#drratio would approve

[havanillas]

hello, science entusiast here! i love your merventurine au and would like to give you some Fun Facts regarding the mammal vs. fish discussion.

taxonomy (the grouping of organisms) is complicated and there are several ways to do it, so i am agreeing with you in that there is no one correct answer.

for example, there exists the 'cladistics' approach to taxonomy, and according to it, fish as a category does not exist at all. this is because cladistics only makes groups by common ancestors based on genetics, and there is not a single common ancestor of all fish.

so, surprisirngly the lungs and live birth are not what causes problems in categorization. there are fishes who have lungs and mammals who lays eggs. biology is a bitch like that.

the most common definition by evolutionary taxonomy (which is an approach that recognizes apparent degree of similarity as well as genetics) for fishes is that they 1) are non-tetrapods, 2) have a backbone of some kind, 3) has gills throughout life, and 4) have limbs in the shape of fins.

the first definition is by far the trickiest part. merfolk clearly doesn't look like they have four limbs, so if you are going based on phenetics (apparent similarity without considering ancestry) then merfolk is definitely fish. but to somewhat appease the cladists, (because i am sure dr. Ratio is not one to stick to just one approach to taxonomy): if you, as the creator of the au, think that merfolk ancestors are fish who developed human traits, then they are non-tetrapods, and therefore, fish. if you think that merfolk are humans who then evolved to be fish-y, then they could be classified as mammals.

the other set of concerns is that merfolk always have human fingers despite having fins, but this is also one of those things you'd want to define from ancestry.

to top it all off, some writers of merfolk stories write that they lose their gills when they go above water, so in that case it depends on where they are at time of observation.

tl;dr, the answer is that, as the creator of the au, i'd say you get to decide whether or not he is fish.

this is so cool holy shit