The reef gillstar emerged as gillstars took to living among wormreefs. These heaping structures provided a refuge from predators, and with their gaping holes for water intake, they were easy enough to enter.
A reef gillstar will usually spend its entire adult life within a single wormreef mound, leaving only if the mound collapses or is otherwise disrupted. They spend most of their time hanging from a polyp using their anal fingers, usually deep in one of the reef's holes or in an interior cavity. There, they filter-feed much like their ancestor, taking advantage of the water flowing in and out the reef. But they're also surrounded by the wormreef's nutritious secretions. Intended to share nutrition throughout the reef and with the reef's juveniles, this abundant pool is easy to absorb and provides much of the reef gillstar's nourishment, without harming their hosts.
The reef gillstar adopted green pigmentation throughout its body, keeping it camouflaged. Their body has become laterally compressed, quite narrow when seen from in front, like many of Earth's reef fish. In the reef gillstar's case, this allows it to squirm between polyps and hide deeper within the reef, whether to hide from predators, to access a different part of the reef, or to escape from a collapsed reef. They can still swim using the anal arm as well.
Otherwise, the reef gillstar has much the same anatomy as the gillstar. They have a usually inverted mouth for absorbing food, eyes like inverted pigment cups, and four chemical receptors near their mouth. They reproduce by clasping their anal fingers and begin life as radial larvae, which is the stage when they'll swim through the ocean before finding a reef in which to live. Their four arms will then differentiate into three adapted as gills and the anal one.
With its simple lifestyle, the reef gillstar has little need for intelligence, but it does have a range of responses to disruptions to its reef and outside predators and threats.