Genetics of Returning Turtles

Amy Bonka wrote this post as a project for Stacy Krueger-Hadfield’s Conservation Genetics course at the University of Alabama at Birmingham. Amy grew up in Florida, completed a BS in Biology with a concentration in Marine Science and Chemistry as well as an MS in Biology from UAB. She is currently pursuing her PhD as a student in Dr. Thane Wibbels’ lab where her research is focused on early lifehistory behaviors of hatchling sea turtles and the dynamics of arribada nesting in the Kemp’s ridley sea turtle. 

How many come home? 

Barbanti et al. (2019) ask this very question about green sea turtles (Chelonia mydas) in the Cayman Islands. This isn’t a silly question as sea turtles return to nest at the beach where they hatched after years in the open ocean. This means that populations in close proximity may actually be separate nesting populations with low levels of relatedness (Shamblin et al. 2012), which can complicate conservation plans for these species. 

In the Cayman Islands, The Cayman Turtle Center was initially established to meet the desires of the local community for turtle meat. Since the 1970’s, one component of this program has been the captive breeding of green sea turtles, as well as a reintroduction program, where green sea turtle hatchlings are released on Cayman beaches. Captive breeding is a part of many species survival plans, though reintroductions from programs is far less common. One of the components of these species survival plans is projected gene diversity, or the projected diversity within the population. In captive breeding and reintroduction strategies, not only is it important to evaluate the health and viability of the breeding individuals, but also the relatedness of the breeders and their offspring, as this component can impact future generations through inbreeding and/or outbreeding depression. 

Barbanti et al. (2019) were the first to evaluate the genetic diversity of the Cayman Turtle Center reintroduction program. Samples were taken from 257 breeding stock females and from 57 wild nesting females. The farm breeding stock samples were split into three groups based on their origins: (i) the original stock that was collected from distinct populations; (ii) C1995, those born at the farm in 1995 and kept for breeding; and (iii) MCF1, those born at the farm between 1986-2002. 

The original founder stock had high diversity and low relatedness, whereas the C1995 group had lower variability and higher relatedness, as may be expected from a single, large replacement. C1995 individuals were kept to increase the numbers of breeders after 81% were lost due to hurricane Michelle. 

For conservation programs that have breeding individuals, continual small replacements are better for the overall genetic variability then a single, larger replacement. Additionally, Barbanti et al. (2019) found that 90% of the sampled wild population is related to individuals at the Cayman Turtle Center (Figure 1).

There were several instances in which farm individuals were the mothers of wild nesting females (Figure 2).

Though it is unclear if those wild nesting individuals are individuals that were released or individuals that escaped during hurricane Michelle, there have been reports of released hatchlings later nesting on the Cayman Island beaches.  We can tell this because some of these hatchling turtles were marked before release by taking a small piece of the bottom shell and switching it with a small piece of the top shell. The top and bottom shells are different colors and this system can be used to mark turtles in a highly visible way (Bell et al. 2005).

There will always be varying opinions when it comes to the topic of keeping animals in captivity, but these data suggest that the release of individuals from the Cayman Turtle Center may be contributing to the increase in nesting females on Grand Cayman. Data from the Cayman Department of the Environmentshows an increase from about 30 nesting green sea turtles in 1999 to about 400 in 2017. Green sea turtles, like all species of sea turtles, nest more than once per season, meaning the actual numbers of nesting females in the population is less than the total number of nests for the season, however these data still shows a dramatic increase over the past 18 years.

So, how many do come home? According to this study, a lot! 

References

Barbanti, A, C Martin, JM Blumenthal, J Boyle, AC Broderick, L Collyer, G Ebanks-Petrie, BJ Godley, W Mustin, V Ordonez, M Pascual. 2019. How many come home? Evaluating ex-situ conservation of green turtles in the Cayman Islands. Molecular Ecology 28:1637-1651. 

Shamblin BM, AB Bolten, KA Bjorndal, PH Dutton, JT Nielsen, FA Abreu-Grobois, KJ Reich, BE Witherington, DA Bagley, LM Ehrhart, AD Tucker. 2012. Expanded mitochondrial control region sequences increase resolution of stock structure among North Atlantic loggerhead turtle rookeries. Marine Ecology Progress Series, 469, pp. 145-160. 

Bell CD, J Parsons, TJ Austin, AC Broderick, G Ebanks-Petrie, BJ Godley. 2005. Some of them came home: the Cayman Turtle Farm headstarting project for the green turtle Chelonia mydas. Oryx, 39(2), pp. 137-148. 

About Stacy Krueger-Hadfield

I am a marine evolutionary ecologist interested in the impacts of seascapes and complex life cycles on marine population dynamics. I use natural history, manipulative field experiments and population genetic and genomic approaches with algal and invertebrate models in temperate rocky shores,estuaries and the open ocean.
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