New colonisers drive the increase of the emerging loggerhead turtle nesting in Western Mediterranean

Abstract

The loggerhead sea turtle (Caretta caretta) is sensitive to climate change and is responding by colonising the Western Mediterranean. To understand the rapid nesting increase in recent years in Spain, we sampled 45 hatchlings from 8 nests between 2016 and 2019. We sequenced a mtDNA D-loop region, genotyped 2291 SNPs using 2bRAD and collected data on clutch size, hatching success, and incubation duration. We confirmed that the colonisation has a Mediterranean and Atlantic mixed origin and we detected that these nests were laid by different females, except for two nests within the same season. Our results suggest that the recent increase in nesting is due to an increase in the number of colonising individuals rather than females born in the same area returning to breed. We hypothesize that this increase in the number of colonisers results from successful conservation efforts, feminisation of the populations of origin and earlier sexual maturation. However, the percentage of offspring females produced in Spain suggests that future returning individuals will aid to the settlement of the new population. These results allow defining the current status of this colonisation although future efforts are needed to detect remigrants to confirm the establishment of a resident population.

Figure 1

Summary of nesting activity in the Spanish coast. (a) Location of sporadic nests recorded in Spain from 1870 to 2019 (N = 22) as coded in Table 1 and Supplementary Table S1. Squares indicate nests laid between 1870 and 2015 in Spain as analysed in a previous study (N = 11). Circles indicate nesting events from 2016 to 2019 (N = 11) analysed in the present study, being the black circles, the nests analysed with 2bRAD sequencing (N = 8). Map created using MAPTOOL (SEATURTLE. ORG Maptool. 2002. SEATURTLE.ORG, Inc. http://www.seaturtle.org/maptool/ 18 Feb 2022). The two foraging areas in the region are highlighted with a black dashed line (Catalano-Balearic Sea) and a grey area (Algerian Basin). (b) Number of loggerhead turtle nests per year (N = 22) and attempts (N = 32) in the Spanish coast (Supplementary Table S1, Supplementary Table S2, adapted from Hochscheid et al.. The lines represent the mean SST in June and July per year in the two foraging areas indicated in the map.

Figure 2

Non-exclusive hypotheses that may explain the increase in nesting activity in the Western Mediterranean basin. On one hand, females produced in past colonisation events could have returned upon maturation and established in the new areas as remigrant females. On the other hand, the increase of the nests could be related to an increase in the arrival of new colonisers. This increase could be produced by the arrival of new females, either due to the increase of the populations due to conservation efforts or due to the feminisation of the origin populations, of because of an early sexual maturation in foraging areas, increasing the chances of laying eggs outside of their origin nesting populations.

Figure 3

Spatial distribution of events detected by year (1990–2019) and the corresponding SST mean temperatures between June and July for each year. Only the SST gradient of the years with presence of nesting events (light blue circles) or attempts (white triangles) is illustrated. The isotherm of 21 °C SST, which is the threshold suggested as critical for the presence of nesting events in the Western Mediterranean is represented with a dashed black line. Maps created using QGIS vs 3.22.9 software (https://www.qgis.org/en/site/).

Figure 4

Heatmap based on the relatedness index among pairs of individual samples based on the 2,291 filtered SNPs obtained with 2bRAD sequencing. Each cell represents the value of the Manichaikul relatedness index between sample pairs. The samples that belong to each nest are indicated in the diagonal arrows. Individuals from nests SP_07 and SP_11 clustered together. Nests are coded as in Table 1.

Figure 5

Multidimensional scaling (MDS) analysis plots based on IBS distance among the 45 individuals included in this study (2,291 SNPs), representing the genetic differentiation of the 8 nests analysed with 2bRAD sequencing on the Spanish coast. (a) 3D scatter plot considering all the individuals (N = 45). (b) 3D scatter plot, considering one individual per nest (N = 8). Nests are coded as in Table 1.