Where are the mermaids?

I'm playing with an ordinary human sim. She's been hanging out with Nalani, and I've also created a household of three mermaids and moved them into town. But neither Nalani or any of these three are ever in mermaid form as far as I can see, not even while swimming.

I hope I'll be able to see them in mermaid form by befriending them, because I would prefer my active sim to stay human.