The shells of large benthic foraminifers (LBF) are key contributors to the development and maintenance of coastal landforms in the Pacific as well as to Paleogene and Miocene carbonates deposited along the Neotethys Seaway and tropical Pacific islands. The current study assessed growth and fecundity of two species, Marginopora vertebralis and Amphistegina lobifera, collected from sites in Viti Levu, Fiji, based on shell diameter, shell weight, fecundity, and survival. Specimens were cultured without supplemental nutrients or food for 15 months under controlled laboratory conditions. Physicochemical parameters, including salinity, alkalinity, pH, and temperature, varied by <5% throughout the experiment. Asexual reproduction by M. vertebralis produced ∼270 offspring per brood, while A. lobifera produced ∼500 offspring per brood. The minimum size at reproduction for M. vertebralis was 15 mm, and A. lobifera reproduction occurred at diameters ≥0.9 mm. These observations were consistent with those of previous studies that predicted asexual fecundity related to parent size. Four non-linear mathematical functions (exponential, Gompertz, logistic, and von Bertalanffy) were compared to describe the age-weight relationship for each species. Results revealed that the logistic model best fits M. vertebralis growth, and von Bertalanffy model best fits A. lobifera growth. The growth model for A. lobifera predicted trends in juvenile growth and maximum size consistent with a previously published von Bertalanffy model based on cultures in which the foraminifers were provided nutrient sources and grew much faster than those observed in this study. These observations support published hypotheses that many LBF are exceptionally well-adapted to extreme oligotrophy, a characteristic that accounted for their dominance as carbonate producers in the Paleogene and Miocene of the Neotethys and Pacific islands.

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