OXYGEN CONSUMPTION AND GILL WATER FLOW IN THE DOGFISH SCYLIORHINUS CANICULA L.

Transcription

1 jf. Exp. Biol. (1968), 49, With 6 text-figures Printed in Great Britain OXYGEN CONSUMPTION AND GILL WATER FLOW IN THE DOGFISH SCYLIORHINUS CANICULA L. BY G. M. HUGHES* AND SHUN-ICHI UMEZAWAf The Plymouth Laboratory (Received 17 April 1968) INTRODUCTION When sharks and dogfish are at rest on the sea bottom, gill ventilation is produced by the action of the branchial musculature and elastic recoil of the visceral skeleton (Hughes & Ballintijn, 1965). Water flows across the gills as a result of an increase in pressure in front of the gills and of suction behind them. During the swimming of some related forms such as the leopard shark, Triakis semifasciata (Hughes, i960) and the sand shark, Carcharias taurus (von Wahlert, 1964), it has been observed that the normal respiratory movements cease when the fish swims above a given speed. Under these conditions ventilation of the gills is achieved by the activity of the red muscles which are known to be mainly concerned during the slow swimming movements of these fish (Bone, 1966; Roberts, 1968). However, ventilation solely by this 'ramjet' method has not been observed in the common British dogfish, Scyliorhinus canicula, so far as we have been able to discover. The present experiments attempted to measure the standard oxygen consumption of this species and to determine the effect of increasing the rate of waterflowpast the fish. The volume of water pumped through the gills and the properties of the gill resistance were studied by measuring changes in flow with alterations in hydrostatic pressure across the gills. Both of these features are involved as the fish changes from the situation when it is pumping at rest on the sea bottom and begins to swim. MATERIALS AND METHODS The dogfish used were specimens of the rough hound Scyliorhinus canicula, L. which had been recently collected and kept in the sea-water circulation of the Plymouth Laboratory during March For most experiments fish in the size range g. were used, but larger specimens of about 600 g. were more suitable for the later experiments. Three methods were used to determine the oxygen consumption of the fish and other aspects of its respiratory mechanism. In the first a small dogfish was kept in a cylindrical tube of about 2 in. diameter and water was passed continuously over thefish. It was found that small dogfish remained very quiet and breathed regularly for prolonged periods when kept in such a situation. The P Oa of the water before and after the experimental chamber was measured with an oxygen electrode (Silver, 1963); Present address: Department of Zoology, Bristol University, Bristol BS 81UG. t Present address: Department of Biology, Kochi University, Japan. 36 Exp. Biol. 49, 3

2 558 G. M. HUGHES AND SHUN-ICHI UMEZAWA theflow rate of the water through the respirometer was also determined. The tube containing the dogfish and the oxygen electrode were kept immersed in a larger tank, and remained at a constant temperature in the range C throughout the experiment. The second two methods were designed to make a direct measure of the volume pumped over the gills by the fish. To do this it was necessary to separate the mouth opening and the exit from the gill slits by means of rubber membranes. In the initial experiments a thin rubber membrane was fixed across the opening of a cylindrical tube and the head was pushed through a hole in the membrane so that the spiracle and mouth were outside the tube, but the external gill slits were inside. In some instances this set-up was quite satisfactory, but usually the fish tended to move out of the apparatus during the course of prolonged observations. A decrease in frequency of respiratory movements was also noted. Accordingly, attempts were made to fix a rubber membrane to the outside of the fish so as to collect all the water leaving the gill slits. Initially the experiments were made using cut portions of rubber balloons and later artificial chambers made from Copydex proved satisfactory. The fixing of these rubber membranes to the outside of the fish was achieved by means of Eastman 910 cement. It was found, however, that these chambers interfere (Hughes, 1966) to some extent with normal ventilation movements of the dogfish, in which elasticity of the branchial region plays an important part (Hughes & Ballintijn, 1965). Experiments using a similar technique carried out by Piiper & Schumann (1967), have been made with larger specimens of Scyliorhinus stellaris. It appears that the skin of this species is more suitable for the attachment of the rubber chambers. Results obtained in the present work were of interest particularly in relation to the resistance to flow of water across the gills with different pressure heads in which the experimental set-up (Fig. 1) was that used by Hughes & Umezawa (1968) for the dragonet, Callionymus lyra. RESULTS (1) The effect of changing the flow rate of water over the fish At low flow rates the oxygen tension of the water breathed by the fish in the respirometer tube tends to fall below saturation, and it is for this reason that the oxygen consumption is much lower (Fig. 2). At flow rates between 50 and 150 c.c./min. a resting level of metabolism was obtained of about 55 c.c./kg./hr. At higher rates of flow this increased further. However, in making determinations of oxygen consumption based upon differences in P Oa and a flow rate, it is clear that with increasing flow rate, and consequently smaller P Oa differences, it is necessary for the accuracy of the P Oa measurements to be correspondingly better. In the present experiment the increase in oxygen consumption measured does seem to be significant, but whether in fact it is as high as the calculations show, is less certain. With increasing flow rates the proportion of the oxygen removed from the water during its passage over the fish correspondingly falls (Fig. 3). At low flow rates it is high, but with averageflowrates ( c.c./min.) it is about 50%, falling to values of 20 % withflowsof c.c./min. During these and other experiments, the regularity of the frequency of respiratory movements was particularly noticeable, often remaining constant at about 50 per min. As measurements of the utilization of the oxygen, i.e. the percentage removed during its passage across the gills, are normally about 40-50%,

3 Oxygen consumption and gill water flow in the dogfish 559 Filter Ventilation volume Fig. i. Diagram of the experimental arrangement used to measure the ventilation volume, utilization and O 0 consumption under varying conditions of inspired P Oa and hydrostatic pressure gradient (A/)) across the gills. The latter was registered on the two water manometers and could be varied by changing the height of the inclined beaker. The closed circulation was filtered in glass wool and maintained by a pump. 80 D175 D225 A D C Dogfish A 40 A 20 - ' i i i Water flowing over fish (c.c./min.) Fig. 2. Scyliorhinus canicula. Relationship between oxygen consumption and the rate at which water flowed past the fish in a respirometer tube. The two lines show the general relationship for two of the specimens used. Each point represents the mean of three or four determinations. The Po a of the water entering the respirometer and its temperature are given for each experiment. 36-2

4 560 G. M. HUGHES AND SHUN-ICHI UMEZAWA which is about the percentage removal at flows between 50 and 100 c.c./min., it seems reasonable to take the oxygen consumption in this range as a normal condition for a fish resting on the sea bottom ' A Dogfish, 21. iii. 65 (175 g C} O 2 consumption (c.c/kg./hr.^ SO Respiratory frequency/min. as x* % oxygen removed by fish A > Flow rate over fish (c.c./min.) Fig. 3. Scyliorhinus canicula. Relationship between the rate of flow of water over the fish in the respirometer tube and its oxygen consumption, frequency of respiratory movements, and the percentage of oxygen removed by the fish Dogfish, 21. iii. 65 (160 g C.) "~~._ Respiratory frequency/min oxygen removed by fish " x - - ~ " Po 2 in water over fish at constant rate (mm. Hg.) Fig. 4. Scyliorhinus canicula. Relationship between changes in the P Oi of the water passed over the fish at a constant rate (90 c.c./min.) and the mean ventilation frequency, percentage oxygen removed by the fish, and its oxygen consumption. Each point is the mean of three or four determinations.

5 Oxygen consumption and gill water flow in the dogfish 561 (2) The effect of lowering the P Qi Using a constant rate of flow of water over the fish, in the range of c.c./min., changes were made in the oxygen content of the water passed over the fish by bubbling nitrogen through it in an exchange column. The results (Fig. 4) showed that with decreasing oxygen there was a rise in the ventilation frequency together with a slight increase in oxygen consumption, but this fell once the P Oa of water was below about 80 mm. Hg. As the oxygen tension was lowered, there was an increase in the percentage of oxygen removed from the water as it passed over the fish. In some experiments Dogfish 26. iii Ap (cm. water) -10 Fig. 5. Scyliorhinus canicula. Effect of changing the hydrostatic pressure across the respiratory system on the rate of water flow through the gills ( x ), and on the oxygen consumption ( ). Positive values for the hydrostatic pressure gradient (Ap) indicate a higher pressure than normal on the mouth side. Ap = o is the normal resting condition. oxygen was bubbled through the exchange column and gave extreme values such as those shown in Fig. 4. It appears that the general trend continues for the ventilation frequency and percentage oxygen removal, but the oxygen consumption of the fish seems to increase markedly. Once again, one has to be careful in such observations because of the relatively small difference in P Oa of the water before and after its passage over the fish. However, the increase in oxygen consumption at higher P Oa levels does seem to be significant. (3) Experiments in which the ventilation volume was measured directly These experiments were carried out on the larger fish and the oxygen consumption was about the same level (22 c.c./kg./hr.) as found in some of the experiments described above when the flow rate was low. The flow over the gills in the resting condition was low here (70 c.c./min.) also. As the hydrostatic pressure across the gills was varied, it was found (Fig. 5) that a linear relationship (65 c.c./cm. H 2 O) existed between the volume pumped by the fish and the change in pressure (A/>). The fish could not continue pumping when the adverse hydrostatic pressure was more than 0-7 cm. of water. As the Ap was changed and there was an increased ventilation volume, so there

6 562 G. M. HUGHES AND SHUN-ICHI UMEZAWA was an increase in the oxygen consumption of the fish notwithstanding a fall in the percentage utilization (Fig. 6). The rate of increase in oxygen consumption with rising ventilation volume falls off gradually. The respiratory frequency remained fairly constant during these changes in A/>, once more at about 50 per min. This is a similar result to that which was obtained using the experimental arrangement in which flow over the fish was increased, although it was not possible to measure the true percentage utilization in that case Dogfish, 26. iii utilization Ox/gen consumption (cc/kg./hr.) Ventilation volume (c.c./min.) Fig. 6. Scyliorhinus canicula. Same experiment as Fig. 5. showing the effect of changing A/) on the relationship between the ventilation volume produced by varying Ap, on the oxygen consumption and the percentage utilization by the gills. When Ap = o, the ventilation volume was approximately 70 c.c./min. A few experiments were carried out in which the P Oa of the ambient water was lowered with dogfish having the chambers attached. The percentage utilization in these cases remained about 67%, although the ventilation volume increased by over 15%. DISCUSSION The results described in the present paper give evidence that the normal resting oxygen consumption of Scyliorhinus canicula is in the range c.c./kg./hr. This agrees fairly well with the mean value obtained by Piiper & Schumann (1967) for Scyliorhinus stellaris which was 40*2 c.c./kg./hr. (range 25 to 55 c.c./kg./hr.). The values obtained for the dogfish in the tube are considered the best because these fish were unanaesthetized and relatively unrestrained and remained at rest in this situation for many hours. It was found necessary to carry out these experiments with the fish facing the direction of the water flow, and therefore experiments in which oxygen consumption was measured with different rates offloware of interest in relation to the changes which occur when the fish starts to swim. At the rates offlowstudied here there was scarcely any change of respiratory frequency, but possibly at higher flow rates some fall in frequency

7 Oxygen consumption and gill water flow in the dogfish 563 might be observed. A rough estimate of the equivalent speed of the fish relative to the flow of water through the respiratory chamber is between 10 and 20 cm./min. for a fish of 40 cm. length. At these low velocities little effect has been noticed on the respiratory frequency in a free-swimming fish, even in those species where the ventilation movements cease at higher speeds. This observation was supported by experiments in which ventilation volume was measured directly and its variation with imposed hydrostatic pressure gradients across the gills. In these cases doubling the ventilation volume had no effect on the respiratory frequency. Changes in frequency were observed, however, when the fish was subjected to a lowering of the oxygen tension. This contrasts with the observations of Murdaugh, Robin & Millen (1966) on Squalus acanthias when flow of water across the gills was increased by increasing its flow through a cannula in the spiracle. A change in respiratory frequency was recorded which varied more or less directly with the rate of water flow through the gills. They observed a threefold increase of frequency as the rate of flow of water was raised from 0-09 to 2 l./min., but the size of the fish used was not stated. Experiments in which the rate of flow of water across the gills was increased in Callionymus and the ray did not show a consistent change in frequency with increase in flow produced by changes in Ap. In Callionymus also there was a marked change in frequency at low P Oa as recorded here but in the opposite direction. The linear relationship between hydrostatic pressure across the gills of Scyliorhinus and ventilation volume is similar to that which it was later found had been observed by Ogden (1945) on Mustelus, and is presumably related to the nature of the gill resistance. The observation that the dogfish is unable to pump against adverse pressure gradients exceeding 0-7 cm. of water contrasts with the ability of Callionymus to pump against 2-0 cm. H 2 O (Hughes & Umezawa, 1968). This difference is possibly due to the absence of an operculum as the body surface outside the parabranchial cavities will be directly subjected to this adverse pressure gradient and will have a greater effect on the flow of water across the gills, especially as expansion of these cavities is normally achieved passively. The values of the ventilation volume measured in the fish was 120 c.c./min./kg. and are lower than the average figures given by Piiper & Schumann (248 c.c./min./kg.) and very much lower than those of Millen et al. (1966) who measured 443 c.c./min./kg. However, apart from the difference in species, the determination here was made at 12 0 C, whereas Piiper & Schumann worked at 15 0 C. when not only would there be less oxygen in the water but also the fishes' metabolic level would be higher. The percentage utilization measured here was correspondingly higher (70 %). The inverse relationship between percentage utilization and the rate of the water flow across the gills seems to be general in all fish and agrees with the analysis presented by Hughes & Shelton (1962) on the importance of considering the O 2 capacities and flow rates of the water and the blood at the respiratory exchanger. Unfortunately no information was obtained in the present work on the rate of flow and oxygen content of the blood. The rise in oxygen consumption as the water flow across the fish is increased might be due to a general increase in blood oxygen levels, both afferent and efferent to the gills. This is supported by measurements obtained in more recent work, e.g. Piiper & Schumann (1967) found that the P Oa in the afferent blood was as low as 11 mm. Hg and increased to 81 mm. Hg during its passage through the gills. Clearly with increasing water flow there is the possibility of a marked increase in both these levels as the P Oe of

8 564 G. M. HUGHES AND SHUN-ICHI UMEZAWA the inspired water was 155 mm. Hg. Such a marked capacity for increasing the oxygen removed from the water was also indicated by the experiments in which the inspired water was highly saturated with oxygen, but further work needs to be done before any detailed conclusions can be drawn from these results. SUMMARY 1. Determinations of the standard oxygen consumption of the dogfish Scyliorhinus canicula, by several different methods, gave values in the range of c.c./kg./hr. at 12 0 C. 2. When the rate of water flow over the resting fish is increased, there is an increase in oxygen consumption, but no marked change in respiratory frequency at the flow rates studied. An increase of respiratory frequency took place when the P Oa of the inspired water was reduced. 3. The ventilation volume of the dogfish was measured by collecting the water in chambers after it had passed over the gills. The resting ventilation volume was about 120 c.c./min./kg. at 12 C. 4. The relationship between the flow across the gills and the imposed hydrostatic pressure gradient (Ap) showed a linear relationship. The fish was unable to continue pumping water across the gills against adverse gradients in excess of 07 cm. H 2 O. 5. When the flow across the gills was relatively low, utilization of oxygen of at least 70% was observed, but with increasing flow rates this fell to between 40 and 50%. We wish to thank Sir Frederick Russell, F.R.S., formerly Director, and the staff of the Plymouth Laboratory for their friendly help during the course of this work. We are also grateful to Dr I. A. Silver for his help in making the oxygen electrodes. G.M.H. was Ray Lankester Investigator when this work was carried out. S.U. was investigator sent abroad by the Ministry of Education, Japan. REFERENCES BONE, Q. (1966). On the function of the two types of myotomal muscle fibre in elasmobranch fish. y. mar. Biol. Ass. U.K. 46, HUGHES, G. M. (i960). The mechanism of gill ventilation in the dogfish and skate. J. exp. Biol. 37, HUGHES, G. M. (1966). Species variation in gas exchange. Proc. roy. soc. Med. 59, HUGHES, G. M. & BALLINTIJN, C. M. (1965). The muscular basis of the respiratory pumps in the dogfish {Scyliorhinus canicula). J. exp. Biol. 43, HUGHES, G. M. & SHELTON, G. (1962). Respiratory mechanisms and their nervous control in fish. Advances in Comparative Physiology and Biochemistry (ed. O. Lowenstein), vol. 1, Academic Press Inc. HUGHES, G. M. & UMEZAWA, S. (1968). On respiration in the dragonet Callionymus lyra. L.J. exp. Biol. 49. S MILLEN, J. E., MURDAUGH, H. V., HEARN, D. C. & ROBIN, E. D. (1966). Measurement of gill water flow in Squalus acanthias using the dye-dilution technique. Am. J. Physiol. 211, MURDAUGH, H. V., ROBIN, E. D. & MILLEN, J. E. (1966). Reflex regulation of gill water flow (ventilation) in Squalus acanthias. Nature, Lond. 211, OGDEN, E. (1945). Respiratory flow in Mustelus. Am. jf. Physiol. 146, PIIPER, J. & SCHUMANN, D. (1967). Efficiency of O 2 exchange in the gills of the dogfish Scyliorhinus stellaris. ROBERTS, B. L. (1968). The coordination of the locomotory movements of_ dogfish. Ph.D. thesis. Cambridge University. SILVER, I. A. (1963). Some observations on the cerebral cortex with an ultramicro, membrane-covered, oxygen electrode. Med. Electron. Biol. Engng 3, VON WAHLERT, G. (1964). Passive respiration in sharks. Naturwissenschaften 51, 297.