In obligate symbioses, the host\u27s survival relies on the successful acquisition and maintenance of symbionts, which can be transferred from parent to offspring via direct inheritance (vertical transmission) or acquired anew each generation from the environment (horizontal transmission). Vertical transmission ensures progeny acquire their obligate symbionts, but progeny encountering an environment that differs from that of their parent may be disadvantaged by hosting a suboptimal symbiont. Conversely, horizontal symbiont acquisition provides hosts the benefit of acquiring symbionts well suited to the prevailing environment, but progeny may fail to acquire their obligate symbionts. Here I show that the coral Stylophora pistillata may garner the benefits of both transmission modes by releasing progeny with maternally derived symbionts that are also capable of subsequent horizontal symbiont acquisition. The algal symbionts (Symbiodinium) present in S. pistillata adults, juveniles, and larvae (planulae) were identified using denaturing-gradient gel electrophoresis (DGGE) and real-time PCR. DGGE confirmed previous reports that in the Gulf of Eilat, Red Sea, shallow water (2-6m) S. pistillata adult coral colonies host clade A Symbiodinium, while deep-water (24-26m) colonies host clade C. Real-time PCR uncovered previously undetected Symbiodinium present at low-levels in some deep, but no shallow water adult colonies. Planulae only inherited the dominant symbiont clade from their maternal colony. While most shallow water juveniles hosted only clade A Symbiodinium, deep-water juveniles either hosted clade C, clade A, or a mixture of both clades. As all planulae analyzed hosted only one symbiont clade, while some juvenile colonies hosted either multiple symbionts, or symbionts not characteristic of the depth in which they occurred, these data support environmental Symbiodinium acquisition, in addition to vertical symbiont inheritance, in the coral S. pistillata. Reciprocal depth transplant experiments of juvenile S. pistillata colonies were executed to monitor potential changes in the Symbiodinium. Hosting physiologically distinct Symbiodinium may allow coral host survival under varied environmental conditions. Therefore, horizontal symbiont acquisition may enable coral species with vertical transmission to acquire advantageous symbionts. In turn, this may provide genetic variation in the symbiosis on which natural selection can act, providing a mechanism for coral adaption to global climate change
