Mortality is high in embryos, but why?
Leatherback sea turtles (Dermochelys coriacea) lay an average of 65 eggs in each clutch, but only about half will successfully hatch. Embryonic mortality is the main cause of this, and less so because of egg infertility. The reason(s) for this high mortality are not yet known but a variety of possible explanations have been put forward.
Previous research done on leatherback sea turtle eggs has found that nest gas concentrations and temperature vary significantly with embryonic development. Within in the nests, gas concentration and temperature gradients develop as embryonic development progresses. Eggs in the center of the nest will be exposed to higher temperatures and lower oxygen tension. This is interesting, coupled with the knowledge that the second half of the incubation period in the nest is when embryonic development and growth are most affected by changes in respiratory gas concentrations. A study done on St. Croix, US Virgin Islands leatherback nests, found both partial pressure of carbon dioxide (pCO2) concentrations and temperature increase as partial pressure of oxygen (pO2) concentrations decrease in the nests, all of which correlated to embryonic death in the clutches. It was found that embryonic mortality typically occurred in later stages of development in St. Croix clutches and hatching success decreased after pCO2 exceeded 490 to 590 ppm and nest temperature exceeded 35ºC. While studies have cited temperatures >33ºC to be the maximum thermal tolerance of leatherback embryos, eggs still hatch successfully even when exposed to temperatures several degrees above 33-35ºC. The St. Croix study contradicts a similar study done on leatherback sea turtle nests at Playa Grande, Costa Rica where early stages of development had the highest mortality when pO2 concentration was high and pCO2 was low. They did find that spatial position of the eggs in the nest was a significant factor in embryonic mortality, where eggs located in the periphery of the nests were significantly more successful than those in the center.
Other abiotic factors have been noted in previous studies to have an effect on nest environments, such as sand particle size, which also influences pO2, pCO2, and temperature. A study done by Mortimer (1990) linked mean particle size with embryonic mortality in green sea turtle (Chelonia mydas) clutches, where coarser particle sizes were correlated with embryonic mortality, potentially due to physiological stress or even nest-caving. Tidal fluctuations and substrate moisture have also been cited as being important factors in hatching success. When taking all of the above factors into consideration, one main theme becomes apparent: all factors and their effects vary greatly depending on the location of the study and nesting site, hence the confusing and often contradictory literature.
Why is this important?
Of the three sea turtle species that have been assessed by the International Union for the Conservation of Nature (IUCN), all are listed as either vulnerable (Olive Ridley, Leatherback) or critically endangered (Kemp’s Ridley). Coastal development infringing on nesting beaches, entanglement in fishing gear, individuals being caught as bycatch, light pollution, egg poaching, ingestion of plastics and so on are reasons for the turtle species’ vulnerability. There are many conservation groups worldwide currently working on ensuring nesting beach habitat is conserved and monitored in order to increase hatching success of sea turtle species and in many cases relocation of eggs to hatcheries. It appears that nest gas concentrations may play an important role in egg hatching success, although how this works is still unclear. Additionally there are large knowledge gaps in temperature limits for all sea turtle species, which should be addressed in future studies. These highlighted factors are all missing pieces to a large puzzle that is ‘how will climate change impact these sea turtle species that depend entirely on terrestrial coastlines in order to reproduce?’. Understanding these pieces will allow for better conservation measures in future, beginning with optimizing egg hatching success.
We will be following this particular field of study and hopefully provide some conclusive evidence in the not-so-distant future.
Howard, R., Bell, I., and D. A. Pike. 2014. Thermal tolerances of sea turtle embryos: current understanding and future directions. Endang Species Res 26: 75-86
Marine Turtle Specialist Group. 1996. Lepidochelys kempii. The IUCN Red List of Threatened Species 1996: e.T11533A3292342. http://dx.doi.org/10.2305/IUCN.UK.1996.RLTS.T11533A3292342.en. Downloaded on 11 January 2016
Mortimer, J. A. 1990. The influence of beach sand characteristics on the nesting behaviour and clutch survival of green turtles (Chelonia mydas). Copeia 1990: 802-817
Ralph, C. R., Reina, R. D., Wallace, B. P., Sotherland, P. R., Spotila, J. R., and F. V. Paladino. 2005. Effect of egg location and respiratory gas concentrations on developmental success in nests of the leatherback turtle, Dermochelys coriacea. Aust J Zool 53: 289-294
Wallace, B. P., Sotherland, P. R., Spotila, J. R., Reina, R. D., Ranks, B. F., F. V. Paladino. 2004. Biotic and abiotic factors affect the nest environment of embryonic leatherback turtles, Dermochelys coriacea. Physiol Biochem Zool 77: 423-432