Abstract
Corals at the lower limits of mesophotic habitats are likely to have unique photosynthetic adaptations that allow them to persist and dominate in these extreme low light ecosystems. We examined the host–symbiont relationships from the dominant coral genus Leptoseris in mesophotic environments from Hawai'i collected by submersibles across a depth gradient of 65–125 m. Coral and Symbiodinium genotypes were compared with three distinct molecular markers including coral (COX1–1-rRNA intron) and Symbiodinium (COI) mitochondrial markers and nuclear ITS2. The phylogenetic reconstruction clearly resolved five Leptoseris species, including one species (Leptoseris hawaiiensis) exclusively found in deeper habitats (115–125 m). The Symbiodinium mitochondrial marker resolved three unambiguous haplotypes in clade C, which were found at significantly different frequencies between host species and depths, with one haplotype exclusively found at the lower mesophotic extremes (95–125 m). These patterns of host–symbiont depth specialization indicate that there are limits to connectivity between upper and lower mesophotic zones, suggesting that niche specialization plays a critical role in host–symbiont evolution at mesophotic extremes.
Highlights
Light attenuation is a primary physical parameter that limits the distribution of coral reefs across depths and habitats [1].2015 The Authors
High-quality sequences of COX1–1-rRNA mtDNA were obtained for all investigated Leptoseris spp. samples (n = 74)
Phylogenetic reconstructions recovered five divergent and highly supported clades, each corresponding to known Leptoseris species previously described in [51]
Summary
Light attenuation is a primary physical parameter that limits the distribution of coral reefs across depths and habitats [1].2015 The Authors. In the case of scleractinian corals, coevolution of the host and symbiont is an important consideration for niche specialization and habitat partitioning, as there are trade-offs between different types of Symbiodinium dinoflagellates and host–symbiont specificities [7,8]. Striking patterns of depth-specific symbiont types have been reported in various coral species [9,10,11,12] and have been linked to differences in photo-physiological responses of different Symbiodinium types from shallow water (less than 14 m depth) dominant reef corals [13], or other depth-related environmental conditions acting synergistically such as temperature, salinity, pH, turbidity and nutrient availability [5,11]
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