Transparency is a powerful defence in open water, where there is nothing to hide behind. Cranchiid squid achieve it with thin, watery tissue and a large body cavity filled with ammonium chloride solution, which is less dense than seawater and provides buoyancy without a swim bladder.
Some organs cannot follow. Vision requires pigment to absorb light, so a functioning eye is necessarily opaque, and so is the digestive gland. Glass squid mount photophores directly under each eye that emit light downward, countering the silhouette from below, and some species orient the digestive gland vertically to minimise its cross-section when viewed from beneath.
A common misreading of laboratory photographs is that these are rare animals. They are widely distributed and abundant enough to feature in the diets of many deep-diving predators, but their bodies rupture in trawls, so intact specimens are unusual. The most famous member, Cranchia scabra, can withdraw its head and arms into the mantle cavity, an option most squid do not have.