A sketch for a sponsored commission of a floating forest endemic species of maceback (paper got creased, sorry). Macebacks are a subgroup of largely herbivorous shimmershiners which have become heavily armoured to defend themselves from marine predators, and a larger, rotund body to facilitate a large gut to more effectively digest marine vegetation.
The urchin-belly has reduced some of this armour because it lives in an isolated ecosystem with few (or no) predators able to harm them as adults, and are one of the largest animals native to the environment.