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Every year during the sea turtle nesting season we conduct nightly surveys between the months of June and October to tag, identify, and examine female green sea turtles that come to shore at TNP to lay eggs. As part of our monitoring program, every season we randomly select 200 individuals to mark their nests to evaluate their hatching and emergence success. For every one of these selected females, we perform a detailed egg count and record clutch size, which is used as the basis to estimate success rates of the nests. Nests are marked discretely by triangulation to prevent poaching. Also, nests are monitored daily throughout the incubation period. Once there is any evidence of hatchling emergence, the nests are exhumed, and hatching and emergence success are determined based on the number of collected empty shells, and any abnormalities in embryos and hatchlings are recorded.

Following the above procedures, during the 2018 nesting season two separate adult female green sea turtles with abnormal coloration were detected while nesting. Although they were not albino, the encountered females presented unusually pale complexions. These turtles were examined in detail, tagged and measured. Curved carapace length (±0.1 cm) was measured three times for each individual with a measuring tape by the same field assistant, until all three measurements were no more than one cm apart. No other abnormalities besides the lack of pigmentation in their skin and carapace were detected. As per our standard operating procedures, each nest site was marked and monitored closely during the incubation period. In addition, the clutches were protected with plastic nets to prevent predation by dogs, raccoons or armadillos. After 60 days of incubation, the nests were exhumed to determine hatching success. The offspring were examined to identify any possible malformations or abnormal hereditary characteristics.

The first leucistic female green turtle was encountered laying eggs on the north end of Tortuguero’s beach, 2.6 km south from the river mouth on 2 October. Four days later on 6 October, the second female nested 3 km south of the first location (see cover photos). Both females completed the nesting process without disturbance. Mean curved carapace length was 108.4 cm for the first and 109.7 cm for the second turtle (±0.2 cm each). Both were within the normal size range of green turtles at Tortuguero. Neither turtle presented an abnormal number of scutes. Each clutch contained over 110 eggs, the average number for the TNP green turtle colony, and had a hatching success of 94.7% and 90.3%, respectively (Table 1). Those values are seemingly higher than the 82.8% mean hatching success reported for this beach in the past (Fowler 1979).

Table 1. Exhumation results and hatchling malformation classification for nests laid by leucistic green turtles at Tortuguero, Costa Rica.

Figure 1. Abnormal hatchlings from clutches of leucistic mothers. a) Hatchlings presenting carapace deformities. b) Contrast between a hatchling with evident lack of pigmentation on its carapace and a hatchling with standard characteristics. c) Hatchlings presenting supernumerary scutes on the rear section of their carapace. d) Leucistic, eyeless, deformed hatchling (dorsal and lateral). Photos: Jaime Restrepo.

During exhumation, we found 125 empty shells, 7 unhatched eggs, and 34 hatchlings left in the chamber of the first nest. For the second nest, we found 112 empty shells, 12 unhatched eggs, and 90 hatchlings remaining in the chamber. We measured the straight carapace length (SCL; ±0.05 cm) with a caliper of each individual hatchling that we observed and did a close evaluation of their body condition, looking for abnormalities or congenital alterations. We obtained an average maximum SCL of 5.20 ±0.20 cm. We compared this with a sample of hatchlings taken from three random nests marked during the season, from females without visible morphological abnormalities. There was no significant difference in SCL between the two groups (Student’s t-test, T = -1.6, df = 38, p = 0.119), and SCL in the leucistic hatchlings ranged from 4.80-5.40 cm. These measurements were seemingly larger than those reported for green turtles in Cyprus hatcheries (Özdemir & Türkozan 2006). Though most of the hatchlings appeared to be normal in size, shape and coloration, we recorded two with malformations of the carapace, where a few marginal scutes were missing (Fig. 1a). We observed a remarkable pale coloration in 18.6% of the hatchlings evaluated (Table 1). The lack of coloration was most evident on the ridge, across the vertebral scutes and on the back of the head (Fig. 1b). It is somewhat common among marine turtles to find one or two hatchlings with pigmentation anomalies from a single clutch; however, the lack of pigmentation in several hatchlings is rare (Perrault & Coppenrath 2019). Supernumerary scutes were detected in four individuals, mainly affecting the posterior carapace (Fig. 1c). This condition is not rare in green turtles, even under controlled laboratory conditions (Özdemir & Türkozan 2006). Scute variation on hatchlings from a single nest estimated for these cases (Table 1), was significantly lower than those presented in similar studies (Özdemir & Türkozan 2006; Zimm et al. 2017). Though scute abnormalities are more common in albino individuals, their presence does not necessarily affect the development or fitness of the hatchlings (Sönmez & Özdilek 2011). In the second clutch, we encountered a live, active leucistic hatchling ready to emerge from the nest. Besides being leucistic, this individual presented several deformities of the carapace and the head. It had an absolute absence of eyeballs and snout (Fig. 1d). Previous reports of albino hatchlings have shown little to no successful emergence in natural conditions (Türkozan & Durmuş 2001; Perrault & Coppenrath 2019), though recently up to one fourth of a green turtle clutch resulted in presumably healthy albino hatchlings (Perrault & Coppenrath 2019). Although this eyeless individual may have had the energy and stamina to crawl to the sea, its sight limitation, head deformities, and coloration made it seemingly vulnerable to predation. In fact, this individual wandered on the beach in circles. For these reasons, we sacrificed the individual by placing it in a freezer at -20°C, as recommended (Lillywhite et al. 2016).

The two encounters with leucistic adult females constitute the first reports of adult, reproductively active green sea turtles at TNP with evidence of leucism, with a brief description of the fate of their offspring. Interestingly, there is a report of an adult “amelanic” loggerhead sea turtle nesting in the South Queensland, Australia (Limpus et al. 1979). Incubation of a few eggs from this female resulted in the production of normally pigmented hatchlings. However, inspection of eggs left in random nests on the beach revealed the occasional presence of dead leucistic embryos that were incapable of tearing the eggshell, presumably due to the lack of the caruncle, a deformity that may have been associated with the lack of coloration (Limpus et al. 1979). These reports not only show that leucistic individuals can survive to adulthood, but that they can become reproductively active and contribute to the population

Acknowledgements. We thank the STC staff and research assistants at the John H. Phipps Biological Station in Tortuguero, Costa Rica, who gave their best effort night and day during the monitoring activities and without whom this report would not have been completed. We would also like to extend our gratitude to the Tortuguero National Park Rangers and staff for their constant support and collaboration. We also thank the Ministry of Environment through the National System of Conservation Areas (SINAC), who allowed us to continue with our research through the internal resolution NAC-ACTO-RE-020-2018; file M-PC- SINAC-PNl-AcTo-O14-18. Finally, to ASOPROTOMA and its turtle spotters with whom every night we share the beach and the turtles, thanks for your vigilant and conscientious work.

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