THE DENSITIES OF CYPRINIDAE

Transcription

1 [333 ] THE DENSITIES OF CYPRINIDAE BY R. MCN. ALEXANDER Department of Zoology, University of Cambridge* {Received 22 October 1958) INTRODUCTION The densities of fish will be expressed in this paper by the sinking factor, defined by Lowndes (1942) as 1 times the ratio of the density of the fish to that of its medium. The term 'flotation pressure' will be used in this paper to denote the amount by which the pressure at which a fish has neutral buoyancy exceeds that to which it is adapted. In a previous paper (Alexander, 1959) a method was described whereby the mean percentage volume and relative sensitivity were determined for a number of species of Cypriniformes. If the mean flotation pressure were known for each species, their sinking factors could be calculated. Consider first a fish whose swimbladder volume depends only on Boyle's Law, and is not controlled by the swimbladder wall. It is adapted to an external pressure P at which its swimbladder has volume v', and it just floats at (P + AP) (i.e. its flotation pressure is AP). At this pressure its sinking factor is 1 and the volume of its swimbladder (v' + Av'). Then, by Boyle's Law, Pv' = (P+AP)(v' AP (1) Av' = - P+AP 1 In the case of a fish whose swimbladder gas is constrained by the swimbladder wall, the change of volume, Av, will be rather smaller, and we must write A AP, Ac,. For small values of AP, the ratio Aw/Aw' is the relative sensitivity. In my experiments, with the possible exception of those on the gudgeon, the mean value of AP for each species is sufficiently small for it to be possible to assume that Ac/Au' agrees with the relative sensitivity to at least one significant figure. Av is the amount by which the volume of the swimbladder, and hence that of the fish, differs from the value for neutral buoyancy. If the total volume of the fish is V * Now at the Department of Zoology, University College of North Wales, Bangor

2 334 R. McN. ALEXANDER the relative difference in the volume of the fish from the value for neutral buoyancy is then, from equation (2) (3) V P+APV Av'' The quotient v'/v is, for a fish whose specific gravity is not far from unity, approximately equal to one-hundredth of the percentage volume of the swimbladder. The relative change in sinking factor, S, is given by From (3) and (4) But AS Av.. S + AS V (4) AS _ AP _' Av S+AS ~ P+APV Av 1 ' S+AS = 1. Therefore the sinking factor of the fish, S, is given by (5) o I AP v' Av\ b = IOOO I I -=; r-= -r,. \ P+APVAv) (6) METHOD Each fish was anaesthetized and placed in urethane solution in the ' Q' flask, and its flotation pressure determined. A mean was taken for each species. This mean, together with equation (6) and the results reported in another paper (Alexander, 1959) sufficed for the calculation of the sinking factor. The results are given in Table 1. Table 1. The sinking factors of Cyprinidae Species Rudd Minnow Tench Bleak Gudgeon Flotation pressure (cm. Hg above manometer zero) -6, , -4-6, -i, -13, , , 2, I, I -9, +4, -2, -6, -3, -7, -7-12, -7, +3. -io. -7 -I, O, - I , -2, -4. -'a. +i. I, 2 -IS, -18, -io -s -6 i -4-7 i -4 Relative sensitivity -41 O o Volume -'4 o IOO8 Planner's wrongly calculated mean density is here corrected. 99 8' F. IO3 IOO3 IOOI IOO2 IOO3 IOOI IOO2 Previous values 11 (Plattner, 1941) 113 (Plattner, 1941) 17 (Popta, 191), 19* (Plattner, 1941) All these species appear normally to be a little, but only a little, denser than water. The present experiments give lower sinking factors for rudd and tench than those found by Popta (191) and Plattner (1941). Plattner determined density by measuring weight and volume directly, usually to three significant figures. In my

3 The densities of Cyprinidae 335 work, and in Popta's, precision was obtained more easily by determining density indirectly by measuring the amount by which it differed from that of water. It is perhaps significant that the gudgeon, which is the strictest bottom-liver among these species (see Jenkins, 1925; Moehres, 194) has the highest sinking factor. The difference is small, however, and I have noticed in aquaria that tench as well as gudgeon spend a great deal of time resting on the bottom. Very much higher sinking factors occur in many species from other groups (see Jones & Marshall, 1953). Table 2. The product of percentage volume and relative sensitivity / /o volume relative sensitivity Product Rudd Minnow Tench Bream Bleak Goldfish Crucian Gudgeon Amiurus S o p-so o-57 o i 3"9 5' " S DISCUSSION All of the Cyprinidae examined, except the gudgeon, have sinking factors very close to 1. In nature the level of neutral buoyancy will probably frequently lie within the vertical range inhabited by the fish. Changes of depth will have very significant effects on the buoyancy of the fish. It will be to the advantage of the fish that these effects be as small as possible. It has been shown in equation (5) that the relative change of sinking factor as a fish moves through a given small depth from the equilibrium level is approximately proportional to the product of the percentage volume and relative sensitivity of its swimbladder. Of two fish having neutral buoyancy at the same depth, that for which this product is greater will have to expend more energy to maintain its station at any other given depth. The values of this product for the species which have been studied are shown in Table 2. The product of percentage volume and relative sensitivity lies between 3-5 and 5-2, except for the bream in which it is only 1-7. This constancy is surprising. It means that there is a tendency for species which require a large swimbladder to avoid the disadvantage of an unusually high rate of change of buoyancy with depth by having a lower relative sensitivity than do species with smaller swimbladders. A statistical test shows that this tendency is real (the correlation coefficient between mean percentage volume and mean relative sensitivity is o-68, giving P < -2). The bream has a considerable advantage over the other species in constancy of density with depth. It would be interesting to know whether this is related to any peculiarity in the behaviour of the bream.

4 336 R. McN. ALEXANDER Bone and Fat The percentage volume of the roach swimbladder is greater by 4-1 than that of the carp (Alexander, 1959). If the remaining tissues of these fish were of equal specific gravity we should expect the sinking factor of the carp to be greater than that of the roach by 41. It is in fact found to be 12, while that of the roach is 13. The lack of any appreciable difference in sinking factor, in spite of the difference in swimbladder volume, must be explained in terms of differences in the constitution of the tissues of the two species. It was thought likely that fat and bone would be important, as a large proportion of the former would tend to give a low sinking factor, of the latter a high one. It therefore seemed appropriate to determine the proportions of these components in a number of species. The fish was killed, roughly dried, and weighed. It was then placed in a beaker of water and boiled lightly. It was skinned, and the skin, with any loose scales, put in one dish. The bones were removed and put in another; great care was taken to miss none of them, and they were stripped of flesh as far as could conveniently be done with forceps. The fish had now been separated into three components: the skin and scales, the skeleton, and the remainder, which we may call flesh. The scales and skeleton were cleaned by leaving them for (usually) two days in hydrogen peroxide solution (about 2 vol.). They were then washed, dried and weighed. The flesh was dried at no C. and ground finely. Both it and the water in which the fish had been boiled were extracted with ether. The ether was evaporated from the resulting solution and the fatty residue weighed. This would include ether-soluble materials other than fats, but not the fats of the skin.* The values for fat content must therefore be regarded as approximate. The results are given in Table 3. The proportions of the components are given as percentages of body weight. of widely different sizes were used to determine whether there is any marked variation of composition with size. It was concluded that there is not. Jacquot & Creac'h (195) give values between -4 and 4-% for the fat content of the edible parts of various Cyprinidae, with one exceptional value of 9-% for a carp in breeding condition. As these values refer to edible parts only they are not strictly comparable to mine. None of the fish I examined contained more than 2% fat. We must consider how the differences in composition which have been found will affect the buoyancy of the fish. For this discussion the specific gravities of the components must be known. Spector (1956) gives the specific gravities of four teleost oils as -925, -9, -93 and -95. The mean of these is about -91. It seems unlikely that much fat was lost with the skin. An entire carp which was dried and extracted gave fat content (1-5 %) within the range of variation found in the skinned and boned specimens.

5 The densities of Cyprinidae 337 Table 3. Composition of Cyprinidae Species fish (g.) skeleton (%) 4" i ' ' scales (%) " O a O 2-2 skeleton + scales (%) i ' fat (%) i-8 i-8 o-8 o-8-7 i-7 i-o The specific gravities of roach, dace and carp skeletons and scales were determined, using a weighing bottle as a density bottle. The weight of the weighing bottle on successive fillings with water was found to be constant within a few milligrams (standard deviation for 1 trials = 7mg.); this constancy was adequate for the determinations, in each of which about 2 g. of material was used. A vacuum pump was useful in removing bubbles of air from the material. The results are given in Table 4. Table 4. The specific gravity of bone s.g. of skeleton i-57, 1-67 s.g. of scales 1-91 i Although there are clear interspecific differences in the specific gravity of bone, there is no appreciable difference between the bone of the skeleton and that of the scales within the species. In the following discussion the specific gravity of roach bone will be taken as 1-9, of dace bone as 2- and of carp bone as i-6. The specific gravity of fish tissues other than bone, scales, fat and swimbladder gas will be taken to be 1-6, a mean value which anticipates the results of some calculations to be presented in Table 5. This value is used for the following calculation only. The volume of 1 g. of fat of specific gravity -91 is 1/-91 ml. The volume of 1 g of tissue of specific gravity 1-6 is 1/1-6 ml. The replacement of 1 g. of such tissue by 1 g. of fat in a fish will increase the volume of the fish by (1/-91) (1/1-6) = o-i6ml. This is equivalent to increasing the volume of the swimbladder by

6 338 R. McN. ALEXANDER -16 ml. The acquisition by a fish of fat constituting 1% of its body weight will reduce the percentage volume of swimbladder required for neutral buoyancy by only -16. The variations in fat content found in Cyprinidae can thus have little influence on the volume of the swimbladder. The volume of 1 g. of bone of specific gravity i-8 (the mean of the specific gravities found) is I/I-8 ml. The acquisition by a fish of bone constituting 1 % of its body weight will increase the percentage volume of swimbladder required for neutral buoyancy by (1/1-6) (I/I-8) = -39. The variations of total bone content found in Cyprinidae must be important in determining the volume of the swimbladder. Table 5. Summary of data on the sinking factors of roach, dace and carp Specie* (a) Tissue (6) Bone Swimbladder Rest Total Bone Swimbladder Rest Total Bone Swimbladder Rest Total Weight (% body wt.) (< IOO S IOO IOO Volume (ml./ioo g. body wt.) S o-o Specific gravi O 1-6 I-OOI i-ooa Weight in water (c d) (% body wt.) s-s + o-i O-2 The above discussion has shown that, in discussing the sinking factors of Cyprinidae, bone must be considered but fat is much less important. In this discussion a value of 1-6 was assumed for the specific gravity of tissues other than swimbladder, bone and fat. In Table 5 the actual values of this specific gravity of ' the rest' in roach, dace and carp are calculated from the data obtained on bone, swimbladders and sinking factors. The figures in bold type are data from this and a previous paper (Alexander, 1959); the remainder is calculated from them or is self-evident. From column (/) of the table we find that the difference in bone content accounts for ( ) = i-1 of the difference in percentage volume of the swimbladder (i-8) between roach and dace. The remainder of the percentage volume difference is due to a small difference in the specific gravity of 'the rest'. The difference in percentage volume between dace and carp is entirely due to a difference in the specific gravity of 'the rest'. The specific gravity of 'the rest' is calculated to be 1-7 in the roach, 1-6 in the dace and 1-4 in the carp. It was thought possible that differences in the specific gravity of muscle might contribute to these differences.

7 The densities of Cyprinidae 339 Muscle The specific gravity of the muscle of roach and carp was determined by means of Hammerschlag's (189) method for the specific gravity of blood, as used by Chen (1931) for the various tissues of the goldfish. A small piece of muscle dissected from the tail of the fish was placed in a mixture of chloroform and benzene. By adding one or other of these substances the specific gravity of the mixture was adjusted until, as far as possible, the muscle neither floated nor sank. It was then determined by means of a hydrometer. The mixture was thoroughly stirred after each addition of fluid by means of a perforated plate stirrer, and care was taken to see that no bubbles of air clung to the muscle. The results are given in Table 6. The mean specific gravity of roach muscle is found to be 1-63 and of carp muscle It is hard to understand how this comes to be, as the fat contents of both are low, and they contain similar proportions of protein (Jacquot & Creac'h, 195; the edible parts of roach contain 19-5% protein, of carp %). This helps, however, to explain the difference found in the specific gravity of the parts other than bone and swimbladder of the two species. Table 6. The specific gravity of muscle Species Weight (g.) s.o. of tail muscle , , , 1*41, SUMMARY 1. The mean sinking factor of each of eight species of Cypriniformes has been determined by a method involving measurement of the pressure at which anaesthetized specimens float. It was found to lie between 11 and 14 for all species except the gudgeon, for which it is , dace and carp have swimbladdere of very different sizes. It was shown that differences in the weight and density of the skeleton and scales, and in muscle density, are important factors in determining the volume of swimbladder gas necessary for neutral buoyancy. Differences in fat content are unimportant in these fish. I wish to thank Dr K. E. Machin and Dr G. M. Hughes for much advice, and the Development Commission for financial support. Exp Blol 36, 2

8 34-Q R. McN. ALEXANDER REFERENCES ALEXANDER, R. MCN. (1959). The phygical properties of the swimbladder in intact Cyprinifonnes. J. Exp. Biol. 36, CHBN, S. C. (1931). The specific gravity of various body-parts, organs and tissues of wild and domesticated goldfish, Carassms auratus. Biol. gen. 7, HAMMERSCHLAO, A. (189). Uber eine neue Methode zur Bestimmung des spezifischen Gewichtes des Blutes. Wien. klin. Wtchr. 3, 118. Referred to by Chen (1931). JACQUOT, R. & CRBAC'H, P. V. (195). Les protides du poisson et lew valeur alimentaire. Notes et rapports de ["Office scientifique et technique des Ptches maritime*. (N.S.), 6, pp. 48. JENKINS, J. T. (1935). The Fishes of the British Isles. London. JONES, F. R. H. & MARSHALL, N. B. (1953). The structure and functions of the teleost swimbladder. Biol. Rev. 3», LOWNDES, A. G. (1943). The displacement method of weighing living aquatic organisms, jf. Mar. Biol. Ass. U.K. 35, MOEHRES, F. P. (194). Untersuchungen tlber die Frage der Wahrnehmung von Druckunterschieden des Mediums (Versuche an Bodenfischen). Z. vergl. Physiol. 38, PLATTNER, W.-A. (1941). fitudes sur la fonction hydrostatique de la vessie natatoire des poissons. Rev. suisse ZL 48, POPTA, C. M. L. (191). fitude sur la vessie aenenne des poissons. Arm. Sci. not. Zool. (ser. 9), ia, SPECTOR, W. S. (ed.) (1956). Handbook of Biological Data. Philadelphia, London.