May 2021

Washers in the swash

Whilst visiting Great Keppel Island, we came across these discs that look like washers on the beach. The large white shells are sand dollars, but we can't figure out what the others ones are? We think these are either shells, coral or perhaps a mineral. Could you please help us out?

Marginopora tests (and two sanddollars), found on Great Keppel Island. Photo: Rod.


The circular objects in your photos are the shells of foraminifera, in this case a species of Marginopora. Foraminifera (“forams” for short) are protozoans (complex single-celled lifeforms) that build an elaborate shell, called a “test”. Most foram species are marine, although a small number are found in freshwater and a few even live in moist soils. The majority of marine species live on the sea floor, while some are planktonic.

When alive each of your Marginopora was a full disc, thickest on the rim and thinnest in the centre. The tests of the dead forams may be tossed about by the surf and the thin centre wears through. The colour of living Marginopora has been described as yellowish-brownish green; this colour is given to them by symbiotic algae (more information below). Beach-washed tests of Marginopora are commonly white to cream-orange and black, depending on staining from iron and manganese oxides. Several species of Marginopora are found in Queensland waters; the ones you found are probably the widespread shallow-water species Marginopora vertebralis. 

Marginopora found at Airlie Beach. Numerous tiny holes in the test are visible under magnification. Photo: Jonathan Cramb.

Like most protozoans, the majority of forams are microscopic, but some are giants and are easily visible with the naked eye. As an example, each of your Marginopora was a single cell; some Marginopora grow to more than two centimetres across. In comparison, the largest cell in the human body is roughly the size of the full stop at the end of this sentence. Giant forams can reach such large sizes by enlisting the help of single-celled algae, which can generate food from sunlight. The forams keep the algae within their own cell which gives them a supplementary source of energy. This means that giant forams tend to be found in shallow waters where there is plenty of sunlight. If this sounds familiar, it’s because some corals do the same thing and even use some of the same species of algae. Just as corals are threatened by rising sea temperatures and acidification, so are forams like Marginopora.

Illustration of a variety of forams by Ernst Haeckel. The disc-shaped form on the right resembles Marginopora, but is actually another giant foram called Nummulites.

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