No abstract

Many symbiotic marine invertebrates acquire free-living Symbiodinium from the environment. Abundance and diversity of free-living Symbiodinium could influence recovery from bleaching, resilience, and the long-term adaptation of host organisms. Although free-living Symbiodinium have been detected in the water column and substrates of coral reefs, their diversity and availability to the hosts are poorly understood. Tank experiments were conducted to test whether asymbiotic coral larvae of Acropora monticulosa acquired free-living Symbiodinium from the water column or sediment to become symbiotic. Treatments included filtered (0.22 µm) seawater (FSW), unfiltered seawater (SW), FSW and sediment, and SW and sediment. Our results showed that greater proportions of larvae in sediment-containing treatments acquired Symbiodinium earlier and had greater in hospite Symbiodinium densities when compared to seawater-only treatments. Additionally, clade A Symbiodinium was only recovered in the larvae from the sediment-containing treatments, whereas clades B and C were recovered from all treatments. Differences in distribution, abundance, replication and motility patterns of Symbiodinium, as well as larval behavior, may have contributed to the observed differences between uptake from the sediment and the water column. However, our results suggest that the sediment may represent an important source of free-living Symbiodinium available for uptake during primary acquisition by coral larvae.KEY WORDS: Free-living Symbiodinium · Acquisition · Coral larvae · Reef sediment · Water column · Acropora monticulosa Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 377: [149][150][151][152][153][154][155][156] 2009 stress events such as bleaching then this mode may be advantageous in the long-term life history of the species. Furthermore, adults of some host species may be able to acquire a new strain of Symbiodinium through secondary acquisition following stress events, such as bleaching (Kinzie et al. 2001). Despite its apparent adaptive advantages, secondary acquisition in scleractinian corals has not been observed, possibly due to experimental limitations. This lack of evidence has lead investigators to suggest that hosts shuffling proportions of different Symbiodinium strains already in hospite is the predominant mode of physiological adaptation in scleractinian corals (Berkelmans & van Oppen 2006). If this is the case, then primary acquisition of Symbiodinium strains is critically important in the entire life history of these hosts. Resilience of symbiotic reef organisms, like corals, may thus be especially reliant upon the symbiont types specifically involved in primary acquisition (Baird et al. 2007).Given the dependence of reef invertebrates on environmental pools of Symbiodinium, it is imperative to understand the diversity and ecology of free-living Symbiodinium and their interactions with potential hosts. Free-living Symbiodinium have been identified in the water column...