Growth and ontogenetic change in juvenile crown-of-thorns sea star (Acanthaster sp.) morphology: Can morphometrics be used as an aging tool?

Abstract The crown-of-thorns sea star (COTS, Acanthaster sp.) is a coral predator that, in population outbreaks, causes major coral loss in Indo-Pacific reefs. Current paradigms explaining the cause of outbreaks focus on the larval and adult stages, while the early herbivorous juvenile stage remains a black box in our understanding of COTS. We followed growth in a large laboratory population of juveniles from settlement to 300 days. Ontogenetic changes in eight traits over time were quantified, including those typically used to track growth in sea stars (total diameter, arm number) and traits not previously quantified (e.g. spine number/type). Combinations of traits were modelled against age to explore their potential as indicators of age. Total diameter exhibited a strong association with age, as did covarying traits: central disk diameter, mouth diameter and arm length. The number of pointed spines was also strongly associated with age, greater in association than arm number. Our results indicate that use of a combination of morphological traits has potential as an indicator of juvenile age. In particular, pointed spines appear to have potential as an age marker for juveniles. Using size-at-age data, we investigated the fit of growth models to estimate age-size relationships. A Gompertz model provided the best fit to the growth/age data and is consistent with the size/time of diet shift (herbivory to corallivory) in juvenile COTS. Addressing knowledge gaps on these juveniles to inform age modelling using morphological traits contributes to the understanding of the biology and ecology of this cryptic life stage.