The role of sea hares as significant algal herbivores on the Southern Great Barrier Reef

Sea hares are cryptic, herbivorous sea slugs present on many coral reefs worldwide. Although they are known to consume high quantities of macroalgae, they are rarely considered or quantified in assessing algal suppression. Their role as algal suppressors is not well understood, therefore this study sought to quantify sea hare herbivory on the abundant red algal genus Laurencia. The sea hares Aplysia dactylomela, Dolabella auricularia and Dolabella sp. were collected and used in herbivory trials in aquaria at Heron Island on the Southern Great Barrier Reef to quantify their rate of consumption. A. dactylomela consumed 76.9 g partially dried weight of Laurencia per 24-h, which was significantly more than the other species, with D. auricularia consuming an average of 22.5 g and Dolabella sp. consuming 37.4 g partially dried weight. Consumption of Laurencia by A. dactylomela increased with body size, with the exception of the smallest animals, which ate up to 12.5 times their body weight. D. auricularia consumed significantly more algae than was lost in control tanks. Although we observed Dolabella sp. feeding on Laurencia, the amount consumed was not significantly different to loss of algae in control tanks. The rate of herbivory on Laurencia by A. dactylomela is comparable or greater than that of more commonly studied reef herbivores such as fishes and urchins, which suggests that they may contribute to algal suppression on coral reefs and further studies are warranted to evaluate their possible role in coral-algal phase shift dynamics.