Sharon E. Hook, Khandaker Asif Ahmed, Demi Cho, Leon Court, Ryan J. Farr, Aisling Fontanini, Alistair J. Hobday, Gunjan Pandey, Rahul Rane, Jenny Su, Thomas Walsh
Many rock oysters have evolved to thrive in the intertidal environment where they experience intense desiccation pressure and wide temperature fluctuations. The tropical natal rock oyster, Saccostrea cucullata, and the black-lip rock oyster, Saccostrea echinata, have higher reported thermal optima and tolerances than the better studied temperate species. However, the genetic basis of their thermotolerance, important for understanding their potential for climate adaptation, has not been characterised. We generated contig-level genome assemblies for two tropical rock oysters, Saccostrea cucullata and S. echinata . The final S. cucullata assembly was 1.2 Gb and comprised 23,868 contigs (contig N50 0.115 Mb), whereas the S. echinata assembly was 0.923 Gb and comprised 424 contigs (contig N50 19.7 Mb). BUSCO analysis (mollusca_odb10, n = 5295) indicated high gene-space completeness for both genomes ( S. cucullata : 93.4% complete; S. echinata : 99.0% complete). K-mer-based genome size estimates indicated that both assemblies exceeded the inferred haploid genome size, consistent with partial retention of alternative haplotypes in highly heterozygous bivalve genomes. Both oyster genomes showed increased orthologues of heat shock proteins and histones compared to temperate species, suggesting potential mechanisms for thermal tolerance and environmental adaptability. In S. cucullata, many of the heat shock proteins were expressed following air exposure, which would include a mild heat stress among other stressors, further reinforcing their role in temperature tolerance. These results suggest both species are likely to be resilient to near term climate stress as well as having the potential to adapt to changing environments, and thus there is potential expansion for rock oyster aquaculture beyond the commonly-farmed Magallana gigas or Saccostrea glomerata species and the further development of a tropical rock oyster industry.