Mystery Object: Lifesavers of the Coral Sea

Identifying obscure and bizarre objects is all in a day’s work for staff in the Queensland Museum Discovery Centre! Today Dr Jonathan Cramb, Information Officer, shares the mystery object of the month.

The mystery

These objects are found in large numbers on some beaches in tropical Queensland. These particular examples are from Airlie Beach on the Whitsunday Coast.

Close examination shows that the objects are covered in tiny holes, and have numerous internal chambers. The objects are made of calcium carbonate, the same material that forms sea shells and corals.

Small disc-shaped objects. What are they? Image credit: Dr Jonathan Cramb

What is it?

The objects 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 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 commonly 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 shown here are probably the widespread shallow-water species Marginopora vertebralis.

Like most protozoans, the majority of forams are microscopic, but some are giants and are easily visible with the naked eye. For example, some Marginopora grow to more than two centimetres across. In comparison, one of the largest cells in the human body (the ovum, or egg) is 0.12 millimetres across.

Numerous tiny pores are visible under magnification. Image credit: Dr Jonathan Cramb

The biggest forams are among the biggest single-celled lifeforms, with some species reaching 20 centimetres across! Some giant forams can reach 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, giving them a supplementary source of energy. This means that most 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 (indeed, forams may supply algae to coral in the aftermath of bleaching events). Just as corals are threatened by rising sea temperatures and acidification, so are forams like Marginopora.

Giant forams such as Marginopora are abundant on some beaches in tropical Queensland. Image credit: Dr Jonathan Cramb

Uncover more mysteries at the Discovery Centre

Visit the Discovery Centre on Level 4 of the museum to see more Mystery of the Month objects on display. Do you have an interesting question or mystery object? Our helpful and knowledgeable staff can answer your questions through our Ask an Expert inquiry service.