🌊 Merfolk Biology 🌊

Merfolk can be generously sorted into four categories depending on appearance/behavior and biology.

Coldblooded Merfolk are Mers that align closer to animals with cold blood such as lizards, fish, arthropods, and invertebrates. Despite the name, some of these Mers may have some ability to thermoregulate parts of their body, but not on the same scale of mammals. These Mers lay eggs and may have gills. Warmblooded Merfolk are Mers that align closer to mammals. They have the ability to completely thermoregulate their body temperature, they will give birth to live young and produce milk, and they need to surface for oxygen. As far as we know, Coldblooded and Warmblooded Merfolk cannot interbreed.
Coldwater Merfolk and Warmwater Merfolk are used to vaguely describe appearances and temperaments. It is observed that Mers that live in colder waters are larger, have duller colors, and low-pitched songs. Conversely, Mers that live in warmer waters are smaller, have brighter colors, and high-pitched songs. Note: These terms are loose and vague, and debates are taking place whether or not to abolish the definitions or to create more subcategories.