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The ventilation flow in the vicinity of the pleopod-pumping thalassinid shrimp Callianassa subterranea in an artificial transparent burrow has been mapped using particle image velocimetry.The flow in the tube in front of the shrimp was unidirectional, laminar and steady, with a parabolic cross-sectional velocity profile.The mean flow velocity was 2.0+/-0.1 mm s(-1).The flow passed the thorax of the shrimp along the lateral and ventral sides.Ventral to the abdomen, the flow was dominated by the metachronally oscillating pleopods.The water around a pleopod is accelerated caudally and ventrally during the power stroke, and decelerated to a much lesser extent during the recovery stroke owing to a reduction in pleopod area.On average, the flow ventral to the abdomen converged towards the small opening underneath the telson, simultaneously increasing in velocity.A jet with a core velocity of 18-20 mm s(-1) entered the area behind the shrimp from underneath the telson, This caused a separation zone with backflow caudal to the telson, Owing to the high rates of shear, the jet diverged and re-adjusted to a parabolic cross-sectional profile within 1-2 body lengths behind the shrimp, showing no traces of pulsation.The metachronal pleopod movements in combination with the increase in flow velocity at the constriction in the tube caused by the uropods and the telson probably prevented pulsation, The energetic consequences of pulsating and steady flows in combination with several tube configurations were evaluated.The results suggested that, by constricting the tube and keeping the flow steady, C.subterranea saves on ventilation costs by a factor of up to six compared with oscillatory flow in a tube without the tail-fan constriction