The effect of bacteria on planula-larvae settlement and metamorphosis in the octocoral Rhytisma fulvum fulvum

Abstract

While increasing evidence supports a key role of bacteria in coral larvae settlement and development, the relative importance of environmentally-acquired versus vertically-transferred bacterial population is not clear. Here we have attempted to elucidate the role of post-brooding-acquired bacteria on the development of planula-larvae of the octocoral Rhytisma f. fulvum, in an in vitro cultivation system employing different types of filtered (FSW) and autoclaved (ASW) seawater and with the addition of native bacteria. A good development of larvae was obtained in polystyrene 6-well cell culture plates in the absence of natural reef substrata, achieving a 60–80% of larvae entering metamorphosis after 32 days, even in bacteria-free seawater, indicating that the bacteria acquired during the brooding period are sufficient to support planulae development. No significant difference in planulae attachment and development was observed when using 0.45 μm or 0.22 μm FSW, although autoclaving the 0.45 μm FSW negatively affected larval development, indicating the presence of beneficial bacteria. Autoclaving the different FSW homogenized the development of the larvae among the different treatments. The addition of bacterial strains isolated from the different FSW did not cause any significant effect on planulae development, although some specific strains of the genus Alteromonas seem to be beneficial for larvae development. Light was beneficial for planulae development after day 20, although no Symbiodinium cells could be observed, indicating either that light acts as a positive cue for larval development or the presence of beneficial phototrophic bacteria in the coral microbiome. The feasibility of obtaining advanced metamorphosed larvae in sterilized water provides an invaluable tool for studying the physiological role of the bacterial symbionts in the coral holobiont and the specificity of bacteria-coral interactions.

Millennia‐old coral holobiont DNA provides insight into future adaptive trajectories

“…Members of the Myxococcota have been linked to disease resistance in extant A. palmata (Rosales et al, 2019), and the presence of this bacterial group in ancient corals highlights the ancestry as well as consistency of beneficial microbial associations with putative insights into the evolution of coral holobiont health. Similarly, Ruegeria species are prevalent coral associates exhibiting a high taxonomic and functional diversity across coral compartments (Luo et al, 2021), with putative beneficial roles for the holobiont in pathogen control (Garren et al, 2014; Miura et al, 2019) and larval development (Apprill et al, 2009; Freire et al, 2019).…”

Embryo and larval biology of the deep-sea octocoral Dentomuricea aff. meteor under different temperature regimes

“…Moreover, the provided rock occupied a very small area compared to the flask walls. Settling on plastic is not uncommon among octocorals ( Lasker & Kim, 1996 ; Freire et al, 2019 ; Carugati et al, 2021 ) but further studies with more settlement surfaces are essential to clarify the settlement requirements of the target species. Pelagic metamorphosis of planulae into polyps has also been reported for many octocorals from shallow tropical ( Ben-David-Zaslow & Benayahu, 1998 ; Lasker & Kim, 1996 ), to temperate ( Linares et al, 2008 ) and deep-sea species ( Sun, Hamel & Mercier, 2011 ).…”

Signalling molecules inducing metamorphosis in marine organisms

“…In particular, isolates of the genus Pseudoalteromonas (γ-proteobacterium) have caught much attention as several strains, including Pseudoalteromonas luteoviolacea , were found to induce larval settlement of the sea urchin Heliocidaris erythrogramma , 31 the coral Pocillopora damicornis , 32 the tubeworm Hydroides elegans 33,34 and the cnidarian Hydractinia echinata . 35 Unspecified Pseudoalteromonas and Alteromonas strains were also reported to induce metamorphosis in the corals Acropora millepora , 36 A. willisae and Rhytisma fulvum , 37 the annelid polychaete Ophelia bicornis , 38 and the mussel Mytilus coruscus . 39 Similarly, abundant Vibrio species, most prominently Vibrio fischerii , were found to induce development processes in eukaryotes within the light-emitting organ of the Hawaiian squid Euprymna scolopes .…”