Light Sandy Beaches Favour Hatching Success and Best Hatchling Phenotype of Loggerhead Turtles

Abstract

We conducted a 5-year field (2017–2021) and laboratory study of the relationship between type of substrate and hatching success, embryonic development, and the quality of hatchlings in loggerhead turtle nests. Our study site, the island of Maio in the archipelago of Cabo Verde, one of the world’s largest loggerhead turtle nesting colonies, displays marked heterogeneity of sand colouration, with dark, mixed, and light sandy beaches. We experimentally incubated eggs, comparing different nesting substrates under standard temperature and humidity conditions. Females nest in all sand types without preference. However, both the field and experimental study revealed a significant difference in hatching success depending on the type of substrate. Substrate of volcanic origin, dark in colour, with a lower amount of calcium carbonate, had a lower hatching success (HS; 30.3 ± 20.2%) compared to substrates of mixed (HS = 46.1 ± 26.5%) or light (HS = 78.1 ± 18.2%) colour. Eggs experimentally incubated in substrate that was light-coloured, with a larger grain size and higher calcium carbonate concentration, produced significantly more and larger offspring. Incubation temperatures were significantly higher in dark substrate, which partially explains the lower hatching success in this type of sand. However, experimental incubation with controlled temperatures consistently showed lower hatching success in dark sand. Thus, we found that not only the temperature, but also the specific characteristics of each substrate determine hatching success. The main predator of eggs and hatchlings (the ghost crab Ocypode cursor) showed no significant differences in abundance or size between different substrate types. Our results indicate that nest site selection between beaches or even within the same beach with different substrate conditions affects hatching success, hatchling physical condition, and subsequently the reproductive success of each female. The results of this study can inform conservation programmes with nest management and controlled incubation in the field and optimise adaptive nest management under future scenarios of rising global temperatures.

Ghost crab predation of loggerhead turtle eggs across thermal habitats

“…This result was slightly higher than the one estimated by Patino-Martinez et al ( 2021) combining several Maio Island beaches but was, however, less impacting than what has been reported in Boa Vista Island, where 98% of clutches were attacked by ghost crabs at the highest-density nesting area (Marco et al, 2015). Predation levels were significantly higher at dark sand beaches, so it does not seem that potentially reduced embryo survival at these warmer beaches (Martins et al, 2020;Patino-Martinez et al, 2022) may be compensated by lower ghost crab predation. A putative explanation for the observed predation levels at dark sand beaches is linked to the negative impact of temperature on sea turtle clutches.…”

“…Here, we used density and diameter of ghost crab burrows as proxies for ghost crab density and size (Valero-Pacheco et al, 2007;Marco et al, 2015;Oliveira et al, 2016). Despite warmer dark sand beaches likely having higher evaporation rates, which could potentially limit the activity of ghost crabs (Lucrezi and Schlacher, 2014), we found no consistent trend on ghost crab density and size structure towards any beach or beach type across years, which is in line with Patino-Martinez et al (2022).…”

“…from the tide line to the upper beach, Patrício et al, 2018) or between beach type (e.g. beaches with dark sand vs. beaches with white sand, Martins et al, 2020;Patino-Martinez et al, 2022), nest site selection has the potential to both enhance or reduce this parameter (Marco et al, 2018;Patrício et al, 2018). Thus, understanding the nest site selection behavior and its consequences for clutch survival is fundamental, particularly for populations of conservation concern.…”

Environmental and pathological factors affecting the hatching success of the two northernmost loggerhead sea turtle (Caretta caretta) nests

“…As shown by the data reported in Table 1 , the two nests showed different hatching success with the lower results (11%) in the most urbanized location (Jesolo Lido) compared to the average (66%) reported in the Western Mediterranean 2 . Human presence and activities, such as the urbanization, the touristic preparation of the beach, and pollution, could have also compromised embryonal survival 24 , 29 .…”

“…Nest 1 area is a heavily urbanized beach with a large tidal range as shown by the distance of the nesting area from the shore (from 2 to 50 m depending on the tide) and a gentle slope artificially created by beach nourishment using sand from inland areas of riverine origin, including silt, clay and organic material to cope with the coastal erosion. Beach nourishment activities, which are constantly performed from early spring in the area of Nest 1, may also have influenced on hatchling success 33 – 29 . Also, the cleaning constantly performed using mechanical systems could have influenced the hatching success by inducing a shallower deposition and reducing the nest dimensions due to soil compaction, as evident by comparing the two different beaches in Table 1 35 .…”