‘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.
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