Subtropical specialists dominate a coral range expansion front

Potential range expansion of scleractinian corals in high-latitude reefs is critically dependent on the coral host-symbiont relationship that determines coral growth and survival. Although increases in coral cover have been observed at higher latitudes, the identities of habitat-building reef corals and their symbionts are underreported. Here, we examine how scleractinian host and symbiont Symbiodiniaceae diversity changes along a tropical–temperate environmental gradient. We use Pocillopora spp. and associated symbiont communities as a model to understand whether they are expanding their range poleward and the role of symbionts in this process. Along the Kuroshio Current, which carries warm equatorial waters northward along the Pacific coast of Japan, we collected coral tissues from 23 (sub)tropical-to-temperate reefs, from southern Iriomote in the Ryukyu Islands (24°N) to northernmost Kushimoto on mainland Japan (33°N). We examined host identities through direct sequencing of the mitochondrial open reading frame (mtORF) and symbiont identities with next-generation sequencing of the internal transcribed spacer 2 (ITS2) region of the ribosomal DNA. Our results show a dramatic reduction of Pocillopora haplotypes and a marked change in dominant symbiont types northward (poleward) from Cape Sata (30°N), Kagoshima. ‘Tropical’ Pocillopora haplotypes were absent from mainland Japan sites. We also demonstrate high host specificity between the subtropical Pocillopora haplotype and Cladocopium symbiont types. Our findings question how common ‘coral tropicalisation’ is, and the location of the coral range expansion front. The specificity of hosts and symbionts in high-latitude corals suggests that high-latitude reefs are unlikely to support the persistence of tropical zooxanthellate corals.