Non-invasive monitoring of the fertilization rate in sea turtle nests: comparison between two techniques

Abstract

Egg fertilization rate is a defining event in the life history of oviparous species. However, for many terrestrial oviparous species, this parameter is only estimated after hatching and emergence of hatchlings, by studying evidence of embryonic development in unhatched eggs. We compared the accuracy of this method with a second technique based on the careful excavation of sea turtle nests within the first 96 hours of oviposition and calculating the proportion of alive eggs, where a clear white spot is externally visible in the shell. Both methods underestimate the fertility rate but we are not aware of any other alternative on-invasive technique. The study was conducted in a nesting population of loggerhead turtles (Caretta caretta) at Boa Vista Island (Republic of Cabo Verde). We found significant differences in the estimation of fertilization rate between both techniques. When inspecting eggs after hatching, researchers significantly overestimated the number of unfertilized eggs and this calculation was not correlated with the estimation based on examination of eggs soon after oviposition. The first estimate was not correlated with hatching success or hatchling phenotype. There was no relationship between the number of viable embryos at oviposition and the hatching success and the hatchling's phenotype. The absence of embryo or its early death had no effect on survival of developing embryos within a nest. Fertilization rate in loggerheads did not show spatial or temporal variation during nesting season. We suggest the implementation in sea turtle nesting monitoring programs of annual less-invasive estimation of egg fertilization rate based on the excavation of a sample of representative nests immediately after the formation of the white spot in the shell of developing embryos.

Effects of postovipositional hypoxia and hyperoxia on leatherback turtle reproductive success and hatchling performance

“…White spot formation did not indicate that individual eggs would develop successfully through to hatching; nearly all eggs formed white spots, even though there was reduced hatching success and higher early‐stage mortality in the hypoxic treatments. This provides further justification for monitoring white spot formation as a noninvasive method for estimating the fertilization rate for a clutch of eggs (Abella et al, 2017; Phillott et al, 2021; Phillott & Godfrey, 2020) because we found that the white spot still formed even when embryos died at early developmental stages. Based on our white spot formation data, at least 97% of eggs collected for our ex situ experiment were fertilized.…”

“…After 3 days (3‐day hypoxia treatment) or 5 days (remaining three treatments), eggs were carefully removed from their Perspex chambers and placed in sand (7 ± 2% gravimetric water content) within normoxic incubators set to 30 ± 1°C. The eggs were monitored three times daily for the formation of the opaque white spot on the upper surface as an indication of development recommencing after oviposition (Abella et al, 2017). While the eggs were being removed from their chambers, two eggs from the hyperoxia treatment were inadvertently rotated.…”

Trace Elements and Persistent Organic Pollutants in Unhatched Loggerhead Turtle Eggs from an Emerging Nesting Site along the Southwestern Coasts of Italy, Western Mediterranean Sea

“…The proportion of unhatched eggs with no visible embryo ranged from 0% to 87% (mean ± sd = 15% ± 19). We have considered these indicative of early embryonic death, given the very high fertilization rate in the loggerhead turtle [ 54 ]. However, we cannot exclude that few of them were unfertile eggs since it is very difficult to discriminate between these two categories at the time of nest excavation, after approximately 2 months of decomposition [ 54 ].…”

“…We have considered these indicative of early embryonic death, given the very high fertilization rate in the loggerhead turtle [ 54 ]. However, we cannot exclude that few of them were unfertile eggs since it is very difficult to discriminate between these two categories at the time of nest excavation, after approximately 2 months of decomposition [ 54 ]. Hatching success and the proportion of unhatched eggs with no visible embryo were strongly correlated (Spearman’s r s = −0.777, p -value < 0.01).…”

Nest relocation of Leatherback turtles (Dermochelys coriacea) decrease the rate of non-developed eggs

“…At egg laying, some fertilized eggs of sea turtles could have already died. Moreover, fertile eggs at laying have no external signals of embryonic development (Abella et al, 2017). Incubation temperature influences several factors in sea turtle egg incubation.…”