Discogypsina vesicularis, copyright Bruce W. Hayward.

Belongs within: Planorbulinida.

Acervulinids: reef forams
Published 21 March 2013

Regular readers of this site will know that, contrary to common belief, not all representatives of the vaguely defined category of organisms known as ‘protozoa’ are too small to be seen with the naked eye. Some are very visible indeed, and many of the more visible forms can be found among the shell-constructing amoeboids known as Foraminifera. One group of giant forams, the xenophyophores, has become fairly famous in internet circles as one of the contenders for the title of ‘largest single cell’, though, as noted elsewhere, the question is kind of a pointless one with regard to xenophyophores. Besides, as described later in this post, xenophyophores may not have even have always been the largest forams.

Encrusting nodules of Acervulina inhaerens from Rhodes, from M. Hesemann.

The Acervulinidae are a family of reef-inhabiting forams belonging among the rotaliids. Juvenile chambers of newly growing acervulinids are arranged in a flat spiral, but chambers of mature specimens may be arranged in one to several layers. The chambers do not have regular apertures, and instead their walls are only pierced by coarse pores (Perrin 1994). Genera and species of acervulinids are distinguished by the presence, arrangement and shapes of layers and chambers, but defining distinctions appropriately is challenging. Acervulinids do not have a determinate ‘adult morphology’; instead, the final adult appearance can be affected by factors such as substrate relief and water movement. Properly identifying acervulinids therefore requires identification of features independent of these external factors.

Living crust of Gypsina plana, photographed by Hal Ray Tichenor.

Acervulinids can be abundant on tropical coral reefs, and may play a not insiginificant role in reef formation as binding organisms. They tend to be particularly prominent in deeper parts of the reef, as they can tolerate lower light levels than other organisms such as coralline algae; in shallower parts of the reef, they are found in more cryptic locations among the coral. Acervulinids may be free-living, or they may be directly attached to their substrate. Their primary food source is benthic diatoms (that they may or may not live with symbiotically), and the abrupt disappearance of the modern Acervulina inhaerens below depths of 130 m probably corresponds to the lower limit of that food source (Bosellini & Papazzoni 2003).

Fossilised nodules from a Solenomeris reef, photographed by Stefano Dominici. Note that Stefano identifies these as Acervulina; due to the complications in distinguishing acervulinid taxa, it remains contentious whether Solenomeris and Acervulina can be reliably separated.

Attached acervulinids may form either nodules or spreading crusts, depending on species and/or growth conditions (Perrin 1994). Such nodules or crusts may have diameters in the millimetre range, but some living species may be within the decimetre range. The most dramatic expression of acervulinid potential, however, was known from the Tethyan region during the Eocene period (the Tethys, for those unfamiliar with it, was the sea that connected the Atlantic and Indian Oceans north of Africa, before the northward movement of that continent closed off the Mediterranean at the eastern end). Here was found Solenomeris ogormani, initially interpreted as a red alga but since reidentified as an acervulinid. Solenomeris was primarily an encrusting form, but large growths would also produce tightly packed branches one or two centimetres in diameter. Over time, Solenomeris formed massive metre-sized domes, and these domes together would form entire reefs stretching over multiple kilometres: reefs formed not of coral, or of algae, but purely of forams!

Systematics of Acervulinidae
    |--Orbitogypsina Matsumaru 1996B-F08
    |--Solenomeris Douvillé 1924B-F08
    |--Borodinia Hanzawa 1940LT64
    |    `--*B. septentrionalis Hanzawa 1940LT64
    |--Ladoronia Hanzawa 1957LT64
    |    `--*L. vermicularis (Hanzawa 1957) [=Acervulina (*Ladoronia) vermicularis]LT64
    |--Sphaerogypsina Galloway 1933C40
    |    `--*S. globulus (Reuss 1848) [=Ceriopora globulus]LT64
    |--Protogypsina Matsumaru & Sarma 2008B-F08
    |    `--P. indicaB-F08
    |--Wilfordia Adams 1965B-F08
    |    `--W. sarawakensisB-F08
    |--Planogypsina Bermúdez 1952LT64
    |    |--*P. squamiformis (Chapman 1901) [=Gypsina vesicularis var. squamiformis]LT64
    |    `--P. acervalis (Brady 1884)S05
    |--Discogypsina Silvestri 1937LT64
    |    |--*D. vesicularis (Parker & Jones 1860) (see below for synonymy)LT64
    |    `--D. discusB-F08
    |--Acervulina Schultze 1854 [incl. Aphrosina Carter 1879]LT64
    |    |--*A. inhaerens Schultze 1854C40
    |    |--*Aphrosina’ informis Carter 1857LT64
    |    `--A. mahabetiS05
    `--Gypsina Carter 1877 [incl. Hemigypsina Bermúdez 1952]LT64
         |--*G. plana (Carter 1876)LT64 (see below for synonymy)
         |--G. howchiniQ72
         `--G. mastelensis Bursch 1947 [=*Hemigypsina mastelensis]LT64

*Discogypsina vesicularis (Parker & Jones 1860) [=Orbitolina concava var. vesicularis, Gypsina vesicularis, Tinoporus vesicularis]LT64

*Gypsina plana (Carter 1876)LT64 [=Polytrema planumLT64; incl. P. miniaceum var. involvaC01, Gypsina melobesioides Carter 1877LT64]

*Type species of generic name indicated


Bosellini, F. R., & C. A. Papazzoni. 2003. Palaeoecological significance of coral-encrusting foraminiferan associations: a case-study from the Upper Eocene of northern Italy. Acta Palaeontologica Polonica 48 (2): 279–292.

[B-F08] BouDagher-Fadel, M. K. 2008. The Cenozoic larger benthic foraminifera: the Palaeogene. Developments in Palaeontology and Stratigraphy 21: 297–418.

[C01] Chapman, F. 1901. On the identity of Polytrema planum of Carter with P. miniaceum var. involva. Annals and Magazine of Natural History, series 7, 7: 82–83.

[C40] Cushman, J. A. 1940. Foraminifera: Their classification and economic use 3rd ed. Harvard University Press: Cambridge (Massachusetts).

[LT64] Loeblich, A. R., Jr & H. Tappan. 1964. Sarcodina: chiefly “thecamoebians” and Foraminiferida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt C. Protista 2 vol. 2. The Geological Society of America and The University of Kansas Press.

Perrin, C. 1994. Morphology of encrusting and free living acervulinid Foraminifera: Acervulina, Gypsina and Solenomeris. Palaeontology 37 (2): 425–458.

[Q72] Quilty, P. G. 1972. The biostratigraphy of the Tasmanian marine Tertiary. Papers and Proceedings of the Royal Society of Tasmania 106: 25–44.

[S05] Semeniuk, T. A. 2005. Fossil foraminiferal assemblages from Pleistocene seagrass-bank deposits of the southern Perth Basin, Western Australia, and their palaeotemperature implications. Journal of the Royal Society of Western Australia 88 (4): 177–190.

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