Morphological Response of Benthic Foraminifera Quinqueloculina seminula to Sediment Grain Size: Results from Culture Experiment

Benthic foraminifera are valuable bioindicators in modern and paleo-environmental studies due to their sensitivity to ecological changes. While sediment grain size is a fundamental factor structuring benthic habitats, its direct impact on benthic foraminifera remains poorly understood, as field studies are often complicated by covarying environmental parameters. To address this knowledge gap, we conducted a 14-week controlled laboratory culture experiment to investigate the biological response of a common benthic foraminifera Quinqueloculina seminula to different sediment grain sizes. Specimens were cultured in three distinct sediment grain size treatments (50, 150 and 250 μm), representing a gradient from fine to coarse sand. Our results demonstrate a clear and significant positive relationship between sediment grain size and the test size of Q. seminula. Furthermore, the species exhibited significant morphological plasticity, with the ratio of length/width increasing from 1.61 in fine sediment to 1.71 in coarse sediment. Individuals of Q. seminula in coarse sediment developed more elongate tests, while those in fine sediment maintained a more rounded morphology. This study provides the first experimental evidence that sediment grain size can drive significant changes in the size and morphology of Q. seminula. The observed phenotypic plasticity likely represents an adaptation to optimize locomotion and energetic allocation in different sedimentary environments. These findings confirmed the importance of sediment granulometry as a key control on foraminiferal biology and have important implications for the use of morphological data in paleoenvironmental reconstructions and modern biomonitoring.