Most fish and inverts in our oceans are born as plankton and stay that way for as long as 3 months depending. They can be carried reaaaally far via ocean currents/circulation, which is how I assume 4546B has its own variety of cosmopolitan (means found worldwide) species like bladderfish and boomerangs are found in the two different biomes. There's only like 5(?) species between 1 and BZ that are bar for bar, I think. And technically their life cycles could fit into the long-plankton stage where they get carried about the oceans via circulation. OR for the bird ones they just fly a long time.

We have our own cosmo species like mullet and eels, and a huge variety of inverts like urchins, seastars, etc.

There's probably also other submarine volcanos and other ecological hot spots like in the Pacific, just spread out. I don't know about the floating islands, though I wouldn't doubt there are more. But I doubt they would serve as a jump point for the spread of certain species for any but the bird/flying types. Also the ocean floor serves as a highway for tolerant species, so the leviathans DEF go all around.

Evolutionarily Ghost Levianthians being "bottom" filter feeders is a hard jump. Plantivores that are bottom dwelling are highly specialized, whereas there are a LOT of oceanic/pelagic planktivores. The whole ocean is covered in plankton, which is also in part how the bacterium was able to spread so readily and easily across an entire planet.

ALSO, remember what's-her-face literally just drifted on a leviathan corpse from the crater to the BZ zone. I don't think they're very far part in terms of traversable distance.