Abstract
The ecological success of shallow-water reef-building corals (Hexacorallia: Scleractinia) is framed by their intimate endosymbiosis with photosynthetic dinoflagellates in the genus Symbiodinium (zooxanthellae). In contrast, the closely related black corals (Hexacorallia: Anthipatharia) are described as azooxanthellate (lacking Symbiodinium), a trait thought to reflect their preference for low-light environments that do not support photosynthesis. We examined 14 antipatharian species collected between 10 and 396 m from Hawai'i and Johnston Atoll for the presence of Symbiodinium using molecular typing and histology. Symbiodinium internal transcribed spacer-2 (ITS-2) region sequences were retrieved from 43 per cent of the antipatharian samples and 71 per cent of the examined species, and across the entire depth range. The ITS-2 sequences were identical or very similar to those commonly found in shallow-water scleractinian corals throughout the Pacific. Histological analyses revealed low densities of Symbiodinium cells inside antipatharian gastrodermal tissues (0–92 cells mm−3), suggesting that the Symbiodinium are endosymbiotic. These findings confirm that the capacity to engage in endosymbiosis with Symbiodinium is evolutionarily conserved across the cnidarian subclass Hexacorallia, and that antipatharians associate with Symbiodinium types found in shallow-water scleractinians. This study represents the deepest record for Symbiodinium to date, and suggests that some members of this dinoflagellate genus have extremely diverse habitat preferences and broad environmental ranges.
Highlights
Mesophotic coral reef ecosystems (MCEs) are coral reefs located below the depth limits of traditional scuba diving (40 m) and extend to the deepest portion of the photic zone, which may be over 150 m in tropical and subtropical regions with high water clarity [1,2,3]
Symbiodinium were identified by genotyping the internal transcribed spacer-2 (ITS-2) region of the nuclear ribosomal cistron and exploring their location in the tissues of a subset of corals using histology and chlorophyll autofluorescence
Our data provide evidence that antipatharians serve as habitat for Symbiodinium types that are commonly associated with some shallow-water scleractinian corals
Summary
Mesophotic coral reef ecosystems (MCEs) are coral reefs located below the depth limits of traditional scuba diving (40 m) and extend to the deepest portion of the photic zone, which may be over 150 m in tropical and subtropical regions with high water clarity [1,2,3]. MCEs are direct extensions of shallow-water reefs; because of their spatial separation from the many anthropogenic and natural stresses that affect shallow-water areas, MCEs have traditionally been considered a de facto refuge for the globally degraded shallow-water ecosystems [2,3,4] Despite their importance, very little is known about the community structure of MCEs and the biological adaptations that allow mesophotic organisms to strive in low-light environments [2,3]. More recent studies using histological techniques [8,17], spectrophotometric chlorophyll measurements [18,19] and molecular approaches [19] have, generally failed to confirm these early reports, with the exception of a single study reporting Symbiodinium in a shallow-water antipatharian (40 m) from Indonesia [6]. Each alignment was manually edited and only identical sequences recovered from two or more independent antipatharian samples included in the downstream analyses. Tentacle tissues were dissected and placed onto microscope slides and viewed under an Olympus Fluoview 1000 laser scanning confocal microscope using a blue light excitation at 430–470 nm, followed by emission through a 500–530 nm long-pass filter
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