Evidence for transoceanic migrations by loggerhead sea turtles in the southern Pacific Ocean

Post-hatchling loggerhead turtles (Caretta caretta) in the northern Pacific and northern Atlantic Oceans undertake transoceanic developmental migrations. Similar migratory behaviour is hypothesized in the South Pacific Ocean as post-hatchling loggerhead turtles are observed in Peruvian fisheries, yet no loggerhead rookeries occur along the coast of South America. This hypothesis was supported by analyses of the size-class distribution of 123 post-hatchling turtles in the South Pacific and genetic analysis of mtDNA haplotypes of 103 nesting females in the southwest Pacific, 19 post-hatchlings stranded on the southeastern Australian beaches and 22 post-hatchlings caught by Peruvian longline fisheries. Only two haplotypes (CCP1 93% and CCP5 7%) were observed across all samples, and there were no significant differences in haplotype frequencies between the southwest Pacific rookeries and the post-hatchlings. By contrast, the predominant CCP1 haplotype is rarely observed in North Pacific rookeries and haplotype frequencies were strongly differentiated between the two regions (F_st=0.82; p=<0.00001). These results suggest that post-hatchling loggerhead turtles emerging from the southwest Pacific rookeries are undertaking transoceanic migrations to the southeastern Pacific Ocean, thus emphasizing the need for a broader focus on juvenile mortality throughout the South Pacific to develop effective conservation strategies.     

The ontogeny of morphological defenses in Kemp's ridley (Lepidochelys kempii) and loggerhead (Caretta caretta) sea turtles

Marine turtles are large reptiles that compensate for high juvenile mortality by producing hundreds of hatchlings during a long reproductive lifespan. Most hatchlings are taken by predators during their migration to, and while resident in, the open ocean. Their survival depends upon crypticity, minimizing movement to avoid detection, and foraging efficiently to grow to a size too difficult for predators to either handle or swallow. While these behavioral antipredator tactics are known, changes in morphology accompanying growth may also improve survival prospects. These have been only superficially described in the literature. Here, we compare the similarities and differences in presumed morphological defenses of growing loggerhead (Caretta caretta) and Kemp's ridley (Lepidochelys kempii) posthatchlings, related species that differ in growth rate, timing of habitat shift (the return from oceanic to neritic locations), and size at maturity. In both species, vertebral spination and carapace widening increase disproportionally as small turtles grow, but later in ontogeny, the spines regress, sooner in ridley than in loggerhead turtles. Carapace widening occurs in both species but loggerheads are always longer than they are wide whereas in Kemp's ridley turtles, the carapace becomes as wide as long. Our analysis indicates that these changes are unrelated to when each species shifts habitat but are related to turtle size. We hypothesize that the spines function in small turtles as an early defense against gape‐limited predators, but changes in body shape function throughout ontogeny—initially to make small turtles too wide to swallow and later by presenting an almost flat and hardened surface that large predators (such as a sharks) are unable to grasp. The extremely wide carapace of the Kemp's ridley may compensate for its smaller adult size (and presumed greater vulnerability) than the loggerhead. J. Morphol. 276:929–940, 2015. © 2015 Wiley Periodicals, Inc.     

The geomagnetic environment in which sea turtle eggs incubate affects subsequent magnetic navigation behaviour of hatchlings

Loggerhead sea turtle hatchlings (Caretta caretta) use regional magnetic fields as open-ocean navigational markers during trans-oceanic migrations. Little is known, however, about the ontogeny of this behaviour. As a first step towards investigating whether the magnetic environment in which hatchlings develop affects subsequent magnetic orientation behaviour, eggs deposited by nesting female loggerheads were permitted to develop in situ either in the natural ambient magnetic field or in a magnetic field distorted by magnets placed around the nest. In orientation experiments, hatchlings that developed in the normal ambient field oriented approximately south when exposed to a field that exists near the northern coast of Portugal, a direction consistent with their migratory route in the northeastern Atlantic. By contrast, hatchlings that developed in a distorted magnetic field had orientation indistinguishable from random when tested in the same north Portugal field. No differences existed between the two groups in orientation assays involving responses to orbital movements of waves or sea-finding, neither of which involves magnetic field perception. These findings, to our knowledge, demonstrate for the first time that the magnetic environment present during early development can influence the magnetic orientation behaviour of a neonatal migratory animal.     

The postembryonic transformation of the shell in emydine box turtles

A key trend in the 210‐million‐year‐old history of modern turtles was the evolution of shell kinesis, that is, shell movement during neck and limb retraction. Kinesis is hypothesized to enhance predator defense in small terrestrial and semiaquatic turtles and has evolved multiple times since the early Cretaceous. This complex phenotype is nonfunctional and far from fully differentiated following embryogenesis. Instead, kinesis develops slowly in juveniles, providing a unique opportunity to illustrate the postembryonic origins of an adaptive trait. To this end, we examined ventral shell (plastral) kinesis in emydine box turtles and found that hatchling plastron shape differs from that of akinetic‐shelled relatives, particularly where the hinge that enables kinesis differentiates. We also demonstrated shape changes relative to plastron size in juveniles, coinciding with a shift in the carapace‐plastron structural connection, rearrangement of ectodermal plates, and bone repatterning. Furthermore, because the shell grows larger relative to the head, complete concealment of the head and extremities is only achieved after relative shell proportions increase. Structural alterations that facilitate the box turtle's transformation are probably prepatterned in embryos but require function‐induced changes to differentiate in juveniles. This mode of delayed trait differentiation is essential to phenotypic diversification in turtles and perhaps other tetrapods.     

Similarity,parsimony and conjectures of homology: The chelonian shoulder girdle revisited

Much has been written about the definition and recognition of biological homology. Homology is usually defined as similarity inherited from a common ancestor (e.g., papers in Hall, 1994). It is recognised through cladistic analysis: Patterson (1982) and de Pinna (1991) have cogently argued that homology can be equated with synapomorphy (a shared evolutionary novelty uniting a monophyletic group). Such identification involves two stages: first, a possible homology is proposed on the basis of morphological similarity. This similarity might be structural, topological, developmental, or any combination thereof. Next, a cladistic analysis is performed, involving the trait in question and all other informative traits identified. If the trait is congruent with the resultant phylogeny, it is accepted as homologous in all taxa which possess it. If the trait is incongruent with the phylogeny, it is interpreted as homoplasious in certain taxa. This has been termed the test of congruence (Patterson, 1982; de Pinna, 1991). Rieppel (1996) has recently suggested that the test of congruence might be circular, and that as a result certain inferences about the evolution of the chelonian shoulder girdle (Lee, 1996) are poorly substantiated. Here I argue that the test of congruence is not circular, and that the disputed conclusions about the evolution of chelonian shoulder girdle can be defended on the basis of parsimony. More generally, I suggest how considerations of parsimony can and should be used to arbitrate between conflicting conjectures of homology that are both congruent with an accepted phylogeny.