[ 553 ] THE SWIMBLADDER AND THE VERTICAL MOVEMENTS OF TELEOSTEAN FISHES I. PHYSICAL FACTORS BY F. R. HARDEN JONES Department of Zoology, Queen Mary College, University of London (Received 20 April 1951) (With Three Text-figures) CONTENTS PAGE Introduction .......... ... 553 The relative volume of the swimbladder . -554 The change in density. .......... 555 The restriction to rapid vertical movements . -555 The restriction to slow vertical movements . -556 The experimental work . 557 Physical factors 558 The resistance offered to the expansion of the swimbladder gas . 558 The percentage volume of the swimbladder ...... 560 Density determinations .......... 561 Rupture of the bladder wall 562 Discussion 563 Summary 564 References 565 INTRODUCTION Moreau (1876) was one of the first to suggest that the vertical movements of a teleost are restricted by the presence of a swimbladder which increases or decreases in volume when the fish moves up or down in the water. The reason for this is that when the fish passes from one level to another it is subjected to a change of hydro- static pressure. If the fish moves above the level at which it is in hydrostatic equi- librium with its environment, the decrease in hydrostatic pressure (1 atm. for about every 10 m. of water) leads to an expansion of the swimbladder. As a result the fish as a whole becomes larger and therefore less dense than the water and tends to rise to the surface. Conversely, if it swims into deeper water the bladder decreases in volume, with the result that the fish displaces less water and tends to sink. A large vertical movement above or below the plane of equilibrium might result in the fish being carried up to the surface or down to the bottom. Too great a pressure reduction might lead to the rupture of the bladder wall. This danger would appear to be a real one; fish taken by line or trawl often arrive at the surface visibly distended and they are described as 'blown'. If they are taken from deep water the enlarge- 554 F- R- HARDEN JONES ment of the bladder may lead to the eversion of the stomach through the moutff (Hickling, 1927) or to a rupture of the bladder wall (Graham, 1924). Moreau believed that this danger would be less for a physostome than for a physoclist, as while the former could get rid of the excess gas rapidly by using the pneumatic duct as an escape valve, the latter could do so by the slower process of gas resorption which may take several hours. He concluded that a swimbladder, and in particular a closed one, would restrict the vertical movements of a teleost by confining it at any given moment to a definite bathymetrical range. There is very little information available from which any conclusions can be drawn about the extent to which the presence of a swimbladder restricts the vertical movements of a teleost. The problem is of interest as it is known that various teleosts undertake diurnal, seasonal or other periodic migrations involving vertical displacements (Murray & Hjort, 1912; Vedel Taning, 1918; Pearse & Achtenberg, 1918; Hickling, 1925; Johansen, 1925; Russell, 1928; Southern & Gardiner, 1932; Allen, 1935; Hasler & Bardach, 1949). Does the swimbladder restrict the magnitude or speed of these movements? Furthermore, it has been suggested that fish may be the scatterers responsible for the diurnal migration shown by the deep scattering layer (Chapman, 1947; Dietz, 1948; Hersey & Moore, 1948; Johnson, 1948; Moore, 1950; Boden, 1950). If an estimate could be made of the restriction that the swim- bladder imposes on vertical movements the information might be of interest in connexion with this and other scattering layers. In the present paper an attempt is made to determine the extent to which the presence of the swimbladder restricts the vertical movement of the perch, Perca fluviatilis Lin. Before passing on. to the experimental work the problem is first considered from a theoretical point of view, and it is shown how the swimbladder might restrict the vertical movements of a teleost and on what factors the extent of the restriction will depend. The relative volume of the swimbladder The average density of a fish, excluding the swimbladder, may be taken as 1-076 (Taylor, 1921). A fresh-water physoclistous fish in hydrostatic equilibrium with water of density 1 -ooo and occupying a total volume of 100 ml. can be shown to have a swimbladder of approximately 7 ml. For if x is the volume of the swimbladder then 1-076 (100 — x)=ioo, # = 7-06, say 7 ml. Similarly, if the fish were in hydrostatic equilibrium with sea water of density 1-026, it could be shown that the swimbladder would occupy a volume of about 5 ml. So when the swimbladder functions as a hydrostatic organ it should occupy about 7% of the total volume of a fresh-water teleost, but slightly less, about 5 %, of that of a marine teleost. It is interesting to note that the figures given by Plattner (1941) for the relative size of the swimbladder in various fresh-water and marine teleosts (viz. Tinea vulgaris 7-7%, Carassius auratus 8-3%, Zeus faber 5-6%, Gadus luscus 4-9%) compare well with those calculated theoretically. 554 F- R- HARDEN JONES ment of the bladder may lead to the eversion of the stomach through the moutlP (Hickling, 1927) or to a rupture of the bladder wall (Graham, 1924). Moreau believed that this danger would be less for a physostome than for a physoclist, as while the former could get rid of the excess gas rapidly by using the pneumatic duct as an escape valve, the latter could do so by the slower process of gas resorption which may take several hours. He concluded that a swimbladder, and in particular a closed one, would restrict the vertical movements of a teleost by confining it at any given moment to a definite bathymetrical range. There is very little information available from which any conclusions can be drawn about the extent to which the presence of a swimbladder restricts the vertical movements of a teleost. The problem is of interest as it is known that various teleosts undertake diurnal, seasonal or other periodic migrations involving vertical displacements (Murray & Hjort, 1912; Vedel Taning, 1918; Pearse & Achtenberg, 1918; Hickling, 1925; Johansen, 1925; Russell, 1928; Southern & Gardiner, 1932; Allen, 1935; Hasler & Bardach, 1949). Does the swimbladder restrict the magnitude or speed of these movements? Furthermore, it has been suggested that fish may be the scatterers responsible for the diurnal migration shown by the deep scattering layer (Chapman, 1947; Dietz, 1948; Hersey & Moore, 1948; Johnson, 1948; Moore, 1950; Boden, 1950). If an estimate could be made of the restriction that the swim- bladder imposes on vertical movements the information might be of interest in connexion with this and other scattering layers. In the present paper an attempt is made to determine the extent to which the presence of the swimbladder restricts the vertical movement of the perch, Perca fluviatilis Lin. Before passing on. to the experimental work the problem is first considered from a theoretical point of view, and it is shown how the swimbladder might restrict the vertical movements of a teleost and on what factors the extent of the restriction will depend. The relative volume of the swimbladder The average density of a fish, excluding the swimbladder, may be taken as 1-076 (Taylor, 1921). A fresh-water physoclistous fish in hydrostatic equilibrium with water of density 1 -ooo and occupying a total volume of 100 ml. can be shown to have a swimbladder of approximately 7 ml. For if x is the volume of the swimbladder then 1-076 (100 — x)=ioo, # = 7-06, say 7 ml. Similarly, if the fish were in hydrostatic equilibrium with sea water of density 1-026, it could be shown that the swimbladder would occupy a volume of about 5 ml. So when the swimbladder functions as a hydrostatic organ it should occupy about 7% of the total volume of a fresh-water teleost, but slightly less, about 5 %, of that of a marine teleost. It is interesting to note that the figures given by Plattner (1941) for the relative size of the swimbladder in various fresh-water and marine teleosts (viz. Tinea vulgaris 7-7%, Carassius auratus 8-3%, Zeus faber 5-6%, Gadus luscus 4-9%) compare well with those calculated theoretically. The swimb\adder and the vertical movements of teleostean fishes 555 The change in density A fresh-water fish in hydrostatic equilibrium at a depth of 20 m. is under a total pressure of 3 atm.; the 20 m. level may be called the fish's plane of equilibrium. According to Moreau, when the fish swims up, the swimbladder and thus the volume of the fish as a whole increases, with the result that its density decreases. Whether or not the gas obeys Boyle's law will depend on the passive resistance that the bladder wall and surrounding tissues offer to its expansion and on the degree to which the fish can actively control the volume of its swimbladder by muscles lying in or about the bladder wall. If resistance is offered to the expansion of the swim- bladder gas, the density change will be less than that which would occur if the gas were to expand unhindered. The restriction to rapid vertical movements Confining the argument to movements which the fish might make above its plane of equilibrium, a distinction can be made between rapid movements characteristic of normal locomotion, and slow movements which occur during the course of diurnal, seasonal or other periodic migrations.