The shallow marine realm is an energetic scenario where strong taphonomic processes influence the distribution of organic and inorganic material. Symbiont-bearing larger benthic foraminifera, which are inhabitants of this environment in tropical, oligotrophic seas, have to provide enough light to their endosymbiotic microalgae for photosynthesis. At the same time, these foraminifera have to resist hydrodynamic forces that are strongest in these environments by constructing tests of different shape, size and density to resist water motion or develop anchoring systems to attach to the substrate. After test release due to death or reproduction, tests may be entrained, suspended and transported depending on their hydrodynamic parameters according to form and intensity of water motion. The hydrodynamic parameters of tests quantify the answer to the energetic input given by the water motion at the sea bottom surface and express the transportability of every test.
The relation between the shape of nummulitids, often expressed in different species names, and water motion allows fascinating results. Concerning larger foraminifera, especially nummulitids, shape variation expressed in taxonomy is highly correlated with hydrodynamics. The distribution of foraminiferal tests on a slope thus depends on their shape responding to water motion, the same as concerning sand grains: coarser sediments are displaced in shallower and higher energetic scenarios, while finer particles are more common in deeper and quieter regions. Because of these correlations, estimations of paleodepth can be based on the foraminiferal shape distribution in the fossil environment. Some equations allow us to obtain the settling velocity and Reynolds number of every test and help to understand the differences of species-specific distributions between living individuals and empty tests. Such differences are most important for interpreting palaeoslopes, intensity and direction of currents by accumulation of foraminifera.