Invisible Records Reveal New Understandings

By HANNAH VANDER ZANDEN

Sea turtles, like all other organisms in the natural world, carry invisible records of their biological history. Researchers simply need to know where and how to look for these records. Stable isotopes are among a growing number of intrinsic markers biologists use to extract information about organisms’ environments without having to observe their actions directly. Considering that sea turtles use a variety of diet and habitat resources throughout their lives, make regular migrations, and can be difficult to encounter (apart from the small fraction of the life cycle during which females come ashore to nest), this methodology has been useful for ecologists and conservationists to make discoveries about the diet and habitats of these sometimes elusive creatures.

Naturally occurring stable isotopes are forms of elements that have differing numbers of neutrons and do not decay through time like radioactive forms. The difference in mass between the light (common) and heavy (rare) forms of the elements can lead to predictable patterns in isotope distributions, which is precisely what gives rise to their utility as a natural biological tracer. In the marine realm, the most commonly used isotopes in ecological studies are carbon and nitrogen, and the otherwise unobservable differences in heavy and light forms of these elements can be measured in small tissue samples using mass spectrometry techniques that are relatively inexpensive. Both carbon and nitrogen are incorporated by primary producers at the base of the food web and then transferred through trophic levels by consumers that assimilate the stable isotopes into their tissues, thus acting as indicators of diet. Because of baseline differences in the isotope signals from different regions in the ocean, stable isotope concentrations can also reflect location. Although these two components may be inextricably linked, this article focuses first on the advances in research on sea turtle diet and habitat use that are gained from stable isotope analysis. In addition, these data can be useful for informing habitat protection and conservation measures. The article then discusses gathering location information.

You are what you eat

One of the most basic interactions an organism can have with its environment occurs through foraging. Organisms are an isotopic representation of what they eat, often with some measurable offset that can be useful in determining trophic level (such as herbivory, omnivory, or carnivory) or the baseline primary producer the organisms use in the food web (such as seagrass or algae). Depending on the tissue sample used (blood, skin, or egg yolk), the record may reflect short or long periods, and in some cases, when the tissue can be subsectioned, such as scute or bone, the sample provides a chronological record through time.

Stable isotope results have been used to document ontogenetic diet shifts (and the lack thereof) in sea turtles. These data have been critical in altering the assumptions regarding green turtles’ diet, specifically by demonstrating that the only herbivorous sea turtle species is not always so. In some cases, green turtles continue feeding carnivorously past the oceanic juvenile stage and even in coastal areas as large juveniles and adults, thus revealing a previously unrecognized trophic role of green turtles in the marine environment.

In habitats where two or more sea turtle species may overlap, stable isotope analysis can be used to define potential dietary competition between species. For example, hawksbill aggregations have been observed to co-occur with green turtles in seagrass habitat, likely as a result of the decline of coral reef habitat, yet the stable isotope values of these two species in the same location indicated that they do not have similar diets, thus eliminating the likelihood of competition in the shared habitat.

In the future, stable isotope sampling also may be able to identify shifts in sea turtles’ diets caused by factors such as habitat decline, oil spills, food availability, and so on. However, recognizing those shifts in sea turtle populations requires first establishing a baseline and then continuing regular monitoring.

Where wert thou?

The geographic variability in stable isotope values within ocean basins has made this approach useful for tracking animal movements in the marine environment. Determining the geographic location where a sea turtle (or its tissues) may have originated relies on understanding isotopic distributions across space, yet large-scale isotopic maps of the oceans have thus far been very limited. Instead, relating sea turtle isotope values with geographic location has been accomplished primarily using the combined approaches of satellite telemetry and stable isotope sampling to identify regions that are isotopically distinct. After these relationships have been validated, then stable isotope analysis alone can be applied to a large number of individuals to track their movements.

For example, stable isotope samples from nesting females have been studied to identify population-level patterns in the different coastal areas or coastal versus oceanic habitats the turtles used. This is an exciting direction for sea turtle research, because the stable isotope approach holds great promise as an inexpensive way of monitoring sea turtle foraging aggregations from the nesting beach, where sea turtles are more easily accessed.

In addition, the sequential records that can be obtained from subsections of scute tissue from the carapace have revealed changes in foraging location through time (such as juvenile green turtles changing from oceanic to coastal habitat) as well as long-term consistency in foraging location used by adult loggerheads and green turtles. Similarly, sea turtle bone exhibits growth rings, and the study of these growth marks (skeletochronology), in combination with stable isotope analysis of the bone sections, has been used to detect the timing of ontogenetic shifts in juveniles to estimate the duration of the oceanic stage and differences in growth rates before and after the habitat shift.

Despite the benefits of stable isotope data, there are limits in classifying turtle movements or origin. First, the period represented in the tissue sample is finite, but the approximate period needs to be known for the sample to be useful. Unlike fish otoliths, which contain a whole life history, sea turtle tissue does not provide a record of the turtle’s entire lifespan. Life history also cannot be obtained in bone from nonliving turtles. Second, only broad regions of habitat use can generally be identified; precise locations of origin cannot be pinpointed. Nevertheless, stable isotope samples from satellite-tracked turtles are very valuable, and the paired data from these two methods may lead to higher resolution maps, or isoscapes, to make finer scale assumptions of likely origin in the future. Stable isotopes in combination with other types of data, such as trace elements or genetics, may also contribute to an increased ability to assess movements and connectivity between nesting and foraging aggregations.

The bigger picture

Stable isotope analysis avoids some of the logistic challenges of working with sea turtles in the otherwise vast and opaque ocean environment. Moreover, sea turtle research using stable isotope analysis has contributed to increasing our knowledge of sea turtle habitat and diet, which can ultimately improve conservation measures. For example, monitoring population trends in foraging aggregations from the nesting beach would allow researchers to know whether particular foraging areas are at risk and to geographically prioritize conservation efforts. Likewise, several studies have used stable isotopes to identify foraging locations of sea turtles and link habitat use to reproductive output, revealing that where turtles forage can contribute to differences in demographic parameters. Having a better handle on what influences these parameters allows us to improve population models.

Above all, these invisible records that sea turtles carry can provide insight into their behavior and ecology. Knowing where we can find them and what they might be consuming in these places can help us to better protect them. Given the advantages of using this method, perhaps one day stable isotope sampling of sea turtles will become as common as applying flipper tags, thus bringing the invisible to light.