The diet and feeding ecology of the green sea turtle (Chelonia mydas) in an algal-based coral reef community

Field: Agricultural and Biological Sciences (Life Sciences)

Nutrition is considered to have an important influence upon the life history of the green sea turtle including regulation of its growth rate, age at sexual maturity, egg production and remigration intervals amongst other influences. It would therefore be expected that the green turtle would select dietary items that would maximise its nutrient intake and balance its energy budget as predicted by optimal,foraging models. \n \nAlthough the feeding ecology of green turtles in seagrass communities has received attention, no study to date has investigated the feeding ecology of green turtles in an algal community. This was the first study to describe the diet and feeding ecology of green turtles foraging in an algal-based community (Heron Reef, Queensland) where seagrasses were absent. \n \nAlgae account for the greatest areal cover of benthic biota on Heron Reef with over four times the areal cover of living corals. Heron Reef supports over 115 species of algae although only seven of these ever exceed more than 2% of the total algal cover. The main component of the algal assemblage is the algal turf which accounts for 56% of the total areal cover of algae. The algal community on Heron Reef is composed of species that are dynamic in distribution and diverse in chemistry. Although no trends or patterns could be detected, there is a significant spatial and temporal variation in the areal cover of algae both within and between reef habitats. The lipid, carbohydrate, ash and energy content of macroalgae species on Heron Reef varied significantly from each other but did not change significantly over time. In contrast, the change in levels of nitrogen was significant over time. \n \nGreen sea turtles on Heron Reef include both resident and migrant turtles. The resident green turtles include animals from all age classes z35 cm in curved carapace length with immature animals accounting for 74% of the resident population. The sex ratio of the resident turtles is not significantly different than 1:1. \n \nAlgae are the most important and almost the exclusive diet item of green turtles of all age classes and both sexes on Heron Island during all seasons. Green turtles on Heron Reef demonstrate diet preference and avoidance of undesirable species. The diet varies significantly both temporally and between age classes although there is no continuity or discernible pattern to these changes. The differences observed between the age classes may disappear when desirable dietary species become available. There are no discernible differences in diet between sexes. \n \nThe diet of individual turtles captured on repeat occasions varied over time although there is no apparent pattern or continuity to this change. Some individuals exhibited considerable shifts in diet while others remained rather constant in their dietary choices. Green turtles of all age classes appear to have a base diet of algal turf but they will opportunistically exploit desirable monogeneric stands of algae when they become available. \n \nOptimal foraging models that predict diet based upon a single variable do not serve as useful predictors of diet in green turtles feeding in complex algal communities. Although diet selection does occur, green turtles on Heron Reef do not select dietary items as an exclusive function of their availability, nutrient, ash, energy or secondary compound profiles. The green turtle appears to select dietary items in response to a complex interdependent matrix of variables that influences the nutritive and energy potential of their diet while reducing the effects of algal secondary compounds. Diet selection is most likely a combination of positive and negative (avoidance) diet selection decisions. \n \nWhen confronted with constantly changing algal chemistry and availability, the green turtle's optimal strategy may be to forage within the heterogenous algal turf. Such a strategy would ensure that at least some superior species were included in the diet while at the same time possibly mitigating the influences of secondary compounds while optimising the benefits of diet mixing. As the composition of the algal turf is dynamic, this strategy would also ensure the inclusion of newly available species in the diet.