Ilha Byrne, Cynthia Riginos , Dean Brookes, Sven Uthicke, Iva Popovic

Metabarcoding is revolutionising the analysis of biodiversity in marine ecosystems, especially as it provides a means of detecting and identifying cryptic life-stages in field samples. The planktonic larval stage of many species underpins the abundance and distribution of adult populations but is challenging to characterise given the small size of larvae and diffuse distributions in pelagic waters. Yet, planktonic larval dynamics are key to understanding phenomena observed in adult populations, such as the boom- and-bust dynamics exhibited by some echinoderms. Rapid changes in echinoderm population density can have significant effects on local benthic ecosystems, for example, outbreaks of the crown-of-thorns seastar (CoTS) on the Great Barrier Reef (GBR) are responsible for declines in coral cover. Here, we used a DNA metabarcoding approach to investigate the spatio-temporal distribution and diversity of echinoderm larvae on the GBR, including CoTS. Echinoderm larvae were found to exhibit seasonal changes in community composition and richness, consistent with expected fluctuations in larval output based on adult spawning periodicity. Furthermore, this study validates the utility of metabarcoding approaches for the surveillance of CoTS larvae, which could prove useful to future monitoring efforts. Our findings suggest that metabarcoding can be used to better understand the life history of planktonic larvae, and analyses combining environmental (e.g., temperature, nutrients) and oceanographic (e.g., currents) data could deliver valuable information on the factors influencing their spatio-temporal occurrence.