Increased number of mucocytes in Aiptasia pallida following bleaching

While many studies have investigated histological changes occurring in cnidarians during bleaching, only a few have focused on continuing changes in tissues during the recovery period. Here, we examine the response of the sea anemone Aiptasia pallida to a transient elevation of water temperature combined with high illumination. Following 30h of exposure to stress conditions (33°C and 1900μE/m2.s), anemones show a significant reduction in their Symbiodinium concentration followed by a progressive recovery over 8 weeks. Histological analyses show an increase in cell proliferation in both ectoderm and gastroderm tissues one day following the stress. This increased proliferation seems to be sustained after 3 weeks before returning to normal after 8 weeks. Moreover, our results show a progressive increase in the number of ectodermal mucocytes over 3 weeks before returning to a normal level after 8 weeks. While the new cells formed in the gastroderm would most likely host new Symbiodinium, the fate of new cells in the ectoderm is still not completely understood. These new cells may contribute to the increased number of mucocytes which could eventually help shift the feeding mode temporarily to a heterotrophic state until restoration of the symbiosis.