Abstract: The invention is an apparatus for the cultivation of marine species, particularly molluscan shellfish such as oysters. The shellfish are cultivated within a rotating cylinder featuring water-permeable walls, and positioned at the water surface by buoyant chambers such that a portion of the cylinder is immersed while the remainder is exposed to the atmosphere. The rotational position of the cylinder is advanced by way of a passive mechanical interaction driven by changes in local water elevations (e.g. tide cycles), thereby creating a repetitive and systematic exposure of the entirety of the cylinder wall to atmospheric desiccation. Furthermore, the rotation of the cylinder promotes a gentle tumbling of the shellfish within, thereby improving product shape and uniformity of growth, as well as enhancing the clearing of accumulating sediments and/or metabolic wastes.