How Fishes Swim

Auckland Star, Volume LXXI, Issue 297, 14 December 1940, Page 2 (Supplement)

How Fishes Swim

! By A. W. B. Powell

rE fact that fishes swim in their natural medium, water, as easily as we walk about on land is obvious—but what is hot quite so obvious is the actqal method by which many differently shaped fish manage apparently effortless propulsion through the water. Fins definitely come into this art of swimming, but to a much lesser extent than most of us imagine. The kind* which rely «nt!r«ly on fins for their motif® power are among the most sluggish of fish. Far more speed is achieved by sinuous movements of a fish's body or the scnlling action of a rakish tail. One of the fastest swimmers, the marine tuna (not the Maori tuns or eel) achieves its great speed entirely by the sculling actios of it* taiL The flns are not only of no asaisttanes in propulsion, but they are disadvantageous in actual swimming and are accordingly either slotted into the body life the blade fits into th e handle of s pocket knife, or pressed flat against the sides into specially shaped depressions, so that the smooth torpedo-shaped

f body offers the minimum of resistance. Even the curvature of the eves in no more convex than the side of the head. One might well ask why the fins have been retained if they do not serve in swimming. They do have a definite , purpose, however, for to the tuna they ' act as both rudders and brakes.. The lifting of one side or pectoral fin, as it , is termed, will cause an instantaneous t almost right-angled turn in the case of the tuna, and tha-.application of both ' pectorals has an instantaneous braking . effect, causing a smother of spray as is t witnessed when one of these fish strikes a trolling spinner. Not Fast Swimmert Of course, a large number of fish do achieve their propulsion by means of movable fins, but these cannot be regarded as fast swimmers. Most of the early fossil apecies, and a few of their 1 modern survivors, had very weak soft fins, which were very inefficient when 1 compared with similar kinds to-day, which have their fins reinforced with . hard spiny rays.

One of the most remarkable natural history discoveries of the century was the trawling of an ancient type of fringe finned fish from South African waters in February, 1939. By a.ll rules of the game this fish should have been extinct since the remote cretaceous era, some 50.000,000 years ago, yet, by some '■ strange chance, thi#> one member has r> lingered on in spite of heavy competition from better adapted and more virile stock. This South African fringe-fin lacks bony support to its fins, all of 1 which are soft and flexible. Distantly it b is related to the equally archaic Queensy land lung fish. l Rhythmic Contortions ' The most simple and most general f form of lioh locomotion is by means of i simple rhythmic contraction of the body X muscles designed to produce regular * contortions of the body, whi<-h cause a i succession of pressures against the water causing the body to be moved forward. A long body is essential to this form of movement, and so we have it to perfection in the eels. > An important aid to swimming is f achieved also by the rapid expulsion of - jets of water from the gill openings » during the process of respiration, and so r we get in some species at least three ; forces acting in unison—sinuous movei ment« of the body, local action by the , fins and water ejections from the gills, i A modification of the principle of body eontor+ion is found in the stingray# and the flat fishes. The body is not moved to any extent, but fringing fins in the case of the flatfish and the edges of the great flukes in the rays are caused to give rhythm undulations, which travel as ripples from head to tail. In effect this is the principle of the marine propeller, or more correctly its more primitive prototype, the marine screw, which was a revolving spiral of many turns. Sea-Horses are Poorest Sea-horses and the trunk or c*>p|*>rfishes of the coral reefs are the poorest of swimmers, for their bodies are quite rigid, and their only method of progression is by means of absurdly small movable fins projecting from their armoured bodies. The highest development in fish locomotion is exemplified by the flyingfishes, which actually leave the water at will and plane by means of large membranous pectoral fins which lock at right angles, whilst the fish is in the air. These fishes achieve their momentum by a vigorous catapulting off the surface by means of a special downward flick of the tail. Actually they cannot fly in the true sense of the word for, whilst in the air they are incapable of prolonging their "glide" unless on alighting they can by means of the tail , project themselves on another plane. The reason for this development of flying fishes is to evade the fast swimming dolphin fish (not the common dolphin or porpoise which is related to the whales). Swimming with fishes, therefore, i« a complicated business, and each style of fish has achieved his own special method designed to suit living conditions.