Diversity of zooxanthellae from corals and sea anemones after long-term aquarium culture

Aquarium systems allow technically sophisticated experiments that promise new opportunities to answer urgent questions about reef coral biology, for instance assessing the responses to decreasing environmental pH and/or increased temperatures. Over recent years, long-term culture and (predominantly asexual) propagation of corals has become possible in such systems. At present however, only limited data are available that clarify whether or not responses of the coral holobiont are dominated by the acclimatization to life in captivity or continue to reflect, for example, taxonomic differences seen in nature. We studied the diversity of the symbiotic algae in corals and sea anemones after long-term aquaculture by analysis of their small subunit (SSU) ribosomal DNA gene. A field sample of Acropora clathrata from the Arabian Gulf which was used as a control contained clade C zooxanthellae. The aquarium corals also harboured clade C symbionts, but sequencing of the SSU DNA suggested that the analysed animals host different subclades. A prevalence of clade C is also found among corals from the Indo-Pacific region, the origin of most of the aquarium samples. An individual of the temperate sea anemone Anemonia sulcata (viridis) contained clade A symbionts, similar to those found in nature, even after nearly 10 years of co-culture with sea anemones (Entacmaea quadricolor) and corals hosting clade C symbionts. The results indicate that the specific host-symbiont association occurring in nature appears to persist over >2 years timescales in captivity, with no mixing of symbionts between hosts maintained in the same aquarium or apparent selection of stress-tolerant symbiont strains such as clade D.