Marine leech parasitism of sea turtles varies across host species, seasons, and the tumor disease fibropapillomatosis.

Fibropapillomatosis (FP) is a tumorous disease affecting all species of sea turtles and is associated with the pathogen chelonid alphaherpesvirus 5 (ChHV5). Hypothesized ChHV5 vectors include the marine leeches Ozobranchus branchiatus and O. margoi, but data on their associations with FP and ChHV5 are minimal. To establish relationships between leech parasitism, turtle hosts, and FP, we compared green and loggerhead turtles from the Indian River Lagoon (IRL), Florida, USA, in terms of (1) the presence or absence of ChHV5 within associated leeches, (2) the association between leech parasitism and host FP status, and (3) seasonal variation in leech presence. We identified 55 leeches collected from green turtles as O. branchiatus and 22 leeches collected from loggerhead turtles as O. margoi. Of 77 sequenced leeches, 10 O. branchiatus and 5 O. margoi were ChHV5 positive. ChHV5-positive O. branchiatus trended towards coming from FP-positive hosts. Using 12 yr of turtle capture data from the IRL, we found that leech parasitism was significantly correlated with FP and capture month in green turtles but not in loggerhead turtles. These results suggest that O. branchiatus and O. margoi may differ in their ability to transmit ChHV5 or to encounter and remain on FP-positive hosts. Alternatively, potential immunological differences between green and loggerhead turtles may explain the observed relationships. This study is the first to provide robust statistical evidence of an association between leeches and FP, as well as seasonal fluctuations in leech presence, in green turtles but not in loggerhead turtles.

Incorporating distance metrics and temporal trends to refine mixed stock analysis.

The distribution of marine organisms is shaped by geographic distance and oceanographic features like currents. Among migratory species, individuals from multiple populations may share feeding habitats seasonally or across life stages. Here, we introduce a modification for many-to-many mixed stock models to include distance between breeding and foraging sites as an ecological covariate and evaluate how the composition of green turtle, Chelonia mydas, juvenile mixed stock aggregations changed in response to population growth over time. Our modified many-to-many model is more informative and generally tightens credible intervals over models that do not incorporate distance. Moreover, we identified a decrease in genetic diversity in a Florida nesting site and two juvenile aggregations. Mixed stock aggregations in central Florida have changed from multiple sources to fewer dominant source populations over the past ~ 20 years. We demonstrate that shifts in contributions from source populations to mixed stock aggregations are likely associated with nesting population growth. Furthermore, our results highlight the importance of long-term monitoring and the need for periodical reassessment of reproductive populations and juvenile aggregations. Understanding how mixed stock aggregations change over time and how different life stages are connected is fundamental for the development of successful conservation plans for imperiled species.