Living Nummulitidae achieve their highest diversity in the subtropical and tropical West Pacific. Although all house symbiotic microalgae, they avoid highly illuminated areas near the water surface, since their flat tests could be easily damaged by the hydrodynamic regime. The preference for calm water conditions extends their depth distribution down to the base of the photic zone. West Pacific Nummulitidae can be differentiated into ten species belonging to six genera according to an ecological species concept. The genus Operculina d’Orbigny is represented by three species. While O. discoidalis (d’Orbigny) prefers a fine-grained bottom under medium light conditions (10% surface intensity), O. ammonoides (Gronovius) prefers a coarser substrate and sometimes can be found on hard bottoms. Light dependence ranges from 1.5% to 68% surface intensity. Less illuminated coarse sands are inhabited by Operculina cf. O. complanata (Defrance), which is the dominant symbiont-bearing foraminifer between light intensities of 0.2% to 12% surface illumination. The genus Planostegina Banner and Hodgkinson demonstrates transitions to the genus Operculina in test form and surface, while the division into chamberlets is similar to Heterostegina. Planostegina operculinoides (Hofker) is distinguished by flat tests and delicate chamberlets. It lives on sandy bottoms restricted to light intensities between 0.45% and 26% surface illumination. The more robust Planstegina aff. P. operculinoides (Hofker) prefers light intensities between 0.4% and 2.7% surface illumination. Planoperculina heterosteginoides (Hofker) shows morphological transition to Operculina cf. O. complanata in developing incomplete septula. This species lives in low illuminated areas (0.3% to 2.5% surface intensity) and prefers medium to fine-grained sands. Heterostegina depressa d’Orbigny spans a broad range in light intensities (2% to 70% surface illumination), and is protected against irradiation by thick tests and a cryptic life mode near the surface. Test construction enables life under strong hydrodynamic regimes. This species lives firmly attached to hard substrates, thus counteracting transportation by water movement. Nummulites venosus (Fichtel and Moll) differs from H. depressa in having undivided chambers. It lives exclusively on coarse sand and avoids high sediment movement, thus starting its distribution beneath the fair weather wave base. According to light intensities, the upper limit may be similar to O. ammonoides (80%), while the lower limit is 2.5% surface illumination. Operculinella cumingii (Carpenter) inhabits coarse to medium sand in deeper regions between 1.2% and 25% surface illumination. Tests of the cyclic, large-sized species Cycloclypeus carpenteri Brady are easily transported due to the thin, plate-like form. The upper distribution limit correlates with the storm wave base, restricting C. carpenteri to depths below 50 m. The lower distribution limit depends on light intensity and is located near the base of the photic zone (0.4% surface illumination).

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