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Thesis Abstract – Osborne (2015)

Title:

Behavioral and Physiological Adaptations to Avoid Desiccation, Starvation, and Lethally High Temperatures During Estivation in the Land Snail Helminthoglypta tudiculata

Author and college:

Teresa Rose Osborne, Pomona College

Date:

May 2015

Degree:

Bachelor of Arts in Biology

Advisor:

Jonathan Wright, Department of Biology, Pomona College

Abstract:

Estivation is a state of dormancy in response to low humidity that terrestrial gastropods enter to avoid hostile summer conditions. While estivating, snails risk desiccation, starvation, and exposure to lethally high temperatures. Strategies to avoid lethal desiccation include sheltering in relatively humid microhabitats, anatomical barriers like epiphragma, and reduced metabolic rate. Respiration decreases due to carbonic acid sequestration and decreased breath frequency. Risks posed by desiccation, starvation, and high temperatures were assessed in the Southern Californian snail Helminthoglypta tudiculata, as were strategies to minimize these risks. It was hypothesized that most H. tudiculata mortality occurs during estivation; that reduced CO₂ flux is an adaptation to aid in water retention; that metabolism decreases most dramatically in early estivation but continues to decrease throughout estivation; that carbon reserves are exhausted before water reserves; and that microhabitat selection is based on local temperature and humidity as well as distance to potential food items.

Live H. tudiculata and H. tudiculata shells were collected from the field, and habitat parameters describing sites with and without signs of H. tudiculata activity were recorded. Data loggers measuring temperature and humidity were also placed in various representative sites. Respirometry techniques were used to determine CO₂ and H₂O fluxes of snails collected in estivation and snail in lab-induced dormancy across several weeks of dormancy. Highest tolerated temperature and lethal temperature were also determined using hot water baths.

H. tudiculata in the field tended not to estivate on the surface of substrates, nor in areas burned by a recent fire. They preferred refugia in cactus-rich areas, in rock piles produced by human disturbances, and under woody debris. Refugia containing live H. tudiculata or shells tended to be wider and deeper than uninhabited refugia. H. tudiculata avoided refugia under tree or shrub canopies, with organic litter substrates, or with gravelly or sandy substrates. It could not be determined whether snails selected microhabitats based on microclimate humidity or food availability. However, it was shown that snails preferred not to estivate on the substrate surface, since snails could survive above 41.3 ± 0.242 °C, and all surface microhabitats measured experienced temperatures above this threshold. By contrast, less than half of subsurface refugia reached lethally high temperatures, suggesting that snails may base microhabitat preference of refuge temperature.

During lab-induced dormancy, H. tudiculata lost 210 ± 37 ng H₂O s^-1 (mean ± SEM) in a 0% RH environment. When this value was adjusted for vapor pressure deficit experienced in refugia, it was predicted that snails could survive 23 ± 9.1 weeks of estivation in the field. This estimate was substantially longer than the longest summer drought in a typical year. Rate of carbon loss decreased throughout dormancy following the function CO₂ flux = 0.0052 L CO₂ week ^-1 * e^-0.16(t). This model predicted that H. tudiculata would not lose more than 0.032 L CO₂ during dormancy. No linear relationship was found between CO₂ flux and H₂O flux observed during the same respirometry recordings, nor did H₂O flux increase when the pneumostome opened, countering previous studies’ claims that suppressed CO₂ flux is a water retention strategy. Two distinct breath patterns were observed in dormant snails, one in which the pneumostome was held closed for extended periods of time punctuated by brief breaths, and another in which the pneumostome was opened and closed rapidly in a series of breaths. RQ during lab-induced dormancy was 0.686 ± 0.0993 during breaths, indicating CO₂ sequestration.

This study helps illuminate the conservation needs of small invertebrates such as H. tudiculata. Though there are several reasons to believe that small fires, like a recent fire at the study site, may have little impact of H. tudiculata population viability, catastrophic wildfires likely pose a serious threat to small invertebrate populations. Since drought length and maximum temperatures within some refugia already challenge snail survival, altered weather patterns due to climate change may make summers less tolerable for estivating H. tudiculata. Habitat preferences reported here also point towards aspects of habitat complexity most important to maintain as part of snail conservation efforts.

For more information:

Contact Jonthan Wright – jonathan.wright@pomona.edu