Sporadic nesting by loggerhead sea turtle can be determining for the future of the species in the Mediterranean

When females do not return to the natal location

The loggerhead sea turtle (Caretta caretta) is a marine species which is present in tropical areas and places with a temperate climate. This carnivorous turtle nests in the coasts of Japan, Oman, Australia, the Caribbean and the east coast of North America, Cape Verde and in the eastern Mediterranean areas (in particular, Greece, Turkey, Cyprus and Libya) and undertakes long migrations to feeding areas such as the western Mediterranean. It comes back to the beach where it previously nested but some female turtles do not return to the nesting place and can nest in new areas sporadically. This phenomenon, which is considered to be exceptional, has been growing over the last years in the beaches of the western Mediterranean.

So far, the philopatry was regarded as the main obstacle for this turtle to colonize new areas. This study reveals for the first time that C. caretta has mechanisms to colonize new areas using the feeding areas as a bridge. This key strategy enabled this species to survive for millions of years, apart from spreading around the world and start colonizing beaches in the western Mediterranean.

“The phenomenon of global warming can turn the current nesting beaches into non-nesting ones. Therefore, the turtle would look for new places to nest”, says the lecturer Carles Carreras, first author of the study and member of the Department of Genetics, Microbiology and Statistics of the UB and IRBio.

Following the turtle through a DNA study

The new study analyses a total of 40 sporadic nesting episodes, recorded in the coasts of Spain, Italy and France, from 1870 to 2015. The researchers applied high-resolution techniques for the study of the mitochondrial DNA and microsatellites of samples from nesting spots.  

“The main limitation to study populations through DNA is the speciesʼ mobility and wide spreading. Therefore, we need a network of international collaborators to get samples from the areas where the turtle nests and therefore have all the necessary pieces to rebuild the puzzle of its population structure”, adds Carles Carreras.

The experts use markers with a certain genetic variability to define the structure caused by the philopatry. The fragments of mitochondrial DNA ─transmitted by mothers only─ bring data on the genetic structure created by females. However, microsatellites ─inherited from both progenitors─ give information on the genetic composition of males too. Using these markers, the study reveals that sporadic nesting turtles came from further populations, such as those in western Atlantic or eastern Mediterranean.

Sex depends on the temperature

In marine turtles, sex is determined by the incubation temperature when the turtle lays the eggs in the nesting beach. Each female turtle can lay more than 100 eggs, which are incubated between 40 and 80 days in the sand. In western Mediterranean, with higher and higher temperatures, a phenomenon of “feminisation” could take place in the nesting beaches, that is, a rise in the number of born females.

“In the common reproduction areas at latitudes with a warmer temperature, there is already a high proportion of female turtles, which reaches the 90% in some cases. This could involve the collapse of these populations if there are no more nesting beaches with suitable temperature conditions”, warns Carles Carreras.

However, in most of the sporadic nests in the western Mediterranean, with colder temperatures, “data shows that there are many male turtles. Sporadic nesting could allow the setting of new populations in western Mediterranean areas, but only if temperatures rise enough in the future to produce females as well”, says the lecturer Lluís Cardona, member of the Department of Evolutionary Biology, Ecology and Environmental Sciences, and IRBio.

“There cannot be colonization without an important production of females, according to the models presented in the new study. However, with the global warming scenarios that are expected to take place in the future, these conditions could take place so that reproductive populations of the loggerhead sea turtle establish in the western Mediterranean”, concludes Cardona, member of the Research Group on Large Marine Vertebrates of the UB.

Tourism and sustainability: more and more difficult

Most turtle nests in the peninsular Mediterranean coasts were found by tourists or swimmers. Once these are found, the priority has always been to protect them and bring them to less busy areas, a task carried out by experts on environmental management. With a more and more important human pressure in the coast, the danger for females who nest sporadically is also getting bigger. Thus, a female that tried up to three times to nest in a beach in Ibiza could not do so due the tourists. Several institutions and entities launched initiatives to study and protect these sporadic nests that appear in the Mediterranean coast.

Regarding the UB, the research teams of the Faculty of Biology and IRBio are unfolding studies to identify the beaches that have the best conditions to host sporadic nests by the loggerhead sea turtle. If the amount of nests grows -as it seems it will be- an integrated management system for tourism on the turtle nesting will be necessary ─it is already successful in other parts of the world─, warn the experts.
 

Images: Carles Carreres and Lluís Cardona (Faculty of Biology - IRBio)