i think it's already a pretty common one, but i have my own twist on it, at the risk of sounding a little weird:

with the headcanon that trolls (especially seadwellers, given differences between landdwelling & seadwelling anatomies) can adopt traits from animals, seadwellers and mutants (or even just anyone particularly lucky/unlucky on the hemospectrum) can not only be born with gills & fins, but not be born with them but develop them later on, or even be born with them but lose them as they develop.

you know, nature vs. nurture + some physical characteristics seem to be based on a wide variety of animals so i wanted to lean into that. also believe that gills & fins vary widely in size, shape, amount, etc. this is depending on birth circumstances (headcanon that seadwellers when born often head for water pools early, though landdwellers can also end up in pools w/o drowning), type/species of aquatic animal that the troll's body is more closely mimicing, as well as the lusus of the troll (shark lusus = higher likelihood of more shark-like gills/fins). though, it is to a certain extent. again, nature vs. nurture.

a lot of times, a mish-mash of different aquatic animals' characteristics happen, and a large amount of seadwelling trolls have a mixture of characteristics, though usually contained to order/family of animal (in rarer cases as specific as genus/species or even as broad as class).

this is basically the reason (excuse) why one of my troll ocs w/ mutant blood & physical characteristics has 3 pairs of gills on their neck and none on their torso, which is also the reason (again, excuse) for them wearing not only a bandana but also a goddamn neckwarmer on alternia. they also have fins short but somewhat tall fins, which they hide with either magnet earrings & headband (the magnets on the headband pull back the earrings and fins, which they can then hide under messy hair) or just a normal cloth headband (which often is in all likelihood just their bandana repurposed for their head instead).

i love making worldbuilding excuses for cool character design :]