Polentatesta
Its ancestor, preyed upon by several fast, voracious descendants and plagued by strong pathogens, went extinct...but not before yielding its final descendant.
It has many defenses against predators: It's fast, skittish, bad-tasting, hard to find (if not attacked), attracts predators if attacked, and too small to be worth eating for the adults of most of its predators. Nonetheless, many Polentatestas are eaten in their largely helpless larval forms, and so survive by high reproductive rates.
Movement
The polentatesta's body is shaped something like an elongated and slightly laterally-flattened teardrop, making it even more hydrodynamic. Whenever a predator shows up, the polentatesta rapidly swims away in a trout-like sub-carangiform style, using its fins to steer. However, to save energy in the resource-poor depths, the polentatesta spends much of its time almost immobile in the water, except for the occasional twitch. While dormant, it turns off its lights, making it hard to find for visual predators.
Senses and Behavior
The sticky membrane on its anterior end (or "nasal patch") has roughly the proportions of an elongated coin. It functions as both the organism's "nose" and "stomach", absorbing and sensing waterborne chemicals, digesting microscopic bits of food (e.g., dead photosagnia falling from upper biomes), and sending nutrients into its interior.
Whenever certain chemicals emitted by the adult Polentatesta's predators (primarily Carpotesta Devoratori descendants) float into the sticky membrane on its nose, it automatically swims away in a simple feedback loop until chemicals from its predator no longer float into its nasal patch. A V-shaped, simple version of a lateral line system by the sides of its nasal patch also allows it to detect nearby predators by movement in the water.
What passes for a "brain", a few threads of mucus-coated chemoreceptor cells (descended from bait-cells), is attached to its nasal patch. It can survive surgical removal of its nasal patch, but consequently acts "brainless": it cannot detect smells and won't swim at all, not even away from predators.
If attacked, it will suddenly flash on its lights. This flash of light can attract bigger predators to eat whatever just attacked it. Its own vision is poor: It can only distinguish between bright flashes of light and dark and distinguish blobs of color. However, in the light-less depths of its habitats, keen vision is not necessary. Like an octopus or a peppered moth caterpillar, it has photoreceptors on its skin (specifically its trunk) that allow it to see without distinct eyes.
Bad Taste
Its most notable development are mucus patches. The nine mucus patches, duplicated from its nasal patch, have little ability to absorb food. However, they do emit a thin, distasteful, mildly irritating yellowish ooze. This ooze shares chemicals (such as dimethyl sulfide and methanethiol) with cooked corn, especially corn polenta, leading to a somewhat similar smell and taste. It also reduces drag on the Polentatesta's body, making it more hydrodynamic, and offers some protection against the various new pathogens that affect descendants of the Carpotesta luceremundare.
A: "Zygote" B. "Brain" C. "Larvae" D. "Neurons"
Reproduction
To mate, Polentatestas swim towards the light of others of their kind and use a sense of smell to get closer. When two polentatestas get close enough, such as by brushing their sides against each other, reproductive clumps next to its body lights dislodge. These reproductive clumps are microscopic (90 µm), vaguely jellyfish-shaped clusters of four cells. The cell clumps have crude tails, modified from their ancestor's baits-on-strings, that allow them to swim (if poorly).
The clumps dislodge, swim together, merge cellular contents, and then copy themselves into something resembling the polentatesta's ancestor, the Carpotesta glutterielux (with the exception of four tiny, simple fins and bigger, fewer lights). A quirk of this process is that polentatesta "embryos" have multiple nuclei.
This glowing, passive food-absorbing, microscopic-but-multicellular ball is effectively a larval form of the species. Starting at 200 micrometers, it grows rapidly. However, as it lacks any real defenses, many are eaten by filter-feeding organisms before reaching adulthood. The young survive (as a whole) largely by sheer numbers.
Physiology and Miscellany
The polentatesta's body has no coelom (body cavity); in fact, it doesn't even have a gut. Nutrients caught in its nasal patch float through its somewhat greasy body fluids, aided by the occasional body twitch.
Its internal cells, which are fairly large (300 µm), are bound loosely together, suspended within its body fluid.
As a footnote, the polentatesta's body is so small, thin, delicate and watery that it breaks apart into an unappetizing, slimy husk if even slightly sauteed.