190

No. 31. Metabolism and Growth in hyperboreus in Relation to its Life Cycle1) By

R o b e r t J. C o n o v e r Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

Experimental and field data attempting to evaluate in the dark at 3° to 7° C. About 150-200 ml of filtered the role of zooplankton in aquatic food chains have sea water were allowed each individual in populations not always been in agreement (Cushing, 1955, 1959), varying from 1 to 30 . They were fed but where both sources of information have been ap­ (usually Thalassiosira fluviatilis) approximately 1200- plied to a single locality over a period of months a 3000 x 106 cells/, once a week when the culture consistent picture of zooplankton- inter­ water was changed. Streptomycin and penicillin were dependence usually results. Thus, Conover (1956) used interchangeably to control bacterial growth. showed that experimentally determined food require­ With this procedure individuals have been main­ ments and grazing rates for Acartia sp. agreed well with tained for over two years and eggs from fertilized the actual phytoplankton organic matter available to females have been raised to adulthood. them in Long Island Sound for over a year. More Respiration was measured using the Winkler water recently Corner (1961) found experimentally that bottle technique described by Conover (1960). This particulate organic matter in the water off Plymouth, method does not agitate the animals and allows for England, is sufficient to promote growth in Calanus replicate measurements ' and statistical analysis of helgolandicus and Menzel and Ryther (1961) observed results. that primary production exceeded the respiratory In feeding experiments, 12 to 25 animals were kept requirements of the zooplankton off Bermuda. Yet in two-litre containers under the conditions described little is known about the actual food intake and effi­ above. cells in experimental and control con­ ciency of energy transformation in marine zooplankton. tainers were counted with a Sedgwick-Rafter chamber In the present investigation, study is made of the at the beginning and end of an experiment. The dry metabolism, food consumption, assimilation, and weight of a cell was determined (1) by weighing a growth of laboratory populations of the large, cold- known number of cells on a tarred membrane filter or water Calanus hyperboreus under controlled (2) by comparing the volume of individual cells laboratory conditions to obtain better insight into (specific gravity = 1 ) measured with an eyepiece aspects of the metabolic and nutritional physiology of micrometer, with the ratio of wet to dry weight for a a zooplankton which are important to large number of cells. Assimilation is estimated as the success in the natural environment. difference in organic matter, determined as percen­ tage loss on ignition at 450°C, between food and faecal Methods material. V % The experimental animals were caught in net hauls For growth studies animals were sorted into repli­ at depths greater than 200 m in the Gulf of Maine cate groups of 12 to 25 animals. The individuals in one group were killed immediately, measured and or in slope waters south-east of Cape Cod. They were maintained both on shipboard and in the laboratory weighed with a microbalance having a sensitivity of ± 10 jug. Replicate groups were then maintained for 3 to 6 weeks during which time food consumption 1) Contribution No. 1260 from the Woods Hole Oceanographic Institution. This research was supported by National Science and respiratory rates were determined. In some ex­ Foundation grants 8913 and 8339. periments replicate groups were starved. At the end 191

28 — ^

26 MAXIMUM OBSERVED INCREASE IN RESPIRATORY RATE W li- FEMALE IN LABORATORY CULTURE 24 INITIAL RESPIRATORY RATE

Q 22 MAXIMUM OBSERVED INCREASE IN RESPIRATORY RATE IN LABORATORY CULTURE ^ 20 INITIAL RESPIRATORY RATE

§ 18 \ ^ 16

1959 1960 1961 MONTH CAPTURED Figure 1. Seasonal changes in the respiratory rates of stages V and female Calanus hyperboreus taken either from the Gulf of Maine or from the slope water. Cross-hatched (lower) portions of the histograms indicate the respiration rate measured immediately or within one week of capture. The solid colour (upper portion) of the histograms represent the maximum observed increment of respiratory increase usually attained after 2 to 4 weeks under laboratory culture. Only initial respiratory rates were available for August 1959 (Gulf of Maine animals), but as they continue the down­ ward trend from high springtime levels shown for the initial values, they were included in the figure. of an experiment, the animals were compared stati­ in April 1959, there was considerable variation in stically with the initial group. measurement from animal to animal, but the mean Fat content was determined by extraction with a respiration for each group, initially 27-32 fi\ O a/cope- micro-Soxhlet apparatus for 24 hours with acetone as pod day for females and 18-24 for stage V, showed no a solvent. Caloric content of a few animals was deter­ pronounced change with time. On the other hand, mined by Dr. Lawrence Slobodkin with a microbomb with animals captured in November 1959, the re­ calorimeter. spiration rate was initially lower than the spring values but increased after two weeks in the laboratory to virtually the same levels as those observed in April Respiration in Calanus hyperboreus (Figure 1). Seasonal variation. Respiration was measured within The seasonal difference in metabolism could not be an hour or two of capture, and subsequent measure­ attributed to differences in size, for the animals cap­ ments were made with the same animals at different tured in November were actually heavier than those times over a period of several months of laboratory captured in April. In these and other experiments, culture under constant temperature and food con­ the correlation between respiration and weight was ditions. In the first experiment with small groups of poor in part owing to the variable amount of stored female and stage V captured in slope water fat, which ranged from 15 to over 70 °/0 of dry weight. 192

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RESPIRATION STAGE Y RESPIRATION FEMALES 26

24 24

20

1960 1960 EXPERIMENT DATE Figure 2. Respiration of fed and starved Calanus hyperboreus captured in the Gulf of Maine (42°50'N, 69°50'W) April 15, 1960. Histograms on the left show the respiration of females and those on the right stage V.

If respiration is compared with fat-free dry weight 1960 near the probable peak of the spring diatom the correlation between respiration and weight some­ bloom (Bigelow, Lillick and Sears, 1940), contained times is improved but there is often a large amount of variation not related to size. 20 Further experiments with C. hyperboreus, both from UNCONDITIONED STAGES slope water and from the Gulf of Maine, indicated a striking seasonal variation in metabolism (Figure 1). O - - CONDITIONED STAGE "2 The respiratory rate at time of capture was high in ■Y»0.8I3X +10.59 the spring and gradually declined until winter. Except in spring, metabolism increased significantly after 2 to 4 weeks of laboratory culture. There also appears to be a downward trend in the maximum observed respiration, but it is not certain that the experiments ■Y = 0.184X +13.08 were always carried out long enough to attain the maximum rate.

Effect o f food on metabolism. The relationship between feeding and metabolism was examined after making two sets of observations. First, freshly caught Calanus hyperboreus are buoyant and remain suspended in the culture water almost motionless. After several weeks they swim about more actively and become denser so that they quickly sink when they stop swimming. As they become more active, they feed more actively and TEMPERATURE °C produce more faecal pellets. These changes in be­ Figure 3. The respiration of stage V Calanus hyperboreus from the Gulf of Maine measured at 2, 5 and 8° C. “Unconditioned” haviour seem to be closely correlated with the mea­ animals had been kept at 3-4° C for three weeks before measure­ sured increase in respiration. ment of their respiration. “Conditioned” animals were kept two Second, animals captured in April of both 1959 and weeks at either 2, 5, or 8° before they were studied. 193 food, but at other seasons the gut seemed to be empty; males had a respiratory rate nearly twice that of the respiratory rates also were higher in April than im m ature females (11-51 and 6-44/d 0 2/mg day, at other seasons (Figure 1). respectively). The respiration of recently layed eggs, Figure 2 shows the respiratory behaviour of fed and 6-78 [A 0 2/mg day, was nearly the same as that of starved female and stage V C. hyperboreus taken in immature females. The high respiration characteristic April 1960. The respiratory rate for both stages kept of reproducing females is undoubtedly related to the without food declined sharply. After 22. June food physiological changes accompanying gonad matura­ was given to both fed and starved animals. The re­ tion. spiratory rate of the previously starved females rose to the same level as that for the fed females. However, Food consumption, assimilation and growth the rate for both fed and starved stage V animals The relationship between respiration, food con­ continued to decline, although fed stage V copepods sumption, assimilation, and temperature for animals captured at other times of the year increased their captured in March 1961 is shown in Table 1. For metabolism after several weeks in laboratory culture. stage V animals respiration and feeding rates were about the same at three temperatures (see also Fi­ Effect of temperature on metabolism. The temperature gure 3). Stage IV animals appeared not to regulate of the deep water where C. hyperboreus is usually found as readily, but this comparison is deceptive because never varies more than 3 or 4° C and can have little the IV’s moulted at the higher temperatures. By effect on seasonal differences in respiration. Since a 30. March almost all IV’s at 5° and 8° had moulted sudden change in temperature would not normally and few were left to compare with the IV’s at 2°C. be a limiting factor, temperature experiments were Measurements of assilimilation, though preliminary, devised to study the effect of a narrow range of tem­ are also interesting. The per cent assimilation was lower peratures on respiration over a short period. than estimates available in the literature for copepods, The effect of 10 days’ acclimatization to temperature but agrees with Richman’s (1958) estimates for is shown in Figure 3. Stage V animals kept with food Daphnia pulex. For C. hyperboreus the m axim um assi­ at 3-4° G for three weeks were divided into replicate milation was only 49-7 °/0 and was usually much lower. groups and respiration measured at 2, 5 and 8°C. The efficiency of assimilation apparently increased The ‘unconditioned’ curve had a Q,10 of 1-76. The with temperature though the total food consumption same groups were then conditioned to the experimen­ remained relatively uniform. Both efficiency of assi­ tal temperature and their respiration measured again. milation and rate of feeding increased with time in the As this second curve (Q_10 = 1-14) had no significant laboratory in a manner similar to respiration rate. slope, C. hyperboreus appears to regulate well over its The culture that became densely contaminated with normal temperature range. bacteria (8°, 6. April) had an unexpectedly low value for assimilation. If the bacteria had attacked the faecal Effect of the breeding cycle on metabolism. During an material, they should have caused an apparent assi­ investigation to test the effect of food on the metabol­ milation rate greater than normal. Possibly the faecal ism of female C. hyperboreus captured in early winter, pellets became a substrate for bacteria which added the expected difference in metabolic rate for fed and their own organic matter to that of the undigested starved groups did not develop. Some individuals in food. Assimilation values for 17. April, determined both groups began to show signs of gonad maturation ; after a single night’s feeding are higher than the others, these had higher rates than the others. The mean for but it is uncertain whether this is coincidental or a unripe females was 15-39 fj\ 0 2/copepod day and for valid argument for bacterial contamination of faecal ripe, 19-83, significantly different a t the 1 °/0 level material in the longer experiments. (VVilcoxen ranked sum). This difference remained for The amount of growth at 2° and 5° C by the several weeks until laying females had become par­ March 1961 stage IV and V animals in relation to tially spent, after which their respiration declined. oxygen consumption and food intake is summarized Apparently gonad development, unlike respiration, in Table 2. Also shown is the growth of stage IV is not directly dependent on food supply, although C. hyperboreus at 5° fed another diatom species, Tha- the number of eggs produced by a female and the lassiosira nordenskioldii. Growth was generally greater length of time she remains ripe are probably related and stage IV animals nearly all moulted at 5°C ; to available food. their food consumption was also higher at 5°C On two occasions respiration of newly captured (Table 1). gravid females was significantly higher than that of Since both food intake and respiration were known, unripe females from the same tow. When respiration measured growth could be compared with expected levels were corrected for weight differences, ripe fe­ growth based on different assumptions regarding the

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T able 1. Oxygen consumption, food consumption, and assimilation for Calanus hyperboreus stages IV and V fed Thalassiosira fluviatilis at different temperatures

Temperature: 2°C ‘5°C 8°C

/«I O j fl\ 0 2 /< 1 0 2 Week c f con- 10s cells/ °/0 assi- con- 103 cells/ °/0 assi- con- 103 cells/ °/0 assi- Average °/0 ending sumed/ animal milated sumed/ animal milated sumed/ animal milated assimilated animal animal animal

1961 Mar. 15 IV 23-80 233 ± 93 - 33-81 246 ± 78 - 49-35 187 ± 112 -- V 54-04 272 ± 92 - 62-23 554 ± 67 - 68-39 409 ± 103 -- Mar. 22 IV 23-80 137 ± 30 33-81 300 ± 59 49-35 296 ± 35 - - 1 16-41 V 54-04 441 ± 20 _ 62-23 565 ± 49 68-39 661 ± 25 Mar. 30 IV 29-61 265 ± 84 55-44 512 ± 41 59-64 615 ± 32 \ 13-0 19-1 29-9 20-5 V 68-39 679 ± 69 / 49-00 735 ± 29 } 75-74 676 ± 26 ! Apr 6 IV 33-84 364 ± 44 63-36 523 ± 35 68-16 833 ± 91 30-2 \ 31-0 \ 29-4 \ 13-42 V 78-16 708 ± 34 1 56-00 584 ± 21 t 86-56 674 ± 73 i (excl. 8 ~ C) Total IV 111-05 999 ± 251 _ 186-42 1581 ± 213 226-50 1931 ± 270 . _ V 254-63 2100 ± 215 — 229-46 2438 ± 166 - 299-08 2420 ± 227 -- Apr. 17 IV-V - - 38-9 -- 49-7 --- 44-3

1 Material at all three temperatures analysed together. 2 Animals became contaminated with bacteria and died. metabolic state of the copepods. Total respiration eaten (Table 1) was then converted into the equivalent from Table 1 has been converted into equivalents of amount of organic matter (Table 2). A weighted mean organic matter burned assuming a fat metabolism assimilation rate was computed from which organic (R. Q_. = 0-7), and carbohydrate metabolism (R. Q_. matter assimilated was determined. Food chain effi­ — 1-0). These values were subtracted from the assi­ ciency is defined as the percentage of the energy con­ milated organic matter to give estimates of growth for sumed which is conserved in animal protoplasm of either fat or carbohydrate metabolism. At 5° C mea­ the next trophic level. Excluding data for stage IV sured growth fell between the estimated values for fat at 2° C, the values ranged from 13-0-1 7-3 °/#. Growth and carbohydrate, but at 2° G it was too large for efficiency is the ratio of growth to energy actually stage V and too small for stage IV. assimilated; where significant growth occurred, effi­ A comparison was also made of measured weight ciences from 51-8 to 90-7 °/0 were obtained. As no losses in starved animals with predicted losses esti­ calorific data were available for the algae, dry weights mated from respiration information. In most cases have been used to compute the efficiencies. These the predicted losses assuming fat as a substrate agreed probably represent underestimates of efficiency, as with measured weight losses, but some measurements the caloric content of adult C. hyperboreus, 7-4 calories/ fell outside the range of predicted values. ash-free mg dry weight, was almost certainly higher The comparison of actual weight changes with than that of the algae. predicted changes based on the possible chemical structure of the substrate oxidized might permit a D iscu ssion rough estimate pf R. Q. for a marine copepod. Most Calanus hyperboreus is known to be an arctic form results argue against a wholly fat or wholly carbo­ but is found in deep water as far south as 30° N (Sars, hydrate metabolism when animals have food and sug­ 1925). Despite its preference for deep water in tem­ gest a R. Q . between 0-7 and 1 -0. Possibly metabolism perate regions it seems to be primarily an herbivore shifts toward fat oxidation under starvation conditions. (Conover, 1960). How is it then possible for an herbi­ Efficiency of growth was also calculated from the vorous organism to live most of its life largely isolated data in Tables 1 and 2. The ash-free dry weight of from its food supply? Possibly the behaviour and Thalassiosira fluviatilis was 1-07 mg/106 cells and that physiological adjustments demonstrated by this spe­ of T. nordenskioldii 0-51 mg/106. The num ber of cells cies in response to its environment are an exaggerated 195

Table 2. Growth, predicted growth, and the determination of growth efficiency for Calanus hyperboreus stages IV and V at 2° and 5°C

Food Growth Organic matter Estimated net chain effi­ oxidized assuming : growth assuming : effi­ ciency ciency Carbo­ Organic Weigh­ Organic Carbo­ Fat Fat Mea­ hydrate matter ted matter hydrate meta­ (col. 9/ (col. 9/ Temp. meta­ sured meta­ Phytoplankton food Stage con­ mean °/0 assimi­ meta­ bolism col. 4) col. 6) °C bolism growth bolism sumed assimi­ lated bolism (mg) x 100 x 100 (mg) (mg) lation (mg) (mg) (col. 6-7) (mg) (mg) (col. 6-8)

(i) (2) (3) (4) (5) (6) (?) (8) (9) (10) (H) (12) (13) Thalassiosira flumatilis. . . . IV 2 1-07 19-6 0-21 005 014 004 016 009 (3-7)» (19-0)8 V 2 2-25 191 0-43 012 0-33 0-391 0-31 0-10 17-3 90-7 IV 5 1-69 20-7 0-35 009 0-24 0-221 0-26 O il 13-0 62-8 V 5 2-61 20-3 0-53 0-11 0-30 0-381 0-42 0-23 14-6 71-7

Thalassiosira nordenskioldii . IV 5 101 270 0-27 009 0-24 0141 018 0-03 13-9 51-8

1 Growth statistically significant. 2 Doubtful values as no significant growth occurred. example of the mechanisms employed by other plank­ dearth in Greenland waters unless it reduced its me­ ton in dealing with environmental in­ tabolism. Conover (1956) showed that the respiratory adequacies. rate of starved Acartia sp. declined in the laboratory. In local waters, adult males appear in the plankton Richman (1958) observed no change in amount of by late November and by December some adult fem­ oxygen consumed by fed and unfed Daphnia when ales have become gravid. At this season stage V compared on a dry weight basis but found a significant animals brought into the laboratory often moult to decline in the respiratory quotient of unfed animals. adults within a few days. A female may become ripe In C. hyperboreus, a shift in R. Q. may accompany and begin laying within two weeks after moulting. decreased respiration. Ripe females are found throughout the winter, but The significance of these adjustments is underlined disappear by the end of March. by the following calculation. If a stage V C. hyperboreus About two months are required from egg to cope- weighing 2 mg ceased to feed in June, at which time podid III under laboratory conditions. The rapidly its respiration rate was lowered from 18 to 10^1 maturing young stages are found in both surface and Oa/copepod day and a wholly fat nutrition adopted, deep water in late winter and spring and all stages by December it would still weigh 116 mg. At a rate have food in their guts in April. The respiratory rate reduced to 7 u\ 0 2/copepod, its total weight loss by the is also high at this time. By May most of the popula­ start of the next spring flowering would be only 50 °/0. tion has reached stage IV or V. By comparison the same animal respiring at 18 ^1 Throughout the warm months there is no apparent 0 2/copepod day with an R. Ç). of 1 would consume feeding in natural populations of C. hyperboreus and itself completely in less than three months. Since a no moulting occurs in late stage copepodids brought weight variation of several hundred per cent for into the laboratory. During this period in the life animals of the same size is common in this species, cycle the metabolism of animals recently taken from there is no reason to believe that a 50 °/0 or even the sea is low and their activity apparently consider­ 75 °/0 weight loss in stored fat reserves would be ably reduced. injurious. The suggestion that plankton animals may reduce The life cycle of C. hyperboreus seems regulated to their metabolic rate during periods of low food avail­ take advantage of a single period of food abundance ability is not new. occupying perhaps 10-20 °/0 of its total life. The time Ussing ( 1938) calculated from the data of Marshall, that C. hyperboreus occurs in surface waters varies from Nicholls and Orr (1935) that Calanusfinmarchicus would locality to locality but seems to coincide with the not be able to survive the winter phytoplankton vernal phytoplankton bloom. In the Norwegian fjords

13' 196

spawning occurs in February and March and upward that of unripe females regardless of previous food vertical migration is completed in April (Wiborg, history. The presence of food is not necessary for 1940, 1954). At weathership M in the Norwegian Sea, ripening of the gonads or successful breeding. C. hyperboreus appears near the surface in May and 6. Assimilation rates for the copepods fed two dia­ June (Østvedt, 1955). At the same station, the spring tom species belonging to the genus Thalassiosira ranged diatom maximum is initiated in May and persists from 13 to about 50 °/0 and were generally less than through early July (Halldal, 1953). In the East Green­ 30 *>/„. land fjords C. hyperboreus breeds in May and is common 7. Growth in stage IV and V animals could be in surface waters during July and August when the measured within four weeks. For those animals show­ water is rich in phytoplankton (Ussing, 1938). ing significant growth, food chain efficiency ranged Since most of the year’s growth must be accom­ from 13—17-3 °/0 and growth efficiency from 518- plished in a few months, high growth efficiency would 90-7 «*/„. be of importance. In the laboratory, a weight in­ crease of 100 °/0 for stage V in three months might 8. Crude respiratory coefficients were computed be expected, and the younger stages should be even from respiration, food intake, and growth data. They more efficient. suggest a mixed metabolic substrate when food is Probably breeding is also dependent on stored re­ available shifting to a fat metabolism under starvation. serves. Sømme (1934) found that egg-laying and ver­ tical migration occurred simultaneously, but Wiborg R eferences (1954) observed breeding in deep water in January Bigelow, H. B., Lillick, L. C., & Sears, M., 1940. “ Phytoplankton and February. Elsewhere reproduction appears to and planktonic protozoa of the offshore waters of the Gulf of Maine. Pt. I. Numerical distribution”. Trans. Amer. Phil. Soc., precede the spring bloom and upward migration by N.S., 31: 149-91. 1 to 2 months. Laboratory experiments showed that Conover, R. J., 1956. “ Oceanography of Long Island Sound, no feeding occurs before naupliar stage V and all the 1952-1954. VI. Biology of Acartia clausi and A. tonsa”. Bull. early stages developed better when food concentra­ Bingham Oceanogr. Coll., 15: 156-233. Conover, R. J., 1960. “The feeding behavior and respiration of tions were low. Breeding, as does the rest of the life some marine planktonic Crustacea”. Biol. Bull. Woods Hole, cycle, seems geared to the food supply so that the 119: 399-H 5. developing generation has reached copepodid I—III, Corner, E. D. S., 1961. “On the nutrition and metabolism of zoo­ ready to put on maximal growth, by the time of the plankton. I. Preliminary observations on the feeding of the marine copepod, Calanus helgolandicus (Claus)”. J. mar. biol. spring bloom. Ass. U .K ., 41: 5-16. Cushing, D. H., 1955. “Production and a pelagic fishery”. Fish. Su m m ary Invest. Lond., Ser, 2, 18 (7). Cushing, D.H., 1959. “On the nature of production in the sea”. 1. The respiration, feeding, and growth of small Fish. Invest. Lond., Ser. 2, 22: 40 pp. laboratory populations of Calanus hyperboreus from the Halldal, Per, 1953. “Phytoplankton investigations from weather­ Gulf of Maine and nearby slope waters have been ship M in the Norwegian Sea, 1948-49”. Hvalråd. Skr., No. 38: studied in relation to certain environmental factors 1-91. Marshall, S. M., Nicholls, A. G., & Orr, A. P., 1935. “ O n the influencing its life cycle. biology of . VI. Oxygen consumption in 2. The respiratory rate of stage V and female C. relation to environmental conditions”. J. mar. biol. Ass. hyperboreus was highest in spring and declined through U .K ., 20: 1-27. Menzel, D .W ., & Ryther, J . H ., 1961. “ Zooplankton in the the warm months to lowest values in late autumn and Sargasso Sea off Bermuda and its relation to organic produc­ winter. tion”. J. Cons. int. Explor. Mer, 26: 249-58. 3. Availability of food was the most important Richman, S., 1958. “The transformation of energy by Daphnia fiulex". Ecol. Monogr., 28: 273-91. factor determining the level of respiration, and highest Sars, G. O., 1925. “ Copépodes particulièrement bathypélagiques” . respiration rates in nature corresponded with the Résuit. Camp. Sei., Monaco, 69: 408 pp. spring phytoplankton bloom. Starved animals had Somme, J., 1934. “Animal plankton of the Norwegian coast waters lower rates than fed animals; the presence of food and the open sea”. Rep. Norweg. Fish. Invest., 4: 1-163. Ussing, H. H., 1938. “The biology of some important plankton brought about increased respiration in animals taken animals in the fjords of East Greenland”. Medd. om Grønland, from the sea during the season when respiration rates 100: 1-108. were low. Wiborg, K. F., 1940. “The production of zooplankton in Oslo- fjord in 1933—1934“ . Hvalråd. Skr., No. 21: 85 pp. 4. The temperature range occupied by the animals Wiborg, K. F., 1954. “Investigations on zooplankton in coastal in the Gulf of Maine is at most 6° C and over this and offshore waters off western and northwestern Norway”. range the metabolism can be more or less success­ Rep. Norweg. Fish. Invest., 11: 1-246. fully regulated independent of temperature. Østvedt, O .J., 1955. “Zooplankton investigations from weather­ ship M in the Norwegian Sea, 1948-49”. Hvalråd. Skr., 5. The respiration of gravid females is higher than No. 40: 1-93. 197

Discussion In answer to questions and comments by Dr. Jør­ gensen and Dr. Yentsch, Dr. Conover agreed that the concentration of phytoplankton in the cultures used was higher than that in the sea even after it had been reduced by the copepods to a tenth of its original density in a week. Dr. Orr commented on the high fat content of 65 °/0 and asked if it was less before feeding. Dr. Conover replied that animals with lower fat content were used in the experiments as they gave better feeding results. He also agreed with Dr. Orr that most of the fat goes to the gonads when the stage V copepods moult and become adults : Calanus hyperboreus does not feed at the time of egg laying.