Quantifying larvae of the coralivorous seastar Acanthaster cf. solaris on the Great Barrier Reef using qPCR

Coral reefs are under threat from a variety of sources including the corallivorous seastar, Acanthaster cf. solaris. (Crown of Thorns Seastar; CoTS). Outbreak prediction is a strategic component of managing the impact of this boom and bust species. Details on the fate and dispersal of planktonic life stages are limited, with CoTS larval stages indistinct morphologically from many other asteroid larvae. Given the similarity of many larvae, quantification of marine larvae stages is a major challenge for marine ecologists. We describe a quantitative polymerase chain reaction (qPCR) assay that enables the enumeration of CoTS larvae in field collected plankton samples. Specific primers for the mitochondrial cytochrome oxidase subunit 1 gene (mtCOI) were developed and validated for specificity and sensitivity. Larval culture experiments with CoTS allowed us to determine the mtCOI copy number per larval stage which aided in relating copy numbers in the plankton to actual larval densities. We found the mtCOI copy number varied 3.6-fold across all CoTS planktonic life stages from unfertilised oocyte to competent brachiolaria and this variation was taken into account when determining CoTS larval densities in field samples on the Great Barrier Reef (GBR). CoTS larvae were detected at many locations in the CoTS ‘initiation box’ on the GBR between Cairns and Lizard Island in December 2014 with the highest mean CoTS larval density at 36.9 (23.7–84.3) CoTS larvae m³ between Rudder and Tongue Reef (16.233°S, 145.643°E). Field negative samples taken outside spawning season along with DNA extraction recovery experiments confirmed our sampling, extraction and assay methods were robust. This method will greatly help further studies on understanding CoTS larval ecology and outbreaks, with methods developed also important tools for other ecologically and commercially important marine species.