Abstract
The chambered nautilus (Nautilus pompilius) encounters severe environmental hypoxia during diurnal vertical movements in the ocean. The metabolic cost of locomotion (Cmet) and swimming performance depend on how efficiently momentum is imparted to the water and how long on-board oxygen stores last. While propulsive efficiency is generally thought to be relatively low in jet propelled animals, the low Cmet in Nautilus indicates that this is not the case. We measured the wake structure in Nautilus during jet propulsion swimming, to determine their propulsive efficiency. Animals swam with either an anterior-first or posterior-first orientation. With increasing swimming speed, whole cycle propulsive efficiency increased during posterior-first swimming but decreased during anterior-first swimming, reaching a maximum of 0.76. The highest propulsive efficiencies were achieved by using an asymmetrical contractile cycle in which the fluid ejection phase was relatively longer than the refilling phase, reducing the volume flow rate of the ejected fluid. Our results demonstrate that a relatively high whole cycle propulsive efficiency underlies the low Cmet in Nautilus, representing a strategy to reduce the metabolic demands in an animal that spends a significant part of its daily life in a hypoxic environment.
Highlights
Chambered nautilus (Nautilus pompilius) perform diurnal vertical movements involving depth changes of 500–600 m
Two different categories of wake structure were identified during jet propulsion swimming: jet mode 1 jets, in which all of the ejected fluid rolls up into an isolated vortex ring, and jet mode 2 jets, where fluid is (a) rsos.royalsocietypublishing.org R
These two jet modes are comparable to the categories of jets described, based on two-dimensional recordings similar to those used here, in free-swimming brief squid [21,22] and more recently quantified using three-dimensional particle image velocimetry in the same species [23]
Summary
Chambered nautilus (Nautilus pompilius) perform diurnal vertical movements involving depth changes of 500–600 m. Posterior flexible funnel orifice anterior wings of funnel mantle cavity [4,5], the ability to extract ambient oxygen via the superficial capillaries in the absence of gill perfusion, and the ability to use oxygen stored in the shell chambers [6] are physiological adaptations that enable Nautilus to survive hypoxia but to maintain sufficient metabolic scope to perform their extensive vertical migrations [7] It is only at PO2 below 50 mmHg, encountered in oxygen deficient water or during retraction of the animal into its shell, that metabolic suppression is required to protect against hypoxia [7]
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