Jet Propulsion and the Evolution of the Cephalopods
The first cephalopods, recognized as such by their possession of a chambered shell with a siphuncle, appeared in the late Cambrian. They are assumed to have been crawling animals. Lightening the shell, made possible by ion and water removal and the consequent appearance of gas in the
chambers would have allowed the animal to escape upwards, using the jet resulting from a sudden withdrawal of the body into the shell. Nautilus still produces a propulsive jet in this manner. It can also generate a low pressure stream using the wings of the funnel to waft a flow through
the gills. It is argued that all the other ectocochleates would have had propulsive mechanisms based on one or other (or both) of the means by which Nautilus develops a jet. One consequence of this conclusion is that none of them were powerful swimmers. The momentum imparted by a jet
depends upon the ejected mass multiplied by the velocity of the jet. But E = ½mv2 so that while slow speed propulsion using the ventilation stream can result in a very low cost of transport any attempt to achieve high speeds is very costly. Only marginal improvements could
be made by lightening or streamlining the shell. More economical fast locomotion could only be achieved by increasing the volume of the mantle cavity, and this meant scrapping or internalizing the shell. It also necessitated the development of a new set of propulsive muscles, in the wall of
the mantle. Mantle musculature now took on both locomotor and ventilatory functions. The two functions have incompatible requirements; for efficient jet propulsion the stream volume should be maximized, while efficient oxygen uptake should minimize the flow. Octopods have optimized oxygen
extraction, the squids, in general are specialized for jet propulsion. In the teuthoids and octopods progressive shell reduction also led to the loss of neutral buoyancy, regained in some forms by ion-substitution in the tissues and an associated return to relatively slow-moving lifestyles.
Jet propulsion remained an extravagant form of propulsion at all but the slowest speeds. Many coleoids, as a result, have moved on to develop yet another locomotor system, replacing jet propulsion by undulant fins.
Document Type: Research Article
Publication date: September 1, 1991
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