Aquatic crustacean-like lifeform that is a natural sailboat. Native to the planet Trees.
Sailfins range in size from about a meter to 3 meters in length. Their soft bodies are enclosed in a hard shell filled with various chambers that can be filled and emptied with water or air to control ballast. They have a series of fins along their underside to control direction and provide some propulsion in calm seas, but their main form of locomotion is a set of three large fins on their backs that act like a ships sails. A complex muscle and tendon structure allows the sailor to turn and orient its sails to catch the wind and to collapse the fins along its back when resting or in rough weather. Sailfins are primarily air-breathers like Terragen baseline dolphins or porpoises and fill much the same ecological niche on their world. However, they are also equipped with moderately efficient gills and can fill their shells with water and collapse their sails to sink beneath the surface and either rest on the bottom or proceed more slowly using their fins.
Sailfins come in a variety of subspecies, some making their living in the shallower coastal waters of the Trees continental shelves, others ranging on great migratory journeys across the oceans, cycling from one hemisphere to the other in a fashion similar to the ancient whales of pre-Expulsion Earth.