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I was recently in Baie d'Oro, Ile des Pins, New Caledonia, and the lagoon had dozens of these mysterious green balls rolling around the sea bed:

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They varied in size from large grape to small grape, with "skin" less than a millimeter thick and seawater inside, which would squirt out if you squeezed them hard enough or (as the kids soon discovered!) burst if you threw them like snowballs.

What are they? My best guess would be some kind of seaweed sac, but I never saw them attached to anything and they weren't air bladders, since the balls did not float. They're also not marimo, which are solid seaweed.

9

From the image, it would seem to be bubble algae. According to Wikipedia,

Valonia ventricosa, also known as "bubble algae" and "sailors' eyeballs," is a species of algae found in oceans throughout the world in tropical and subtropical regions. It is one of the largest single-cell organisms.

Valonia ventricosa typically grow individually, but in rare cases they can grow in groups. They appear in tidal zones of tropical and subtropical areas, like the Caribbean, north through Florida, south to Brazil, and in the Indo-Pacific.Overall, they inhabit virtually every ocean throughout the world, often living in coral rubble. The single-cell organism has forms ranging from spherical to ovoid, and the color varies from grass green to dark green, although in water they may appear to be silver, teal, or even blackish. This is determined by the quantity of chloroplasts of the specimen. The surface of the cell shines like glass.

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