Heart rate as a proxy for estimating oxygen consumption rates in loggerhead turtles (Caretta caretta)

Heart rates of air-breathing diving animals can change on a short time scale due to the diving response during submergence. Heart rate is used frequently as a proxy for indirectly estimating metabolic rates on a fine time scale. However, most studies to date have been conducted on endothermic diving animals, and the relationships between metabolic rates and heart rates in ectothermic diving animals have not been well studied. Sea turtles are unique model organisms of diving ectotherms because they spend most of their life in the ocean and perform deep and/or long dives. In this study, we examined the relationship between heart rates and metabolic rates in captive loggerhead turtles, Caretta caretta, to estimate oxygen consumption rates during each dive based on heart rates. The oxygen consumption rates (V̇_O2: mlO₂ min⁻¹ kg⁻¹) and average heart rates (f_H: beats min⁻¹) were measured simultaneously in indoor tanks at water temperatures of 15–25°C. Our results showed that oxygen consumption rate was affected by heart rate and water temperature in loggerhead turtles. Based on the collected data, we formulated the model equation as V̇_O2=0.0124f_H+0.0047T_w - 0.0791. The equation can be used for estimating fine-scaled field metabolic rates in free-ranging loggerhead turtles. The results of this study will contribute to future comparative studies of the physiological states of ectothermic diving animals.

Summary: The relationship between oxygen consumption rate and heart rate in the loggerhead turtle.

Junk food: Interspecific and intraspecific distinctions in marine debris ingestion by marine turtles

The pervasiveness of marine debris is now considered one of the most persistent changes in marine environments. This study reports marine debris ingested by green sea turtles Chelonia mydas and loggerhead sea turtles Caretta caretta that stranded along the eastern coast of the Sharjah Emirate in the United Arab Emirates. We observed that both green and loggerhead sea turtles frequently ingest (Frequency of Occurrence: 75.0% and 57.1% respectively) high quantities of marine debris, particularly plastics. The results suggest that green sea turtles are more likely to ingest soft items such as threads and sheets while loggerheads are more likely to ingest hard items. When considering the quantity, frequency and nature of ingested marine debris as well as the physiology of specific species and age classes, green sea turtles, particularly younger specimens, ingest the greatest amount of marine debris.

Epibionts Reflect Spatial and Foraging Ecology of Gulf of Mexico Loggerhead Turtles (Caretta caretta)

Sea turtles are exposed to numerous threats during migrations to their foraging grounds and at those locations. Therefore, information on sea turtle foraging and spatial ecology can guide conservation initiatives, yet it is difficult to directly observe migrating or foraging turtles. To gain insights into the foraging and spatial ecology of turtles, studies have increasingly analyzed epibionts of nesting turtles, as epibionts must overlap spatially and ecologically with their hosts to colonize successfully. Epibiont analysis may be integrated with stable isotope information to identify taxa that can serve as indicators of sea turtle foraging and spatial ecology, but few studies have pursued this. To determine if epibionts can serve as indicators of foraging and spatial ecology of loggerhead turtles nesting in the northern Gulf of Mexico we combined turtle stable isotope and taxonomic epibiont analysis. We sampled 22 individual turtles and identified over 120,000 epibiont individuals, belonging to 34 macrofauna taxa (>1 mm) and 22 meiofauna taxa (63 μm–1 mm), including 111 nematode genera. We quantified epidermis δ13C and δ15N, and used these to assign loggerhead turtles to broad foraging regions. The abundance and presence of macrofauna and nematodes did not differ between inferred foraging regions, but the presence of select meiofauna taxa differentiated between three inferred foraging regions. Further, dissimilarities in macrofauna, meiofauna, and nematode assemblages corresponded to dissimilarities in individual stable isotope values within inferred foraging regions. This suggests that certain epibiont taxa may be indicative of foraging regions used by loggerhead turtles in the Gulf of Mexico, and of individual turtle foraging and habitat use specialization within foraging regions. Continued sampling of epibionts at nesting beaches and foraging grounds in the Gulf of Mexico and globally, coupled with satellite telemetry and/or dietary studies, can expand upon our findings to develop epibionts as efficient indicators of sea turtle foraging and spatial ecology.

Projected shifts in loggerhead sea turtle thermal habitat in the Northwest Atlantic Ocean due to climate change

It is well established that sea turtles are vulnerable to atmospheric and oceanographic shifts associated with climate change. However, few studies have formally projected how their seasonal marine habitat may shift in response to warming ocean temperatures. Here we used a high-resolution global climate model and a large satellite tagging dataset to project changes in the future distribution of suitable thermal habitat for loggerheads along the northeastern continental shelf of the United States. Between 2009 and 2018, we deployed 196 satellite tags on loggerheads within the Middle Atlantic Bight (MAB) of the Northwest Atlantic continental shelf region, a seasonal foraging area. Tag location data combined with depth and remotely sensed sea surface temperature (SST) were used to characterize the species’ current thermal range in the MAB. The best-fitting model indicated that the habitat envelope for tagged loggerheads consisted of SST ranging from 11.0° to 29.7 °C and depths between 0 and 105.0 m. The calculated core bathythermal range consisted of SSTs between 15.0° and 28.0 °C and depths between 8.0 and 92.0 m, with the highest probability of presence occurred in regions with SST between 17.7° and 25.3 °C and at depths between 26.1 and 74.2 m. This model was then forced by a high-resolution global climate model under a doubling of atmospheric CO₂ to project loggerhead probability of presence over the next 80 years. Our results suggest that loggerhead thermal habitat and seasonal duration will likely increase in northern regions of the NW Atlantic shelf. This change in spatiotemporal range for sea turtles in a region of high anthropogenic use may prompt adjustments to the localized protected species conservation measures.

Movement patterns of large juvenile loggerhead turtles in the Mediterranean Sea: Ontogenetic space use in a small ocean basin

Mechanisms that determine how, where, and when ontogenetic habitat shifts occur are mostly unknown in wild populations. Differences in size and environmental characteristics of ontogenetic habitats can lead to differences in movement patterns, behavior, habitat use, and spatial distributions across individuals of the same species. Knowledge of juvenile loggerhead turtles' dispersal, movements, and habitat use is largely unknown, especially in the Mediterranean Sea. Satellite relay data loggers were used to monitor movements, diving behavior, and water temperature of eleven large juvenile loggerhead turtles (Caretta caretta) deliberately caught in an oceanic habitat in the Mediterranean Sea. Hidden Markov models were used over 4,430 spatial locations to quantify the different activities performed by each individual: transit, low-, and high-intensity diving. Model results were then analyzed in relation to water temperature, bathymetry, and distance to the coast. The hidden Markov model differentiated between bouts of area-restricted search as low- and high-intensity diving, and transit movements. The turtles foraged in deep oceanic waters within 60 km from the coast as well as above 140 km from the coast. They used an average area of 194,802 km2, where most individuals used the deepest part of the Southern Tyrrhenian Sea with the highest seamounts, while only two switched to neritic foraging showing plasticity in foraging strategies among turtles of similar age classes. The foraging distribution of large juvenile loggerhead turtles, including some which were of the minimum size of adults, in the Tyrrhenian Sea is mainly concentrated in a relatively small oceanic area with predictable mesoscale oceanographic features, despite the proximity of suitable neritic foraging habitats. Our study highlights the importance of collecting high-resolution data about species distribution and behavior across different spatio-temporal scales and life stages for implementing conservation and dynamic ocean management actions.

Loggerhead sea turtle (Caretta caretta) diving changes with productivity, behavioral mode, and sea surface temperature

The relationship between dive behavior and oceanographic conditions is not well understood for marine predators, especially sea turtles. We tagged loggerhead turtles (Caretta caretta) with satellite-linked depth loggers in the Gulf of Mexico, where there is a minimal amount of dive data for this species. We tested for associations between four measurements of dive behavior (total daily dive frequency, frequency of dives to the bottom, frequency of long dives and time-at-depth) and both oceanographic conditions (sea surface temperature [SST], net primary productivity [NPP]) and behavioral mode (inter-nesting, migration, or foraging). From 2011-2013 we obtained 26 tracks from 25 adult female loggerheads tagged after nesting in the Gulf of Mexico. All turtles remained in the Gulf of Mexico and spent about 10% of their time at the surface (10% during inter-nesting, 14% during migration, 9% during foraging). Mean total dive frequency was 41.9 times per day. Most dives were ≤ 25 m and between 30-40 min. During inter-nesting and foraging, turtles dived to the bottom 95% of days. SST was an important explanatory variable for all dive patterns; higher SST was associated with more dives per day, more long dives and more dives to the seafloor. Increases in NPP were associated with more long dives and more dives to the bottom, while lower NPP resulted in an increased frequency of overall diving. Longer dives occurred more frequently during migration and a higher proportion of dives reached the seafloor during foraging when SST and NPP were higher. Our study stresses the importance of the interplay between SST and foraging resources for influencing dive behavior.

Effect of maternal foraging habitat on offspring quality in the loggerhead sea turtle (Caretta caretta)

Exploring a trade‐off between quantity and quality of offspring allows differences in the fitness between alternative life histories to be accurately evaluated. We addressed the mechanism that maintains alternative life histories (small oceanic planktivores vs. large neritic benthivores) observed in a loggerhead sea turtle (Caretta caretta) population, which has been suggested to be environmental, based on the lack of genetic structure and a large difference in reproductive output. We examined whether maternal foraging habitat affects offspring quality, by measuring the morphology, emergence success, and righting response of hatchlings following incubation in a common open sand area over the whole nesting season at Yakushima Island, Japan, and by recording early growth and survival of offspring that were reared in a common environment at a Japanese aquarium. Furthermore, we tested whether sea turtles adjust egg size in response to temporal shifts of the incubation environment. There were no significant differences in any hatchling traits between oceanic and neritic foragers (which were classified by stable isotope ratios), except for clutches laid during the warmest period of the nesting season. There were also no significant differences in the growth and survival of offspring originating from the two foragers. The size of eggs from both foragers significantly increased as the season progressed, even though the rookery had heavy rainfall, negating the need to counteract heat‐related reduction in hatchling morphology. In comparison, the sizes of adult body and clutches from both foragers did not vary significantly. The results further support our previous suggestions that the size‐related foraging dichotomy exhibited by adult sea turtles does not have a genetic basis, but derives from phenotypic plasticity. Adjustment in reproductive investment may be associated with: (1) predation avoidance, (2) founder effect, and/or (3) annual variation in egg size.

Specialists and generalists coexist within a population of spider-hunting mud dauber wasps

Individual foraging specialization describes the phenomenon where conspecifics within a population of generalists exhibit differences in foraging behavior, each specializing on different prey types. Individual specialization is widespread in animals, yet is understudied in invertebrates, despite potential impacts to food web and population dynamics. Sceliphron caementarium (Hymenoptera: Sphecidae) is an excellent system to examine individual specialization. Females of these mud dauber wasps capture and paralyze spiders which they store in mud nests to provision their offspring. Individuals may make hundreds of prey choices in their short lifespan and fully intact prey items can be easily excavated from their mud nests, where each distinct nest cell represents a discrete foraging bout. Using data collected from a single population of S. caementarium (where all individuals had access to the same resources), we found evidence of strong individual specialization; individuals utilized different resources (with respect to prey taxa, prey ecological guild, and prey size) to provision their nests. The extent of individual specialization differed widely within the population with some females displaying extreme specialization (taking only prey from a single species) while others were generalists (taking prey from up to 6 spider families). We also found evidence of temporal consistency in individual specialization over multiple foraging events. We discuss these findings broadly in the context of search images, responses to changing prey availability, and intraspecific competition pressure.

Biochemical indices and life traits of loggerhead turtles (Caretta caretta) from Cape Verde Islands

The loggerhead turtle (Caretta caretta) is an endangered marine reptile for whom assessing population health requires knowledge of demographic parameters such as individual growth rate. In Cape Verde, as within several populations, adult female loggerhead sea turtles show a size-related behavioral and trophic dichotomy. While smaller females are associated with oceanic habitats, larger females tend to feed in neritic habitats, which is reflected in their physiological condition and in their offspring. The ratio of RNA/DNA provides a measure of cellular protein synthesis capacity, which varies depending on changes in environmental conditions such as temperature and food availability. The purpose of this study was to evaluate the combined use of morphometric data and biochemical indices as predictors of the physiological condition of the females of distinct sizes and hatchlings during their nesting season and how temperature may influence the physiological condition on the offspring. Here we employed biochemical indices based on nucleic acid derived indices (standardized RNA/DNA ratio-sRD, RNA concentration and DNA concentration) in skin tissue as a potential predictor of recent growth rate in nesting females and hatchling loggerhead turtles. Our major findings were that the physiological condition of all nesting females (sRD) decreased during the nesting season, but that females associated with neritic habitats had a higher physiological condition than females associated with oceanic habitats. In addition, the amount of time required for a hatchling to right itself was negatively correlated with its physiological condition (sRD) and shaded nests produced hatchlings with lower sRD. Overall, our results showed that nucleic acid concentrations and ratios of RNA to DNA are an important tool as potential biomarkers of recent growth in marine turtles. Hence, as biochemical indices of instantaneous growth are likely temperature-, size- and age-dependent, the utility and validation of these indices on marine turtles stocks deserves further study.

A mechanism that maintains alternative life histories in a loggerhead sea turtle population

Intrapopulation variation in habitat use is commonly seen among mobile animals, yet the mechanisms maintaining it have rarely been researched among untrackable species. To investigate how alternative life histories are maintained in a population of the loggerhead sea turtle (Caretta caretta), cumulative reproductive output was evaluated and compared between small planktivores inhabiting oceanic areas (with water depths > 200 m) and large benthivores inhabiting neritic areas (depths < 200 m) that sympatrically nested at Yakushima Island, Japan, from 1986 to 2011. In total, 362 nesting females sampled in three different years were classified into the two foraging groups based on stable isotope ratios in egg yolks. There were significant differences between the two foraging groups in most recorded life history parameters (clutch size, clutch frequency, breeding frequency, and remigration intervals), with the exception of emergence success. We did not find evidence of life history trade-offs, nor age-related changes in fecundity. Over the 26-year study period, we calculated a 2.4-fold greater reproductive output for neritic foragers than for oceanic ones, accounting for breeding and clutch frequency. Temporal consistencies in stable isotope ratios and remigration intervals within females suggested that female Japanese loggerheads show fidelity to respective foraging habitats throughout the adult stage. The large difference in productivity between the two groups was unlikely to be offset by the difference in survival during the period from aboveground emergence to first reproduction, suggesting that oceanic foragers have a lower level of fitness than neritic ones. Together with an absence of genetic structure between foraging groups, we infer that alternative life histories in a loggerhead turtle population are maintained by a conditional strategy.

The complete mitochondrial genome of the loggerhead turtle Caretta caretta (Testudines: Cheloniidae): genome description and phylogenetic considerations

The marine turtle Caretta caretta is a widely distributed species that is facing critical population decline, especially in the Mediterranean rookeries. Molecular markers, such as mitochondrial DNA (mtDNA) sequences, are of great importance for the description and monitoring of turtle migratory populations. The complete sequence of the C. caretta mitochondrial genome is presented here. The genome comprises 16,440 base pairs, containing 37 genes (13 protein-coding genes, 22 tRNA genes, and 2 rRNA genes), and a control region, all organized similar to the majority of vertebrate mitogenomes. MtDNA length polymorphism and heteroplasmy were observed among, and within, individuals due to the variable size of a microsatellite repeat residing at the 3' end of the control region. The use of the above repeat as a marker for individual fingerprinting is discussed. Furthermore, phylogenetic analyses among Testudines based on complete mitogenomes, as well as among marine turtles based on partial mtDNA sequences, are considered.

Smaller longer, larger shorter: energy budget calculations explain intrapopulation variation in remigration intervals for loggerhead sea turtles (Caretta caretta)

Adult female loggerhead sea turtles ( Caretta caretta (L., 1758)) show a size-related behavioural and trophic dichotomy within several populations: oceanic planktivory by small females and neritic benthivory by large females. Within a Japanese population, smaller females have longer remigration intervals (the intervals between successive nesting years) than larger females. We hypothesized that this is attributed to the size-related foraging dichotomy, and calculated energy budgets and food requirements for the small and large females. Total reproductive energy costs for the small and large females were 1003 and 434 MJ, equivalent to 4043 kg of planktonic items and 275 kg of benthic items, respectively. The minimum durations necessary for accumulating the total reproductive energy for the small and large females were 430 and 26 days, equivalent to remigration intervals of 2 years and 1 year taking into account the nesting and migration durations and seasonal reproduction. Therefore, it was theoretically demonstrated that the intrapopulation variation in remigration intervals for Japanese loggerheads could be caused by the difference in their feeding habits, implying that remigration dynamics in a sea turtle population that shows a foraging dichotomy is regulated by feeding conditions not in a single habitat but in multiple habitats.