdelloids were found in a subset of rock pools at Hueco Tanks State Park and Historic Site, El Paso Co. TX. Both DOC and red pigmentation are known for their ability to absorb and attenuate UVR. To determine whether the presence of pigmented bdelloids was correlated to DOC concentration, I sampled 12 rock pools and found that bdelloids were more prevalent in rock pools with lower concentrations. From these results, I inferred that in habits with low DOC concentrations, bdelloids increase pigmentation to offset the damaging effects of UVR. To further determine the photoprotective qualities of red pigmentation, I cultured Philodina in the laboratory to obtain individuals with varying levels of coloration. Individuals were exposed to three levels of UVR (low, corresponding to local winter levels; mid, corresponding to fall levels; high corresponding to summer levels) in both the active and dormant form. I varied the length of dormancy since it is known that bdelloids become inactive in response to environmental stressors and that the longer they remain dormant, the less likely they are to recover. Comparing highly pigmentated (HP) to non-pigmented (NP) Philodina, HP bdelloids were twice as likely to survive UVB exposure. When bdelloids were desiccated for one day, the HP treatment was four times more likely to survive UVB exposure than the NP treatment. However, odds of surviving decreased as desiccation time increased. These results provide additional support for hypothesized the photoprotective capabilities of the red pigmentation. The consequences of transgenerational UVB radiation exposure were investigated by exposing three non-consecutive generations of Philodina to the same levels of UVB and determining survival of the parental, F2, and F4 generation. Life table experiments were used to quantify responses to repeated UVB exposure. Interestingly, both lifespan and net reproductive rate increased in low (117%, 233%) and mid (50%, 205%) UVB treatments when compared to the control. Similar responses to these UVB levels were seen in the F5 generation offspring. These results suggest that Philodina has locally adapted to regional UVB intensities. Lastly, I determined changes in gene expression after UVB exposure in both HP and NP Philodina. In HP bdelloids, ~50 genes showed differential expression while in the NP treatment >10,000 genes showed differential expressed when exposed to regional levels of UVB radiation. These observations further support the photoprotective properties of the pigmentation seen in Philodina. In this study, I have demonstrated that the pigmentation observed in Philodina provides effective photoprotection and that it may be a viable means to mitigate UVR-induced damage in invertebrates inhabiting shallow waters. Furthermore, I have provided evidence for local adaptation of Philodina to regional levels of UVB radiation. These findings could potentially serve as a model for understanding how other invertebrate species might respond to shifts in UVR levels as the climate changes.




Recieved from ProQuest

File Size

179 p.

File Format


Rights Holder

Maribel J Baeza

Included in

Evolution Commons