Active macroalgal habitat selection in the large benthic foraminifera Marginopora vertebralis

Marginopora vertebralis are large benthic foraminifera (LBF) found commonly in the low-lying reef flats on the Great Barrier Reef. They form photo-symbiotic associations with microalgae and through calcification, are essential contributors to reef sediments. While many studies have noted M. vertebralis as being epiphytic, evidence supporting this lifestyle and data on deliberate habitat selection is lacking. Here we investigated M. vertebralis -macroalgal associations and habitat preferences. Through field sampling campaigns, we found significant associations with the calcifying macroalgae Amphiroa foliacea , Halimeda opuntia and H. macroloba on the reef flat. Subsequent preference experiments confirmed A. foliacea and H. opuntia as selected macroalgal habitats of choice. We also found strong aversion toward the calcifying brown algae Padina tenuis , emphasising the deliberate nature of the selection. The clear preference for living, structurally dense, calcifying macroalgae, implies that these associations confer some biological or ecological benefit to M. vertebralis . These findings highlight the need to understand the role of M. vertebralis- macroalgal associations in M. vertebralis ecological resilience and how it may shape the fate of M. vertebralis in a future marine environment.