Flatfishes (Pleuronectiformes) are characterized by their asymmetrical skulls, with both eyes on one side of the head. Flatfishes lay with their “blind” side facing the sediment and “eyed” side facing upward. The urohyal is an ossified tendon of the sternohyoideus. In flatfishes, this bone forms a channel between the eyed and blind gill chamber for water to flow through. Micro-CT scans of 56 flatfish species were used to quantify the morphological variation of the urohyal. We created a phylomorphospace that illustrates urohyal shape diversity. We then recorded gill chamber ventilatory pressures from five flatfish species. We examined the relationship between asymmetrical ventilatory pressures, urohyal shape, and ecology. Asymmetrical ventilatory pressures were observed in each species during above sand breathing, burial, and below sand breathing. Degree of asymmetry was not proportional to variations in urohyal shape. We expected to see a higher degree of asymmetry in species whose urohyals formed larger canals for shunting. This data shows us that the degree of asymmetry in ventilatory pressure is not influenced by the shape of the urohyal.