My fantasy world has a species of hominids that evolved to become semi-aquatic - their lifestyle is similar to that of seals & sea lions, spending much of their time in the water.

Now - given that, to my understanding, marine food chains tend to rely much more on secondary producers than primary ones (ie most sea animals eat fish, plankton, etc rather than plants), these aquatic humanoids have a diet consisting mainly of fish and other seafood, with the occasional seaweed or other algae to spice things up. Once they learn to build rafts they might be able to sustain some small floating gardens - but these would probably come with several constraints and not do much to replace meat as the staple of the diet.

That being the case - what kinds of nutrient deficiencies would these humanoids be most susceptible to? I'm sure its possible that over the course of evolution they'd develop slightly different metabolism from humans - but I doubt that enough time has really passed for them to actually subsist on meat alone with no issues (if I'm wrong about this, someone let me know and I'll happily have my not-mermaids become obligate carnivores!), so I'm curious what they'd need to do to fill out the rest of their diet ad stay healthy.

  • $\begingroup$ Is there a reason you're assuming 'adapt to a carnivorous diet' takes longer than 'adapt to a semi-aquatic lifestyle'? Sounds like they've already undergone significant evolutionary changes pretty much by definition... $\endgroup$
    – Toby Y.
    Sep 5, 2022 at 3:52
  • $\begingroup$ That's a great question. The thing is, their physiological adaptations actually aren't that significant, at least externally. It's mostly things like lung capacity/oxygen storage, possibly webbed hands/feet. Even tho I mentioned seals, I doubt my humanoids even have fused legs - theyre just much better swimmers than other humanoids. They can probably even still interbreed with land-dwelling humans. Plus,... it'd just be interesting. If they have to go out of their way to fill certain parts of their diet, it could have interesting implications for their culture once they develop one. $\endgroup$
    – Nascence
    Sep 5, 2022 at 4:35
  • $\begingroup$ Eskimos do exist, and they are real humans. Before ultra-modern times they ate almost exlcusively animal products. $\endgroup$
    – AlexP
    Sep 5, 2022 at 7:56
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    $\begingroup$ @Nascence don't underestimate the necessary changes for them to become sufficiently effective fish hunters that they can live off of hunting fish. $\endgroup$ Sep 5, 2022 at 16:29
  • $\begingroup$ @Blueriver this is a good point - I should probably put some thought into how they hunt and how much of it could have been accomplished with primitive hominid tools and how much would require biological adapation. $\endgroup$
    – Nascence
    Sep 10, 2022 at 21:44

2 Answers 2


Eat it Raw

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Your aquatic people are like the Native Americans who live up in the Arctic Circle and eat a carnivorous diet.

It is out of fashion to call these people Eskimos, but in fact the reason they do not all die of scurvy/diabetes/vitamin deficiency is because some of the meat and blood is consumed raw. Raw meat contains more carbohydrates and nutrients than cooked meat.

Your aquatic people are fine to eat mostly fish and shellfish and whales and sealions, provided they eat some of it raw.

This is a necessity if you are on a long expedition days away from the shore. You must eat your catch raw as there is no way to cook it.

This opens up an interesting plot point. Perhaps raw food is seen as low-brow the same way oats and brown bread are seen as for the lower classes, despite the fact that they are better for you. Once the aquatic people started becoming a wealthy civilization, they moved to cities, stopped eating raw food and started getting scurvy and diabetes.

  • $\begingroup$ This is the kind of thing I was looking for! I don't know how I forgot about arctic cultures like the Inuit when I've definitely read about them extensively before. The reliance on raw meat would definitely put interesting constraints on their lifestyle and culture (and answer how they could possibly cook food at sea - there's no need to bother). $\endgroup$
    – Nascence
    Sep 10, 2022 at 21:42

Here's the thing about adaptation. It happens through steps and stages, and it affects multiple aspects of an organism.

Suppose the ancestors of your aquatic beings were land dwelling. Then, as they gradually moved over to ocean dwelling, they would also develop the ability to live with the diet available.

Humans need vitamin C in their diet. Many animals can make their own. Fish, for example.

Suppose a land animal lacked the ability to produce a particular vitamin. And suppose that vitamin was not available in the food it could easily get in the ocean. Then evolving into a sea-going animal would be difficult. They would either need to spontaneously develop the gene (or have it re-express if it previously existed in an ancestor). Or they would have to get it from some form of food in the ocean.

For example, vitamin C can be obtained from kelp.

So your critters would step by step evolve to the ocean. Either they would find foods that included enough of what they needed. Or they would have (or develop or re-develop) the ability to manufacture chemicals they needed. Or they would stop evolving at a point they could still survive.

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    $\begingroup$ Good point. Maybe this should have been more of a question about what barriers they would have faced in evolving an aquatic lifestyle. kelp being a source of vitamin C would probably be a big help to them. $\endgroup$
    – Nascence
    Sep 5, 2022 at 5:07

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