VOLUMETRIC STUDIES OF THE FOOD 
 AND FEEDING OF OYSTERS ^ ^ > 
 
 From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXVIII, 1908 
 
 Proceidi7igs 0/ tlie fourth International Fishery Congress 
 
 Washington, igo8 
 
 WASHINGTON 
 
 : : GOVERNMENT PRINTING OFFICE 
 
 1910 
 
VOLUMETRIC STUDIES OF THE FOOD 
 AND FEEDING OF OYSTERS ^ ^ ^ 
 
 From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXVIII, 1908 
 Proceedings of the Fourth International Fishery Congress : : Jlashington, ipoS 
 
 \'\W\r' 
 
 ^x. 
 
 WASHINGTON :::::: GOVERNMENT PRINTING OFFICE 
 
 : : 1910 
 

 BUREAU OF FISHERIES DOCUMENT NO. 719 
 
 Issued May. 1910 
 
 D. OF D. 
 
 IV;A/ ic 1910 
 
I 
 d 
 
 VOLUMETRIC STUDIES OF THE FOOD AND FEEDING 
 OF OYSTERS 
 
 By H. F. Moore 
 
 Assistant, U. S. Bureau of Fisheries 
 
 Paper presented before the Fourth International Fishery Congress 
 held at Washington, U. S. A., September 22 to 26, 1908 
 
VOLUMETRIC STUDIES OF THE FOOD AND FEEDING 
 OF OYSTERS. 
 
 By H. F. MOORE, 
 Assistant, United States Bureau of Fisheries. 
 
 Economically considered, probably the most important direct interrela- 
 tion between a marine animal and plants is that existing between the oyster and 
 its food. We have in the United States alone an industry valued at $18,000,000 
 per annum, which is immediately dependent upon the supply of microscopic 
 vegetation in our bays and estuaries, a vast food resource useless to man in its 
 original state, but of great present and still greater potential value when trans- 
 substantiated into the flesh of oysters, clams, and other moUusks. 
 
 Various investigations have shown that about 95 per cent of the food of the 
 oyster consists of diatoms and that most of the remainder is composed of other 
 equally minute plants or organisms on the more or less debatable borderland 
 between plants and animals. The oyster obtains these microscopic organisms 
 by drawing feeble currents of water between the open shells, straining them 
 through the exceedingly minute orifices in its gills, and passing the filtrate by 
 ciliary action into its mouth, which lies ensconced between two pairs of fleshy 
 palps close to the hinge of the valves. Though the currents induced are feeble 
 they are constant, and during the course of twenty-four hours the water thus 
 minutely strained is many times the volume of the oyster. 
 
 It is common knowledge among oystermen and oyster growers that differ- 
 ent localities differ markedly in their powers or capabilities for growing and fat- 
 tening oysters, and the results of various researches have shown that these 
 diversities are correlated with the amount of food available to the sessile oys- 
 ters. A deficiency may be due to a natural poverty of the waters, to an over- 
 population of oysters, or to an absence of currents sufficient to carry the food 
 within reach of the feeble external currents set up by the oysters themselves. 
 Frequently all three of these factors are found to be involved where oysters grow 
 slowly and fail to fatten. 
 
1298 BULLETIN OF THE BUREAU OF FISHERIES. 
 
 Certain enthusiasts, some of whom should know better, have held forth the 
 prospect of a time when the entire available bottom of our bays and sounds 
 would be planted in oysters as densely as are the comparatively small areas now 
 utilized. They fail to consider the fact that the natural fertility of the waters 
 imposes some limit upon the production of oyster food, and that a vast increase 
 in the oyster population, such as their imaginations contemplate, would undoubt- 
 edly exceed the limits which nature has set. 
 
 The microscopic vegetable life of our brackish bays and sounds is probably 
 as abundant as it is capable of becom.ing under existing conditions. It is depend- 
 ent primarily upon the quantity of certain mineral salts in solution, and is as 
 strictly limited by the conditions as is the crop yield of a given area of land by 
 the available salts in the soil. The soils can have their fertility artificially 
 increased, but though experiments conducted by the author for the Bureau of 
 Fisheries have shown that the same expedient is partially successful for limited 
 areas of inclosed water, it can never be applied to open waters, as the fertilizer 
 would be speedily carried away. In this connection, however, it is an interesting 
 speculation whether our coastal waters are not to-day richer in fertilizing salts 
 than they have been in the past. The denudation of our forest lands, the 
 erosion due to faulty agriculture, the artificial fertilizers carried away from 
 cultivated fields during periods of heavy rainfall, and the discharge of sewage rich 
 in organic matter have undoubtedly added much to the available fertilizing 
 content of our coastal waters, to the advantage of their microscopic vegetation. 
 
 The question of food supply, its availability, and the quantity required for a 
 given area planted in oysters is one of vital importance to the oyster culturist. 
 Of the total oyster supply of the United States, about five-eighths, valued at 
 over $10,000,000, is produced on planted beds, and the future growth of the 
 industry is dependent upon the increase of the area of private bottoms under 
 culture. With the extension of the planting industry to new localities and the 
 inevitable congestion in places naturally favorable for growing and fattening 
 oysters, the value of definite data upon this subject will be greater in the future 
 than in the past. 
 
 Empirical methods involving actual planting to determine the suitability of 
 a locality are expensive and often wasteful, and operators with small capital are 
 frequently deterred from taking the risk. Even though the work on a small 
 scale may prove successful, an increase to a large commercial basis may overtax 
 the food supply to such an extent as to make the growth of the oysters slow and 
 their fattening impossible. A number of cases of this kind have come to the 
 author's attention, the most noteworthy being in Lynnhaven Bay, where the 
 increase in the area planted, though the quantity per acre is exceedingly small, 
 has made it almost impossible to fatten oysters properly on certain bottoms 
 formerly satisfactorj'. 
 
FOOD AND FEEDING OF OYSTERS. 1 299 
 
 As the economic importance of the subject merits, it has frequently been the 
 matter of investigation and has probably attracted more attention from biolo- 
 gists than has any other direct correlation between marine plants and animals. 
 The nature of the oyster's food was long ago determined and the work of the last 
 twenty years has been hardly more than confirmatory of that which preceded it. 
 
 Dean appears to have been the first to attempt the quantitative determination 
 of the oyster food available in the water. He employed a chemical analysis 
 of the water to determine the albuminoid ammonia content, assuming that the 
 results would indicate the comparative food values of different regions. 
 
 Subsequent investigators have recognized the grave defects in this method, 
 and, including myself, have all followed the general method of Rafter. Water 
 specimens of definite volume, usually i liter, have been collected either by means 
 of a stoppered bottle or jug, from which the cork is pulled after it has been sunk 
 to the bottom, or by a specially designed metal cylinder constructed on essen- 
 tially the same principle. The suspended matter in the specimen, a large part 
 of which often consists of sand, mud, and debris, is then concentrated in, say, 
 ID cubic centimeters of water by filtration through sand or precipitation in an 
 Erlenmeyer flask after the addition of a few drops of formalin. A definite quan- 
 tity of the filtrate is then removed after agitation and the food organisms counted 
 in a Rafter cell, the calculated number of such organisms per liter being regarded 
 as an expression of the food value of the water. 
 
 This method has two defects, the first of which is that the water specimen is 
 not drawn from the stratum tenanted by the oyster, but solely from a height of 
 about 12 inches above the bottom. It would be possible to correct this defect 
 by using a shorter, broader bottle or specimen cup, but as the water flows rather 
 slowly into the necessarily narrow inlet, there would enter with it a considerable 
 quantity of material stirred up when the instrument strikes the bottom. As 
 the amount of this material would vary with the bottom, the impact, Mid the 
 currents, a more serious source of error would arise and the results would become 
 worthless. 
 
 To obviate these difficulties I have designed the type of bottle illustrated in 
 text figures i to 5. It consists essentially of a brass barrel of a capacity somewhat 
 over I liter, two conical valves, and a tripping device. The lower valve is fixed 
 at a height of 2 inches above a broad base, which prevents the instrument from 
 sinking in soft mud, but the barrel and upper valve slide freely on a central 
 column or rod. The instrument is set by engaging the lug (F) over the inclined 
 surface (G) of the stirrup or tripping device (CDEG), which suspends the 
 upper valve (B) and the barrel (A) so as to leave a gap of 2 inches between the 
 two valves and their respective seats, the stirrup being maintained in position 
 by tension on the cord by which the instrument is lowered. By rotating the 
 cam (H) so as to pinch the cord between it and the collar on top of the upper 
 
I300 
 
 BULLETIN OF THE BUREAU OF FISHERIES. 
 
 valve, the instrument may be locked in the set position, but it is automatically 
 unlocked when it is raised by the cord. 
 
 As the instrument is lowered there is a free flow of water through the barrel, 
 so that at any given time its contents are taken from the stratum in which it 
 
 ^/<f//i7//a/7. 
 
 s5<Tc//o/^f. 
 
 rests. When bottom is touched the tension on the cord is relaxed, the tripping 
 device instantly releases the upper valve and the barrel suspended from it, and 
 they fall into their respective seats, inclosing a sample of water before it can be 
 contaminated by the stirred-up bottom deposits. As the barrel is lo inches 
 
FOOD AND FEEDING OF OYSTERS. 
 
 1 301 
 
 Fig. 3 . — Cross section 
 at A. figure 2. 
 
 long, the water inclosed is a vertical column of the stratum lying between 2 
 inches and 12 inches above the bottom, and as the currents do not flow over 
 the beds in horizontal strata, but roll over and over, this specimen is regarded 
 as a fair sample of that in which the oysters are bathed. 
 
 The instrument is now used in Massachusetts, Maryland, Virginia, and Loui- 
 siana, and actual tests have shown that it takes a water speci- 
 men much cleaner and freer from mud and extraneous materials 
 than do the instruments previously employed. 
 
 The other defect of the old method of determining the food 
 value of oyster-producing waters arises from the practice of using 
 the number of diatoms or organisms per liter as the measure 
 of their productiveness. It is well known that diatoms, which 
 usually constitute upward of 95 per cent of the food of oysters, 
 differ greatly in size and the species vary in comparative abund- 
 ance in different regions and from season to season in the same locality. When a 
 numerical expression is employed, it follows therefore that a multitude of small 
 organisms may give an apparent superiority to a water specimen as compared 
 
 with another containing a 
 o/) /e/yo/"^ smaller number of a spe- 
 
 cies of vastly larger size and 
 much greater aggregate vol- 
 ume, and my own ex- 
 perience has shown cases 
 where this error amounted to 
 nearh' 400 per cent. The 
 method is attended with 
 grave error as applied to 
 even limited regions and is 
 wholly untrustworthy as a 
 basis of comparison between 
 widely separated localities. 
 It gives seemingly quanti- 
 tative results, but these, not 
 being volumetric, are decep- 
 tive. 
 
 Direct volumetric deter- 
 mination can not be made 
 on account of the presence 
 of considerable volumes of sand, mud, and extraneous debris in the filtrate, 
 these materials greatly exceeding the food organisms in volume. Grave 
 attempted to overcome the difficulty by listing the food organisms by species, 
 
 SaM3/77jy/<rfr<i/' ^ 
 
 Details of tripping device of water specimen cup in figures i and : 
 
I302 BULLETIN OF THE BUREAU OF FISHERIES. 
 
 but this arrangement, though an advance on previous work, is not capable of 
 comparative use, and any error in identification, not unHkely to occur with 
 persons not diatomists, would be misleading to future investigators. 
 
 To overcome these difficulties I have for several years used the following 
 indirect method, which has given satisfactory results. The diatoms and other 
 food organisms are collected and counted, as before indicated, and are listed by 
 species, although their identification by their correct names is not essential. 
 Careful outline camera lucida drawings are made of the zonal and valvular 
 aspects of a number of specimens of each species, and their cubic contents are 
 calculated by geometric methods from planimeter measurements of the draw- 
 ings. The average of a number of such calculations will give the average rela- 
 tion of the volume to the product of length, breadth, and thickness of the 
 species. Using this relation and the average of a number of micrometer meas- 
 urements of the specimens themselves, a simple calculation will furnish an 
 approximately correct expression of the average volume of the species in the 
 region under investigation. If these volumes be employed as multipliers into 
 the numbers of the respective species, determined from the counts in the Rafter 
 cell, we have an approximately correct volumetric expression for the amount 
 of the food content of each specimen of water. As the most convenient unit of 
 measurement I have adopted Van Heurck's "c. d. m." (o.oi millimeter), the 
 unit of volume being the cube of this, "cu. c. d. m." (0.000,001 cubic milli- 
 meter) . The following is an illustration of the data required for each species : 
 
 Synedra commuiata (Matagorda Bay) ; average length, 4.7 c. d. m. ; breadth, 
 0.5 c. d. m.; thickness, 0.5 c. d. m.; volume = 0.6 (1 X b X t) =0.7 cu. c. d. m. 
 
 This method sounds elaborate in its narration, but has not shown itself to be 
 cumbersome in practice, and, moreover, it appears to be the only method so 
 far proposed which gives data of real value. The results are directly compar- 
 able with those obtained in other waters or with those reached in the same 
 waters at different seasons. Five hundred or 600 determinations have been 
 made in the past two years, and, for reasons shown below, the procedure gen- 
 erally was found to require but little more labor than the older misleading and 
 less accurate method. 
 
 In oyster investigations it is customary to take a large number of water 
 specimens at adjacent stations, and as the nature of the food content of each 
 varies in quantity rather than in the character of the organisms, the measure- 
 ments of eight or ten species will apply to all water samples from the locality. 
 Only those organisms need be measured which examinations of the stomach 
 contents of the oysters show to be important as food. The counts have to be 
 
FOOD AND FEEDING OF OYSTERS. 
 
 1303 
 
 made, whatever method be employed. In Matagorda Bay, where the present 
 method was first used, about 150 specimens of water were examined and the 
 additional time required was not over 10 per cent. The following table shows 
 the results and the manner of tabulation, as well as the differences in results 
 attained by the numerical and the volumetric methods : 
 
 Food Value of Waters op Matagorda Bay." 
 [Roman figures indicate volume of organisms, or food value. Bold-face figures indicate number of organisms.] 
 
 No. 
 
 Species. 
 
 A. 
 
 Between 
 
 Sand and 
 
 High 
 
 Mound 
 
 signals. 
 
 B. 
 Between 
 
 High 
 
 Mound 
 
 and Lake 
 
 'signals. 
 
 C. 
 
 Between 
 
 Mad Island, 
 
 West, and 
 
 Lake 
 
 signals. 
 
 D. 
 Between 
 
 Shell 
 Island and 
 Mad Island 
 
 reefs. 
 
 E. 
 
 Between 
 
 Dog Island 
 
 and Shell 
 
 Island 
 
 reefs. 
 
 F. 
 Between 
 Dog Island 
 and Pa- 
 vilion 
 signal. 
 
 
 
 i 
 
 500 
 
 121,80s 
 3,483 
 
 1,750 
 
 ^■37! 
 
 '■m 
 2I 
 
 250 
 
 
 917 
 
 '■* 4,081 
 17.760 
 2.960 
 
 8<6 
 4. 160 
 416 
 1.688 
 675 
 6=5 
 125 
 125 
 125 
 
 250 
 
 2,000 
 1,000 
 
 li5o 
 
 100 
 
 '"4.650 
 
 
 
 lineatus _ _ 
 
 141,470 
 4.042 
 
 6,413 
 
 683 
 
 1,660 
 166 
 
 1. 145 
 458 
 
 '^i'lSS 
 
 3 
 
 
 3.500 
 
 
 2,000 
 
 
 
 
 
 
 'Too 
 i5o 
 
 100 
 426 
 
 5'?S2 
 
 6 
 
 arenaria 
 
 Amphora ovalis 
 
 500 
 
 '■5S0 
 
 '/ 
 
 
 
 
 8 
 
 
 
 
 9 
 
 
 
 
 
 
 125 
 375 
 
 
 
 225 
 350 
 
 500 
 
 
 
 292 
 
 125 
 
 1,250 
 
 
 
 
 
 
 
 1 
 I 
 
 { 
 
 { 
 
 5, 600 
 8.000 
 
 250 
 
 6,650 
 9,500 
 
 68,340 
 3,417 
 
 IS',000 
 
 i',083 
 
 675 
 
 sei^o 
 ■1^0 
 
 8. 155 
 11.650 
 
 "^'650 
 
 18,160 
 
 908 
 
 J.-iS2 
 
 14 
 
 
 875 
 250 
 
 
 16 
 
 
 
 
 
 
 
 200 
 300 
 
 
 
 
 { 
 
 1,250 
 
 'kill 
 
 600 
 
 292 
 
 8.7SO 
 1,260 
 
 250 
 
 8.7SO 
 1,250 
 
 1,000 
 
 
 
 
 
 
 Total volume, or food value _ _ 
 
 Total number of organisms 
 
 
 
 
 
 
 219.342 
 23,108 
 
 273.861 
 
 26,416 
 
 271. 743 
 
 30,393 
 
 284.050 
 51,750 
 
 177.640 
 20,083 
 
 166, 52s 
 
 13,260 
 
 
 208.527 
 259. 774 
 190,049 
 
 287,737 
 345. 200 
 188,450 
 
 239.024 
 267.000 
 314.412 
 
 259.875 
 274.350 
 317,625 
 
 159,937 
 177.900 
 193. 770 
 
 89,92s 
 254.92s 
 '53. 72s 
 
 
 Middle of bay 
 
 
 
 
 
 I From Survey of Oyster Bottoms in Matagorda Bay, Texas, by H, F. Mo 
 
 of Fisheries Document 61 
 
I304 
 
 BULLETIN OF THE BUREAU OF FISHERIES. 
 
 Food Value of Waters of Matagorda Bay — Continued. 
 
 No. 
 
 Species. 
 
 G. 
 Between 
 
 Pavilion 
 and Three- 
 mile sig- 
 nals. 
 
 H. 
 Between 
 Three-mile 
 and Seven- 
 mile sig- 
 nals. 
 
 I. 
 Between 
 Seven- 
 mile and 
 Grass 
 signals. 
 
 J. 
 Between 
 Grass and 
 Dressing 
 
 Point 
 signals. 
 
 K. 
 
 Live Oak 
 Bay. 
 
 L. 
 
 Above 
 
 Dressmg 
 
 Point. 
 
 
 rt 
 
 / 
 
 875 
 
 292 
 
 163.310 
 4,666 
 
 "Mo 
 
 Ji2 
 
 '■•ilo 
 
 166 
 
 792 
 
 ''5,666 
 ' 2,667 
 
 1.458 
 
 583 
 
 200 
 
 118,720 
 3,392 
 9, 150 
 1,525 
 
 '''isi 
 
 
 
 3 
 
 
 157. 500 
 4,500 
 
 140,000 
 4,000 
 
 
 
 3 
 
 
 583 
 
 
 
 
 
 
 
 
 
 
 
 ^'iio 
 
 
 
 5 
 
 
 
 ■250 
 
 
 
 i',000 
 
 830 
 
 166 
 
 3. 750 
 1,.500 
 
 3.7SO 
 750 
 
 •2',167 
 1, 66s 
 
 
 
 
 
 333 
 
 
 
 8 
 9 
 
 166 
 125 
 166 
 166 
 167 
 642 
 917 
 sSi 
 166 
 
 1,000 
 167 
 125 
 
 166 
 
 
 
 
 750 
 
 260 
 
 
 
 
 492 
 
 883 
 
 583 
 
 lO 
 
 teniii'^ciimum 
 
 
 1,500 
 125 
 
 962 
 1,375 
 
 \Mo 
 
 '°'500 
 250 
 250 
 
 750 
 166 
 
 I. 793 
 541 
 
 750 
 
 883 
 
 
 angulata major 
 
 Synedra coramutata -- 
 
 sp -- 
 
 
 13 
 
 14 
 
 1.686 
 2,408 
 4.812 
 1,375 
 
 208 
 100 
 517 
 
 14.581 
 2,083 
 
 2\MS 
 V,55o 
 
 I. 166 
 1,666 
 
 '3,166 
 
 
 
 
 
 
 sp- -- 
 
 Pyxilla sp 
 
 542 
 167 
 125 
 
 
 333 
 
 
 750 
 2,250 
 
 =ilo 
 
 
 
 500 
 
 19 
 
 
 
 2.62s 
 375 
 
 \,tu 
 
 11,669 
 1,667 
 
 Total volume, or food value 
 
 Total number of organisms 
 
 
 178.275 
 14.750 
 
 194.600 
 
 13.832 
 
 260. s8o 
 15,540 
 
 167. 792 
 15,207 
 
 'WMh 
 
 181.586 
 16,964 
 
 
 1S1.050 
 186,599 
 189.67s 
 
 162. 900 
 .82.925 
 238.349 
 
 191. 700 
 357. 650 
 232. 250 
 
 182,470 
 152.525 
 168.674 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 In the study of the food actually consumed by the oyster, it has been 
 regarded, heretofore, as sufficient to remove the stomach contents by means of 
 a pipette inserted in the mouth or through an incision in the body walls. This 
 method extracts but an indeterminate portion of the undigested food in the 
 stomach, a considerable proportion remaining in the folds of that organ and in 
 the wide openings of the hepatic ducts, and it removes practically nothing of 
 the intestinal contents. For quantitative work the method is very defective, 
 and it is useless as a basis for those studies of food consumption and the rate of 
 feeding which must have an important place in the oyster investigations of the 
 future. 
 
 In order to remove the entire contents of the alimentary canal, I am now 
 using the apparatus illustrated in figure 6, which is essentially a combination of 
 stomach pump and enema, effectually irrigating the entire digestive tube. It 
 consists of a reservoir (A), connected by a flexible siphon tube with a glass 
 canula (B) ligated in the rectum, and of an aspirator (C) connected through the 
 medium of a vial or test tube (D) with another canula (E) inserted in the mouth. 
 
FOOD AND FEEDING OF OYSTERS. 
 
 1305 
 
 The canulas are made of glass tubing, and their tips are held for a moment in a 
 
 Bunsen flame to produce a burr, which prevents their slipping from the ligature. 
 
 The operation of the apparatus is as follows: The reservoir (A) is lowered 
 
 until the water surface is about level with the stage F. The oyster is carefully 
 
 removed from its shell and placed on the stage, its rectum is slit for a distance of 
 about one-eighth inch from the anus to facilitate the insertion of the canula, 
 which is ligated in position by means of a needle and thread. The oral canula, 
 which has a wider opening, is inserted in the mouth and ligated by means of a 
 
I3o6 BULLETIN OF THE BUREAU OF FISHERIES. 
 
 needle and thread carried through the tissues. The pinch cock (G) is then 
 released on the siphon of the aspirator, which exhausts the air from the vial or 
 tube (D), draws out some of the stomach contents, and causes a slight collapse 
 of the walls of the alimentary canal. The reservoir (A) is then raised until a 
 flow of water is established through the rectum with a resultant slight turgescence 
 of the intestine. There is thus established a current of water running into the 
 rectum, through the intestine, and out of the mouth, carrying with it eventually 
 the entire alimentary contents, which collect in the tube (D). To facilitate the 
 dislodgment of the more or less impacted faeces, the intestine is occasionally 
 gently tapped with the handle of a scalpel or dissecting needle. With one appa- 
 ratus about six oysters per hour can be opened and operated on, and dissection 
 shows the entire alimentary canal to be freed of contents. The contents of the 
 tube are treated with a few drops of preservative and are concentrated, by pre- 
 cipitation and the removal of the supernatent water, to a standard volume of 
 5 or ID cubic centimeters, after which the organisms are counted by the Rafter 
 method and the volume calculated as previously described. It is usual to take 
 the average of five specimens as the measure of the food content of a given lot of 
 oysters. 
 
 For studying the rates of feeding of oysters under different environmental 
 conditions, I have recently used the following experimental methods, which have 
 been found effective : 
 
 Pieces of sheet rubber (dentists' " rubber dam ") about 8 inches square, called 
 " aprons, " are prepared by cutting out of the middle semicircular " windows " of 
 about 2 inches radius, over which pieces of no. 25 bolting cloth are cemented 
 with a thick ethereal solution of rubber. A slit about 5 inches long is cut in the 
 rubber parallel to and about J^ inch below the long diameter of the window. 
 
 A number of oysters, about 5 inches long, are then thoroughly scrubbed with 
 a brush, washed in fresh water, the shells covered with a thin layer of Portland 
 cement so as to fill all cavities and smooth irregularities in their surfaces, and 
 thoroughly dried in the air. 
 
 Each is then inserted in the slit in the middle of an " apron " in such position 
 that the edges of the slit approximate the line running from the dorsal side of 
 the hinge to the point of insertion of the gill at the edge of the mantle. The 
 edges of the slit are then pasted to the shell with rubber solution, care being taken 
 to provide a small fold in the apron at the lip of the shell, to carry it around the 
 dorsal side of the hinge so as not to interfere with the opening of the valves, and 
 to see that there are no gaps between the shell and the rubber at any point. 
 
 Security of adhesion can be promoted by first giving the proper parts of the 
 shell several coats of thin rubber solution, the final cementing being performed 
 with a thick paste made by squeezing the ether-softened crude rubber through 
 cheese cloth and reducing it to the desired consistency by shaking it in ether. 
 
FOOD AND FEEDING OF OYSTERS. 1307 
 
 The oysters prepared in accordance with the foregoing description are then 
 placed for about three days in filtered sea water, renewed morning and night, 
 at the end of which time they are practically purged of food and usually gaping 
 with hunger. The intestinal contents of five are then determined by means of 
 the apparatus and methods already described. 
 
 The remaining oysters are now placed each in a 6-inch Petri dish, the shells 
 resting on a wire support to raise them above the bottom and lying so that the 
 deep valve is downward and in such position that the cloacal or excurrent cham- 
 ber of the oyster lies below the apron and the oral or incurrent chamber above 
 it. The " apron " is then confined to the sides of the dish by means of a rubber 
 band or cord, and a layer of sand is placed over the window and the surround- 
 ing rubber to serve as a filter, as shown in figure 4. A piece of cheese cloth tied 
 over the sand will prevent its being washed away by currents or disturbed by 
 inquisitive fishes and crabs. 
 
 When the oysters thus prepared are transferred to their natural environment 
 they are as free to open their valves and feed as if they had never been removed 
 from their beds; the oral chamber is in unobstructed communication with out- 
 side waters while the excurrent chamber discharges into the Petri dish, where the 
 faeces are retained while the expelled water passes through the filter. Oysters 
 prepared as described have been kept under close observation under otherwise 
 natural conditions and appeared to feed as freely and normally as neighboring 
 specimens that had never been disturbed. The fseces drop into the dish in a 
 little heap of demicylinders, while extraneous matter was excluded by the apron 
 and filter. 
 
 At the end of three and six days, respectively, lots of five of these oysters are 
 taken up, their intestinal contents removed by the method already described and 
 added to the faeces collected from the dishes. As about 95 per cent of the food 
 consists of diatoms whose tests pass unchanged through the alimentary canal, 
 it is evident that by calculating the volume of the combined food organisms of the 
 faeces and alimentary canals by the methods described and deducting the 
 volume of the residual intestinal contents, as determined from the lot of five 
 starved check oysters, we can arrive at a volumetric expression of the average 
 rate of feeding. Determinations of the diatomaceous content of the surrounding 
 water made at intervals during the experiment supply the data for the necessary 
 correction to be applied for dead diatom frustules ingested by the oysters under 
 experiment. 
 
 It is perhaps not necessary to use starved oysters for these experiments, but 
 they have been used in order to insure the prompt commencement of feeding as 
 soon as returned to the water, unstarved specimens sometimes "sulking" 
 after repeated handling. A check upon possible error due to any abnormal 
 
I308 BULLETIN OF THE BUREAU OF FISHERIES. 
 
 appetite at the beginning of the experiment is provided by comparison of the 
 results obtained in the two lots fed for different periods. 
 
 My first experiments were with bolting cloth aprons, but it was found diffi- 
 cult to keep them covered with sand, especially close to the edges of the dish, 
 and also to secure good adhesion between the shell and the apron. These 
 defects in some cases permitted the infiltration of fine mud and other extraneous 
 matter. Such difficulties have been obviated by the use of sheet-rubber 
 " aprons " as described. The latter have been in use for four or five months and 
 have proved satisfactory, but there has not been time for the tabulation of the 
 quantitative results. 
 
 It is believed that the apparatus and methods of research above described 
 furnish for the first time efficient instruments for strictly qtiantitative studies 
 of the food and feeding of oysters and similar mollusca, and they also furnish 
 data for determining the amount of water filtered through the gills. In addition 
 to the scientific interest attaching to the studies it is believed that they will lay 
 the foundation for valuable economic data. As is well known to those who have 
 made a study of the oyster fisheries, much time and money is lost in futile 
 attempts to grow oysters in localities which eventually prove unsuitable. In 
 many cases these failures are due to a paucity of food, the oysters failing to 
 fatten. If there could be determined the minimum food unit requisite under 
 varying conditions of bottom, currents, and density of oyster population, the 
 waste of time, money, and effort in useless planting could be largely prevented. 
 
 As a preliminary to the determination of such unit, it is necessary to deter- 
 mine with accuracy the relations existing between the oysters and the plants 
 which constitute their diet. We must know the exact relation existing between 
 the food consumed by oysters which are rapidly growing and fattening and by 
 those which are not. We must determine how much more food an oyster will 
 consume in strong currents than when living in sluggish waters equally rich per 
 unit of volume, and it will be necessary to learn also the water food content 
 required to supply the minimum requisite under varying conditions of current. 
 
 The experiments already conducted have shown that all of these data can 
 be obtained with considerable accuracy by the means above described, and by 
 conducting further research in regions such as Lynnhaven Bay, where the 
 quantity of oysters on the bottom over considerable areas can be approximately 
 arrived at, they can be given concrete application. The fornmlation of the 
 desired unit will require much patient research and observation, the study of 
 currents, of the behavior of oysters under various natural conditions, and pos- 
 sibly of the reproductive activity of diatoms and other food organisms, but it is 
 believed that we are now in possession of instruments which warrant an attempt 
 at the solution of the problem. 
 
BuL. U. vS. B. F., 1908. 
 
 Plate CXXV 
 
mSsm,