./ Vs^w* iU.\^t Glass_5. n^M_ hokJ}lA5_ NUTRITION OF OYSTERS: THE NATURE OF THE SO-CALLED "FATTENING" OF OYSTERS By Philip H. Mitchell From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXXV, 1915-16 Document No. 860 : : : : : : : : : : : : : Issued March ij, roiS PRICE, 5 CENTS Sold only by the Superintcndcut of Documents. Government Printing Office, Washington, D. C. WASHINGTON ::;;:: GOVERNJIENT PRINTING OFFICE 191S ■wagTJp!* D« of D. APR 20 1918 NUTRITION OF OYSTERS: THE NATURE OF THE SO-CALLED "FATTENING" OF OYSTERS. By PHILIP H. MITCHELL. Contribution from the United States Fisheries Biological Station, Woods Hole, Mass., and the Biological Laboratory of Brown University. INTRODUCTION. The term "fat" as applied to oysters refers in a popular sense to their appearance. When in that condition the meats look plump and have in the body portions a milky appearance not unlike emulsified fat. The juice running out of the meats, however, shows the opalescence characteristic of glycogen solutions. This, together with the fact that glycogen is shown by analysis to be especially abundant in some specimens of oysters and to vary greatly in different specimens, suggest the possibility that glycogen ma)^ be the chief if not the only substance increased in oysters when they become "fat." In a previous paper" it was shown that glycogen shows seasonal variations in oysters and that an increase of glycogen accompanies favorable feeding conditions. It was also shown that glycogen storage not . only accompanied the nonnal feeding process, but could occur as the result of assimilation of sugar in solution in the water utilized by oysters. This paper presents evidence in the fonn of chemical analyses'* of oysters in varying nutritive conditions to show that the amount of glycogen present is the only material which marks a notable difference between "fat" and "lean" oysters. VARIATIONS OF PROTEIN IN THE OYSTER COMPARED WITH THOSE OF GLYCOGEN. The percentage of glycogen or protein in whole oyster meats is not a useful index as to their nutritive condition or their value as human food. The great variations in the proportion of water present in oysters are obvious causes of apparent variations in other constituents when figured as percentages. It goes almost without saying that the results of analyses must be expressed in terms of percentage of dry substance. An equally important variable is the salt content. Ash determinations made under com- paraljle conditions have shown in these analyses variations from 14 to 37 per cent of the dried weight. It is therefore necessary in comparing determinations of glycogen, o "Nutrition of oysters: Glycogen formation and storage." Bull. Bureau of Fisheries, vol. xxtcv, 1915-16. pp. 151-161. t" Part of the analytical work presented in tliis paper was done by .A.. E. Barnard. 27S99°— l.S 479 48o BULLETIN OF THE BUREAU OF FISHERIES. protein, etc., in oysters to express results in terms of percentage content of the ash-free solids. Glycogen and nitrogen, the latter to be used as an index of the amount of protein, were determined in many specimens of oysters of varying nutritive conditions. Some oysters were analyzed immediately after removal from their natural habitat, others after treatment in various artificial ways. The results of a series of analyses are given in Table i. The arrangement is in decreasing order of the amounts of glycogen in the ash-free solids. Table i. — Comp.\rison of the Glycogen and NriROGEN' Content or Oyster Meats. Dried meat Ash-free solids. Dried meats. Ash-free solids. Experi- ment ment No. Glycogen Nitrogen Ash con- Glycotreu Nitrogen No. Glycogen Nitrogen Ash con- Glycogen Nitrogen content. content. tent. content. content. content. content. tent. content. couteut. Prr cent. Per cent. Ptr cent. Per iiill- Per cent. Per cent. Per cent. Per cent. Per cent. Per cent. I iS-SS 7-31 17.40 2... 46 8.86 17 7-44 8.80 28.64 10- 45 12-33 3 17- 6l 7.98 19. 10 21.76 9.87 18 6- 19 7-86 35- 70 9- 63 12.22 I.'. 51 8.61 21. 20 15- 88 10.93 19 6.07 7-86 32-15 8.94 II. 61 4 lo. 54 7.92 29.30 14-89 11.20 20 6.17 8-21 30. 22 8-8j II. 78 S lo. 51 8.04 28.94 14-79 II. 20 21 5.07 8.08 33-52 7-63 12.17 6 g. yo 7.50 33.00 14. 76 11. 20 22 4.85 8.05 34- 20 7-38 12. 22 7 10. s6 7.96 27-78 14. 61 11.03 23 5-37 9- 03 26. 40 7-29 12. 26 S 9.18 7- 17 35-42 14.22 11. 10 24 5-02 8. 19 30.60 7-25 11.82 10.65 8. 8i 23-30 13-91 II. 50 = 5 4- 55 7.46 35-75 7-09 11.62 lO 10.80 y.jS 21.97 13-85 11. 90 26 4-03 7-58 37- 72 6.49 12.17 11 8.69 7- 13 35- 60 IJ- 51 11.08 27 4- 20 S. 04 34- 20 6.40 12. 22 12 10. 76 9. 20 19-50 13- 38 11.42 28 4-34 8. ;6 lO- lO 6.23 12-57 S. 76 7.96 27-55 12- 10 11.00 29 3-78 7-73 35.60 5-8S 12.02 ■4 9. =6 9.OJ 21. 38 11. 77 11.48 30 3-51 7-48 3-60 5-64 12.01 IS 7-59 7-32 34-00 11.50 11. 10 31 3-82 8.76 31-62 5- 59 12. 11 i6 7-8S 8.6s 29-41 11. 11 12.24 32 1-93 8.02 36-75 3- 05 12.69 Examination of Table i shows that as the percentage of glycogen in the ash-free solids decreases the percentage of nitrogen, similarly computed, tends to increase. There is not a regular mathematical relationship between the two sets of figures, but many of the irregularities would fall within the limits of experimental errors. At any rate, the series shows strikingly that protein, as indicated by nitrogen determina- tions, does not increase in oysters as an accompaniment to glycogen storage. In spite of their long continued growth, oysters, indeed, have some tendency toward nitrogen equilibrium. Like the higher animals, oysters not only store glycogen in preference to protein when food is plentiful, but also use glycogen to protect them- selves from loss of body protein when food is scarce. Evidence of this is shown by a more detailed examination of some of the results recorded in this table. Eleven of these results are segregated in Table 2. They were selected because in each case pre- vious experiments, recorded in the first paper" of this series, showed that changes in glycogen amounting to 10 per cent or more had occurred in periods from 2 to 14 days. The various abnormal feeding conditions causing these sudden fluctuations in glycogen content are explained in Table 2. That the comparatively small variations in the nitrogen percentages in ash-free solids are merely due to the glycogen fluctuations can be seen from the computations of the percentage of nitrogen figured not only on an ash-free but also glycogen-free basis. These results are sufliciently uniform to show that sudden variations in the food supply of oysters are not accompanied by changes in their protein content. u " Nutrition of oysters: Glycogen fonnatimi and storage." Bull. Bureau uf Fislieries, vol. xx.xv. 1915-16, pi). 151-161. NUTRITION OK OYSTERS: NATURK OK THE " K ATTENING " OK OYSTERS. 48 1 Table -Comparison of Glycogen and Nitrogen of Oysters which .Show Si^dden Changes IN Glycogen Due to Abnormal Feeding Conditions. Extieri- nient No. I'ed dextrose do Fed chopped seaweeds Starved in fdtered water Starved iu partly purified water. . Fed dextrose do do Ill polluted water 4 days Changed from salt to fresh water . In polluted water 14 days Ash-free solids. C'.lycOKfil auiltiit. Per cent. 14. 76 I4-6I 13- yi 13-51 8.94 8. S3 J- 63 6.23 5-59 3- OS NiUoReu content. NitroKeu ill asli' free and ulycogen- free solids. Per cent. 11. 20 11-03 1 1. 10 II- so 11.08 11. 61 11.78 12. 17 12.57 12. II 13-69 Per cent. 13- M 12.94 12.95 12.81 12.75 12.92 13- l6 13.40 12.84 13. Cy Changes in the proportion of protein present, aside from the unifonn increase due to growth, no doubt occur in the oyster. An instance is shown by examination of certain of these resuUs. Those in Table 3, chosen because they represent analyses made very soon after the oysters were taken from their natural habitat, show marked differences in their nitrogen content. This is true even when figured on a glycogen-free basis. That seasonal changes are responsible for this is indicated by the fact that oysters taken in July and August, which include the spawning season, tend to show a higher proportion of nitrogen than those taken in November. Further work would be required to give an adequate explanation of this, but the suggestion that accumulation of egg and sperm materials, together with heightened metabolism of reproductive glands, may l)e the explanation is obvious. Table 3. -Comparison of Glycogen and Nitrogen of Oysters wmcii Had Not Been Surjected to Abnormal Experimental Conditions. ICxpcrimeiit No. 4- 16 18 Dale when taken from water. Nov. 15 Nov. 15 Aujj. 20 Aug. 7 Aug. 10 July 7 Ash-free solids. Glycogen content. Per cent. 22. 52 21. 65 14.89 II. II 9- 63 7-29 Kitrogeu content. Per cent. 8.86 9.87 II. 20 12.24 Nitrogen in ash-free and gly- cogen-free solids. ■ cent. 11.46 12.O0 13-17 13.76 13- 53 13- 17 ICxperiinent No Date when takL-n from water. July 27 26 1 July 29 =7 ' July 19 29... July 20 30 ' July 22 Ash-free solids. Glycogen content. Per cent. 7.09 0.49 6- 40 5-86 5-64 Nitrogen content. Nitrogen in a;-Ii- f rec and glycogen- free solitK. Per cent. 11.62 • cevt. 12-54 13. 00 13.03 12.79 VARIATIONS OF FAT IN THE OYSTER COMPARED WITH THOSE OF GLYCOGEN. The storage of fat in oysters, as detected by ether extraction of the dried meats, was also investigated. In the previous paper " the suggestion that fat might be formed from dextrose was tentatively made. It was based, however, on only two experiments and is not substantiated by the results of 13 analyses reported in Table 4 below. These later u Bull,, Bureau of Fisherie'., vol, xxxv, 1915-16. pp. 155-161. 482 BULLETIN OF THE BUREAU OF FISHERIES. experiments were made with very careful technique. The oyster meats were dried at low temperature — some of them in vacuum desiccators — to constant \veight and the ether used for extraction was rendered anhydrous by distillation over sodium imme- diately before use. The seeming increase of fat reported for one of the earlier experi- ments may have been due to the difficulty in maintaining ether in an anhydrous con dition in the moist atmosphere of Woods Hole where the analysis was made. The re- sults given in Table 4 do not show in the amounts of ether extract obtained any regu- larity or any relationship to glycogen. A number of other fat determinations on oysters have been made during the progress of this work. These are not included in this table because glycogen was not determined on the same specimens. In no case, however, did the ether extract amount to more than 3.50 per cent of the dried meats. A series of analyses reported by Atwater" gives higher figures, ranging from 6.50 to 10.97 per cent, with an average of 8.78 per cent for 34 analyses. .\s those determinations were not made with the use of anhydrous ether, they are hardly comparable with the ones reported in this investigation. Tabi.i! 4.— Comp.\rison op the Glycogen and Fat Content of Ovster Meats. !-!xperiliU'nt Nil. Gly- Fat (ether Fat ia ash-free extract) iu dried meats. ash-free sohds. Per cent. Per ceKt. Ptr cent. 22.46 2.90 3- SI 21. ;6 3.16 3.91 14. 22 2.04 3-16 13.31 I-5I 2.28 8.94 2-93 4-32 8. S3 1.19 I. 71 li.xpcrinieiit Ko. Gly- cogen in ash-free sohds. anl. --t>3 7.33 6- 40 6.23 5-SS 5 :;9 Fat (ether extract) in dried meals. Per cent. 2' 33 2.2b I. 41 2.SS I. 72 1.47 1. ;o Fat iu ash-free soHds. Per cent. 3-44 2- IS The oysters showing the high glycogen content were the ones which presented a "fat" appearance. Indeed, the two specimens yielding the highest glycogen figures were selected for analysis by practical oystermen and chosen from beds of oysters deemed to be in the best marketable condition. The conclusion that glycogen is the real nature of the "fat" does not rest alone on the results recorded in the preceding tables. During the past two years glycogen determinations have been made on many samples of oysters in connection with this work. However, only those for which ash and either nitrogen or fat, or both, have been determined also are tabulated here. Of the other specimens it has been noticeable that the higher the glycogen the "fatter" the oysters appeared. Six samples from Lynnhaven Bay, Va., and Narragansett Bay, R. I., con- sidered by the trade to be in good marketable condition, contained glycogen varying from 15.5 to 22,8 per cent of the dried weight and from 20 to 27,9 per cent of the ash- free solids. DISTRIBUTION OF GLYCOGEN. The distribution of glycogen in the bodies of oysters of average "fatness" was inves- tigated. About 50 oysters were opened innnediately after removal from the water, about September 15, when glycogen formation is rapid. All the juice was drained off o Atwater. W. O.: "The chemical composition and nutritive value of food fishes and aquatic .invertebrates," Report of United St.nes Commission of Fish and Fisheries, iSSS. pp. 679-868. NUTRITION OF OYSTERS: NATURE OF THE "FATTENING" OF OYSTERS. 483 from the shell contents and evaporated to dryness. The gills and mantles were dis- sected off from each meat, mixed together, dried, and gromid. vSimilarly, the adductor muscle was separated and prepared. The remainder, or body, of the oyster containing the liver, digestive system, etc., was dried and ground into one preparation. Glycogen determinations on the four parts of the oyster thus obtained are reported in Table 5. These show little or no tendency for glycogen to diffuse out into the shell liquor of the oyster, and indicate that like higher animals oysters can store glycogen in all tissues but more especially in the liver, for the so-called liver is the chief organ in the part designated as the body of the oyster. T.\BI.E 5. — DiSTRIBUTIO.V OF GLYCOGEN IN OVSTERS. Tarts. Bods' Gills and mantles. Muscle Oyster liquid Gh'coccn in dried sub- stance. Per cmt. 37.60 12. 69 8. SI (") Ash in dried sub stance. Glycocea in ash- free soUds. Per crvl. Per cent. 12.71 .11.61 18. ,11 T>- '=^^ 10.77 9-53 71.0 " Too low to be accurately determined. CONCLUSIONS. 1. Protein and fat do not accumulate in oysters when they atlain the condition known as "fat." This is in marked contrast to the accumulation of glvcogcn which must be regarded as the chief storage substance for oysters. "Fat " oysters are glycogcu- rich oysters. Investigations and practical procedures looking to improvements in mar- ketable value of oysters must take into account the importance of those nutritive condi- tions favoring glycogen fonnation. 2. The glycogen storage occurs more or less in all tissues of the oysters but is espe- cially prominent in the region of the liver. 1 '" LIBRARY OF CONGRESS 002 894 061 '