Deepsea Mer:

The Deepsea are “ugly” by aesthetic, but a peaceful and mostly pacifistic subtype that dwell near the bottom of the ocean and are unable to rise due to the level of pressure their bodies are used to. Deepsea mer have life spans of up to a thousand years and are absolutely giant. Like most creatures that live near the bottom of the sea, they tend to grow until they die and after hundreds of years, they’re massive — easily four to five times the size of Shallows. They’re far more plentiful around the world than Shallows.

Deepsea mer have no hair, but instead sharp cartilage that sits on their head to protect their head and brain. They lack ears and noses as we know them, but have useless vestigial remnants of them leftover from when they were Shallows. Their skin is gray-white and translucent with all their veins and organs visible, but they’re bioluminescent and capable of lighting up in more colors than the human eye can see. They have eyes that are deeply set into their skulls and they’re mostly blind, only able to pick up colors and patterns from the bioluminescence which they use for communication.

Deepsea mer have a full society near the bottom of the sea. They have a culture where the shallows are nomadic and don't remember theirs, and are mostly pacifistic (except for, like, food. they still eat meat). Deepsea live near the trenches at the bottom of the ocean. They’re deeply entwined culturally. They don't raise their offspring individually like Shallows mer. They took “it takes a village” to its logical extreme. Anyone and everyone can and is expected to look after children that need guidance. They're raised in groups in a kind of "orphanage" and are educated until they age out. Once they're adults, they're expected to become a productive member of society.

Deepsea mer are massive. They grow until they die (just like a lot of deep sea creatures do). You can tell the age of a deepsea mer just by looking at it. Their culture is a lot more advanced than Shallows, and while it looks utopian, it's extremely rigid. It's basically a dystopian society where the government controls knowledge and education to conform to their standards of usefulness. Deepsea mer have multiple sexes — male, female, dualsex*, and various gender identities.

* Please note, I am intentionally using the word "dualsex" because intersex does not accurately describe them. They have fully function male and female sex organs, whereas intersex people are a spectrum of functionality. Thank you!