Ripplegut
The Ripplegut is a simple, watery filter-feeder, which moves up into the sunlight zone at night to feed.
The ribbing of its stomach (technically not a gut) has a distinct rippled pattern. The cilia in its stomach are just barely below the visibility of the human eye, and move constantly, if fairly slowly. Due to the cilia and shininess, its guts always seem a little blurry and maddening to look at.
Its "skin" (a thin, primitive surface tissue) is nearly transparent. However, in life, it constantly emits low levels of blue light from light emitters dotted nearly everywhere on its body. While it only has cyan (sky blue) and medium-blue lights left, by varying which ones are activated and how many, it can match most blue backgrounds. Whether at various times of day in the sunlight zone or in the twilight zone, it is nearly invisible. Indeed, it is so well-camouflaged that, against pure blue backgrounds, no would-be predators at its time of evolution can spot an adult...but some find it anyway, either from bumping into it or by using other senses.
One way to detect it in especially food-rich waters is to watch a swirl of particles being drawn into the gyre of its mouth. If it gets too close to the surface of the water, clouds of purple Photosagania descendants tint the water slightly purple, making it unable to match. This is also a problem (to a lesser extent) under a background of abundant green photosynthesizers (e.g, adorbalgae).
Its petal-like fins are pretty stiff, and are used to steer. Their big, flat surfaces can tilt downwards when the Ripplegut swims up, making it more hydrodynamic, or move outwards to make them harder to swallow for predators. The fins are made mostly of Type 1 collagen, making them chewy and almost rubbery.
All of its external surfaces have guanine crystals embedded, helping it not only match the surrounding water in color and shade, but also reflectivity.
Its thicker skin and fins make it very slightly denser than water, but its system of air pockets help it regulate buoyancy and so not sink.