Merpeople around the world have their own individual quirks. While Icthyocentaurs are generally quite welcoming to visitors Orca-mer have no such kindnesses and while Orca-mer and Shark-mer have no compunctions against eating other merpeople and sharks respectively, Seahorse-mer generally eat similarly to their aquatic parts, and abhor the idea of eating sharks or other merpeople.
There are also differences in how they interact with one another, not solely whether they might occasionally indulge in cannibalism, but things such as the Orca-mer preference to the visits of Icthyocentaurs to all others, and the difference in trading patterns between sirens, open ocean surface merpeople and coastal surface merpeople.
It is important to know as well, when greeting Rusalka, not to mention the most recent indiscretion of the Rusalka-River merpeople, while with the Ningyo it is considered good manners to ask for an Amabie to translate thee songs they use to communicate.
These unique aspects of cultural and ethnic behaviours and differences amongst different groups of merpeople are listed in this volume, with each sectioned out, and appointed their chapters, based on alphabetic location.
— Excerpt from An Introduction to the Cultures of Merpeople by Arielle DuPont
(For underwater-witchery for the advent requests series. I hate that I have to include this but PLEASE DO NOT REMOVE THE IMAGE SOURCE OR MY CAPTION.)