Three experiments were carried out with laboratory reared Acanthaster planci larvae. In all experiments larvae were transferred to culture dishes containing 80 ml of artificial seawater with no thyroxine added (controls) and thyroxine added at different concentrations. Larval density was less than 1 larva/ml. Unicellular algae were supplied to control and thyroxine cultures at the same density. All cultures were maintained in a 27°C constant temperature room. Larvae at various stages of development, but with most being at the mid-brachiolaria stage, were transferred to control culture dishes and culture dishes containing thyroxine at one of three concentrations (10^-6M, 10^-7M, 10^-8M). The stage of larval development was recorded after 26 hours.The effect of thyroxine on larval settlement was investigated by adding coral fragments with crustose coralline algae on their surfaces to control cultures and cultures exposed to two concentrations of thyroxine (10^-7M, 10^-8M) to induce settlement. The experiment was terminated 3 days later when all of the larvae in some of the culture dishes had settled. Coral fragments were removed so that all surfaces could be inspected microscopically and the number of settled (attached, metamorphosing) larvae and juvenile starfish counted. To more precisely quantify development acceleration by thyroxine, early-brachiolaria-stage larvae were transferred to control dishes and dishes containing one of three thyroxine concentrations (10^-7M, 10^-8M, 10^-9M). After 2 days larvae were measured and the development stage of each larva recorded.

Thyroxine accelerates adult rudiment development in sea urchin larvae. This study was undertaken to examine whether larvae of the crown-of-thorns starfish, Acanthaster planci, exhibit a similar response to thyroxine.