Researchers at the Okinawa Institute of Science and Technology have revealed how squid and cuttlefish survived the Cretaceous-Paleogene mass extinction by retreating to oxygen-rich deep-sea refuges. Their analysis of newly sequenced genomes shows these cephalopods originated in the deep ocean over 100 million years ago, followed by rapid diversification into shallow waters. The findings, published in Nature Ecology & Evolution, provide the first comprehensive evolutionary tree for decapodiform cephalopods.
Dr. Gustavo Sanchez, a staff scientist at OIST's Molecular Genetics Unit, led the study that combined three newly sequenced squid genomes with global datasets. The work resolved long-standing debates on the evolution of squid and cuttlefish, known as decapodiform cephalopods. Sanchez noted, 'With our new genomic information, we have been able to resolve some of the mysteries surrounding their origins.' The team constructed an evolutionary tree covering nearly all major lineages through a five-year global collaboration, including the Aquatic Symbiosis Genomics Project funded by the Wellcome Sanger Institute. A key inclusion was the genome of the rare ram's horn squid, Spirula spirula, whose spiral shell had previously confused classifications, according to co-author Dr. Fernando Á. Fernández-Álvarez of the Spanish Institute of Oceanography. The study traces the group's origins to the mid-Cretaceous period, about 100 million years ago, in deep ocean habitats. Little diversification occurred for tens of millions of years, fitting a 'long fuse' model. Around 66 million years ago, the K-Pg extinction wiped out three-quarters of species, including dinosaurs, but cephalopod ancestors endured in protected deep-sea pockets amid surface ocean acidification. As ecosystems recovered, with coral reefs reemerging, squid and cuttlefish rapidly diversified into shallow coastal areas. Sanchez explained, 'Following the initial lineage splits in the Cretaceous, we don't see much branching for many tens of millions of years. However, in the K-Pg recovery period, we suddenly see rapid diversification.' Prof. Daniel Rokhsar, head of OIST's Molecular Genetics Unit, highlighted the potential for future research into their unique traits like dynamic camouflage and neural complexity.