300 Million years of coral treaders (Insecta: Heteroptera: Hermatobatidae) back to the ocean in the phylogenetic context of Arthropoda.

Among hundreds of insect families, Hermatobatidae (commonly known as coral treaders) is one of the most unique. They are small, wingless predaceous bugs in the suborder Heteroptera. Adults are almost black in colour, measuring about 5 mm in body length and 3 mm in width. Thirteen species are known from tropical coral reefs or rocky shores, but their origin and evolutionary adaptation to their unusual marine habitat were unexplored. We report here the genome and metagenome of , hitherto known only from its type locality in the South China Sea. We further reconstructed the evolutionary history and origin of these marine bugs in the broader context of Arthropoda. The dated phylogeny indicates that Hexapoda diverged from their marine sister groups approximately 498 Ma and that Hermatobatidae originated 192 Ma, indicating that they returned to an oceanic life some 300 Myr after their ancestors became terrestrial. Their origin is consistent with the recovery of tropical reef ecosystems after the end-Triassic mass extinction, which might have provided new and open niches for them to occupy and thrive. Our analyses also revealed that both the genome changes and the symbiotic bacteria might have contributed to adaptations necessary for life in the sea.