Here's a list of some of the wajinru's characteristics. I find it interesting that whales are considered their second mothers in the story, but the wajinru's morphology is more inspired by fish. Obviously, I don't think we're supposed to think so hard about this, but certain details did confuse me. feel free to add more details!

depth of habitat they most likely live below the 1.5km level in the ocean. When Yetu kills the frilled shark, she mentions they usually don't come down that low, so that's close to 1.5km. and she stated that if she swam up past that shark, she'd bump into other wajinru. light doesn't penetrate the water past 1km either so that explains the darkness described by her that she finds normal. I guess their usual range is 1km to 2km.

range of ecological roles it doesn't seem like all wajinru live in the same area, they're kind of spread out. combined with the fact her mother has a favourite fish, the wajinru probably take on different ecological roles. some live closest to the border between the twilight zone and midnight zone (1km below surface), some live deeper. Yetu is able to swim all the way down to hydrothermal vents, and those are 2.5km or lower. And she was able to survive in a tidal pool and at the ocean surface, though her body isn't used to the change in pressure. so even though the wajinru have different niches, it doesn't seem it's due to biological limitations.

I guess it's preferences related to their individual ancestry. They have their shared ancestors as pups from pregnant African women from the slave trade, but obviously there are many different countries, regions, and cities in Africa with different cultural practices. It seems like the wajinru, despite not keeping their memories for majority of the year, still keep their human mother's traditions alive in some form.

electroreceptors Yetu mentions the electricity in water, which means the wajinru have an acute ability to detect the electrical waves from other species and the earth (so she should have never felt lost when swimming away, but whatever). They probably have ampullae of Lorenzini (sharks have them around their snouts and along the lateral line) and they're basically these jelly spots with tiny hairs inside that detect electromagnetic signals (and the direction of the wave moves the hair in different ways so the movement is converted into different voltage signals - it's super cool). So wajinru probably have lateral lines too. Those might be the glowing spots along her side in the cover photo. Yetu perhaps has more lines, making her more sensitive to the signals. And these lines are visible up close (you can see them in some fresh water fish if you go fishing, so I like to think she has similar "tattoos" as Oori

reproduction I think their reproduction makes sense in terms of survival - I'm not an expert on fish reproduction so take this understanding with a grain of salt. Multiple partners being the norm is probably to increase the genetic diversity of future populations and there are sharks that participate in similar methods. The pups would basically be half siblings. It seems like they give live birth (no eggs were mentioned) so there's a great deal of parental care involved in raising young since pups don't pop out ready to survive on their own. Their social nurturing is probably similar to whales (who are their second mothers) as there are whale species that have "grandmothers" that help in raising pups even though they're past reproductive age (it's really cool if anyone ever wants to look it up)

also wajinru being hermaphrodites is an interesting detail. a lot of plants and animals have both egg producing and sperm producing organs and there are papers detailing the ecological drivers that caused that. I think it's related to dispersal and resource allocation, but it's been a long time since I read those papers so I'm not sure. It does make sense though, the ocean is gigantic and there are going to be some wajinru that are alone so they could technically self-fertilize (there's also a shark species that can basically make a close daughter!).

tail morphology also interesting that the art on the cover gives the wajinru a fish tail instead of a dolphin/whale tail. The different size of the top and bottom of the tail remind me of thresher shark, so wajinru have a heterocercal tail. But I also can't tell if the tail structure has soft flappy parts (like more flowy/showy fins that some fish have for sexual selection) or if stiff like a shark tail or has rays like bony fish. they have webbing between their fingers so maybe there's webbing between the top and bottom tail lobe, but that would be dumb because you need it to be sturdy to provide enough thrust when swimming.

cartilage they're also all cartilaginous?? That's too much of a change from human to mermaid imo. I personally think the wajinru pups (like generation zero) should have been closer to dolphins than fish. Or make them bony! There are bony fish! Or make sharks also maternal figures?!