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Supplementary image showing a rhombill from above.

Name: Rhombill (latin name pending)
Creator: Cube67
Ancestor: Twilight Gill
Habitat: LadyM Twilight Zone, South LadyM Temperate, South LadyM Subtropical Ocean, Rhino Subtropical Ocean, East Glicker Subtropical Undersea Meadow, Glicker Temperate Undersea Meadow, Glicker Temperate Undersea Meadow, Glicker Temperate Coast, West Glicker Subtropical Coast, East Darwin Temperate Coast, East Darwin Subtropical Coast, Dixon-Fermi Temperate Coast
Size: 30 cm (non-W mating types), 24 cm (W+ mating types)
Support: Soft-Bodied (Hydrostatic Skeleton)
Diet: Planktivore (<2 mm)
Respiration: Passive (External Gills)
Thermoregulation: Ectotherm
Reproduction: Sexual budding (ingested gametes, 121 mating types)


Following the recent extinction event, the amount of phytoplankton in the waters of Sagan IV drastically decreased, to the point where many species of macroscopic floating flora went extinct. While many smaller filter-feeders were safe, their populations were still affected, especially in the case of less efficient filter-feeders. Enter the gills, a group of planktivores which have remained relatively unchanged for 70 million years, despite the fact that their feeding method lacks a filter-feeding mechanism or any way to actively suck in water in large quantities. This combined with their slow movement and lack of streamlining has made it tougher for the average gill to survive. Thus, the rhombill has undergone some important adaptations to become more competitively viable.

Locomotion

The rhombill is more streamlined than its ancestors, with the front arm being truncated to reduce drag in the water. The thin membranes connecting their arms have gotten thicker, being more permanently supported by muscle rather than temporarily extensible hydrostats. These membranes are relatively seamless with the arms, with the lateral arms themselves being wider and stronger as well. This allows the rhombill to use a more efficient manta-like method of swimming all the time, as opposed to traveling mainly with the previously paddle-like lateral arms. The increased speed and swimming efficiency of the rhombill allows it to more easily seek out plankton-rich areas, as well as aiding in escape from predators.

Feeding

While being faster to search for food is important, being able to eat that food is even moreso. The rhombill’s ancestors were unable to manually draw food into their feeding bristles, with the bristles’ small size making it unlikely for much plankton to simply float in anyhow. The rhombill has evolved to overcome both of these issues. The feeding bristles themselves are a little wider and fewer in number, with the entrance pores being fairly large and mostly clustered at the tip of each bristle. While this does help the rhombill feed, this trait alone cannot account for the nutrient-poor conditions common when the rhombill evolved. This is what led to the development of the rhombill’s more advanced feeding system: biogenic pipettes.

Between the back of each bristle and the front of its corresponding esophagus, a muscular pharynx has evolved, closed off by simple valves on each end. When the rhombill is in a spot with abundant food, it opens the bristle-pharynx valves and draws in the plankton-rich water by expanding the pharynges. When the pharynges are full, it closes the bristle-pharynx valves, opens the pharynx-esophagus valves, and squeezes the pharynges to push the water further down the esophagi, where peristalsis then takes over. While this system may seem somewhat complex, all of the musculature involved was derived from the muscles already being used for peristalsis, with the pharynx and valves themselves being derived from “bloated” and “pinched” sections of esophagus respectively.

Reproduction and Genetics

Given the decreased number of gills in general thanks to the recent extinction event, the incompatible result of a rhombill accidentally mating with a rhombill of the same mating type became even more costly. This resulted in the evolution of a new mating protein encoded by the secondary mating type gene: protein W. This results in eleven varieties of gamete protein gene 1 (A, B, C, D, AB, AC, AD, BC, BD, CD, none), and eleven varieties of gamete protein gene 2 (W, X, Y, Z, WX, WY, WZ, XY, XZ, YZ, none). When multiplied together, this setup yields 121 different mating types. Peculiarly, the gene encoding for the new W protein also encodes for a mild form of neoteny; rhombills that grew from gametes bearing the W protein are able to reproduce earlier in life and have a smaller adult size. These traits make this subgroup of rhombills less likely to die before reaching sexual maturity, in addition to reducing their competition with other rhombills.

Offspring almost always bud from the underside of the posterior arm, where they are well-protected and cause minimal drag. Babies may also occasionally bud from the gills, where they are able to receive ample oxygen for more rapid development. Reproductive budding from the lateral arms, anterior arm, or back is generally selected against due to the risk of the developing offspring being targeted by predators or torn off prematurely. Rhombills are able to instinctually “count” the number of developing offspring attached to their body via a hormonal system. Rhombills that have recently lost a growing larva enter a state of mourning, wherein they have a decreased drive to eat and reproduce. This aids in natural selection for rhombills that are better able to protect and care for their offspring.

This organism replaces the twilight gill in its range.

This post has been edited by Cube67: Feb 14 2023, 04:43 PM

Side question: should this replace anything? I'd think it might, since the gills are pretty inefficient at their lifestyle already (somewhat weird filter feeding method, passive breathing, soft exterior), have competition, and are facing the current low-oxygen event. (I finished most of this description a while ago, I forget which gills this overlaps the range of).

Yes, feel free to do some gill replacement! They evolved when they had no competition in their size range, so they weren't supposed to be that efficient yet. Either way, by the end of the Week I'll probably want to go over and declare some outcompetition extinctions for gills.

I really like how this design uses the gill features to create a more streamlined and effective design.

Added the detail that this replaced the twilight gill in its range.