They look like unremarkable polished stones but sailor’s eyeballs are actually one of the most unique organisms on the planet
Sailor’s eyeballs – often more prosaically referred to as ‘bubble algae’ – are found in almost all tropical and sub-tropical waters, particularly among mangroves and coral reefs.
The surface of the bubble is dark green and so smooth that it appears to shine like a polished stone, for which it is often mistaken. In fact, Valonia ventricosa is one of the largest single-cell organisms in the world.
Although it appears as if one would burst like a balloon if prodded with a sharp object, Sailor’s eyeballs are far from hollow. The structure of the alga is coenocytic, a process whereby the cell’s nuclei divide – like a regular multicellular organism – but remain within a single exterior membrane.
As a result, whereas most single cells are invisible to the naked eye, Sailor’s eyeballs can reach a diameter of up to 5cm across.
They have been spotted as deep as 80m but as it is a type of algae and therefore requires sunlight to synthesise, it is unlikely to be found any deeper.
Valonia ventricosa has been widely studied, as its large size lends itself to the study of how cell walls function, but despite its unique appearance and status as one of the largest unicellular organisms on the planet, it is often overlooked by divers as unremarkable.
