Foraminifera: Single Cells Known for Shells, Some Coin-Sized

Bench card

Name

Foraminifera (foraminifers), single-celled organisms known for the shell they build, called a test

Group

phylum Foraminifera, kingdom Chromista in the GBIF Backbone (not animals); GBIF 8376456

Actual size

shells (tests) of adult drifting Globorotalia menardii 150–1,500 µm [10⁻⁴–10⁻³ m]; under 2 mm is the "normal" test size of the smaller seafloor and drifting kinds; tests of larger seafloor kinds reach 13 cm living and 19 cm fossilSource: Rillo et al. 2020, https://doi.org/10.1002/ece3.6792; Hohenegger et al. 2019, https://doi.org/10.1038/s41598-019-44646-w

Size in this image

not measurable from this image

Lives in

mostly on or in the seafloor; about 50 modern species drift as plankton; some in fresh water and soilSource: Holzmann et al. 2024, https://doi.org/10.1038/s41597-024-04137-8

Seen with

naked eye for fossil nummulites of 5 and 30 mm; light microscope, or a stereomicroscope in reflected light, for the smaller kindsSource: Sremac et al. 2024, https://doi.org/10.3934/microbiol.2024030; Hohenegger et al. 2019, https://doi.org/10.1038/s41598-019-44646-w

Color in images

native: fossil shells, no stain; shells from a sediment trap, no stain stated; the Haeckel plate is printed in one inkSource: figure captions on this page

Key source

Holzmann et al. 2024, Scientific Data 11:1292 (what they are, species counts); Hohenegger et al. 2019, Scientific Reports 9:8286 (sizes, the pyramid block)

foraminifera-bench.csv (CSV) · where each value comes from, and its license (JSON)

Data credits for this card

Data: GBIF Secretariat 2023, GBIF Backbone Taxonomy, Foraminifera (taxon key 8376456), checklist dataset https://doi.org/10.15468/39omei, CC BY 4.0, retrieved 2026-10-25; Rillo MC, Miller CG, Kucera M, Ezard THG 2020, Ecology and Evolution 10(20):11579–11590, CC BY 4.0, https://doi.org/10.1002/ece3.6792, retrieved 2026-10-25; Hohenegger J, Kinoshita S, Briguglio A, Eder W, Wöger J 2019, Scientific Reports 9:8286, CC BY 4.0, https://doi.org/10.1038/s41598-019-44646-w, retrieved 2026-10-25; Holzmann M, Nguyen NL, Angeles IB, Pawlowski J 2024, Scientific Data 11:1292, CC BY 4.0, https://doi.org/10.1038/s41597-024-04137-8, retrieved 2026-10-25; Sremac J, Bošnjak M, Firi KF, Šimičević A, Aščić Š 2024, AIMS Microbiology 10(3):644–673, CC BY 4.0, https://doi.org/10.3934/microbiol.2024030, retrieved 2026-10-25

Are foraminifera animals?

No. Each foraminifer is one cell, and the GBIF Backbone Taxonomy lists Foraminifera as a phylum in the kingdom Chromista, not among animals. In a 2024 reference dataset, Holzmann and colleagues call them protists, or microbial eukaryotes, set apart by net-forming threads of cytoplasm called granuloreticulopodia and by a test that is calcareous, agglutinated from gathered particles, or organic.

The name means "hole bearers", Sremac and colleagues note in a 2024 review of marine microfossils. Tests vary in shape, size, number of chambers and chemistry: commonly calcite, but also aragonite, gathered particles, chitin or, rarely, silica. Holzmann's group counts 8,896 modern and 39,976 fossil species, citing the World Register of Marine Species as of 2024. It adds that DNA surveys have turned up many unknown species, most of them soft-walled or naked (shell-less) single-chambered kinds that shell-based research had largely overlooked.

Most are benthic, living on or in the seafloor; about 50 modern species drift as plankton, and others live in fresh water and soil. Their shells also keep a record: by Sremac and colleagues' account, oxygen isotopes in foraminiferal tests are used to estimate past sea temperatures.

How big are foraminifera?

The "normal" test size for the smaller seafloor and drifting kinds is under 2 mm, and the larger seafloor kinds surpass it, Hohenegger and colleagues wrote in 2019. In a 2020 study of 3,799 museum shells, Rillo and colleagues found adult shells of the drifting Globorotalia menardii ranging from 150 to 1,500 µm, a tenfold spread within one species.

Those shells sit among familiar small things. A human hair is typically 50–100 µm wide (Li and colleagues, 2023), diatoms are usually 20–200 µm (Piegari and colleagues, 2019), and sand grains run from 63 µm to 2 mm on the grain-size scale VanDusen and colleagues used in 2012. A 150 µm adult G. menardii shell is about the size of a 139 µm male wasp, the smallest known parasitoid insect in Polilov's 2015 account.

At the other end, the tests of larger seafloor foraminifera, which house algae in warm shallow seas, reach 13 cm in living species and 19 cm as fossils. Hohenegger's group cites both figures from earlier records. Xenophyophores, giant deep-sea foraminifera, build tests up to 24 cm from seafloor particles (Gooday and colleagues, 2020). From a 150 µm shell to a 24 cm test is a factor of 1,600, computed for this page; only a log scale shows both on one chart.

Slide the diagram sideways to read its labels, or open it full size.

What were Strabo's stone lentils?

Strabo, born at Amasia in Pontus, lived under the emperors Augustus and Tiberius. In Book 17 of his Geography he describes the stones:

"One extraordinary thing which I saw at the pyramids must not be omitted. Heaps of stones from the quarries lie in front of the pyramids. Among these are found pieces which in shape and size resemble lentils. Some contain substances like grains half peeled. These, it is said, are the remnants of the workmen's food converted into stone; which is not probable." Strabo, Geography 17.1.34, in W. Falconer's translation (London: Bohn, 1857)

So the petrified-food story was hearsay, and Strabo judged it unlikely; he compared the pieces with lentil-like pebbles on a long hill at home, near Amasia. His translators added a note in 1857: Niebuhr had reported small petrified lentil-shaped bodies in these stones, like shells he collected at Bushir, and to Clarke they looked like the petrified remains of some unknown animal.

The stone can now be named. Nummulites is a genus of large seafloor foraminifera with coin-shaped tests, and nummulitic limestone, made up of them, was the main building stone of the Egyptian pyramids, by the account of Sremac and colleagues, who cite a 2018 magazine article. Members of the genus built up large beds of carbonate rock, Hohenegger and colleagues note, and one species, N. venosus, still lives off Okinawa.

Their 2019 paper shows the surface of a block from the pyramid of Khufu, also called Cheops, built of Nummulites gizehensis tests (spelled ghizehensis in the paper): many 5 mm tests of one generation, called A, and 30 mm tests of another, called B.

Strabo gave no measurements, so the pieces he saw cannot be matched one by one to these shells. The claims line up like this (foraminifera-strabo-claims.csv (sources, version and license, JSON) Data: Strabo, Geography 17.1.34, translated by H. C. Hamilton and W. Falconer, vol. 3 (London: Henry G. Bohn, 1857), public domain, https://www.gutenberg.org/ebooks/44886, retrieved 2026-10-25; Hohenegger J, Kinoshita S, Briguglio A, Eder W, Wöger J 2019, Scientific Reports 9:8286, CC BY 4.0, https://doi.org/10.1038/s41598-019-44646-w, retrieved 2026-10-25; Holzmann M, Nguyen NL, Angeles IB, Pawlowski J 2024, Scientific Data 11:1292, CC BY 4.0, https://doi.org/10.1038/s41597-024-04137-8, retrieved 2026-10-25; Sremac J, Bošnjak M, Firi KF, Šimičević A, Aščić Š 2024, AIMS Microbiology 10(3):644–673, CC BY 4.0, https://doi.org/10.3934/microbiol.2024030, retrieved 2026-10-25; GBIF Secretariat 2023, GBIF Backbone Taxonomy, checklist dataset https://doi.org/10.15468/39omei, CC BY 4.0 (Foraminifera, taxon key 8376456), retrieved 2026-10-25):

Does a big shell mean a big cell?

In chambered foraminifera, the cell normally fills its shell. They build the test by adding a new chamber at every growth step, and during undisturbed life the cytoplasm fills the chambers completely, Hohenegger and colleagues write; chamber volumes therefore trace the growth of one cell. For living Nummulites venosus off Okinawa, a species that houses diatoms as partners, they estimated an expected life span of 18 months.

Drifting species grow the same way, one chamber at a time, until they reproduce, when the cell dies (Rillo and colleagues, 2020).

The living cell also reaches past its shell. Foraminifera move on reticulopodia, threads of cytoplasm that join into networks with cytoplasm flowing both ways, Greco and colleagues explain in a 2023 study. In culture, one Neogloboquadrina incompta held itself in place with ectoplasmic roots, thick strands of the cell's outer cytoplasm, 515–885.7 µm long, three to more than five times its own size; twig-like strands of N. pachyderma reached up to four times the size of the shell.

At the giant end the reverse holds. A xenophyophore's test can reach 24 cm, but its branching cell body occupies only a small proportion of the test volume, Gooday and colleagues wrote in 2020, so the 24 cm measures a shell, not a cell. How Big Is a Cell? sets that test beside the largest single cells.

Slide the diagram sideways to read its labels, or open it full size.

How can you tell a living foraminifer from an empty shell?

Not always by eye. In a 2018 paper, Ross and Hallock describe Amphistegina gibbosa slipping into a dormant state in which living individuals looked just like dead ones: no extended pseudopods, no attachment, no visible waste.

Stains help, and the color they add is borrowed color, not the cell's own. Rose Bengal stains proteins and can color dead cytoplasm as well as live, Ross and Hallock note. CellTracker Green becomes fluorescent only after esterases, enzymes common to living cells, cut it, so a cell must be alive to make it glow.

Slide the diagram sideways to read its labels, or open it full size.

That check can be fooled too. In their experiments, 48% of specimens killed by heat and kept 48 hours in non-sterile seawater showed some fluorescence, which the authors tie to microbes growing in dead shells. As of that 2018 paper, the fluorescence readings did not settle whether a still Amphistegina was dormant or dead.

Sources

1. Holzmann M, Nguyen NL, Angeles IB, Pawlowski J. "BFR2: a curated ribosomal reference dataset for benthic foraminifera." Scientific Data 11:1292 (2024). https://doi.org/10.1038/s41597-024-04137-8 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
2. Rillo MC, Miller CG, Kucera M, Ezard THG. "Intraspecific size variation in planktonic foraminifera cannot be consistently predicted by the environment." Ecology and Evolution 10(20):11579–11590 (2020). https://doi.org/10.1002/ece3.6792 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
3. Hohenegger J, Kinoshita S, Briguglio A, Eder W, Wöger J. "Lunar cycles and rainy seasons drive growth and reproduction in nummulitid foraminifera, important producers of carbonate buildups." Scientific Reports 9:8286 (2019). https://doi.org/10.1038/s41598-019-44646-w Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
4. Strabo. The Geography of Strabo, vol. 3, translated by H. C. Hamilton and W. Falconer (Books 7 onward by Falconer). London: Henry G. Bohn, 1857. Book 17, chapter 1, section 34, and the translators' notes 858–859. Project Gutenberg eBook #44886. https://www.gutenberg.org/ebooks/44886 Read: full text of Book 17, 1.34 and its notes.
5. GBIF Secretariat. GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei, accessed 2026-10-25; Foraminifera (phylum, kingdom Chromista), usage key 8376456; Globorotalia menardii (d'Orbigny, 1865), 4382036; Nummulites Lamarck, 1801, 5428655; Nummulites gizehensis (Forskål, 1775), 8375775; Nummulites venosus (Fichtel & Moll, 1798), 7845048; Neogloboquadrina incompta (Cifelli, 1961), 7775876; Neogloboquadrina pachyderma (Ehrenberg, 1861), 7576430; Amphistegina gibbosa d'Orbigny, 1839, 8426787; Globigerinoides ruber (d'Orbigny, 1839), 8107071; Dicopomorpha echmepterygis Mockford, 1997, 1367990. Read: dataset record (species match and species records). Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
6. Sremac J, Bošnjak M, Firi KF, Šimičević A, Aščić Š. "Marine microfossils: Tiny archives of ocean changes through deep time." AIMS Microbiology 10(3):644–673 (2024). https://doi.org/10.3934/microbiol.2024030 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
7. Li C, Du Y, Chen H, Han X, Wu W, Kong X, Zhang C, Yuan X. "Transmission infrared micro-spectroscopic study of individual human hair." Journal of Biomedical Optics 28(11):116501 (2023). https://doi.org/10.1117/1.JBO.28.11.116501 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
8. Piegari G, De Biase D, d'Aquino I, Prisco F, Fico R, Ilsami R, Pozzato N, Genovese A, Paciello O. "Diagnosis of Drowning and the Value of the Diatom Test in Veterinary Forensic Pathology." Frontiers in Veterinary Science 6:404 (2019). https://doi.org/10.3389/fvets.2019.00404 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
9. VanDusen BM, Fegley SR, Peterson CH. "Prey Distribution, Physical Habitat Features, and Guild Traits Interact to Produce Contrasting Shorebird Assemblages among Foraging Patches." PLoS ONE 7(12):e52694 (2012). https://doi.org/10.1371/journal.pone.0052694 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
10. Polilov AA. "How small is the smallest? New record and remeasuring of Scydosella musawasensis Hall, 1999 (Coleoptera, Ptiliidae), the smallest known free-living insect." ZooKeys 526:61–64 (2015). https://doi.org/10.3897/zookeys.526.6531 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
11. Gooday AJ, Durden JM, Smith CR. "Giant, highly diverse protists in the abyssal Pacific: vulnerability to impacts from seabed mining and potential for recovery." Communicative & Integrative Biology 13(1):189–197 (2020). https://doi.org/10.1080/19420889.2020.1843818 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
12. Greco M, Westgård A, Sykes FE, Ezat MM, Meilland J. "Uncovering hidden structures: previously undescribed pseudopodia and ectoplasmic structures in planktonic foraminifera." Journal of Plankton Research 45(4):652–660 (2023). https://doi.org/10.1093/plankt/fbad031 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
13. Ross BJ, Hallock P. "Challenges in using CellTracker Green on foraminifers that host algal endosymbionts." PeerJ 6:e5304 (2018). https://doi.org/10.7717/peerj.5304 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

Cite this page

Ants, Cells & Atoms. “Foraminifera: Single Cells Known for Shells, Some Coin-Sized.” Published 2026-10-25. https://antscellsandatoms.com/microscopic-life/foraminifera/. Accessed (add the date you read it).

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25 October 2026

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