Two Deep Ones

The Deep One picture below is based on the descriptions in “Dagon” and “The Shadow over Innsmouth” by H. P. Lovecraft, but focusing on the carven images in “Dagon”. It is portrayed as a humanoid wielding a harpoon as in “Dagon”, but fish-gilled and froglike to fit the “fish-frog” mentions in “Shadow”. This is closer to how I tend to imagine Deep Ones. To be interfertile with humanity — and produce fertile offspring! — means that Deep Ones must be genetically very close to humanity. Their gills are probably artificially engineered, either by some spawn of Cthulhu freed from imprisonment or from an ancient human civilization; in the Cthulhu Mythos setting, there have been many advanced civilizations - human and alien - before ourselves. Besides the gills, the other differences are genetically minor: webbed feet, partially webbed hands, claws (which might be merely narrow claw-like nails). The Deep One in this picture is slightly disproportioned and has only two toes on each foot; disabilities that would be impairing on land by limiting movement would not be so in the sea.



The next image (black and white) is more like the common depictions of Deep Ones. This type of Deep One draws on “Shadow over Innsmouth” but ignores the more human-like descriptions in “Dagon”. (Deep Ones are sometimes illustrated - the d20 Call of Cthulhu gaming book an especially egregious example - with heavy, armor-like scales over their entire bodies. This is incorrect. Lovecraft clearly states in “Shadow” that Deep Ones are mostly ’slimy and slippery’, with scales only on ‘the ridges of their backs’.)
Perhaps this bulkier, frog-like Deep One is a blubber-heavy adaptation to cold waters, while the beings of “Dagon”, dwelling in tropical seas, did not need it.
I imagine (if any scientific justification is desired) that Deep Ones can grow to enormous sizes because - as water dwellers whose weight does not pull on their bodies - genetic abnormalities that cause gigantism are not selected against, as they would be on land.