Use of microsatellites to compare solitary vs arribada nesting olive ridley turtles (Lepidochelys olivacea) along the Eastern Pacific coast of Costa Rica
A trait unique to Lepidochelys is a nesting behavioral polymorphism where some females nest solitarily and others nest en masse in events termed arribadas in which groups of 100 or more female turtles emerge synchronously to lay their eggs on a single beach. Olive ridley sea turtles (Lepidochelys olivacea) utilize both nesting strategies along the eastern Pacific coast of Costa Rica; arribada s occur on Playa Ostional and Playa Nancite and solitary nesting occurs on beaches along the entire eastern Pacific coast. This species has undergone a demographic reduction including a massive take by sea turtle fisheries that occurred in Mexican and Ecuadorian waters during the 1960s and 1970s within the distribution range of the turtles sampled for this study. Nesting ecology studies have suggested, but not confirmed, that solitary and arribada turtles belong to separate populations. Using eight polymorphic microsatellite loci, I assessed the fine-scale genetic structure of olive ridleys nesting along the eastern Pacific coast of Costa Rica to determine whether arribada olive ridley sea turtles show genetic signatures distinct from solitary nesters and to determine if this sample displays the signals of a population which has recently experienced a bottleneck. No significant structure was detected using the program STRUCTURE regardless of nesting strategy or nesting beach, which is the expected pattern for a marine species with high dispersal potential and a promiscuous mating system. However, cryptic structure was detected among the sample set using the program adegenet . Estimates of FST also supported differentiation between these clusters. This is consistent with findings from previous studies using microsatellites, where no genetic differentiation was found comparing turtles utilizing differing nesting strategies across similar geographic distances, yet relatively weak structure was found at larger scales in the Pacific along the Mexican coast. Furthermore, in contrast to the large bottleneck signature detected in Mexican nesting populations, this study did not detect evidence to support a recent population bottleneck in this area due to demographic events. These results include a lack of heterozygosity excess, no evidence of a mode-shift either globally or in any adegenet groups, and a very weak bottleneck signal using the M-ratio test. The protection of habitats supporting each behavior is necessary to ensure the genetic diversity vital to the long-term persistence of this species and must be considered in recovery plans.^
Frank V. Paladino, Purdue University.