This is not pebbles, it's the egg sheath of a marine mollusk called the Brazilian vortex snail (Adelomelonbrasiliana). This conch will lay eggs in the rich

It's not pebbles, it's the egg sheath of a marine mollusk called the Brazilian vortex snail (Adelomelon brasiliana). This conch lays the eggs in the sheath of the nutrient liquid rich in sugar and protein, allowing the eggs to slowly absorb, hatch and grow inside.

Brazilian turbulus snails usually lay their egg sheaths in shallow seas near the coast, and in the event of strong winds and waves, these egg sheaths can be swept onto the beach by the waves, just like you can see in the pictures.