Though it may be more due to the results of my current growing conditions than anything else, I have been pleasantly surprised to find that this species has taken hold as probably my favorite rainbow plant so far. Should I get larger spaces to grow in the future where the giants have an appropriate amount of room, that may change, but for a small area the rest can't quite seem to beat this guy. With access to a little gibberellic acid, this is almost more of a beginner's plant than its weedy relative B. liniflora is. Certainly, it's more floriferous, and just as adaptable to cramped conditions that the rest of the genus seems to just about hate. Of course, I may also be lucky with the form I have, as it's quite thoroughly earned the "robust" title it came with.
B. aquatica is, like the rest of the annual rainbows, a Northern Australian native found in the second most northern point on the continent, near the city of Darwin (interestingly enough, the same area is home to the Humpty Doo locality of D. burmannii famous for its red color). In the same size range as both B. liniflora and rorida, this species tends not to grow much more than a foot in height under standard terrestrial circumstances. However, this species also doesn't tend to grow in normal circumstances to begin with. As the name suggests, this rainbow plant thrives in very, very wet conditions, growing in places that may be entirely underwater during the rainy season and for some time afterward. Plants adapt to this by growing lanky, scrambling stems that might reach 2 feet or more whose leaves will support the uppermost 6 inches to a foot above the water surface. Even more interestingly (and seen in my care), the seed pods will not open fully when mature until they come in contact with water. This releases the seeds rapidly afterward, allowing them to be washed away to colonize broader areas. Ironically, like most other rainbows those seeds still need the chemicals from fires to germinate, so somehow they must go from floating in water to surrounded by burnt vegetation.
The color of this species is also quite distinct; B. liniflora tends to be green throughout, maybe tinging a little reddish or purplish on extremities in strong light, but B. aquatica unless in low light starts off very, very red or purple.
The form that I grow also possesses a very sturdy, thick stem not seen in its relatives, and relative to its size also thicker than any of the larger species like B. filifolia or guehoi. While even a relatively small liniflora or guehoi might begin scrambling and leaning on other structures/plants for support while just a few inches tall, the stem of this species keeps it sitting perfectly upright to well over a foot high. Were my shelves taller, or if I tried growing them outside, I don't doubt they might just remain fully erect to 18 inches or more. Truly impressive on this too is that once they hit about 6 inches tall, they can start sending out an army of flowers that the plant must also support. Flower stalks are relatively short, and seem to have a tendency to curl around near the stem (which may help support so many), but can still appear in the dozens, so long as they aren't all pollinated (after which they rapidly drop their petals).
B. aquatica possesses the darkest purple flowers of any of the species I've successfully grown so far, and quite interestingly is also the only species that possesses tentacles that just barely extend off the leaf surface; most others have long tentacles often of various lengths across their surfaces, but these are short, uniform, and rather closely resemble the appearance of the linear-leaved Pinguicula like P. filifolia, elongata, or medusina. Another attractive quality is that this species can self-seed, and do so without severe inbreeding issues. However, the flowers tend to still need manual pollination to set seeds, and better counts and offspring can still be had with crossing, so having at least a couple specimens in a pot is always best. Plus that way you get even more flowers.
One last note: no issues with relatively small or shallow pots with this species. Don't have a lot of room, and want something other than the standard liniflora? Definitely look to this one as something to try out. Here in the shop, there are plenty of seeds as well.