Brick Shev
The Brick Spev splits from its ancestor, living in the more crowded twilight zone environs. It lives a slow-paced life, catching microbes or detritus that stick to its pseudopod-like feeding tentacles.
Brick Shevs' first stage resembles a smaller version of the Goliath Shev. However, once they find a suitable location (especially small hills) and others of their kind, they become sessile. (The picture shows one individual for clarity.) Their leglike pseudopods link into the shells of others of their kind and fuse, and their upper pseudopods lengthen and taper, becoming feeding tentacles. Their brick-like shells, over years, gradually merge together with a slimy "mortar", taking the shapes of small walls and lumpy ‘fortresses’.
In time, they form a unique arrangement: a Twilight Zone 'reef', the first of its kind. As they no longer need to move at this stage, they can pick up stronger, heavier minerals for their protection. The exact composition of their shells depends on where they are in the seafloor, but it can contain unusually high (if still small, in absolute terms) amounts of heavy metals, to the point predators (such as the Greater Knightworm) who try to eat a lot of Brick Shevs may poison themselves.
When space is limited (such as on a hill) newcomers must glue themselves to the top of the colony. As the newcomers grow, they can make it harder and harder for the Brick Shevs beneath them to feed, and even lead to starvation. While they are fused into a sort of colony through their tentacles, they are ill-equipped to share nutrients. As such, Brick Shevs blocked from feeding may starve, leaving big (<30 mm) empty pockets inside their bricklike bodies.
In 15-20 Saganian years, the reef can grow up to two meters, though half a meter is the most common size. With little ability to control the shape of the reef, the Brick Shev colony may grow to impractical, fanciful shapes, such as sloppy bridges to nowhere. This and the brittle bodies (by human standards) of younger Brick Shevs leave their colonies vulnerable to breaking apart in rare earthquakes or when large fauna swim into them.
Their tentacles, in addition to microbes, catch the spores and microscopic young of many Twilight Zone organisms.