Cerith snail eggs?
			 
			 
			
		
		
		
		According to Ron Shimek, cerith have long-lived (a few weeks) planktonic and feeding larvae, and so passing through the larval stage to give juvenile snails is uncommon in aquaria, but possible especially if you feed phytoplankton. 
		
	
		
		
		
		
		
	
		
		
	
	
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