Redescription of the skull of the Australian flatback sea turtle, Natator depressus, provides new morphological evidence for phylogenetic relationships among sea turtles (Chelonioidea)

Chelonioidea (sea turtles) are a group where available morphological evidence for crown-group relationships are incongruent with those established using molecular data. However, morphological surveys of crown-group taxa tend to focus on a recurring subset of the extant species. The Australian flatback sea turtle, Natator depressus, is often excluded from comparisons and it is the most poorly known of the seven extant species of Chelonioidea. Previous descriptions of its skull morphology are limited and conflict. Here we describe three skulls of adult N. depressus and re-examine the phylogenetic relationships according to morphological character data. Using X-ray micro Computed Tomography we describe internal structures of the braincase and identify new phylogenetically informative characters not previously reported. Phylogenetic analysis using a Bayesian approach strongly supports a sister-group relationship between Chelonia mydas and N. depressus, a topology that was not supported by previous analyses of morphological data but one that matches the topology supported by analysis of molecular data. Our results highlight the general need to sample the morphological anatomy of crown-group taxa more thoroughly before concluding that morphological and molecular evidence are incongruous.

Nesting ecology of flatback sea turtles Natator depressus from Delambre Island, Western Australia

Nest site selection is likely to be important for the fitness of sea turtle populations, but data on the environmental drivers of nest site selection and other important parameters like nest site fidelity and inter-nesting and remigration intervals are limited. We address these questions using data on flipper tag resightings and track counts from flatback turtles (Natator depressus) from Delambre Island in Western Australia collected over 2–3 weeks each nesting season across six nesting seasons. The median inter-nesting interval was 13 days (range 9–17 days) and the mean±s.d. remigration interval was 1.99±0.95 years. Turtles had around 10% probability of returning to the same sector of the beach (150-m-long sections). The median distance between subsequent emergences (whether false crawls were included or not) was ~450m. The number of turtles both emerging and successfully nesting was higher when air temperature and humidity were lowest and emergences increased slightly with tide height. Sector of the beach was by far the strongest predictor of nest site, with turtles showing preference for the less exposed side of the island. The results of this study will assist with future monitoring of this population and the management of threats related to coastal development and activities.

Does behaviour affect the dispersal of flatback post-hatchlings in the Great Barrier Reef?

The ability of individuals to actively control their movements, especially during the early life stages, can significantly influence the distribution of their population. Most marine turtle species develop oceanic foraging habitats during different life stages. However, flatback turtles ( Natator depressus ) are endemic to Australia and are the only marine turtle species with an exclusive neritic development. To explain the lack of oceanic dispersal of this species, we predicted the dispersal of post-hatchlings in the Great Barrier Reef (GBR), Australia, using oceanographic advection-dispersal models. We included directional swimming in our models and calibrated them against the observed distribution of post-hatchling and adult turtles. We simulated the dispersal of green and loggerhead turtles since they also breed in the same region. Our study suggests that the neritic distribution of flatback post-hatchlings is favoured by the inshore distribution of nesting beaches, the local water circulation and directional swimming during their early dispersal. This combination of factors is important because, under the conditions tested, if flatback post-hatchlings were entirely passively transported, they would be advected into oceanic habitats after 40 days. Our results reinforce the importance of oceanography and directional swimming in the early life stages and their influence on the distribution of a marine turtle species.