Ecotoxicology of Organic and Inorganic Pollutants in Chelonians. Marine Turtle Risk to Pollutant Exposure

Marine turtles are oviparous vertebrates subdivided in seven existing species widespread in most of the oceans. They are suffering from human activities and especially from fishery by-catch, egg harvesting, adult poaching, degradation of their habitats, environmental pollution and climate change. While direct threats (e.g. fishery by-catch) are often less challenging to identify and predict, indirect threats associated with environmental pollution often induce more insidious effects that can take longer to manifest and be more significant and lasting. Understanding the marine turtle risks to pollutant exposure is critical because a) pollutants are persistent and ubiquitous in the environment, b) all the marine turtle species are listed on the Red List of Threatened Species by the IUCN and c) pollutants were indicated to adversely threaten the marine turtles’ survival, especially the developing individuals. Despite decades of investigations and evidences of significant pollutant threats, few data is still being available for marine turtles.
The present study proposed to assess pollutant exposure in the green Chelonia mydas and hawksbill Eretmochelys imbricata marine turtles nesting in Guadeloupe (French West Indies FWI, Caribbean Sea). Trace elements (e.g. selenium, mercury and cadmium) and organohalogen pollutants OHP (e.g. polychlorobiphenyls PCBs and chlordecone) were determined in dermis collected from the nesting females and in their eggs. A broad range of pollutants was detected in these tissues among which chlordecone. This was quite interesting because of the past history of chlordecone in the FWI. Results suggested that the green and hawksbill marine turtles fed on contaminated foraging ground, accumulated chlordecone in their body and then transferred it into their eggs during the egg formation. Both Guadeloupean marine turtle species appeared less exposed to OHP and trace elements than other marine turtle populations, except for other green marine turtle colonies (i.e. trace elements). The developing embryo risks to pollutant exposure were evaluated as those for the Guadeloupean inhabitants that consume marine turtle eggs. Little threat may be expected for the Guadeloupean inhabitants while some pollutants may affect the marine turtle embryos’ survival of both species (i.e. p,p’-DDE, cadmium, mercury and selenium).
The present study was not the first to arise concern about embryo risks to selenium exposure as suggested by previous reptilian studies including marine turtles. As part of the present study, the toxicokinetics and dynamics of selenium were thus approached under laboratory conditions by using the slider turtle Trachemys scripta as model candidate. Juvenile turtles dietary exposed to selenium effectively accumulated selenium in their tissues but appeared unaffected by the exposure. Indeed, their body condition and antioxidant system were unaffected over the feeding trial. This was unexpected since the dietary levels used in the present study were indicated to induce sublethal effects in birds and other reptilian species. Selenium toxicity was initiated by oxidative stress leading to unusual production of oxidant species such as reactive oxygenated species. Therefore, turtles could tolerate high selenium levels due to specific trait of life (e.g. ability to deal with oxygen introduction after anoxic conditions associated with hibernation and/or diving).
It is challenging to transpose results obtained from laboratory animals exposed to controlled conditions to wild individuals exposed to many environmental factors, even if species are closely related. The development life-stage further greatly affects the sensitivity of individuals to pollutant exposure. Nevertheless, the green and hawksbill marine turtle embryos could also tolerate high selenium exposure. This would contradict the risk assessment conducted in the present study but would be possible considering the food habits of both marine turtle species. These species are feeding on seagrasses and sponges which may expose the marine turtles to natural toxic compounds. Consequently, they could have developed adaptive strategies to deal with toxics in response to pressure at their foraging ground. To date, more works are needed to better understand the metabolism of selenium in turtles as well as to properly determine toxic thresholds of selenium for marine turtles.
Finally, nondestructive collection techniques were tested for their suitability in assessing the turtle exposure to pollutants in both field and laboratory conditions. Keratinized tissues (i.e. carapace and skin) were proposed as promising tools and should warrant further investigations in researches aiming at the conservation of marine turtles.
The present study provides several firsts such as the first baseline levels of a) pollutants in green and hawksbill marine turtles nesting in an area not investigated yet (i.e. Guadeloupe), b) OHP in the marine turtle dermis, c) OHP in hawksbill turtle eggs and d) chlordecone in marine turtle tissues. The first toxicological data on e) selenium kinetics and dynamics were further provided in turtles.