Ontogeny and ecological significance of metabolic rates in sea turtle hatchlings

Background

Sea turtle hatchlings must avoid numerous predators during dispersal from their nesting beaches to foraging grounds. Hatchlings minimise time spent in predator-dense neritic waters by swimming almost continuously for approximately the first 24 h post-emergence, termed the ‘frenzy’. Post-frenzy, hatchling activity gradually declines as they swim in less predator-dense pelagic waters. It is well documented that hatchlings exhibit elevated metabolic rates during the frenzy to power their almost continuous swimming, but studies on post-frenzy MRs are sparse.

Results

We measured the frenzy and post-frenzy oxygen consumption of hatchlings of five species of sea turtle at different activity levels and ages to compare the ontogeny of mass-specific hatchling metabolic rates. Maximal metabolic rates were always higher than resting metabolic rates, but metabolic rates during routine swimming resembled resting metabolic rates in leatherback turtle hatchlings during the frenzy and post-frenzy, and in loggerhead hatchlings during the post-frenzy. Crawling metabolic rates did not differ among species, but green turtles had the highest metabolic rates during frenzy and post-frenzy swimming.

Conclusions

Differences in metabolic rate reflect the varying dispersal stratagems of each species and have important implications for dispersal ability, yolk consumption and survival. Our results provide the foundations for links between the physiology and ecology of dispersal of sea turtles.

Sea turtle hatchling locomotor performance: incubation moisture effects, ontogeny and species-specific patterns

Incubation conditions are critical in determining numerous traits in reptilian neonates. This is particularly significant in species with low offspring survival such as sea turtle species, because of the extremely high predation rates that hatchlings face during their initial dispersal from nesting beaches. Hatchlings that develop in suboptimal nest environments are likely to be smaller, slower and more susceptible to predation than hatchlings from optimal nest environments. Previous studies have focused on the effects of temperature on hatchling traits, but few have investigated the effects of moisture concentrations, despite moisture levels in nests influencing hatchling size, sex, incubation duration, and hatching success. Here, we incubated eggs of three sea turtle species at various moisture levels and tested the terrestrial and aquatic locomotor performance of the resultant hatchlings during the frenzy and post-frenzy period. We also compared and evaluated the ontogeny of early locomotor performance for each species over the first months of life. Drier incubation conditions produced hatchlings that crawled more slowly and took longer to self-right than hatchlings from wetter incubation conditions. There was no difference in swimming performance associated with moisture treatments. We suggest that moisture in the nest environment during incubation may influence hatchling performance via their initial hydration levels. Thus, nest moisture influences terrestrial performance (i.e., escaping from the nest and dispersing across the beach), although upon entering the ocean hatchlings have the opportunity to rehydrate by drinking and thus, differences in locomotor performance associated with moisture treatments cease.

The ontogeny of sea turtle hatchling swimming performance

Sea turtle hatchlings experience high mortality rates during dispersal. To minimize time spent in predator-dense waters, hatchlings typically undergo a period of hyperactivity termed the ‘frenzy’, characterized by almost continuous swimming for ~24 h. Research has focused on swimming performance during the frenzy, but our understanding of changes in swimming performance post-frenzy is limited. Thus, we measured green turtle (Chelonia mydas) hatchling swimming performance during the frenzy and post-frenzy when the turtles were 4, 12 and 24 weeks old. Using load cells, we recorded thrust production, stroke rates and the time turtles spent performing various swimming gaits. We found that the proportion of time spent powerstroking and the thrust generation per powerstroke were the main determinants of overall swimming performance. Older, larger turtles generated more thrust per stroke, but the proportion of time spent powerstroking throughout the entire swimming trial did not differ among age groups. Hatchlings have been thought mainly to use currents to reach nursery foraging grounds, and our findings suggest that hatchling swimming might also play an important role in directing hatchlings to optimal nursery habitats, supporting recent studies. Additionally, turtle size is positively related to swimming performance in post-frenzy turtles, suggesting that faster-growing turtles might have fitness advantages over slower-growing turtles.

The effects of extended crawling on the physiology and swim performance of loggerhead and green sea turtle hatchlings

Following emergence from the nest, sea turtle hatchling dispersal can be disrupted by artificial lights or skyglow from urban areas. Misorientation or disorientation may increase exposure to predation, thermal stress and dehydration, and consume valuable energy, thus decreasing the likelihood of survival. In this study hatchlings were run on a treadmill for 200 or 500 m to investigate the physiological impacts of disorientation crawling in loggerhead (Caretta caretta) and green (Chelonia mydas) sea turtle hatchlings. Oxygen consumption, lactate production and blood glucose levels were determined, and swim performance was measured over 2 h following crawls. Crawl distances were also determined for hatchlings that disoriented on the Boca Raton beach in Florida, with plasma lactate and blood glucose sampled for both properly oriented and disoriented hatchlings. Green and loggerhead hatchlings rested for 8-12% and 22-25% of crawl time, respectively, both in the laboratory and when disoriented on the beach, which was significantly longer than the time spent resting in non-disoriented turtles. As a result of these rest periods, the extended crawl distances had little effect on oxygen consumption, blood glucose or plasma lactate levels. Swim performance over 2 h following the crawls also changed little compared with controls. Plasma lactate concentrations were significantly higher in hatchlings sampled in the field, but did not correlate with crawl distance. The greatest immediate impact of extended crawling as a result of disorientation events is likely to be the significantly greater period of time spent on the beach and thus exposure to predation.

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.

Influence of industrial light pollution on the sea-finding behaviour of flatback turtle hatchlings

Context Numerous studies show that artificial light disrupts the sea-finding ability of marine turtle hatchlings. Yet very little has been published regarding sea-finding for flatback turtles. Given the current industrialisation of Australia’s coastline, and the large potential for disruption posed by industrial light, this study is a timely investigation into sea-finding behaviour of flatback turtle hatchlings. Aims We investigate sea-finding by flatback turtle hatchlings in relation to ambient light present in areas of planned or ongoing industrial development, and evaluate the fan and arena-based methods that are frequently used for quantifying hatchling dispersion. Methods Using a combination of methods, we assessed the angular range and directional preference of sea-finding hatchlings at two key flatback turtle rookeries, Peak and Curtis Islands, during January–February 2012 and 2013, and at Curtis Island in January 2014. Relative light levels at each site were measured using an Optec SSP-3 stellar photometer, and moon phase, moon stage and cloud cover were also recorded. Key results We found no evidence of impaired hatchling orientation, and observed very low levels of light at Peak Island. However, at Curtis Island, hatchlings displayed reduced sea-finding ability, with light horizons from the direction of nearby industry significantly brighter than from other directions. The sea-finding disruption observed at Curtis Island was less pronounced in the presence of moonlight. Conclusions The reduced sea-finding ability of Curtis Island hatchlings was likely due to both altered light horizons from nearby industry, as well as beach topography. Both methods of assessing hatchling orientation have benefits and limitations. We suggest that fan-based methods, combined with strategically placed arenas, would provide the best data for accurately assessing hatchling sea-finding. Implications Sky glow produced by large-scale industrial development appears detrimental to sea-finding by flatback turtle hatchlings. As development continues around Australia’s coastline, we strongly recommend continued monitoring of lighting impacts at adjacent turtle nesting beaches. We also advise rigorous management of industrial lighting, which considers cumulative light levels in regions of multiple light producers, as well as moon phase, moon-stage, cloud cover and time of hatchling emergence. All these factors affect the likelihood of disrupted hatchling sea-finding behaviour at nesting beaches exposed to artificial light-glow, industrial or otherwise.

Blood concentrations of lactate, glucose and corticosterone in dispersing hatchling sea turtles

Natal dispersal of sea turtles is an energetically demanding activity that is fuelled primarily by aerobic metabolism. However, during intense exercise reptiles can use anaerobic metabolism to supplement their energy requirements. We assessed anaerobic metabolism in dispersing hatchling loggerhead and flatback turtles by measuring the concentrations of blood lactate during crawling and at different times during the first four hours of their frenzy swim. We also measured concentrations of blood glucose and corticosterone. Blood lactate (12.13 to 2.03 mmol/L), glucose (6.25 to 3.8 mmol/L) and corticosterone (8.13 to 2.01 ng/mL) concentrations decreased significantly over time in both loggerhead and flatback hatchlings and no significant differences were found between the species. These results indicate that anaerobic metabolism makes a significant contribution to the dispersal phase of hatchling sea turtles during the beach crawl and the first few hours of the frenzy swim.