Fungal colonisation of sea turtle nests in eastern Australia

This thesis reports on a study of fungi found in nests of green (Chelonia mydas), loggerhead (Caretta caretta), flatback (Natator depressus) and hawksbill (Eretmochelys imbricata) sea turtle nests at Heron Is., Wreck Is., Mon Repos, Peak Is. and Milman Is., eastern Australia, examined during the 1995/96-1998/99 nesting seasons. Egg mortality and fungal colonisation of eggs were significantly greater in loggerhead turtle nests at Heron Is. and Wreck Is. than in green turtle nests at the same two rookeries, and also greater than in loggerhead, green, flatback and hawksbill turtle nests at other rookeries. Three species of fungi, Fusarium oxysporum, Fusarium solani and Pseudallescheria boydii were frequently isolated from failed eggs in nests of all turtle species at all rookeries. The fungi are all ubiquitous soil species, and probably originated from the nest substrate. However, there was some evidence for acute, intra-seasonal oviductal contamination of eggs with fungi, accumulated in the cloaca and oviduct during nesting behaviour. The Pisonia grandis forest and high seabird density, present only at Heron Is. and Wreck Is. of all the rookeries investigated, did not appear to harbour fungi colonising sea turtle eggs at these locations. Within the sea turtle nest, fungi first appeared on an egg that had failed from other (natural) causes. Spores and hyphal fragments in the sand are likely to be disturbed during the nesting process and may settle on the exterior of sea turtle eggs. Experiments showed that the infection of viable eggs of all sea turtle species with fungi is possibly inhibited by the anti-fungal properties of mucus secreted during oviposition, egg albumen and the dense ultra-structure of the eggshell. However, if an egg dies the lytic enzymes and acids produced by F. oxysporum, F. solani and P. boydii could allow penetration of the eggshell and utilisation of egg contents. Using this nutrient source, hyphae could then expand to adjacent, viable eggs. The linear growth rate of all fungi varied with the thermal and hydric conditions during incubation. Embryo mortality as hyphae spread across a viable egg is probably due to inhibition of the respiratory surface area or possibly to calcium deprivation. Analyses of hatchlings from nests with different levels of fungal colonisation suggested that hatchlings emerging from nests that have a high percentage of eggs colonised by fungi should have a similar fitness to those from nests without fungi. Any nest characteristic that enhances egg failure, particularly early in incubation, will markedly accelerate fungal invasion of the sea turtle nest. Each additional failed egg greatly increases the likelihood of a focus of growth being available for fungi. The increased vulnerability of loggerhead nests at Heron Is. and Wreck Is. to egg failure and subsequent colonisation by fungi is likely to be to due characteristics of the nest. Significantly high substrate conductivity at Heron Is. and Wreck Is. is likely to impose osmotic stress on loggerhead eggs, which are smaller than green turtle eggs in adjacent nests, and result in higher egg mortality. Once fungal invasion of the nest is established, equivalent linear growth of fungus will cover a greater surface of the egg and allow faster access to adjacent eggs in loggerhead turtle nests than green turtle nests. This results in a significantly lower hatch success of loggerhead turtle nests.