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«By Rachel R. George, B.S. THESIS Presented to the Faculty of University of Houston- Clear Lake In Partial Fulfillment Of the Requirements For the ...»

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TEXAS DIAMONDBACK TERRAPIN (Malaclemys terrapin littoralis)


Rachel R. George, B.S.


Presented to the Faculty of

University of Houston- Clear Lake

In Partial Fulfillment

Of the Requirements

For the Degree



December, 2014


TEXAS DIAMONDBACK TERRAPIN (Malaclemys terrapin littoralis) by Rachel George



George Guillen, Ph.D., Chair __________________________________________

Cynthia L. Howard, Ph.D., Committee Member __________________________________________

Richard L. Puzdrowski, Ph.D., Committee Member __________________________________________

Dennis M. Casserly, Ph.D., Associate Dean __________________________________________

Zbigniew J. Czajkiewicz, Ph.D., Dean Dedication To David and Cheryl George, Parents that taught me to love and cherish nature Acknowledgements I would like to express my appreciation to Dr. George Guillen for giving me the opportunity to work with this unique and fascinating turtle, and for guiding me through the research and writing process. I would also like to thank Dr. Richard L. Puzdrowski and Dr. Cynthia L. Howard for their help.

I would like to thank my parents, David and Cheryl George, for supporting me and encouraging me through all of the many obstacles I faced.

I would like to thank all of the help I had in sampling especially, Bryan Alleman, Natasha Zarnstorff, Mandi Moss, Laila Pronker, Amanda Anderson, Richard Blackney, James Yokley, Steven Curtis, and Micheal Lane for long hours in the field helping me collect my data. Also, I would like to thank Dr. Mustafa Mokrech for the extensive help with the habitat surveys and ArcGIS analysis.



TEXAS DIAMONDBACK TERRAPIN (Malaclemys terrapin littoralis) Rachel George, M.S.

The University of Houston- Clear Lake, 2014

Thesis Chair: Dr. George Guillen

The Diamondback Terrapin is the only turtle in North America adapted to brackish water. The terrapin’s range extends from Cape Cod, MA to Corpus Christi, TX and exhibit considerable latitudinal variation in life history attributes. Terrapin have small home ranges, but they can be difficult to locate, especially in Texas. Therefore little is known about the entire life history of terrapin. The objective of my study was to define what physical habitat attributes are associated with nesting terrapin, and when do terrapin potentially nest in Galveston Bay, TX. I used two lines of evidence including habitat surveys of known nesting areas and follicle development to accomplish these objectives.

There is limited previous information on populations of terrapin in Galveston Bay, and terrapin have been observed nesting at each of our two study sites where we conducted

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330R and ArcGIS software was used to help collect and analyze geospatial data on multiple variables associated with predicted nesting habitat characteristics, including shell hash zone width (6-14 m), elevation, vegetation beyond shell hash, and sediment size and composition. Based on my assessment, two continuous areas were identified and delineated as possible nesting areas on Shell Island and seven possible nesting areas were delineated on South Deer Island. Each of these sites had high elevation (above 0.3255 m), high to medium shell hash zone width and high to medium levels of vegetation. Follicle size data were collected with a Sonosite® ultrasound from six different sites within Galveston Bay. Follicle development data were analyzed to identify seasonal nesting patterns. Based on follicle development trends, pitfall trap captures, and previous observations of terrapin nesting, nesting season was defined as starting from April to early June. Habitat attributes will be used in the future to define areas that most likely support nesting in the Gulf Coast.

vi Contents




Nesting Season

Nesting Habitat

The Reproductive Process

Previous Studies of Terrapin in Texas



Study Site

Environmental Assessment

Capture Methods

Crab Traps

Land Searches

Radio Tracking

Pitfall Traps - Nesting Terrapin

Environmental and Biological Data Collected at Time of Capture

Data Collected from Potentially Nesting Terrapin Captured in Pitfall Traps............... 29 Nesting Habitat Surveys

Data Analysis

Geospatial Analysis

Data Analysis



Follicle Data


Nesting Habitat

Nesting Season


Future Research

vii Literature Cited


Appendix 1.

Appendix 2:

Appendix 3.

1. Regression Analysis

2. One-Way ANOVA

3. Follicle Presence

viii List of Tables Table 1: Summary of surveys on South Deer and Shell Island.

Table 2: Comparison of goodness of fit between models using AIC and r 2

ix List of Appendix

1.1.Regression Analysis: Image J- Follicle Measurement versus Carapace length (mm) 82

1.2.Regression Analysis: Follicle Measurement versus Body condition

1.3.Regression Analysis: Follicle Measurement versus Weight (kg)

1.4.Regression Analysis: Follicle Measurement versus Log weight

1.5.Regression Analysis: Follicle Measurement versus DOY (Day of Year)................. 83

1.6.Regression Analysis: Follicle Measurement versus Tide

1.7.General Regression Analysis: Follicle Measurement versus DOY, Tide

2.1.One-way ANOVA: Follicle Measurement versus Location

2.2.One-way ANOVA: Follicle Measurement versus Month

3.1.Binary Logistic Regression: Binary versus DOY, Weight (kg)

3.2.Binary Logistic Regression: Binary versus DOY, Carapace length (mm)................. 88 x List of Figures Figure 1. Possible terrapin scrape.

Figure 2. Galveston Bay with all sites marked.

Figure 3. Shell Island.

Figure 4. South Deer Island.

Figure 5. Bolivar Flats.

Figure 6. Greens Lake.

Figure 7. North Deer Island.

Figure 8. Sportsmans Road.

Figure 9. Labeled terrapin for aid in measurement.

Figure 10. Ultrasound machine with coupling gel and female terrapin.

Figure 11. Examples of output from ultrasound.

A= Egg, B= ImageJ follicle measurement, C= Follicle without measurement, D= Follicle with measurement from internal calipers.

Figure 12. Sokkia 330R Total Station (left) and target prism (right).

Figure 13. Identified points taken with the total station.

Figure 14. Shell hash zone width of Shell Island.

The area with the pitfall trap shows the preferred width of shell hash being used to define predicted nesting habitat................... 38 Figure 15. Elevation of Shell Island. Pitfall trap marks the preferred elevation being used for the predicted nesting habitat.

Figure 16. Vegetation density classes of Shell Island beyond shell hash.

Vegetation classes are defined as follows: Low- 0-49% vegetation cover, medium- 50-74% vegetation cover, and high- 75-100% vegetation cover. Vegetation classes of areas beyond the shell hash pitfall trap vicinity were used for the predicted nesting habitat vegetation requirements.

Figure 17. Sediment cores taken from Shell Island showing sediment composition.

Sediment core near pitfall trap shows the sediment composition used for defining predicted nesting habitat characteristics.

Figure 18. Shell hash zone width of South Deer Island.

Figure 19. Elevation of South Deer Island

Figure 20. Vegetation classes of South Deer Island.

Vegetation classes are defined as follows: Low- 0-49% vegetation cover, medium- 50-74% vegetation cover, and high- 75vegetation cover.

Figure 21. Sediment cores and sediment size composition of South Deer Island.

........... 46 Figure 22. Comparison of sediment size classes from Shell Island and South Deer Island.

The green box indicates the 95% confidence interval for the median.

Figure 23. Comparison of sediment size classes from Shell Island and South Deer combined.

The green box indicates the 95% confidence interval for the median............ 48 Figure 24. Potential nesting sites on Shell Island.

xi Figure 25. Potential nesting sites on South Deer Island using variable ranges defined from pitfall trap areas at Shell Island.

Figure 26. Fitted line plot of maximum follicle measurement and carapace midline Estimated follicle size (cm) = 0.

8294 + 0.003183 Length Mid (mm) Carapace.............. 52 Figure 27. Fitted line plot comparing maximum follicle length to weight. Estimated follicle size (cm) = 1.273 + 0.1357 Weight (kg).

Figure 28. Fitted line plot comparing maximum follicle measurement to Log10 weight.

Estimated follicle size (cm) = 1.406 + 0.4297 Log weight.

Figure 29. Fitted line plot comparing maximum follicle measurement to body condition.

Estimated follicle size (cm) = 1.421 + 0.00089 Body condition. Body condition was previously defined as W (g)/ L3 (mm) *100000.

Figure 30. Box plot of maximum follicle measurements from each site.

Box size proportional to sample size. Bolivar was excluded due to low sample size (n=1)........... 54 Figure 31. Comparison of means of maximum follicle measurements from each site with 95% confidence intervals. The pooled standard deviation was used to calculate the intervals.

Figure 32. Fitted line plot of maximum follicle measurement versus day of year estimated maximum follicle size (cm) = 1.

684 - 0.001175 DOY.

Figure 33. Fitted line plot comparing high tide to maximum follicle measurements;

Estimated maximum follicle measurement (cm) = 1.538 - 0.4120 tide hh (m)................ 56 Figure 35. Box plot of maximum follicle measurements from 2012 to 2013

Figure 36. Boxplot comparing overall mean maximum follicle size to each month.

Box size is proportional to sample size.

Figure 37. Mean plot with 95% confidence interval for the mean maximum follicle measurement for all sites.

The pooled standard deviation was used to calculate the intervals.

–  –  –

Introduction Background The Diamondback Terrapin1 (Malaclemys terrapin) is in the family Emyidae which contains seven sub-species distributed in estuaries from Cape Cod, MA to southern Texas (Glenn and Hauswaldt, 2005; Roosenburg, 1994). The Texas Diamondback Terrapin (Malaclemys terrapin littoralis) bears the sub-species epithet littoralis.

Terrapins get their name for the diamond shaped scutes on their back. Researchers have attempted to use the concentric rings on the scutes to estimate the age of a terrapin.

Unfortunately, when terrapin shed these scutes the concentric rings begin to smooth out, so older terrapin cannot be aged reliably using this method (Roosenburg, 1991). The maximum life span of terrapin is unknown but thought to be as long as 50 years, with little known about the first few years of the life (Roosenburg, 1991). It is the only species of turtle uniquely adapted to living in estuaries. They exhibit, however, latitudinal variation in microhabitat use within their range (Glenn and Hauswaldt, 2005).

Estuaries are located in between the ocean and upstream rivers. Typically, they are semi-enclosed, and have a continuous exchange of water with the open ocean (Roosenburg, 1994). An estuary is a unique habitat because of the mixing of freshwater inflow and marine water. This creates a gradient in salinity and suspended sediments, creating a dynamic physicochemical environment within the estuary (Pritchard, 1967).

Variation in the amount of freshwater inflow and precipitation can alter the salinity

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gradient in an estuary (Pritchard, 1967). Astronomical and wind influenced tides can reinforce or partially neutralize the influence of the factors listed above (Pritchard, 1967).

Terrapin are the only reptile found in estuaries that are known to have a functional salt gland, an exocrine gland that aides the kidney by producing excretions containing higher concentrations of salt than sea water (Davenport and Macedo, 1990). Terrapin need a balance of Na+ and Cl- ions to prevent diffusion of unwanted or essential fluids in or out of the body, which is controlled by gradients that are regulated by the salt gland (Davenport and Macedo, 1990). Dunson (1970) confirmed the existence of the terrapin’s salt gland by transferring terrapins from freshwater to 3.3 % NaCl solution and recording an increase in electrolyte concentration of whole blood. However, Davenport and Macedo (1990) states that the terrapin’s salt gland is aided by behavioral osmotic control because the gland is not as effective as in true marine reptiles such as sea turtles.

Terrapins will drink water from surface layers of freshwater overlying more dense saline water and from pooled rainwater (Davenport and Macedo, 1990). Dunson (1970) reported that terrapin have been collected from Maryland to Florida where found in salinities between 11 and 32 parts per thousand.

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