DAPI-based vital staining reveals entry of heterologous zooxanthellae into primary polyps of a vertically-transmitting soft coral

Many cnidarians form an obligatory mutualism with photosynthetic algae (zooxanthellae) of the genus Symbiodinium. Break-down of this symbiosis leads to bleaching, and often death, of the host. Coral hosts may survive such break-down of symbiosis by exchanging their algal types, provided they can host multiple symbiont types. The soft coral Litophyton crosslandi (Octocorallia, family Nephtheidae) transmits its symbionts vertically to its sexual progeny during oogenesis. This has been considered a "closed symbiosis", in which the host retains the same single zooxanthellae type throughout its entire life span. In this work we examined the possible entry of heterologous zooxanthellae into primary polyps of a vertically transmitting host. Primary polyps were challenged with DAPI-stained heterologous clade C zooxanthellae, allowing their visualization and distinction from the homologous clade A ones. DAPI-stained zooxanthellae were found in the epidermis, and in some cases even in the gasterodermis, of the polyps. The entry of heterologous symbionts and their survival inside host tissue suggests that the latter may indeed possess the ability to host different algae. These findings also suggest an entry pathway via the epidermis that, prior to this study, has not been described for vertical transmitters. These results further contribute to the understanding of coral ability to cope with stress-induced bleaching.