@cohanf
Thanks! I had no idea. It’s such a strange plant that I kind of thought it was another succulent. Also surprised I haven’t killed it yet with the self-watering pot. 
@sollat They are kind of weird aren't they? (that's the fun, of course..lol) I think the natural forms would already be odd, then horticulture selects extra weird forms and colours.. There are a some Opuntioids (+ another similar group) that even have small persistent leaves, and even a genus in another subfamily- Pereskia- that look like regular woody, leafy shrubs (if a bit thorny or slightly succulent).
@sollat I just did some reading (since it's been years since I was following cactus theory closely) and interestingly, there is debate- even after some molecular and genetic study in recent decades- as to whether those cacti with leaves represent the early state of cacti, or whether in fact they started as small leafless succulents and those with leaves developed them later!
@cohanf
The look of it was quite surprising at the nursery. I’ll try anything once. These three sit nearest to the heater, keeping weird company.
@sollat fun to see 😊 You probably already know those are vestigial leaves which many Opuntia grow temporarily on new growth (the pads are stems, not leaves).