Phenotypic plasticity is an important driver of species resilience. Often mediated by epigenetic changes, phenotypic plasticity enables individual genotypes to express variable phenotypes in response to environmental change. Barramundi (Lates calcarifer) is a protandrous (male-first) sequential hermaphrodite that exhibit plasticity in length-at-sex change between geographic regions. This plasticity is likely to be mediated by changes in DNA methylation (DNAm), a well-studied epigenetic modification. However, region-specific relationships between length, sex and DNAm in sequential hermaphrodites were previously unreported. To investigate these relationships, here we compare DNAm in four conserved vertebrate sex-determining genes in male and female barramundi of differing lengths from three regions of northern Australia. Despite a strong association between increasing length and male-to-female sex change, the data reveal that DNAm becomes more sex-specific (rather than more female-specific) with length. Significant differences in DNAm between males and females of similar lengths suggest that female-specific DNAm arises rapidly during sex change, rather than gradually with growth. The findings also reveal that region-specific differences in length-at-sex change are accompanied by differences in DNAm, and were concurrent with variability in remotely sensed sea temperature and salinity. Together, these findings provide the first in situ evidence for epigenetically and environmentally mediated sex change in a protandrous hermaphrodite, and offer significant insight into the molecular and ecological processes governing the marked and unique plasticity of sex in fish.

Stings from certain species of cubozoan jellyfish are dangerous to humans and their seasonal presence in tropical marine waters poses a significant risk to coastal communities. The detection of cubozoans is difficult due to high spatial and temporal variation in their occurrence and abundance. Environmental DNA (eDNA) has the potential to detect rare species and therefore offers potential to detect cubozoans, not only pelagic medusae, but presence of cryptic polyp life-stages. The objective of this study was to validate the use of eDNA as a viable detection method for four cubozoan species (Chironex fleckeri, Copula sivickisi, Carybdea xaymacana and Carukia barnesi). Species-specific primers were developed for each of these four cubozoans and an eDNA approach validated utilising both laboratory and field trials. Laboratory DNA degradation experiments demonstrated that C. sivickisi DNA degraded quickly but could still be detected in sea water for up to 9 days post-jellyfish removal. Positive detection was found for C. fleckeri, C. xaymacana and C. sivickisi medusae in the waters surrounding Magnetic Island, Queensland, in the Austral spring/summer (September-January). Based on visual surveys there was a poor relationship between concentration of eDNA and abundance of jellyfish. Positive eDNA amplification was also shown near the substratum when C. sivickisi medusae were absent. This can only be explained by the detection of polyps. Consequently, eDNA is an effective tool to detect both the medusae and polyps of cubozoans. This approach provides the means to reduce the risk of envenomation to swimmers and enhance our knowledge of cubozoan ecology.