Common Crestgill
The Common Crestgill replaced its ancestor, becoming optimized for subterranean detritivory. Its gills are fused onto its body, creating distinct “crests” made of gill material and helping prevent them from being tangled and damaged while digging.
The Common Crestgill has developed a pair of reproductive organs from the end of its intestine to optimize its reproduction. This also allows it to actively give birth rather than let its babies swim out on their own. This has helped allow its babies to develop more because they could grow more before they had to be birthed, now being born as miniature adults. Instead of feeding its unborn babies mostly waste, it takes the more optimal route of secreting nutrients for them from glands inside the wombs. In the males, in place of the wombs are an internal pair of testes where sperm is produced.
The Common Crestgill has no more need for filter-feeding, being able to live exclusively off of detritus. Like its ancestor, it pulls bits of detritus into its mouth by inverting its mouth, partially enveloping the food, and pulling it in; it does not do this with sediment, however, as it has developed the ability to simply dig with its face like an earthworm. As a result, its cloaca has shrunk back down to a reasonable size, no longer needing to deal with such large amounts of waste.