So. I have a theory. I dont know what the actual distances or times are, but i believe that a chasmfiend, after pupation, and after becoming bigger, moves to the reshian sea we read about in words of radiance ( With rysn). The similarities, along with the mention of the soul of the Reshi Greatshell Relu-na and the spren that saved her, are leading me to conclude that the chasmfeinds we hear and see in the books become huge, and move into the ocean. unfortunately, we have no knowledge of the early life and birth of the Chasmfiends, or the Greatshells we see in the Reshi trading scene. I would like to hear your alternate theories, and what proves / disproves mine. I feel like the greatshells and chasmfiends will become more prevalent in later books and i would like to know more about them. (besides, the ecology of roshar is amazing and why wouldnt you want to know more)
Also, is the Greatshell closer to a chull? or is a Greatshell closer to a chasmfiend (behavior and anatomy wise)