Because viscous forces are related to surface area, drag effects on swimming are relatively greater for small organisms than large. Small animals swim slowly compared with large ones. In fish, swimming speeds are roughly proportional to length. Based on experimental observations, Bainbridge (1958) formulated the following equation relating speed, length and rate of tail beat in teleosts:
where V = swimming speed in cm s-1, L = fish length in cm, and f = tail beats per second s-1.
The effects of viscosity are greatly increased where movements generate turbulence; hence the advantages of streamlining. The best streamlined form for preserving laminar flow is circular in cross-section with a rounded forward end tapering aft to a point. Fast-swimming cephalopods impelled by jet propulsion adopt a shape close to this ideal streamlining. The majority of fish are elliptical rather than circular in cross-section because they swim by lateral oscillations of the trunk, lateral flattening giving extra thrust (Gray, 1968). However, the fastest fish such as large tunas are more circular in cross-section, their lateral flexures being confined mainly to the narrow posterior end of the body, and thrust is obtained chiefly from the large caudal fin. Certain tuna are reported to achieve bursts of speed exceeding 20 ms_1, but normally they cruise at slower speeds of 1-2 m s^1.
Swimming efficiency varies with drag, thrust and efficiency of energy conversion. Generally the speed at which a fish can swim the greatest distance for a given supply of energy appears to be about one fish length per second irrespective of size. Efficiency of energy conversion decreases at speeds above and below this optimum.
The limitations on swimming speed inherent in small size may be one factor setting a lower limit on the size of fishes. Few teleosts have adult sizes less than
2.5 cm. Other disadvantages for fish of very small size include the difficulties of osmocontrol with a relatively large surface area, and reduced capacity for egg production.
Recent research at Woods Hole Oceanographic Institute in Massachusetts, USA, has provided new data on those features of fish propulsion that result in the high swimming efficiencies seen in many fish and in dolphins (Triantafyllou, 1995). The work is primarily aimed at the development of robotic free-swimming craft which will need a very efficient means of propulsion if constraints of energy storage are to be overcome. To this end they have made considerable progress in constructing a free-swimming 'robo-tuna' of aluminium and lycra, and have added to our understanding of drag, thrust and turbulence.
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