Question of the month

Pirate’s Eyeball

Is this some sort of egg sac that I found on a beach in North Queensland?

Answer

Dried remains of a ‘Pirate’s Eyeball’ on a beach in north Queensland. Photo: Marisa Giorgi. Most single-celled lifeforms are microscopic, but this one is (relatively) enormous! Photo: Marisa Giorgi.Live Pirate’s Eyeball. Photo: Gary Cranitch, QM.

What you have found is commonly known as a Sailor’s or Pirate’s Eyeball (other common names are Bubble Algae or Sea Grapes). Despite its gruesome common name this is in fact a common alga found in many parts of the world. The scientific name is Valonia ventricosa. The often clear composition with a dark spot gives it the appearance of an eyeball, though often they are green in appearance. Underwater they can appear blue or black.

One of the extraordinary aspects of your specimen is that it is one of the largest one celled (or ‘unicellular’) organisms on the planet. This means that the whole ball is a single cell. They are usually singular but are sometimes found in clumps and range from oval to spherical and can occasionally grow as large as a tennis ball. Your specimen has very little chlorophyll (the green pigment found in plants and algal cells that creates energy from sunlight through photosynthesis) so it’s not very green in appearance. These incredibly interesting squishy balls are usually found in shallow waters; on the Great Barrier Reef they are common on beds of coral rubble.

Another fascinating element of your specimen is apparent if we consider the nitty gritty of cell construction. The biological ‘machinery’ within a cell works best at very small scales, so a cell can’t usually grow as large as this without doing things a bit differently. A Pirate’s Eyeball contains several cytoplasmic domains with individual nuclei and chloroplasts. If you accidentally break it open, then it may form several other Pirate’s Eyeballs because of all those individual nuclei.

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