Twilight Gill
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The twilight gill rises from the depths that its ancestors inhabited. They have adapted to better respond to predators and, as a result, have nearly replaced the midnight gill within LadyM's midnight zone; however, the midnight gill can still be found ascending there to take advantage of the greater oxygen. The twilight gill, meanwhile, has adjusted its internal pressures and is comfortable traveling through a wide vertical range but can no longer withstand the abyss. They have also spread via the poles into the Vailnoff Ocean.
The overall body plan remains much like their ancestor's. Four limbs extend from a central body, one in front, one behind, two to the side. The side arms serve as fins and have become wider and more paddle-like. The whole body operates with a hydrostatic system, integrated with their circulation of pinkish-violet blood, distributed by three hearts. A branched panoply of gills, colored a bright rose from the presence of hemerythrin-using blood, extends between the side limbs and the back one. Filter-feeding bristles extend between the side limbs and front one.
These filter-feeding bristles are now positioned to face more directly forward, growing largely from the side arms, with just a few extending out of the base of the front arm. These bristles have developed into hollow tubes with many openings to let in microbes and other organic particles in the water, like the falling corpses of surface microbes. These hollow bristles are too small, however, to let in any food larger than about 250 micrometers. Each bristle is, effectively, a mouth, leading to digestive tracts within the arms that in turn meet in the stomach. Waste goes through another tract through the back limb, leading to an anus at the far end.
At their great size, no predators eat twilight gills whole. However, they still must worry about hit-and-run attacks from the likes of spectrestars and horrorstars. These are especially dangerous for a soft-bodied organism with significant vulnerable spots. They've adjusted the position of the digestive pouch, which by now can be considered a stomach. It's still in the center part of the body, but it's more centrally located rather than bulging up and forward as before. They also have fatty deposits around the stomach and hearts. The membranes between adjacent limbs now extend a little further; most importantly, they can expand these by engoring them with fluid and coil up their gills. This is by no means complete protection, but it does make it harder to attack the gills when they're in this defensive position. When extended, these membranes can also ripple to provide additional propulsion, like manta ray fins.
Twilight gills now reproduce exclusively sexually. They still eject gametes when they detect fellows of their species, but rather than passing through their anus, they have an additional, much smaller hole on the underside of the back arm. The gametes come with hard outer casings and are swept up by the filter-feeding bristles and ingested. The casings ensure that the gametes remain undigested as they work their way into the body.
Self-fertilization was a problem, however, as they often ingested their own gametes. The twilight gill has prevented this by developing mating types. The gamete casings may come with certain proteins, which serve no purpose but to signify mating type. A twilight gill will reject any casings from its own mating type. Two genes determine mating type: The first encodes proteins labelled A, B, C, and D; the second encodes ones labelled X, Y, and Z. The former results in eleven varieties (A, B, C, D, A&B, A&C, A&D, B&C, B&D, C&D, none), and the latter seven (X, Y, Z, X&Y, X&Z, Y&Z, none). Multiplied together, this makes 77 mating types, meaning that twilight gills can mate with the vast majority of individuals they meet. These sex genes do not currently result in any additional distinctions.
After mating, the young grow by budding. These buds form at the base of the back or front limbs; they can no longer form on the side limbs, where they would disrupt balance. Like other asterzalians, the young first develop radially, but the front and back quickly become distinguished; they separate from their parent after the gills and bristles form.
The front limb is further developing to act as a head. In addition to the four pairs of chemoreceptive patches, arrayed from the base body to near the end of the front arm, they have developed two pairs of eyespots, one pair near the top of the main body, and one pair at the tip of the front arm. These are only capable of spotting light and its direction. The front arm also retains its row of green vibration-sensing bristles. It is possible to adjust the shape of the front arm by moving fluids, thus extending it to investigate something, or retracting it for streamlining or to protect it. The brain has grown within the base of the arm and is further cushioned by fatty tissue deposits, such that there is a noticeable bulge there, just in front of the main body. Their range of behaviors is still quite simple, however; just keep swimming, mate when other twilight gills are around, avoid hit-and-run predators when possible, and keep vulnerable spots away from them when they get too close. They use the increased brainpower to manage their greater range of senses.