Resilience and adaptation of COTS larvae to development in oligotrophic tropical waters: maternal provisioning and larval cloning

Abstract

Explanations of crown-of-thorns sea star (COTS, Acanthaster) outbreaks have focussed on the larval stage and the terrestrial runoff – larval starvation hypothesis – whereby eutrophic runoff from floods generate plankton blooms increasing larval food and success. In contrast to this notion, we found that the larvae of COTS are highly resilient to food scarcity. This is likely an adaption to life in oligotrophic (low nutrient) tropical seas as well as due to the extensive energetic reserves provided by their unusually large eggs. We reared eight populations of COTS larvae all from different parents in course filtered lagoon water (60 µm FSW) to remove potential predators but retain nutrients typical of reef waters. Over 21-25 days the larvae grew to the advanced larval stage. The energetic buffer provided by the eggs allowed for an extended period when the larvae developed in the absence of food augmentation but in parallel would be competent to avail of patches of phytoplankton as they may encounter in nature. This is the longest facultative feeding period for any sea star species with a feeding larva. There was some difference in the larval populations in how long they lasted as well as in the propensity of larval cloning – where the larvae fragment and regenerate to make more of themselves. Although the larvae achieved an advanced stage in the absence of exogenous food it was clear that an algal food source is needed to achieve the final settlement stage. Our findings reinforce the resilience of the larvae of COTS to food scarcity and show that they do not require nutrient augmentation generated by runoff to reach the advanced larval stage.