Fallen Pillars: The Past, Present, and Future Population Dynamics of a Rare, Specialist Coral-Algal Symbiosis

Abstract

Rare and ecologically specialized species are at greater risk of extinction. Rarity in terms of low genotypic diversity may be obscured in sessile foundation species that can reproduce asexually. Asexual propagules are often only distinguishable from sexual recruits through molecular tools. Thus, molecular markers are necessary to assess genotypic variation and population structure in clonal organisms such as corals. The global decline of corals has necessitated marker development for improved conservation of rare coral species. We infer past demographic changes, describe modern population structure, and quantify asexual reproduction of the uncommon Caribbean pillar coral, Dendrogyra cylindrus and its endosymbiotic dinoflagellate, Symbiodinium dendrogyrum using de novo microsatellite markers. Results show that D. cylindrus comprises three distinct populations in the Caribbean whereas the symbiont was differentiated into four populations. Thus, barriers to gene flow differ between host and symbiont. In Florida, host and symbiont reproduced mainly asexually, yielding lower genotypic diversity than predicted from census size. Models of past demographic events revealed no evidence of historical changes in population size, consistent with the geological record of D. cylindrus indicating it has been rare for hundreds of thousands of years. The most recent global thermal stress event triggered a severe disease outbreak among D. cylindrus in Florida, resulting in a precipitous population decline. Projections indicate a high likelihood that this species may become locally extinct within the coming decades. The ecosystem consequences of losing rare coral species and their symbionts with increasingly frequent extreme warming events are not known but require urgent study.