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Hatching success in olive ridley mass nesting arribada beaches is typically low. We conducted a short study to understand whether incubation temperature in dry months accounts for exceedingly low hatching success at Ostional Beach, Costa Rica, a mass nesting rookery. We measured in situ incubation temperatures in nests from 4 arribadas recorded in October to December 2008, and January 2009. Mean incubation temperatures for all months exceeded the upper lethal limit of 35°C, and no nests produced hatchlings when incubated at this or higher temperatures. Embryo development was inversely related to mean incubation temperature. Hatching success was low (2%) for the study period, and only 5 of 37 marked nests produced hatchlings. Mean incubation temperature for successful nests was < 35°C. Since incubation temperatures of 32°C and higher recorded during the gonadal thermosensitive period were above the mean pivotal temperature of 30.5°C, the few hatchlings produced were presumably female. Incubation temperatures were significantly higher during the second and third trimesters of incubation during all months as a result of metabolic heating. However, during January to March when embryos did not develop, higher incubation temperatures of in situ nests relative to controls indicated that heating was a result of microbial activity associated with egg decomposition. Our study demonstrates that after the onset of the dry season, incubation temperatures at this beach become lethal.

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Several studies have suggested that significant embryo mortality is caused by microbes, while high microbial loads are generated by the decomposition of eggs broken by later nesting turtles. This occurs commonly when nesting density is high, especially during mass nesting events (arribadas). However, no previous research has directly quantified microbial abundance and the associated effects on sea turtle hatching success at a nesting beach. The aim of this study was to test the hypothesis that the microbial abundance in olive ridley sea turtle nest sand affects the hatching success at Ostional, Costa Rica. We applied experimental treatments to alter the microbial abundance within the sand into which nests were relocated. We monitored temperature, oxygen, and organic matter content throughout the incubation period and quantified the microbial abundance within the nest sand using a quantitative polymerase chain reaction (qPCR) molecular analysis. The most successful treatment in increasing hatching success was the removal and replacement of nest sand. We found a negative correlation between hatching success and fungal abundance (fungal 18S rRNA gene copies g-1 nest sand). Of secondary importance in determining hatching success was the abundance of bacteria (bacterial 16S rRNA gene copies g-1 g-1 nest sand). Our data are consistent with the hypothesis that high microbial activity is responsible for the lower hatching success observed at Ostional beach. Furthermore, the underlying mechanism appears to be the deprivation of oxygen and exposure to higher temperatures resulting from microbial decomposition in the nest.

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