Hinged Shellstar

The hinged shellstar split off from the shellstar in more ways than one—what was once a single shell with four distinct lobes is now split into four separate individually movable shells. This allows it to move to keep its balance or return to an upright position when disturbed, as well as seal itself off better when it hides from a predator. This is not its only change, however.

With an increased size and slightly greater energy demand, it was advantageous for the hinged shellstar to develop the ability to consume larger meals. While it can still filter-feed using the bristle-like filter baits on its arms, it now has the ability to grab larger morsels—whether small free-swimming organisms or bits of larger dead ones—using its arms and pull them into its center where it secretes digestive enzymes to break them down so it may absorb them into its cells. It detects potential meals using a combination of sensory input from its eyes and currents flowing over its filter baits to pinpoint the size and distance of something passing overhead. This helpfully supplements the microscopic food it already absorbs using its filter-baits. It has no specific maximum length of the prey it consumes, other than practicality; in the case of flat or soft-bodied fauna, such as many asterzoa, even ones longer or wider than itself can potentially be compressed or folded to fit inside its primitive stomach. When it has digested all that it can, it ejects indigestible parts by briefly opening up and inverting its stomach, letting ocean currents do the rest.

The hinged shellstar is otherwise much like its ancestor. Its shell is made of calcium carbonate, it reproduces by releasing spore-like gametes into the water, and it has a brief free-swimming life stage before it develops its shell and sinks to the ocean floor. It has lost the ability to reproduce by fragmentation, as fragments may accidentally digest themselves and would usually starve, not to mention the high risk to the parent if its digestive enzymes are spilled by the process.