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Role of ingesta particle size in the green turtle grazing strategy, ontogenetic diet shifts, and responses to seagrass declines

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Abstract

Populations of the green turtle (Chelonia mydas), a mega-herbivore that consumes seagrasses, are recovering worldwide. Understanding green turtle adaptations to herbivory and responses to changes in seagrass availability will be critical to interpreting plant–herbivore interactions as green turtle populations continue to rebound. Ingesta particle size and diet composition of two green turtle foraging aggregations (Bermuda, 32.3° N, 64.8° W; U.S. Virgin Islands [USVI], 17.8° N, 64.6° W) in the Northwest Atlantic (NWA) were evaluated to assess the prevalence of herbivory across foraging sites and life stages, determine if there is an optimum ingesta particle size, and evaluate green turtle responses to changes in seagrass availability. Both aggregations were herbivorous (> 90% seagrass/algae) across size classes (straight carapace length, SCL). Ingesta particle size (mean ± SD) did not differ between Bermuda (2.6 ± 1.4 cm) and the USVI (2.3 ± 1.2 cm). Of seagrass leaves ingested, 20–30% were 1.7 cm in length, indicating a potential optimum for maximizing digestion rates. Turtle size (SCL) had a significant effect on particle size in Bermuda (p = 0.01, R2 = 0.16) (35.1 ± 9.9 cm SCL) but not in the USVI aggregation, which was comprised of larger turtles (49.0 ± 6.1 cm SCL). In Bermuda, there was no apparent response to the declines in seagrass availability. Ingesta particle size and volume of seagrass leaves did not decline from 2015 to 2019, nor was there an increase in volume of seagrass roots and rhizomes. These results indicate herbivory is prevalent across size classes at two NWA foraging sites and ingesta particle size has important implications for optimizing the green turtle grazing strategy and facilitating ontogenetic diet shifts to herbivory in juveniles. Ingesta particle size is a valuable tool for assessing green turtle responses to seagrass declines that should be interpreted within the context of population demographics.

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Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request, and in Griffin et al. (2020).

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Acknowledgements

This work was supported by the University of Florida (UF) Archie Carr Center for Sea Turtle Research through funds from the Jeff and Monette Fitzsimmons Fund and Disney Conservation Fund. Sampling in Bermuda was supported by the Atlantic Conservation Partnership, Bermuda Zoological Society, Sea Turtle Conservancy, Bermuda Aquarium Museum and Zoo, Helen Clay Frick Foundation, and Florida Fish and Wildlife Conservation Commission. Sampling in the USVI was supported by the U.S. Geological Survey (USGS) Natural Resources Preservation Program. Funding for the long-term study at Union Creek, Great Inagua, Bahamas was provided by the Disney Conservation Fund and U.S. National Marine Fisheries Service. We are grateful to the staff and volunteers at Bermuda Aquarium Museum and Zoo and to the students of the Bermuda Turtle Project field course for assistance with necropsies and collection of diet samples from Bermuda turtles. We thank the National Park Service at Buck Island Reef National Monument, and USGS staff members Andrew Crowder, Mike Cherkiss, Thomas Selby, Andre Daniels, David Roche, and Devon Nemire-Pepe for field assistance and sample collection in the USVI. Sampling in the Bahamas would not have been possible without the assistance of the Bahamas National Trust (BNT) and BNT wardens on Great Inagua: H. Nixon, R. Burrows, J. Nixon, S. Nixon, and T. Major. In addition, Morton Bahamas Ltd., U.S. Coast Guard Air Station Clearwater, and the people of Mathew Town (Inagua) provided invaluable logistical support. We are grateful to the Bahamas Department of Marine Resources for their support and permits to conduct research in The Bahamas. We thank Emma Long and Keyla Correia for their assistance with processing diet samples at UF. We appreciate the constructive feedback provided by Meg Lamont, Jeffrey Seminoff, and Lyndsey Howell that improved the manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Funding

This work was supported by the University of Florida (UF) Archie Carr Center for Sea Turtle Research through funds from the Jeff and Monette Fitzsimmons Fund and Disney Conservation Fund. Sampling in Bermuda was supported by the Atlantic Conservation Partnership, Bermuda Zoological Society, Sea Turtle Conservancy, Bermuda Aquarium Museum and Zoo, Helen Clay Frick Foundation, and Florida Fish and Wildlife Conservation Commission. Sampling in the USVI was supported by the U.S. Geological Survey (USGS) Natural Resources Preservation Program. The long-term study at Union Creek, Great Inagua, Bahamas was funded by the Disney Conservation Fund and U.S. National Marine Fisheries Service.

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Authors and Affiliations

  1. Archie Carr Center for Sea Turtle Research and Department of Biology, University of Florida, Gainesville, FL, 32611, USA

    Alexandra G. Gulick, Alan B. Bolten & Karen A. Bjorndal

  2. Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, St. Petersburg, FL, 33701, USA

    Anne B. Meylan

  3. Natural Sciences Collegium, Eckerd College, St. Petersburg, FL, 33711, USA

    Peter A. Meylan

  4. United States Geological Survey, Wetland and Aquatic Research Center, Davie, FL, 33314, USA

    Kristen M. Hart

  5. Bermuda Zoological Society, Flatts, Bermuda

    Jennifer A. Gray & Gaëlle Roth

Authors
  1. Alexandra G. Gulick
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  2. Anne B. Meylan
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  3. Peter A. Meylan
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  4. Kristen M. Hart
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  5. Jennifer A. Gray
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  7. Alan B. Bolten
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  8. Karen A. Bjorndal
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Contributions

AGG, ABB, and KAB conceived and designed the study. AGG, ABM, PAM, KMH, JAG, GR, ABB, and KAB collected the data and contributed to the planning and implementation of the study. AGG, ABB, and KAB analyzed the data. AGG wrote the manuscript with contributions and final approval from all authors.

Corresponding author

Correspondence to Alexandra G. Gulick.

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Conflict of interest

The authors declare no conflicts or competing interests.

Ethics approval

Sampling in Bermuda for this project was permitted by the Government of Bermuda Department of Conservation Services (License 15-07-27-28) and the Bermuda Department of Natural and Environmental Resources (Licenses 16-07-27-54, 17071007, 2018071309, and 2019061105). International transportation of diet samples from Bermuda to the United States was carried out under CITES export permits from the Bermuda Department of Natural and Environmental Resources (16BM0008, 18BM0002, 18BM0005, 19BM0009) and CITES import permits from the U.S. Fish and Wildlife Service to the Florida Fish and Wildlife Conservation Commission (16US758093/9, 18US48288C/9) and the University of Florida Archie Carr Center for Sea Turtle Research (19US724540/9). Sampling in the USVI was permitted by the National Marine Fisheries Service (Permits 16146 and 20315, issued to K. Hart), National Park Service IACUC (USGS-SESC2014-02), U.S. Geological Survey IACUC (WARC\GNV 2017-04), and the National Park Service at Buck Island Reef National Monument (BUIS-2011-SCI-0012, BUIS-2014SCI-0009, BUIS-2016-SCI-0009, issued to K. Hart). The long-term study in Union Creek, Great Inagua, Bahamas was conducted under permits issued by the Bahamas Department of Marine Resources.

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Gulick, A.G., Meylan, A.B., Meylan, P.A. et al. Role of ingesta particle size in the green turtle grazing strategy, ontogenetic diet shifts, and responses to seagrass declines. Mar Biol 168, 157 (2021). https://doi.org/10.1007/s00227-021-03965-1

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