I know Renaissance painters don’t just up and put stuff in their paintings for the heck of it. AND MULTIPLE ONES DID IT. There is symbology at work here. The ‘shell’ means something, other than a container for water.
The only thing that pops into my head is ‘Birth of Venus’:
https://ih1.redbubble.net/image.413937159.2113/mp,840x830,matte,f8f8f8,t-pad,1000x1000,f8f8f8.u6.jpg
https://en.wikipedia.org/wiki/The_Birth_of_Venus
Symbology:
John is pouring out the OLD Paganism (Venus) and starting the NEW era of Christian religion.....................
Perhaps those artists did hold that thought. In heaven, you can ask those who are there. Then, let us know your findings............
