I noticed an amazing thing about these today. When I first saw them they were completely clear but when I nudged it the edges became white. This defense mechanism is used by Corolla sea butterflies as well, but I can't think what advantage it gives the animals.
Doing some research it seems this is called blanching and another siphonophore (Hippopodius) does this too. (https://hal.sorbonne-universite.fr/hal-02951249/document)
Mackie & Mackie 1967 stated that the opaque edges are permanent for Bassia and I can't find any later literature which mentions this behavior so maybe it is a previously unknown behavior.

This is a GO-SEA absence observation.