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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:

enter image description here

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.

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    May be better on biology.stackexchange.com/questions/tagged/…?
    – Martin Smith
    Commented Jul 9, 2017 at 14:40
  • 2
    "balls floating around" vs. "the balls did not float". Which is it?
    – oerkelens
    Commented Jul 9, 2017 at 15:50
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    @oerkelens Clarified.
    – lambshaanxy
    Commented Jul 9, 2017 at 20:26
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    It looks like bubble algae but I would suggest Earth Science might be a good place to ask.
    – Giorgio
    Commented Jul 9, 2017 at 21:36
  • @Dorothy I'm pretty sure you're right, please add that as an answer!
    – lambshaanxy
    Commented Jul 10, 2017 at 1:57

2 Answers 2

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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.

enter image description here

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They might be a kind of cyanobacteria called nostoc that sometime grows in round balls.

nostoc

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.townofchapelhill.org/home/showdocument%3Fid%3D28866&ved=2ahUKEwjZsqq98-_oAhUO2qwKHfl_CHwQFjAAegQIAhAB&usg=AOvVaw3l9D3Iz3S3r6Ycasco7VSe

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  • Interesting, but nostoc appear to be much smaller and more fragile.
    – lambshaanxy
    Commented Apr 18, 2020 at 4:14

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