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Climate change threatens symbiotic cnidarians’ survival by causing photosymbiosis breakdown in a process known as bleaching. Direct effects of temperature on cnidarian host physiology remain difficult to describe because heatwaves depress symbiont performance, leading to host stress and starvation. The symbiotic sea anemone Exaiptasia diaphana provides an opportune system to disentangle direct vs. indirect heat effects on the host, since it can survive indefinitely without symbionts. We tested the hypothesis that heat directly impairs cnidarian physiology by comparing symbiotic and aposymbiotic individuals of two laboratory subpopulations of a commonly used clonal strain of E. diaphana, CC7. We exposed anemones to a range of temperatures (ambient, +2°C, +4°C, +6°C) for 15–18 days, then measured their symbiont population densities, autotrophic carbon assimilation and translocation, photosynthesis, respiration, and host intracellular pH (pHi). Symbiotic anemones from the two subpopulations differed in size and symbiont density and exhibited distinct heat stress responses, highlighting the importance of acclimation to different laboratory conditions. Specifically, the cohort with higher initial symbiont densities experienced dose-dependent symbiont loss with increasing temperature and a corresponding decline in host photosynthate accumulation. In contrast, the cohort with lower initial symbiont densities did not lose symbionts or assimilate less photosynthate when heated, similar to the response of aposymbiotic anemones. However, anemone pHi decreased at higher temperatures regardless of cohort, symbiont presence, or photosynthate translocation, indicating that heat consistently disrupts cnidarian acid-base homeostasis independent of symbiotic status or mutualism breakdown. Thus, pH regulation may be a critical vulnerability for cnidarians in a changing climate.
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A symbiotic oyster in a shrimp burrow: phylogenetic position of Anomiostrea within the Ostreidae (Bivalvia)
ABSTRACT The ostreid genus Anomiostrea Habe & Kosuge, 1966, is monotypic for A. coralliophila Habe, 1975, which is known as a symbiont inhabiting the burrow of the ghost shrimp Neocallichirus jousseaumei (Nobili, 1904), but despite this unusual habit among oysters its phylogenetic position within the Ostreidae remained unknown. Using specimens collected from two distant localities of the Indo-Pacific, Oman and Japan, we compared shell morphology of these specimens with the holotype and assessed their phylogenetic relationships based on DNA sequence data from two mitochondrial and two nuclear genes. The genetic distance in cytochrome c oxidase subunit I between the specimens from Japan and Oman was 5.5%, indicating substantial geographic differentiation. Our molecular phylogenetic results suggest that A. coralliophila is sister to Ostrea algoensis G. B. Sowerby II, 1871, an oyster from rocky shores in South Africa, and both are closely related to other Ostrea species. This confirms assignment of Anomiostrea to Ostreinae and suggests that this burrow-wall symbiont evolved from typical rocky-shore oysters. Moreover, A. coralliophila was not monophyletic with another symbiotic ostreid Ostrea permollis G. B. Sowerby II, 1871, nor with other symbiotic oysters, indicating that the symbiotic habit evolved multiple times in the Ostreinae.
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- Award ID(s):
- 1856245
- PAR ID:
- 10548716
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Journal of Molluscan Studies
- Volume:
- 90
- Issue:
- 4
- ISSN:
- 0260-1230
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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