Many planktonic foraminifera have light tests for the reasons described by Mike above. However, some planktonic foraminifera such as globotruncanids, truncorotaloidids and globorotaliids can have thicker walls as they have a diurnal feeding cycle which means they must retain both negative and positive buoyancy to travel up and down the water column in deep water.
Also, many larger benthic foraminifera must have thick, robust, test walls to enable them to withstand the high hydrodynamic energy of shallow-water environments. Reefal benthic foraminifera that occur at or around fair-weather wave base (FWWB) are subject to the highest energies and must have thick walls to prevent abrasion from rolling around on the seafloor.
Miliolid foraminifera found within shallow-water environments also have very thick microgranular test walls to protect them from the harmful UV radiation of the sun penetrating shallow, clear water.
Examining the test morphologies, or morphogroups, of assemblages of foraminifera is a powerful tool in aiding interpretations of depositional environments within rock samples.
Thank you David and Mike. I understood the reason of hardness of planktonic and benthic foraminifera but i could not understand the reason of ornamentation and deep water currents are much slower than surface water currents. So planktonic foraminifera should be in more hydrodynamic environment ?
Planktonic foraminifera may also have complex ornamentation, examples include tubulospines on species of Hantkenina and digitate extensions on Globigerinoidesella fistulosa. On benthic foraminifera, ornamentation such as grooves and spines act to help the organism stay bedded within soft sediment to prevent it being moved by currents or wave energy. This is similar to the role of spines/grooves in some benthic bivalves. Spines may also provide some limited protection against predation in both benthic and planktonic foraminifera, although there is evidence that the presence of spines does not deter all predators from attacking prey.
Moreover, it may be depend on the chemistry of water. At low latitude ocean or seas of passive margin, the shelf may reach to 350 km width with high biogenic productivity that can able to change the chemistry of sea water itself with richness of carbonates that enable benthos that dominate at shallow water shelf to accumulate its skeletal with more dense carbonate skeleton with thick carbonate wall than those of floating away from shelf that posses little opportunity to build its skeleton than benthos one. As Prof Dr Kenneth said above its functional morphology so this idea it may be one of many factors affect on the building their skeletons.
The selectivity of these foraminiferal 'stone-masons' that Kenneth describes is quite astonishing. It reminds me of recent papers by Sabbatini et al. (2016) and Pearson et al. (2018) which describe preferences of some species of agglutinated foraminifera to singularly select heavy mineral grains or bioclasts of a single species of planktonic foraminifera, respectively. How they do this, I have no idea! Astounding nonetheless.
See:
Pearson, P.N., Expedition, I.O.D.P. and Party, S.S., 2018. A deep-sea agglutinated foraminifer tube constructed with planktonic foraminifer shells of a single species. Journal of Micropalaeontology, 37(1), pp.97-104.
Sabbatini, A., Negri, A., Bartolini, A., Morigi, C., Boudouma, O., Dinelli, E., Florindo, F., Galeazzi, R., Holzmann, M., Lurcock, P.C. and Massaccesi, L., 2016. Selective zircon accumulation in a new benthic foraminifer, Psammophaga zirconia, sp. nov. Geobiology, 14(4), pp.404-416.
David, do not forgot the complication of the internal endoskeleton
I recently came across this paper that may be of interest to the functional morphology of test structures:
Dumitriu, S.D., Dubicka, Z. and Ionesi, V., 2018. The functional significance of the spinose keel structure of benthic foraminifera: inferences from Miliolina cristata Millett, 1898 (Miliolida) from northeast Romania. Journal of Micropalaeontology, 37(1), pp.153-166.
Planktonics need light shells to remain buoyant while benthonics need heavy shells.The ornamentations are not for beauty but adaptational, to increase the density that is necessary in benthonic mode of life.
Obianuju P. Umeji
- Badges
- Science topic