Sunlight Gill
A population of twilight gills rose from the chilly depths of the twilight zone into the still-chilly but brighter sunlit zone around the southern pole. There they found an environment free of significant predation and full of microbes to feed on. They continued their steady growth, reaching an unprecedented adult length of 70 centimeters.
Their filter-feeding bristles were always quite limited, as they could only ingest food through tiny openings in the bristles themselves, serving as a plethora of microscopic mouths. The sunlight gill has changed the bristle structure to make a much wider size range possible, which makes it easier to sustain themselves. They've repurposed genes responsible for the branching in their gills—not too difficult, as the gills and bristles form from the same original structures. Now they have several primary bristles, able to accommodate food up to two millimeters and serving as the main areas for intake. Secondary branches on the bristles serve two functions. Many are much like the ancestral bristles, trapping microbes among them and absorbing them through tiny openings. Others are stiffer and grind together to crunch hard-shelled food like rainbowhedrons and young rainbowts. The primary bristles have slight mobility too, mainly used to enhance this grinding action, though they can also angle the bristles toward a cloud of microbes if they see or smell one. This mobility occurs at the base, using the same hydrostatic system as the rest of the body.
The sunlight gill's overall body plan is little changed from the twilight gill. They have a central body with four arms: one to each side and one in front and back. The side arms serve as broad paddles, the back arm contains an anus, and the front arm contains a brain at its base and an array of sensory equipment. It supports a pair of cup eyes near the tip and a pair above the base, four pairs of chemoreceptive patches along its length, and a row of vibration-sensing bristles. The back arm and side paddles support the branching gills, bright rose from the hemerythrin blood. With the higher oxygen content near the surface, the gills have shrunk slightly, and the hind arm is shorter too, no longer needing to accommodate quite as large an array. Similarly, the paddles and front arm support the filter-feeding bristles. The central body contains a stomach, protective fatty deposits, and three hearts, which distribute pinkish-violet blood throughout the whole organism.
The reproductive system is almost the same as the twilight gill's. They eject gametes through an opening beneath the hind arm, which they take in through their filter-feeding bristles. With hard outer casings, the gametes remain undigested until they arrive at the reproductive organs, and they refrain from grinding action while mating. Once fertilized, the sunlight gill grows a bud on the front or back arm, which develops into a new sunlight gill. The bud detaches after its gills and bristles are developed.
The only difference comes in the proteins in the gamete casings. As before, these proteins signify mating types, and the sunlight gill rejects any gametes belonging to its own mating type (thereby preventing self-fertilization). They have two genes to determine these proteins, one encoding A, B, C, and D, and the other encoding X, Y, and Z. Unlike the twilight gill, the first gene no longer has a null variant; all gametes must have the A, B, C, or D protein. This means the first gene has ten varieties (A, B, C, D, AB, AC, AD, BC, BD, CD), not eleven, making a total of seventy mating types when multiplied by the second gene's seven varieties.
The sunlight gill has a simple lifestyle, with little need for significant intelligence. They paddle languidly along, picking up food in their bristles while barely noticing it, approaching others of their kind to mate, and paying little heed to much else.