Cupotesta
This offshoot of the vinagobs spread northward into temperate, then polar climes. They continued their filter-feeding lifestyle but had to switch to a diet of mainly adorbalgae. Indeed, in the coldest months and northernmost reaches, they rely on the adorbalgae's antifreeze, absorbing it into their own bodies. Notably, the change in diet meant that the ribbed stomach now took on a greenish rather than purplish hue.
The most significant change came in their reproduction, however. Vinagobs were capable of spawning in times of stress. In the new environment, all times were times of stress, and the cupotesta transitioned to using exclusively sexual reproduction. However, their sparse population density meant that mates could be hard to find. They made two adaptations for this. One, they can store received sperm within their bodies. Two, the top row of fins evolved into hectocotyli: specialized sperm-transferring arms. Each hectocotylus is forked at its tip, allowing it to clasp another cupotesta's hectocotylus and thereby transfer sperm. When not in use, these fold up onto the cupotesta's side.
These arms, as well as the expanded fins in the second row, operate with a hydrostatic system. The fins are effective for rapid movement. In addition to mating, they can also use the hectocotyli to clasp with other individuals to form chains, whether horizontal or vertical.
Another significant change was the development of eye patches. Not yet true eyes, each patch consists of a series of eye spots, each sensitive to a different range of color. As a result, they can't make out an image of their surroundings, but they can detect which colors are present and on what side of the body. This is most useful for making out each other's colorful bioluminescent displays, which are important in attracting a mate (the more colors, the better). There are two rows of three eyes, spaced evenly; between each eye in the upper row are two hectocotyli. Between each eye in the lower row are two fins (for a total of six fins and six hectocotyli).
This has also led to the development of simple ganglia, clusters of nerves, beneath each eye patch, connected by a neural net.
The overall body structure is otherwise similar to their ancestors. Their bulk is a large stomach, which contains ribbon-tailed detriti microbes who aid digestion. Air pockets exist throughout the body, connected to the stomach, which can expand and contract to suck food in and to adjust buoyancy. The yellowish reproductive clumps now exist only above the hectocotyli, where they produce and store sperm and eggs. These are also connected to pores that eject offspring. The body is still fairly insubstantial.