1. 1.

    Neutrally buoyant, ultrasonically telemetering, differential pressure transducers were attached to Nautilus pompilius L. to record mantle cavity pressures while the animals were induced to swim in a flume in response to food.

  2. 2.

    Synchronized videotapes and computer data files showed that jet pressures (p, Pa) increased linearly with swimming speed (μ, ms−1), p=1565u-8.

  3. 3.

    Because of their stereotyped responses to food, Nautilus could be induced to produce similar pressures in 51 respirometers, where oxygen consumption (VO2 ml kg−1) increased as a power function of pressure, VO2=3.85p0.584

  4. 4.

    Combining these equations gave an oxygen consumption-speed relationship, VO2=283u0.584. We used this equation to show that the cost of transport for Nautilus is dramatically lower than that for squid and, at speeds below 0.05 ms−1, even lower than that of an undulatory swimmer, the salmon.

  5. 5.

    Calculated power inputs and outputs suggest that squid have increased their power density (W kg−1) 100-fold over their ectocochleate ancestors, and that Nautilus is very efficient in its low-speed, low-energy environment.

  6. 6.

    These laboratory ‘calibrations’ are a basis for field studies using telemeteredpressure data to develop energy budgets for this unique living fossil as a reference for comparing the energetic requirements of ancient and modern seas.

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