‘Shell-ebrations’, it’s World sea turtle day (16th June 2019), the day of Dr. Archie Carr’s birthday, who is considered widely to be ‘the father of sea-turtle biology’. As an American herpetologist, ecologist, and a pioneering conservationist, Dr. Carr focused his career on sea-turtle research, during a time in which little was known about their biology (1959-1987). Currently, 50 years later, nearly half (45%) of the world’s sea turtle populations are threatened with extinction, according to the International Union for Conservation of Nature (IUCN); therefore, it has never been more important to keep Dr. Carr’s legacy alive to study, protect and conserve these species.
Life-history traits of sea turtles in the Mediterranean
To date, four species of sea turtle have been recorded in the Mediterranean (Table 1), including: loggerhead (Caretta caretta), leatherback (Dermochelys coriacea), green (Chelonia mydas) and hawksbill turtle (Eretmochelys imbricata; Panagopoulos et al., 2003; Casale et al., 2018). Each species has been classified as threatened by extinction on a global scale by the IUCN:
The following information, unless cited otherwise, has been taken from: www.IUCN.org, www.Medasset.org, and www.conserveturtles.org. Sea turtles have a varied diet, ranging from molluscs (e.g. octopuses, squids, and snails), corals, tunicates (sea squirts), crustaceans (e.g. crabs), sea urchins, sponges, jellyfish, seaweeds and seagrasses. Adult loggerheads are mainly carnivorous, adult green turtles are herbivorous and fully-grown leatherbacks and hawksbills are omnivorous. Loggerheads are the smallest species reaching 90 cm when fully grown, followed by hawksbills at 114 cm, while greens reach 150 cm, and leatherbacks are the largest, potentially growing to 175 cm. Hawksbill sea turtles perform maximum dives to 91 m (Blumenthal et al., 2009) and loggerheads have been recorded diving to a maximum of 110 m, green turtles to 233 m, and leatherbacks can reach depths of over 1000 m. Throughout their lives, sea turtles are naturally predated upon by different animals: Eggs can be dug up and consumed by foxes, lizards, jackals, and weasels, while hatchlings can be eaten by gulls, crabs, lizards, snakes, or cormorants, and even adults can be caught by large fish, sharks and occasionally seals. Currently, loggerheads and green sea turtles use more than 50 nesting sites across the Mediterranean, while leatherbacks and hawksbill sea turtles do enter the Mediterranean Sea to feed, but no nesting has been documented yet.
A closer look at loggerhead turtles (Caretta caretta)
A fundamental element of a species’ survivability is its genetic diversity, the more diverse a species is the more it can withstand changes in the environment (Allendorf, 1986; Lande, 1988). Tolve et al. (2018) completed analysis on mitochondrial DNA (Deoxyribonucleic acid; building blocks of complex cells that are housed in the mitochondrion of a cell which is ultimately the powerhouse of metabolism in complex cells) for 488 stranded or incidentally-captured loggerhead turtles across the Adriatic Sea. No significant genetic divergence between locations was observed; however, Greek and Crete populations provided the most abundant contributions to the Adriatic-mixed stock. Contrastingly, Bertuccio et al. (2019) found a high Libyan contribution to the Adriatic Sea, conducting similar analysis on 93 loggerhead turtles. Interestingly, five distinct haplotypes (term used to describe equivalent of a genotype for mitochondrial DNA) were documented, with two being endemic (found nowhere else) to the Mediterranean rookeries, suggesting the Mediterranean populations had derived from their north Atlantic conspecifics; however, data were subject to bias due to low sample size and a higher frequency of samples taken from Libya. Significant differences in physiological adaptations between the north Atlantic and Mediterranean populations have also been suggested by Marn et al. (2019). These studies highlighting life-history traits, ecology, population genetics, and evolutionary history are vital in the conservation of this species.
Threats & Conservation
Globally, threats facing sea turtles range from climate change, coastal development, light pollution, bycatch, sea litter, to intentional killing (Dobbs, 2001; Casale, 2010; Lutcavage et al., 2017; Arreola and Fulton, 2018; Casale et al., 2018; Delgado, 2018). Potential effects of climate change may affect sea turtles at different life-history stages from flooded nests due to rising sea levels (Daniels et al., 1993; Fuentes et al., 2010), to varying temperatures leading to skewed sex-ratios (Laloë et al., 2016; Bókony et al., 2019). Sex is determined by overall temperature during egg development, with cooler nests (<28 °C) producing males and warmer nests (>28 °C) producing females (Patel et al., 2016; Sandoval et al., 2017; Santos et al., 2017). Artificial lights (i.e. street lights, city light pollution) can disorientate hatchlings on their way out to sea (Lohmann et al., 2017; Celano et al., 2018) and disturb adults, emerging onto beaches to lay eggs (Lutcavage et al., 2017; Hu et al., 2018). Unintentional capture by fisheries (bycatch) is a serious issue facing all marine megafauna species (Reeves et al., 2013; Lewison et al., 2014; ICES CIEM, 2016; Hamilton and Baker, 2019) and it has been estimated that ca. 132,000 sea turtles are captured annually as bycatch in the Mediterranean causing ca. 44,000 deaths every year (Casale and Heppell, 2016).
Efforts to monitor, protect, and restore environments and individual sea turtles are being made across the Mediterranean. Loggerheads are the most common sea turtle species in the Mediterranean and an emblematic species to Greek seas. As such, they are widely protected by law (Mazaris et al., 2017; Casale et al., 2018). In particular their nesting site on Sekania is legally designated an area of absolute protection (Presidential Decree 966/1999), with only scientific research allowed in the National Marine Park of Zakynthos (Katselidis et al., 2013; Casale et al., 2018). In Greece, sea turtles use beaches on Zakynthos, Kyparissia, Lakonikos, Rethymno, Chania, Messenia, and Koroni throughout May–August to lay their eggs, before hatchlings appear from July–October, making their break for the ocean (Casale et al., 2018; Hatase and Omuta, 2018; Zampollo et al., 2018). Two Greek organisations are involved with the protection of sea turtles, ARCHELON and MEDASSET, both engaging volunteers and local communities to learn more about these protected species, their biology and conservation. ARCHELON tagged 12 individual females in June 2019, increasing the dataset of a long-term loggerhead tagging project, running since 1982 in conjunction with an annual nesting-activity monitoring program. You can even follow these 12 individual sea turtles here. MEDASSET are also trialling a program to detect marine litter using drones. This non-invasive pilot study will be vital in targeting areas of increased litter and possibly identifying entangled animals in distress, enabling a quick rescue. Ongoing campaigns like this span the Mediterranean including the turtle paradise, Cyprus. It seems sea turtles have won over the public and policymakers in Greece, with conservation initiative involvement increasing in popularity with tourists and locals, which is ‘turtlley’ awesome.
Allendorf, F.W. (1986): Genetic drift and the loss of alleles versus heterozygosity. Zoo Biology 5, 181-190.
Arreola, L., and Fulton, J. (2018): Microplastic pollution in the ocean affecting marine life and its potential risk to human health.
Bertuccio, V., Costantini, F., Angelini, V., Furii, G., Gobic, K., and Abbiati, M. (2019): Haplotype and biometric patterns in loggerhead turtles from the Adriatic foraging ground. Journal of Sea Research 147, 1-9.
Blumenthal, J., Austin, T., Bothwell, J., Broderick, A., Ebanks-Petrie, G., Olynik, J., Orr, M., Solomon, J., Witt, M., and Godley, B. (2009): Diving behavior and movements of juvenile hawksbill turtles Eretmochelys imbricata on a Caribbean coral reef. Coral Reefs 28, 55.
Bókony, V., Milne, G., Pipoly, I., Székely, T., and Liker, A. (2019): Sex ratios and bimaturism differ between temperature-dependent and genetic sex-determination systems in reptiles. BMC evolutionary biology 19, 57.
Casale, P. (2010): Sea turtles in the Mediterranean: distribution, threats and conservation priorities. IUCN.
Casale, P., Broderick, A.C., Camiñas, J.A., Cardona, L., Carreras, C., Demetropoulos, A., Fuller, W.J., Godley, B.J., Hochscheid, S., Kaska, Y., Lazar, B., Margaritoulis, D., Panagopoulou, A., Rees, A.F., Tomás, J., and Türkozan, O. (2018): Mediterranean sea turtles: current knowledge and priorities for conservation and research. Endangered Species Research 36, 229-267.
Casale, P., and Heppell, S.S. (2016): How much sea turtle bycatch is too much? A stationary age distribution model for simulating population abundance and potential biological removal in the Mediterranean. Endangered Species Research 29, 239-254.
Casale, P., and Tucker, A. (2017): Caretta caretta (amended version of 2015 assessment). The IUCN Red List of Threatened Species 2017: e. T3897A119333622.
Celano, L., Sullivan, C., Field, A., and Salmon, M. (2018): Seafinding revisited: how hatchling marine turtles respond to natural lighting at a nesting beach. Journal of Comparative Physiology A 204, 1007-1015.
Daniels, R.C., White, T.W., and Chapman, K.K. (1993): Sea-level rise: destruction of threatened and endangered species habitat in South Carolina. Environmental management 17, 373-385.
Delgado, A.M. (2018): Thoughts and threats: understanding the current sea turtle landscape in southern Belize. University of Alabama Libraries.
Dobbs, K. (2001): Marine turtles in the Great Barrier Reef World Heritage Area: a compendium of information and basis for the development of policies and strategies for the conservation of marine turtles. Great Barrier Reef Marine Park Authority.
Fuentes, M., Limpus, C., Hamann, M., and Dawson, J. (2010): Potential impacts of projected sea‐level rise on sea turtle rookeries. Aquatic conservation: marine and freshwater ecosystems 20, 132-139.
Hamilton, S., and Baker, G.B. (2019): Technical mitigation to reduce marine mammal bycatch and entanglement in commercial fishing gear: lessons learnt and future directions. Reviews in Fish Biology and Fisheries, 1-25.
Hatase, H., and Omuta, K. (2018): Nest site selection in loggerhead sea turtles that use different foraging areas: do less fecund oceanic foragers nest at safer sites? Journal of Zoology 305, 232-239.
Hu, Z., Hu, H., and Huang, Y. (2018): Association between night-time artificial light pollution and sea turtle nest density along Florida coast: A geospatial study using VIIRS remote sensing data. Environmental Pollution 239, 30-42.
ICES CIEM (2016): Bycatch of small cetaceans and other marine animals- review of national reports under council regulation (EC) No.812/2004 and other infomation. ICES CIEM. 1-6 pp.
Katselidis, K.A., Schofield, G., Stamou, G., Dimopoulos, P., and Pantis, J.D. (2013): Evidence-based management to regulate the impact of tourism at a key marine turtle rookery on Zakynthos Island, Greece. Oryx 47, 584-594.
Laloë, J.-O., Esteban, N., Berkel, J., and Hays, G.C. (2016): Sand temperatures for nesting sea turtles in the Caribbean: Implications for hatchling sex ratios in the face of climate change. Journal of Experimental Marine Biology and Ecology 474, 92-99.
Lande, R. (1988): Genetics and demography in biological conservation. Science 241, 1455.
Lewison, R.L., Crowder, L.B., Wallace, B.P., Moore, J.E., Cox, T., Zydelis, R., McDonald, S., DiMatteo, A., Dunn, D.C., Kot, C.Y., Bjorkland, R., Kelez, S., Soykan, C., Stewart, K.R., Sims, M., Boustany, A., Read, A.J., Halpin, P., Nichols, W.J., and Safina, C. (2014): Global patterns of marine mammal, seabird, and sea turtle bycatch reveal taxa-specific and cumulative megafauna hotspots. Proceedings of the National Academy of Sciences 111, 5271-5276.
Lohmann, K.J., Witherington, B.E., Lohmann, C.M., and Salmon, M. (2017): Orientation, navigation, and natal beach homing in sea turtles: The Biology of Sea Turtles, Volume I. CRC Press, pp. 121-150.
Lutcavage, M.E., Plotkin, P., Witherington, B., and Lutz, P.L. (2017): 15 human impacts on sea turtle survival. The Biology of Sea Turtles 1, 45.
Marn, N., Jusup, M., Catteau, S., Kooijman, S.A.L.M., and Klanjšček, T. (2019): Comparative physiological energetics of Mediterranean and north Atlantic loggerhead turtles. Journal of Sea Research 143, 100-118.
Mazaris, A.D., Schofield, G., Gkazinou, C., Almpanidou, V., and Hays, G.C. (2017): Global sea turtle conservation successes. Science advances 3, e1600730.
Mortimer, J., and Donnelly, M. (2008): Eretmochelys imbricata. The IUCN Red List of Threatened Species 2008: e. T8005A12881238.
Panagopoulos, D., Sofouli, E., Teneketzis, K., and Margaritoulis, D. (2003): Stranding data as an indicator of fisheries induced mortality of sea turtles in Greece. FIRST MEDITERRANEAN CONFERENCE ON MARINE TURTLES, pp. 202.
Patel, S.H., Morreale, S.J., Saba, V.S., Panagopoulou, A., Margaritoulis, D., and Spotila, J.R. (2016): Climate impacts on sea turtle breeding phenology in Greece and associated foraging habitats in the wider Mediterranean region. Plos One 11, e0157170.
Reeves, R.R., McClellan, K., and Werner, T.B. (2013): Marine mammal bycatch in gillnet and other entangling net fisheries, 1990 to 2011. Endangered Species Research 20, 71-97.
Sandoval, S., Gómez-Muñoz, V.M., and Porta-Gándara, M.Á. (2017): Expansion of the transitional range of temperature for sea turtle Lepidochelys olivacea from sex ratio data at controlled incubation temperatures. Herpetology Notes 10, 63-65.
Santos, K.C., Livesey, M., Fish, M., and Lorences, A.C. (2017): Climate change implications for the nest site selection process and subsequent hatching success of a green turtle population. Mitigation and adaptation strategies for global change 22, 121-135.
Seminoff, J. (2016): Chelonia mydas. The IUCN Red List of Threatened Species 2004: e. T4615A11037468.
Tolve, L., Casale, P., Formia, A., Garofalo, L., Lazar, B., Natali, C., Novelletto, A., Vallini, C., Bužan, E., Chelazzi, G., Gaspari, S., Fortuna, C., Kocijan, I., Marchiori, E., Novarini, N., Poppi, L., Salvemini, P., and Ciofi, C. (2018): A comprehensive mitochondrial DNA mixed-stock analysis clarifies the composition of loggerhead turtle aggregates in the Adriatic Sea. Marine Biology 165, 68.
Wallace, B., Tiwari, M., and Girondot, M. (2016): Dermochelys coriacea. The IUCN Red List of Threatened Species 2013: e. T6494A43526147.
Zampollo, A., Azzolin, M., Arcangeli, A., Buoninsegni, J., Paraboschi, M., Crosti, R., Mancino, C., and Giacoma, C. (2018): Employing ferry as platform of observation for monitoring Loggerhead sea turtle (Caretta caretta) distribution in the Adriatic-Ionian Region. 2018 IEEE International Workshop on Metrology for the Sea; Learning to Measure Sea Health Parameters (MetroSea), 8-10 Oct. 2018, pp. 140-144.