Subtropical specialists dominate a coral range expansion front

Area of Science:

• Marine Biology
• Coral Reef Ecology
• Symbiosis

Background:

• Coral host-symbiont relationships are crucial for coral growth and survival, influencing potential range expansion.
• Increases in coral cover at higher latitudes have been observed, but the identities of corals and their symbionts are not well documented.
• Understanding these relationships is key to predicting coral responses to climate change and poleward range shifts.

Purpose of the Study:

• To investigate changes in scleractinian coral host and Symbiodiniaceae symbiont diversity along a tropical-temperate environmental gradient.
• To use Pocillopora spp. and their symbionts as a model to assess poleward range expansion.
• To determine the role of symbionts in the potential range expansion of corals.

Main Methods:

• Collected coral tissues from 23 reefs along the Kuroshio Current, spanning from southern Iriomote (24°N) to northern mainland Japan (33°N).
• Identified coral hosts by sequencing the mitochondrial open reading frame (mtORF).
• Determined symbiont identities using next-generation sequencing of the internal transcribed spacer 2 (ITS2) region.

Main Results:

• A significant reduction in Pocillopora haplotypes and a shift in dominant symbiont types were observed north of Cape Sata (30°N).
• Tropical Pocillopora haplotypes were absent from mainland Japan sites.
• High host specificity was found between subtropical Pocillopora haplotypes and Cladocopium symbiont types.

Conclusions:

• The findings challenge the extent of 'coral tropicalization' and the location of coral range expansion fronts.
• High host-symbiont specificity in high-latitude corals suggests limited capacity for tropical zooxanthellate corals to persist.
• Poleward expansion of tropical corals may be constrained by specific host-symbiont associations.