The brownble is a hybrid of the bristlesquid and brittlesquid. Over time their spawning of eggs cross bred and developed into an extremely different looking offspring. Much of these mutations were harmful while a few were helpful. Those that were eventually led to the brownble. Unlike its ancestors it was now brown in color to blend in with the mud which was very successful. And was having spines all over its body rather than just on the arms. Some of which formed an anchor on the end of it so it is hard to pull out of the mud by predators.
Their breathing was the most extreme which lead to to ultimately replacing both of their ancestors. Rather than siphons it has 2 "tentacle-gills". These allow it to breath while half submerged under the mud.Before they had to constantly breathe through the mud or come out to breathe properly, but now they can stay immobile for hours. Their mouth too is now a long tube which can reach up to the tentacles. Each of the arms has a bristle just on the end of it to filter-feed. Once filled it will bring it to its mouth to suck out whatever it caught.
Even the eyes have improved. Before they were primitive eye spots which were constantly covered up in the mud. Now they are on primitive eye stalks which can peak out from the dirt. However not all its adaptations were helpful. Without its siphons it now has to walk across the river bottom to get places. This was a fair trade off for all the other adaptations it has gained.