What are these "balls" that we found in Uruguay?

If they haven't exploded yet, in addition to liquid, you will find some embryos inside.

They are very popular among Uruguayan coasts, that lead to wrongly associating them with shark eggs or turtle eggs.

Commonly, they are known as huevos de caracol negro (EN: black sea snail), voluta negra (because of its family: Volutidae) or ovicápsulas con embriones de caracoles (ovicápsula: from Latin: ovum: egg and caps?la.

Technically speaking: Adelomelon brasiliana.

This yellowish capsule, it's filled with food supplements for the embryos, it is located on the seabed and has an oval form, being this a unique feature between sea snails worldwide. After strong winds, many of them remain on the coast, with very low chance of surviving because of the depth where this specie raises. After approximately 2 months, eggs hatch in between 10 and 30 embryos.

• http://www.mgap.gub.uy/sites/default/files/multimedia/1907_caracol_negro_vf_0.pdf
• https://en.wikipedia.org/wiki/Adelomelon_brasiliana
• http://www.marinespecies.org/aphia.php?p=taxdetails&id=384671

Picture from http://www.mgap.gub.uy/sites/default/files/multimedia/1907_caracol_negro_vf_0.pdf

I'm from Uruguay, and we typically call these "turtle eggs" although I'm not sure if the term is accurate.

We call them here Agua Viva (Live or living water), AFAIK its irritant, so be careful, I wouldn't touch it.

What I found is that is some kind of relative of jellyfish, look for Medusozoa.