In well-aerated aquaria, Plecostomus appears to rely on gill irrigation alone. If aeration is stopped, these fishes complement gill irrigation with regular air breaths at the water surface. At 22C the inhaled air is retained in the well-vascularized stomach of adults, usually for 7 to 12 min. The expulsion of gas through the opercular and oral apertures occurs while the fish is submerged. Both opercular cavities have a membranous lining which is interrupted only at the gill clefts and opercular outlet. The behavior of the fishes varies cyclically with gastroventilation. They also make four basic types of excursion to and from the water surface.
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Norman Gradwell (1971) studied this question.