CHAPTER X

ORGANS OF DIGESTION AND FOOD OF THE OYSTER

l'a/ItMouth_. _•• • •••_. "' • ••_. ._ __ 2UIEBopbagua md stomach •• ._____________________ 219OUtric sh1e1d. __ • • • •__ • .__ 222Crystalline style •• • • • • •__ • ._.____ 22ll

Formatlon • ••_. __ • ••_. ' • •__ 22llCbemJcaI oomposttIOD_. • •__ •__ ._. ., •__ .__ 22ll

MldlUt and rectum•• • __ • __• • • ._. •• ._ 22llDI&est1ve dlvertIcula__ ._._•• •••__ • • • .___ _ 22llAlimentary treot and formllilon of tecelI ••••• •• •••_ 22ll

Dl&est\oD•• _. • • _._••_. • __ • • • • • • __ 22ll

The pH content oUhe cut md stomach._. ._•••• •__._. •• __ 230Abeorpt\oD otfood by the &111I and mmtle ._ •• __ • •• • 231Food md teed!DI-.----•••--. _. •__ • ••__ • ••• __ ._______ 281

ArtI1IcIal feedlng. •• • __ ._. ._ •••_._ •• __ __ 234Bibliography._. __ • •_. • • •• __ •_•••• • • •__• 286

The system of organs concerned with the in­gestion and digestion of food and elimination offeces consists of the mouth, short esophagus,stomach, crystalline style sac, digestive diver­ticula, midgut, rectum, and anus (fig. 197). Withthe exception of a short section of the rectum, theentire alimentary canal lies within the visceralmll.88 and is completely immobilized by the sur­rounding connective tissue. In the absence ofperistaltic movements the food is moved from themouth toward the anus exclusively by the strongciliary motion of the epithelial lining of the system.I t is difficult to reveal all anatomical dete.ils of thealimentary tract by dissecting the tissues. Betterpreparations can be obtained by making casts oflatex or other suitable materia.l injected throughthe esophagus or via the rectum and left until itsets. Yonge (1926a) used for this purpose a warm,concentrated solution of gelatin. Awati and Bai(1931) employed a mixture of paraffin and resincolored with carmine. Satisfactory results wereobtained in the Bureau's shellfish laboratory byusing red or yellow latex injected into the esop­hagus through a wide mouth pipette with Ito rubberballoon. The preparation was immediately placedin 5 percent formalin in which the latex sets andremains tough and llexible for a long tilDe. Castsof the entire alimentary canal or of its part13 canbe obtained in this way.

MOUTH

The mouth is a compressed U-shaped slit be­tween the two lips (fig. 104) and is lined withcolumnar ciliated epithelium set on a narrow basa.lmembrane. The epithelial cells of the mouth areta.ller than those of the labial pa.lps and containonly a few mucous glands. In the surroundingconnective tissue are large vesicular cells, num­erous muscle fibers, and blood spaces which areoccasiona.lly filled with leucocytes. Leucocytesare also found in narrow spaces between the tissuecells and on the surface of the mantle lining fromwhich they are discarded.

ESOPHAGUS AND STOMACH

The esophagus, a short, funnel-shaped, anddorso-ventra.lly compressed tube, is lined withepithelium similar to that of the mouth. It leadsto the stomach, which occupies a centra.l positionwithin the visceral mass (fig. 197). The stomachis an irregularly shaped, large sac (figs. 198 and199) with several outgrowths or pouches. At theentrance of the esophagus the wa.ll of the stomachforms an anterior chamber, a, which leads into abroader posterior chamber, b (figs. 198 and 199).An oblique outgrowth or pouch oaIled the caecum,c, is the most conspicuous structure which arise8from the ventral side of the anterior chamber.Both the anterior and posterior ends of the caecumare curved and form the anterior and posteriorappendices (a.ap., p.ap.). The larger posteriorappendix is a strip curved ventrally and towardthe right of the stomach. The configuration andrelative sizes of the appendices vary but thestructures are recognizable in &It the cuts. Agroove aIoog the wall of the caecum leads to theopening of the midgut (m.g.) and eervee for BOrtingof food (Yonge. 1926&). On the left side belowthe caecum. the wall of the stomach bulges out toform a broad pyloric caecum (p.c.) , which leadsto &. long outgrowth alongside the midgut, thecryBta1line sac. (erA).

219

cI--+_+--__

oi,---'-----'----'------'----r!SoCentimeters

FIGURE 197.-Digestive system of the oyster, C. virginica, drawn from the disllected preparation after the injection oflatex. The right outer labial palp WIlll out off to expoee the esophagus. The parenchymal til!l8uee over the stomach andintestine were removed. an.-anUSj cl.-eloac&; cr.s.-crystalline style sac; dig. div.-digestive divertieuJaj int.­intestine (mid-gut) j oe.-esophagusj r.-rectum; st.-stomach.

Three groups of wide ducts emerging from thewall of the stomach lead to the digestive diver­ticula. Two of them (fig. 199. d l , d2) originateat the anterior chamber and one (ds) from theposterior chamber.

The internal lining of the anterior chamberforms a number of irregular ridges and furrowscovered with ciliated epithelium. A broad ridgeseparates the anterior from the posterior chamberand apparently dirocts the food particles. Theleft ventral wall of the posteriorcbamber iscovered by a translucent membrane, the gastricshield (fig. 200), which lies directly opposite the

220

opening of the long sac occupied by the crystallinestyle (cr.s.).

Ciliary tract8 of the stomach lining are verycomplex. Detailed observations on the OOUl"8e

followed by food particles after they enter thestomach were made for O. td1lliB And Mya, arena.rin.by Yonge (1923, 1926&), who studied them bycarefully cutting oft the wall and adding finepowdered carborundum or aquedag to the exposedsurface. In general the pattern of ciliary move­ments in the stomach of the Amerioi.n oyster issimilar to that of O._~i8. The direction ofciliary beat along ditfereut ridges and channels

J'IB!l --ANI).~ 'SDVlCE

Centimeters

~--cr.s.

FIOURE 199.-Latex cast of the stomach. crystalline stylesac, and esophagus of a large C. uirgiflie4 viewed fromthe right side. The visceral IJUUI8 was disaected, and theinjected parts were left in their natural position. Drawnfrom a preparation preeerved in 5 percent formalin.a.-anterior ohamber; b.-posterior chamber; or.s.­crystalline style sac; dt , dJ , d.-dllcts leading to the diges­tive diverticulaj m.g.-midgut; oe.~phagua.

centimeters

aDe·

a.ap. r}, dz.cFap a

bbIJ

?c. d3

FIGURE 198.-Latex cast of the stomach, crystalline stylesac, and esophagus of a large C. oirginica, viewed fromthe left side. Upper part of the esophagus is slightlydistended by injection. The visceral mass was di88eCtedand the injected parts left in their natural position.Drawn from a preparation preserved in 5 percent for­malin. a.-anterior chamber; a.ap.-anterior appendixof the caecum; b.-posterior chamber; c.-eaecumjor.B.-crystalline style sac; d.p.-dorsal poUch; d.­group of duets of the digestive gland; m.g.-midgut;p.ap.-posterlor appendix of the caecum; p.c.-pyloriccaecum.

brings the food from the esophagus to the caecumwhere the food. materials are separated accordingto size. SoIne of the largerpartieleeeD~ themidgut may be voided without .being. d~tedwhile the smaller particles are pusbAtd towardthe gastric shield. Other grotipso£.ciliarioDductthe particles toward the duet8 teaamg to thedigestive divel'f,iwl.... The dUCUI l>r&neh out intOa large number at smaltet'paMage8 that,.ramiCyandextend deep into tbemass ofdivertictda;

Nearly t.he.~ntire~tHlrfaee.ill· the stomach

is covered by ciliated epitbelium; only the areasunder the gastric shield and near the posteriorend of the stomach are noneiliated. The epithe­lium is ofeolumnar type with very long cilia,whiehare ~ieul&l'ly prominent on the ridgeS(fig.20i). The height of the ce1Ja gradually.decreues. tmvard the. caecum. Mucou ee1Ia areahuDdant; pat'tieuladynea.r the- junction with the-lIlidgut, .aDdp~ytes ~ ·n~ b.etwe&n.th6epitbelial cella and in the 'underlying ..Connee­tift tU.ue. There is no· weIl-developed mUllCula:rlay«UBder the :epit,h~ial1iDing,butaIeWenlOOtb

0R<Wt!l...~~ ....AmJ.~·:Qt· THE...0T8T&Il.~~··U··

investigators, assumes that the shield is formedby the droplets secreted by the epithelial cells.No evidence in support of this view can be foundin the histological preparations of the stomachsof O. edulis and C. virginica. No droplets couldbe seen in the sections of stomach, and no otherindication of the secretory activities of these cellscould be found. Yonge (1926a) thinks that theshield is very likely formed by the fusion of thecilia and in support of this view points out thatthe structure is attached to the epithelium by finethrea.ds which transverse the substance of theshield and resemble the cilia. Indistinct trans­verse striation can be seen in the sections of thestomachs of C. tJirginica fixed in osmic acid andstained with iron hematoxylin (fig. 202). Thequestion could be settled by electron microscopy,which would reveal the structure of the cilia if thelatter are present within the shield substance. Sofar no such studies have been made.

The shield is not destroyed by boiling in a 40percent solution of potassium hydroxide. Treat­ment with iodine followed by a strong solution ofzinc oxide gives the deep violet coloration that ischaracteristic of the color reaction for chitin(Zander reaction). These facts support Berkeley's(1935) findings that the material of the shield ofthe common Pacific coast clam, the Pa.cific gaper(Schizothaerus nutalli nutalli Conrad), is made ofchitin and contains no chondrinlike constituent.

In C. angulata Leenhardt (1926) described thetorch bearing cells near the edges of the areaoccupied by the gastric shield. The function ofthe cells is not known. They are not found in myprepa.rations of C. flirginica. and are not mentioned

.'."'~~FIGURB 201.-CroIiB ieouon' oft.be waD uftlm stomach.

~H"~D"",:

..·.A!m:'WILIIWfS.PRVICE

Centimeters

222

FlOUR!) 200.-Gastric 8hield viewed in its natural poeitionin a dilI8ected stomach. The poeition of the crystallinestyle i8 indicated by the dotted line. Drawn from anunpreserved preparation.

muscle fibers may be found under the basement.membrane. In general, the histological pictureof the stomach of an adult C. virginica is similarto that described for the spat of this species byShaw and Battle (1957), C. angulata by Leenhardt(1926), and O. chilensis and O. edulis by Dahmen(1923) and Yonge (1926a) respectively.

GASTRIC SHIELDThe stomach wall in front of the openings to

the midgut and style sac is covered. by a thinbut tough, irregularly shaped membrane (fig. 200)made of translucent and slightly striated material.The structure, named the gastric shield by Nelson(1918), rests on a prominent epithelial ridge ofnarrow columnar cells with oval nuclei, rich inchromatin (fig. 202). The cells are devoid ofcilia. The shield is made of two portions ofdifferent size, joined together by a narrow middlepiece (fig. 200). The thicker portion of the shieldlies over the peak of the ridge and is underlined bythe tallest cells in the area. On both sides ofthe peak the epithelium flattens and at the edgeschanges into the typical ciliated lining of thestomach. The surface of the shield is roughenedby the remnant8 of food particles embedded in it.

The origin of the shield has not been fullyexplained. Obviously, it is tire product of theunderlying cells, but the process. of its formationhas not been studied sufficiently. One view,advanced by Gutheil (1912) and shared by 8OUl6

Q.

b.

6 Microns

FIauu 202.-Two C1'OM aectroDB of the wall of the stomaoh of C. vl"mU:o under the gutric 8hield. A~the thiabetportion of the shield. Domn, hematoxyIin-eoein etain. B-cross section near the periphery of ,be Ihie1d. Osmicaeid, iron bematoxylUl. The 8UI1ace of the shield is rough due to embedded and partially JfOQDd food~.Note eroes striation oBbe shield viaible in B. .

by Shaw and Be.ttle (1957) in their work ont.hemicroscopic ana.tomy of the digestive tract of thisspecies.

The function of thesbieid is·to provide a·bwfor grinding of food by the rotatinghe8d of thecrystalline style.

CRYSTALLINE. STYLE

The posterior wall ofthe··stomach·leads to·anelo~ out«rowth or Me which extends a con;'siderable distance along ·the V8Iltl'al·armoftbevi8ceral~· (fig. 197, er.s:, and ~'203) on·the·~tero-veDtra1aide of· the ·adalWt&rinuaole;· A

narrow slit joins the sac over nearly its entirelength to the midgut; near the entrance to thestomach the two structures are sepa.ra.ted. Thesac is slightly twisted around the midgut andoccupies a somewhat dorsal position, while themidgut forms the ventral portion of the commonstructure (figs. 198, 199). A cross section of thesac and midgut shows (fig. 204) that the twochannels are separated in the middle by a narrowslit compressed by the two protruding lobes ortyphlosoles. In figure 204 the style sac is at thetop; its lumen is usually larger than that of themidgut (lower part of the figure). This relation­ship between the style sac and midgut is similarto the topography of this organ in O. chilensis(Dahmen, 1923), O. edulis (Yonge, 1926a), Mya(Edmondson, 1920), Ensis (Graham, 1931a),Mytilus edulis (Sabatier, 1877), M. latus and M.magella.nicus (Purdie, 1887), and A'Mdonta (Nel­son, 1918). In the old literature the structure wascalled a "tubular stomach" by Saba.tier (1877),and" pyloric appendix" by Purdie (1887), na.meswhich have not been accepted in malacologica.lliterature.

0.5CentImeters6

FIOUU 203.-CrystalHne stylell of C. tnTgln1ca (left)and C. gigaa (right) in their Datural position. Drawnfrom live preparatioDs. The wall of the stomach andof the crystalline style sac has been dissected.

Mi Jlimetera

FIGUU 204.-TranaVerBe eec:tiOD of the style sac (upperpart) and midgut (lower part). Too crystaDine styleis absent. Ka~, hemawxyUn-eo8La.

The style sac is lined with densely packedcylindrical cells that have large oval nuclei andlong cilia measuring about 20 p. The intracellularfibrillar apparatus is well developed. Phagocytesand mucous cells are se&ree. The basal membranerests on a thin layer of collagenous fibers; circulartnuecles are sparsely a.rr&nged, 8.8 in the stomach,

224

and there is no distinct muscular layer. Theepithelial cells of the two lobes (typblo8oles) ofthe sac and midgut grwU6lly change hom robust,long cells to shorter ·e&Dawith $lD&JlerQili:a, typicalfor the lining.of the J.Didgut. .The .DlUCOUaoollsare more abundantiD the.midgut tJuLninthe sac.

.P$H .um...:wJLDldft .&lBVICE

The connective tissue lU'Ound the sac and underthe typhlosoles consists of typical vesicular cells.

In actively feeding oysters the lumen of the sacis occupied by a gelatinous rod with a bulging headprotruding. inside the stomach (fig. 203) and thepointed tail extending to the distal part of the sac.The color of the style varies from greyish whiteto deep yellow and brown, depending on the typeof food consumed by the oyster. The head isusually darker than the rest of the style becauseof the food particles wrapped lU'Ound it.

Inside the sac the style is rotated by the ciliaryaction of the epithelium. The rotary motionwas originally suggested by List (1902) in hiswork on mussels, but the demonstration that therotation actually takes place in Anodonta andModiolWl was made by Nelson (1918). Accordingto Yonge (1926a), the large cilia of the groove ofthe sac of O. edulis move in such a way as toproduce a slow clockwise rotation of the stylewhen seen from the stomach. There is, however,a tract of cilia on the side of the larger typholsolewhich beats in the direction of the stomach andpresumably pushes the style forw.a.rd. Foodparticles tha.t enter the sac are carried by thecurrents down the gut but some of them tanglein the substance of the style, are wrapped lU'Oundit, and carried back to the stoma.ch. This process,observed by Nelson (1918, 1925), Allen (1921),and Orton (1924), may be significant for the bi­va.lves in which, like in Ostrea, the style sac is indirect communication with the midgut.

As the style rotates and rubs against the gastricshield, aiding in mixing and grinding food particlesit slowly dissolves in the gastric juice and yieldsdigestive enzymes.

FORMATION

The crystalline style is not a permanent struc­ture. In oysters removed from water and left inthe air the style dissolves in a. short time. Thisobservation, reported for O. edtdi8 by Orton (1924),has been confirmed for C. wginiea. and C. gigru.At room temperature of 21° to 22° C. the crystal­line styles of the .American specie8 removed fromthe sac and left exposed 00 air completely dissolvewithin 45 to 60 minutes. In the body of theoysters (C. fJirginiM) taken out of water the styledisappea.rs in 2 to 3 hours. The a.bsence of thestyle is frequently observed in nonf~oysters.The -symptom is useful, but not entirely reliablebecause-under certain conditiona tbeatyle Ula1'bepresent in·oysters wbicb.do not take. food. Qb-

servations made in winter in the Woods Holelaboratory showed that in late December, at tem­peratures varying from 5.40 to 5.7° C. about 4 outof 10 oysters had crysta.lline styles. No trace offood was found in these oysters, which were ex­amined within a few minutes after they had beentaken out of water.

Yonge (1926a) states that in O. edulis the styleis a.!ways present in hea.!thy oysters, even whenthey are starved, and is absent only under ab­norma.! conditions and in disea.sed. specimens.

The style must be the product of secretion butinvestigators do not agree on the manner and siteof its formation. List (1902), Nelson (1918),Edmondson (1920), and Mackintosh (1925) thinkthat the style is secreted by the narrow cells of thesmaller typhlosoles but do not present conclusiveevidence in support of this view. For fresh­water Anodonta, Gutheil (1912) demonstrated thepresence of vesicula.r granules lU'Ound the nucleiof the epithelial cells of the sac and probablyinterpreted them correctly as a sign of activesecretion. No such granules were found, however,in the histological preparations of O. edulis (Yonge,1926&) and in my slides of the sac of C. uirgin~a.

Evidence of the secretory activity of the style sacwas produced by Yonge (1926a) by injecting 0.5percent solution of iron 8&CCha.ra.te into the ad­ductor muscle, washing the animals, and thendissecting and fixing the sac at 2-hour intervals.The sections were treated by potassium ferricy­anide in acid solution to demonstrate the presenceof iron by Prussian blue reaction. Fine bluegranules indicative of the presence of iron salt werefound in the cytoplasm above the nuclei and be­tween the cilia. of the epithelial cells. No ironwas detected in the epithelium of the midgut orof the larger typhlosoles, although some of themetal was present in the epithelia.l cens of theminor typhlo801e. The experiments may indicatethe secretory function of the epithelial eella, butthey cannot _be considered as evidence of theformation of the style from the 88Cl'8ted granulea.

CHEMICAL OOMPOsmoN

.Analysis of the eryBtaIJint, style of tlJrdiummade by Bar.rois (1889, 1890) showed the followingcomposition: wa.ter 87.11 percent; solid otpDieDlBJ,ter 12;03. percent; solid inorganic matter 0-86percent. The orga.nie eompoo.ent. of the style wuconsidered 1;0 be a globulin with tfteee ofmQOUS

- or chondrinIike substance. .~ey (1936) dem-

onstrated that the styles of four species of bivalves(~ gigas, Mya arenari4, Schizot/uuMt8nuttalii, and Saxidomm {figante'U8) in addition toprotein, yield, on acid hydrolysis, glucionic andsulphuric acids and a hexamine, the essentialconstituents of both mucin and chondrin. Theease of the hydrolysis and the solubility of thestyle materials indicate that mucin rather thanchondrin is involved. The variations in the solu­bility and the quantitative differences in thechemical composition of the styles suggest, ac­cording to Berkeley, that the less readily solublestyles contain larger quantities of mucin.

All examined styles are carriers of certainenzymes which they yield upon dissolution. Therole the styles play in digestion is discussed later(see p. 230 of this chapter).

MIDGUT AND RECTUM

The portion of the intestine between the stomachand the rectum is called the midgut. It beginsat the ventral wall of the stomach next to theopening of the crystalline sac and runs parallel tothe sac as far as"ita distal end, then turns sharplybackward parallel to its previous course (fig. 197,in t.). The ascending branch of the intestinemakes a loop that completely encircles the stomachand continues as a descending branch which endswith the rectum and anus (r., a.).

Throughout its entire length the midgut ischaracterized by a well-developed typhloBOlewhich extends along its inner wall (fig. 205).The gut is lined with columnar ciliated epithelium;there is an abundance of mucous cells and ofwandering leucocytes. The muscular layer isabsent.

The rectum (fig. 197, r.) runs along the dorsalside of the heart. In this respect the oysterdiffers from many other bivalves (sea mussels,clams, fresh-water mU88els) in which the rec­tum runs through the heart. The structure ofthe rectum is similar to that of the midgut; themain difference is the disappearance of the well­developed typhlosole near the anal region wherethe lining is thrown into numerous small folds(fig. 206). A distinct feature of the rectum is acircular layer of smooth muscles which, however,do not form a sphincter at the anus (fig. 207).According to Leenhardt (1926), the anal sphincteris present in the Portuguese oyster. The surfaceepithelium of the &nal region is well developedand abounds in mucous cells.

226

DIGESTIVE DIVERTICULAThe stomach is surrounded by an irregularly

shaped ma.ss of dark tissue which has been calledthe "liver" or "hepatopancreas." Its color variesfrom light yellow to dirty green IUld dark brown.In most cases the color is not visible through awhite or creamy layer of connective tissues richin glycogen.

Yonge (l926b) has shown that assimilation andintracellular digestion tQ.kes place in this m888 ofdarkly colored tissues; it has none of the knownfunctions of the liver or pancreas. He named it"digestive diverticula", a term that correctlydescribes its role.

The digestive diverticula are made of a largenumber of blind tubules emptying into severallarge ducts which lead to the interior of thestomach. The structure of the tubules is simple.In cross section (fig. 208) they are usually roundwith a lumen in the form of a cross. The tubulesare surrounded by connective tissue in whichmuscle fibers are absent. The digestive cellswhich form the interior of a tubule are large andwell vacuolated, with large nuclei at their base.Food vacuoles can be seen in them during feeding.At the corners of the "cr08l'l" of the lumen oneusually finds crypt:e of young eeDs with darkstaining protoplasm, large and compact nuclei,and indistinct cell boundaries. Cells from thesecrypts replace the old cells that are cast off. Thedigestive cells of the American oyster are non­ciliated, but the cells in th~ diverticula of otherbivalves (Teredo) have been reported to havecilia (Potts, 1925). Yonge (1926&, 1926b) believesthe cilia are present in the tubules of eduli8 butprobably retract 80 rapidly that they cannot beseen in the fragments of tissues pressed by acover slip. I was not a.ble to detect them in C.virginica. Pha.gocytee are very a.bundant betweenthe cells and in the surrounding connectivetissue.

The ducts that connect the tubules with thestomach are circular in eroes section and arelined with ciliatedepithe1ium (fig. 209)... Theirlumen is, however, in'egular due to the variationsin the height of the epithe1ia1 ceDa. The epithe­lium is similar to thatpof the stomach .and con­tains many mucousgla.nds and: pbagoeyUs.

AUMENTARY-TRACT AND FORMATIONOF··F£CE-8·

Food ingested by theoyaf.«"is ]Jievei! tbroughthe alimentary canal by the "oiBari~otthe

nSiI-~ •...~.-_.i:D'ncJ:

o Microns

FIGURE 205.-CrOl8 section of the midgut. Bonin, hematoxyiiD-eosin.

epithelium. There is no peristaltie motion sincethe muscular layer of the intestines is eithera.bsent or poorly devel()ped, and the feces a.redischarged in a. continuoU8 ribbon which is cani.edaway by the cloacal current and eventually settles.The time required for food to paee f,hrough thetmtire intestinal tract can be measured by record­ing the time between addition ofa 8U8~ ofcarmine or yeast to the gills and _the appearanceof the rad or white particles in the feees~ Ther&te of passage m.turallydepem:1son the lengthof the intestinal tract and the rate of-feeding.

oRa.uts --OF DroZS'rION ANI)-}IOOD OF -'l'HJ!l -OnrrBR

In large oyst.ere (&bout 10 by 6 em.) kept in run­ning sea. water of about 15° to 16° C. the timerequired for food to pass through the entire intes­tinal tract varied from 90 to 150 Plinute8. Thelength. of -~ inteetm&1 t1'&eta of the oysters usedin -these tests was measured on latex casta whichwere left in situ and exp:oeed by dissecting thetiMues above them. The ~ngthsof the alimentarytrt.of.$W$'8 &8 follOWB:

_In a&_~yflter JileU~g 11 by Gcm._. lU em.IDan~ r.neuurinIIO.Oby 7~ cm~_.11.1om.

Inan~~uriD&Uby6om_. 1.2;00111.In an oySter meuunnl H.i by ~.:J __ ~_ '12.8 ...

TABLE 30.-Rate of formation of fecal ribbonI (in em.) inC. virginica during feeding in laboratorfl sea tDlJUr, Wood.Hole

FIGUBII: 207.-~ IeOtion Gf ~he "DUB and ad­jacent portion of \he reetum. N. the ·ffJcal mM8

inside and the ab8enoe·.of .. ~;.·BoWD, hema­toxylin-eoein.

0.5Millimeters

6

Date TIme ~mpera- ~of Rate ofture rib formatlaD

MtRlAJu ·0 c.. C..I.r.May 18 ___________ • ______ 108 16.6 23 7.718 __________________ 17 16.0 3.2 11.318__________________102 16.0 9.8 6.8May 111__________ •_______

70 16.2 14. 2 12. 2Ill __________________ 123 16.0 II. a 4.6May 28 __________________ 80 16.7 7.2 7.2

Fecal ribbons of oysters contain many livecells-diatoms, dinoflagellates, yea.st, and otherswhich are not killed by the gastric and intestinaljuices and can be recultured.

DIGESTION

The digestion and absorption of food in theoyster are primarily an intracellular process whichtakes place in the digestive diverticula. This wa.sdemonstrated by Yonge (1926a, 1926b) in a seriesof carefully executed feeding experiments in whichthe solutions of iron saccharate, suspension ofcarmine powder, oil emulsion, and dogfish bloodcorpuscles were fed to European oysters. Heproduced convincing evidence that very smallfood particles are absorbed by the cells of thedigestive diverticula, while the diatoms and other

FIGURE 206.-CrOll8 section of the rectum near the ana.lregion. Bouin, hematoxylin-eoein.

The rates of discharge of fecal ribbons observedon actively feeding oysters in laboratory sea waterof 15.00 to 15.7 0 C. are given in table 30. Theaverage of the observed rate was 8.1 em. per hour.Assuming that the average length of the intestineswas 12.8 em., the estimated time of passage offood through the entire alimentary tract was 95minutes.

The feces of the oyster are voided from therectum as a compact and slightly flattened ribbonof sufficient consistency to withstand the velocityof the cloacal current. In an actively feedingoyster the ribbon is maintained in a. horizontalposition along the axis of the current, but beingheavier tha.n the Se&. water it sinks down to thebottom as BOon as the cloacal current slows downor ceast'S. lArge masses of fecal ribbons accumu­late on the bottom a short distance from theopening of the cloaca. The ribbon remains intactfor 2 to 3 days until it is disintegrated throughdecomposition and mechanical disturbance. Theappearance of the fecal masses of the oyster istypical and can be recognized by their shape. Itwa.s shown by Moore (1931) that specific identi­fication of fecal pellets can be made fora numberof marine invertebrates.

228

Microns

FIGURE 208.-Cr088 seotion of a single digestive divertic­ulum. Note the orypts of young cells at the corner ofthe cross of the lumen. Kahle, hematoxylin-eosin.

algae of larger size are ingested by phagocytes.Yonge's work fully confirmed the idea, first ex­pressed by Saint-Hilaire (1893), that the digestivediverticula are the organs of absorption. Hefound no evidence of any secretion from the di­verticula and demonstrated the importa.nce ofphagocytes in the digestive processes. Since thework of the earlier investigators is fully discussedby List (1902) and more recent investigations aresummarized in several pa.pers of Yonge (19268.,1926b), the reader interested in the history of theproblem is referred to these publica.tions.

The digestion of food also takes place in thestomach where several digestive enzymes arepresent. On the basis of our knowledge, whichadmittedly is not complete, the process of di­gestion seems to take the following course. Afterbeing sorted several times by various mechanismsof the gills a.nd labial pdps, the food particlesenter the stomach where the sorting oontinuesand the larger· particles are brobn by the com­bined action of the crystalline style rotatingagainst the gastric shield and the cllemical aeUonof enzymes which dissolve from theetyle. Verysmftll particles are pushOO by the ~I\ throughthe ducts into the digeBtive tubules where theyare taken .. into the vacuoles· of .the digestive·ceUs

ORGANS·"6I' DiQ£S1'loN •.A!m. FOOD OF .THE OTSTJtR

a.nd are acted upon by the enzymes of these cells.Usable material is ingested by the pha.gocytes oris stored in the surrounding connective tissue.Indigestible substances like colloidal carbon ofindia ink are expelled. Some of the food particles,especially of larger size, are engulfed by thephagocytes which abound in the digestive tract.Circulation of food in the ducts is maintainedby the ciliated cells.

The stomach contains free enzymes which aredissolved from the crystalline style. The mostactive among them are the a.mylase and gly­cogenase which digest starch a.nd glycogen.Yonge's experiments (1926a, 1926b) showed thatthe optimum activity of oyster amylase is atapproximately pH 5.9. Purification by dialysisor with a.bsolute ethyl alcohol inactivates theenzyme, but its action ca.n be restored by theaddition of chlorides or bromides. Besides thesetwo enzymes, the style contains a completeoxidll8e system. The presence of oxidases inthe extract of styles was first demonstrated byBerkeley (1923) in the Pacific coast clam, Saz­idomU8 giga'TlhU8, in rock cockle, Paphia staminea,and in sofwhell clam, Mya arenaria. Thisfinding lead Berkeley to advance a theory thatthe crystalline style represents a reserve of oxygenand is a factor in the anaerobic respiration ofmollusks. The theory is not supported bysufficient evidence a.nd has not been accepted bythe students of molluscan physiology.

The presence of oxidases in the styles of Ostrea

FuJUU 209.-G1'(ll!8 section of.tbe dllCt leadiDgto thedi~iv~diveriicula.K.hIe,hematoll.yl~.

was confirmed by Yonge (1926a) and Graham(1931a, 1931b). The enzymes were obtained bygrinding the styles with sand and extracting for2 to 3 days in distilled water with a small amountof toluol as an antiseptic. For testing, the5-ml. samples of 1 percent extract of style weretreated with 2 ml. of hydrogen peroxide and 12drops of 1 percent pyrogallol. After 5 minutesthe sample turned dark red-brown. The ex­tract produced color even in the absence of hydro­gen peroxide, indicating the presence of a completeoxidase system. Reactions with guaiacum and2 percent hydroquinine were less pronouncedthan with pyrogallol.

Sawano (1929) reported the presence of buty­rase, an enzyme that clots milk, in the styles ofO. circ'Umpida but his observation remains un­confirmed.

Extracts of digestive diverticula contain alarge array of sucroclastic enzymes which acton starch, glycogen, sucrose, maltose, lactose,raffinose, and on some glucosides. The amylase,which converts starch into dextrin and dextrininto maltose, is present both in the style and inthe digestive diverticula of the oyster. It has,however, different optima; the style amylaseacts best at pH 6.0, whereas the enzyme fromthe diverticula has an optimum at pH 6.4 (Sawano.1929).

The proteolytic enzyme of O. edulis is absentin the gut but can be found in the extract of thediverticula. It acts very slowly and has two pHoptima at 3.7 and 8.5 when casein is used as asubstrate. With gelatin the optima are 4.1 and8.5.

Cellulase, the enzyme which hydrolizes cellulose,has not been found in the digestive extracts of theoyster. It must be assumed, therefore, that theoysters are unable to digest cellulose. Thepossibility is not excluded, however, that thisenzyme may be present in the bacteria and fungiwhich happen to be in the gut. The presenceof cellulase in mollusks has been established forthe gastropods Heliz and Linnaea and for the woodboring bivalve Teredo.

Fats are hydrolized to fatty acids and alcoholsby the action of lipase. Yonge (1926a) dem­onstrated the presence of this enzyme byfeeding the oysters an emulsion of olive oil stainedred with Nile blue sulphate and watching thechange of red color into blue as the digestionproceeded. Oil is ingested by phagocytes and is

230

carried by them through the tissues, the gradualchange of color serving as an index of the actionof lipase. From the observation that the dropletsof oil found free in the stomach retain the redcolor, Yonge deduced that free lipase is absentin the gastric juice. These findings are contra­dicted by the observations of George (1952) whoshowed that in O. virginica and in Mytil'U8 thehydrolysis of neutral fats takes place extra­cellularly in the stomachs and that lipase can beextracted from the crystalline style. According tohis observations, droplets of olive or peanut oilstained scarlet red with Sudan I or Sudan IIIare not deposited in the tissues. It is knownthat in mammals and birds the stained fat maybe stored in the bodies (Gage and Fish, 1924).Several possibilities may be considered: (a) thatthe stained fat is rapidly metabolized; (b) thatit may be deposited in connective tissue in minutequantities undetectable under the light microscope;and (c) that the mollusks are unable to utilizethe peanut and olive oil because of the differencesbetween the fatty acids of these oils and theunsaturated fatty acid of their natural food. Sofar no experimental evidence has been presentedin support of any of these possibilities (George,1952) and further studies of the problem of fatdigestion in bivalves are needed.

pH CONTENT OF GUT AND STOMACH

The digestive fluids found in the alimentarytract are acid. The most acid conditions exist inthe stomach (average pH 5.5) due to the dis­solution of the crystalline style, which has a pHof 5.2 (5.4 in starved animals), and, accordingto Yonge (1926a), is the most acid subetance in thagut. In the absence {)f the style the pH of thAstomach fluids increases. This has been demon·strated on oysters with clamped sheila, kept for 6days out of water. Under these conditi()~ thepH of the stomach rose from 6.67 to 6.14 while thepH of the liquid in the mantle cavity decreaseddue to the accumulation ofearbon dioxide from6.7 to 6.14. It is significant that the acidity inthe stomach caused by the dissolution of the styleapproximates the {)ptimum (pH 5.9) for the actionof the style's amylase. The pattern of·. pH dif­ferences in various parte of the ~ntary tractslUI shown by Yongeis as foBow: eeophagus 5.6­6.0; stomach 5.4-~:6; style 5.2j mi4gu~ 5.5-6.0;rectum 5.8-6.3. The pH of the' extracts of thestyles of O. wgi:n.w,determined "bT"plaeing the

.J'ISliI.'~ ..'~ ".f.DVICE

stylea acroBS the one-drop electrode, was found byDean (1958) to vary between 5.8 and 6.0.

Extracts of the digestive diverticula of O.edulis have pH values from 5.6 to 5.9; the varia­tions are probably associated with the resting andactive stages of digestion. The styles of G.virginica contain a heat-labile substance, probablyan enzyme, which has the ability to attack certainalgal cells only during the dissolution of the styleor within a very short period after the dissolution.This has been reported by Dean (1958) whoobserved rapid disintegration of Oryptomonas cellsin buffered sea water (pH 6.0) containing styleextract. Monochrysis sp. were immobilized bythe extract while Isochrysis sp. were not affectedand were able to swim near or even touch thestyle. Dean thinks the "enzyme" may be aprotease, lipase, or amylase. The observed resultsmay be interpreted as the differences in the reaist­ance to digeation by different species of algae usedby the oyster as food.

It has been pointed out in support of the im­portance of extracellular digestion that fragmentsof partially disintegrated large diatoms (Goscin­odiscus, Melosira, Skeletonema) are frequentlyfound in the stomachs of O. virginica (Nelson,1934), but the question of the significance ofextracellular digestion in bivalveB has not beensettled. Weak proteolytic action was found inthe stomach of the giant clam, Tridacna, thepearl oyster, Pimtada (Mansour-Bek, 1946,1948), and in the crystalline style extract of O.virginica (Chestnut, 1949) and strong amylo­lytic activity in the stomach of the oyster wasdemonstrated by a number of investigators.Oysters apparently have a great capacity toutilize materials rich in carbohydrates.

ABSORPTION OF FOOD BY GILLS ANDMANTLE

The idea that the exposed surfaces of bivalves,particularly the gills, palps, and mantle, absorbthe organic matter dissolved in sea water (Ranson,1926, 1927) is not substantiated by experimentalevidence. In experiments with O. eduli8 Yonge(1928) has shown that the oyster a.baorbs glucosefrom the Witter but that this absorption takesplace through the alimentary eanaland digestivediverticula. No absorption W8$ rooordedin .tooanimals in which the access of water to the eSopha­gus was prevented by stuffing the InOuth with waxplugs. Glucose may be absorbed, how6ver,by

ORGANS OP' &!GE8TION AND .FOOD OF THE OTSTZR

the phagocytes which accumulate on the surface ofthe mantle. The results of Yonge's observationswere confirmed by Koller (1930) in his experimentswith MytUU8 edulis and Mya arenaria.

Since phagocytes normally aggregate on thesurface of the mantle and gills, it is possible thatthe oyster may absorb the substances preeent inthe surrounding media by means of these wander­ing cells. Yonge admits this possibility in the caseof oysters fed iron saccharate, and I observed thatthe particles of iron oxide added to the water inwhich I kept G. mrginica were ingested by thephagocytes of the gills and transported to thedeeper parts of the body.

FOOD AND FEEDING

The study of food of the oyster has attractedthe attention of many investigators who examinedthe stomach contents and recorded the variety oforganisms found in it. One of the earliest obser­vations was made more than a century and a halfago by Reade (1844, 1846), who was "induced" toexamine the contents of the stomach of Britishoysters and the "well known ciliary currents tnthe fringes of the oyster." His curiosity was wellsatisfied, for he found "myriads of living nomads,the Vibrio also in great abundance and activity,and swarms of a conglomerate and cilia.ted livingorganism, which may be named Volvoz ostrea.M1L8,somewhat resembling the Volvoz globator, but soextremely delicate a structure, that it mustbe slightly charred to be rendered permanentlyvisible." He listed also a number of commondiatoms, silicoflagellates, and desm.ids which hecalled "Infusoria." It is impossible to gueee thetrue identities of the "Vibrio" and "Volvox."

Since the oyster is a filter feeder it is. naturalto expect that the contents of its aIimentaryeanalwould reflect the material suspended in water.Many of the investigators were unduly impressedby the occurrence of one or several species in thestomach and because of their abundance consideredthem to be of primary importance in the oysterdiet. Opinions based on such examina.tions re­ferred to the fonowing forms found in the Europeanoyster as important food materials: NQ,~jv.Bijormi8 v. om-eari, Griin. (Puy~gur, 1884);desmids, minute animals, and dead organic matter(Hoek, 1883); bottom diatoms N~iapumtaUz.,N. acumiMta,. N. Bigtn4, GrammatlJphimJ o«anica,andDipl.oMis bomhus var.demutri.ata, the latterspeCies being considered ofspeeial importAMefOf

281

E 0 8

15.l<--'--~-"----'---':lbCenlimeteB

:.. A

o

FIGURE 21O.-Moore's method for washing out the stomachand intestinal eontent of the oyster. A-reeervoir withsea water" B-canule inserted in \he rectum; C­aspirator; D-collecting vessel; E-canu1e inserted intomouth of the oyster; F-iliphon of the reaervoir; G­siphon of the aspirator; O-Oyster. Upper insert 0­shoWl! the detailil of the method of itlllerting canules Band E into the oyster.

G

was made by Savage (1925), whose work remainsthe most valua.ble contribution to the study ofthe problem. He used Moore's (1910) method ofwa.shing the entire alimentary canal; this techniqueis diagramma.tically shown in figure 210. Twocanulae are introduced, one into the anus, B, andthe wider one, E, into the mouth. Rubber tubingconnects the anal canule with the siphon F in­serted in glass container A filled with sea water.The oral canule leads to a smaIl collecting veeeelD which is connected to the aspirator bottle C., .By regulating the flow of water from the a.spJte.torC the alimentary canal ma.y be washed out withoutdamaging the digestive tract. The volume of thecollected material is measured ILIld the collectedmicroorganisms identified ILIld counted. By thismethod Sa.vage (1925) sampled at regular intervalathe stomach contents ofBritish oysters and analyzedthroughout the year the aeasooal. llucluations inthe a.bunda.nce of different epecles ofa.lgae.Heconsidered that the following ·di.ms were ·themost important food items of the British (Oxford)oysters: NitzscAWlla:J'Gf'M, Pleur~8p"CWci.odisC1L8 sp., ~i4 &p., wM~ sp~

The most signifiea.nt eoncluaion ..mad8by&n.geis that the grea.terputOfthefood fo1md.m.tM •.oysters examined. by .mm eonSi8ied·ef:arcaDk.~

J'l8HAHDW'lf••'~·232

fattening of oysters (Rinard, .192~). A:ne.n~biologists made similar observations m C. vtrgm?Ca.McCrady (1874) concluded that "diatoms ~dspores of algae" constitute the food of Carolinaoysters; Lotsy (1893) found that in th~ JamesRiver, Va., "oyster lives almost exclUSively ondiatoms"; according to Smeltz (1898), the naturalfood of Florida oysters "can be supplementedby . . . the pollen of our pine trees an~ the bloomof our palmetto", (p. 307) but no eVIdence waspresented that pollens were found in the stomachsor that they can be digested by the oysters. Theflourishing and fattening of oysters in DelawareBay was attributed by Nelson (1947) ~o the abun­dance of the diatom Skektonema, which he called"the most valuable of all diatoms in the food ofoysters in New Jersey waters." In an earlierpaper (1923b) and in the Report of the NewJersey Agricultural Experiment Station for theyear 1924 (Nelson, 1925), he emphasized thesignificance of nannoplankton which "comprise byfar the largest part of the food of the oyster" andat times is composed of small fla.gellates and otherminute forms which may comprise 80 to 90 percentof the stomach's contents. Since no planktonanalysis was IIl8de by Nelson of the Delaware Baywater at the time of the Skektonema bloom, theconclusion that the species is "the most valuable"requires corroboration.

Moore (1910) found tha.t eight species of diatomsconstituted 98 percent of the total amount of foodin the alimentary tract of TexB.8 oysters and thatorganic detritus also might play ~n important p~rtin nutrition. Experimental studies of the feedmgof oysters made by Martin (1927b) showed nosignificant differences between the average in­creases in size of young oysters which were fedpure cultures of the diatoms-Nitzschia palea,Amphora cojJeaejormis, Nitz8chiapoleaceae ,AmplwracojJeatjormiB va.r. linea.ta, and one species of greenalga, Gloeocystis vuiculosa. No check was made onthe amount of food added to the water and theexperiment lasted only 4 weeks. Water waschanged only once during this period. Because ofthe obvious deficiencies in the experimentaltechnique no definite conclusions could be madefrom these observations. Martin also suggestedthat zoospores of Enteromorpha and other algae(VItia, Monostroma, Ectocarpus, and PylaieUa)form an important element in the food of planktoneaters (Martin, 192780). A comprehensive in­vestigation of the food of the Europea.n oyster

detritus and that "the animate food (i.e., livingmicroorganisms) never exceeded 10 percent of thetotal" (by volume). Be also advanced a hypoth­esis wmch, however, lacks experimental con­firmation, that growth of Oxford oysters was duemainly to the inanimate food (detritus) and thatfattening was caused by diatoms (Nitzschiellalongissima f. parrJa). Be found no evidence ofselection of food by the oysters and commentedthat the actively feeding oyster appears to ingestanything that it can capture.

The extreme .view that phytoplankton is of nodirect significance a.s food of O. edulis in Danishwater wa.s expressed by Blegvad (1914), whoclassified this mollusk a.s a "pure detritus eater."Phytoplankton, according to his view, contributesto the food only as part of the detritus after thedeath of the algae.

Petersen and Jensen (1911) attributed greatimportance to eel grass, Z08tera, as a possiblesource of food for bottom organisms. On thebasis of their observations Sparck (1926) ex­perimented with O. edulis, which he kept in atank with sea water to which he added a liberalsupply of old brown Zomra. Examination of thestomach contents of these oysters showed manyspecies of 1ia.gellates and some .l<J8tera detritus,but the quantity of the latter was by no meansgreater than in the oysters from the naturalbottoms in the fjord. Decaying Zo8tera probablyfertilized the water and stimulated the growth ofthe plankton. Danish investigators emphasizedthe fact that pentosan released from the decayingZostera is a principal 90urce of organic food forbottom invertebrates. The substance is ap­pa.rently uBele88 to oysters because they are unableto digest it, as has been shown by Yooge's experi­ments (192611.). The question of the extent ofutilization by the oyster of the organic detrituswhich is always present in ita natural environmenthas not yet been settled.

Naked flagellates and infusoria are frequentlyfound in the contents of the alimentary tract.Under the influence of gastric fluids these formsare rapidly destroyed and, therefore, ~annot beenumerated with any degree of certainty. Thesanu~problem applies to the baCteria whieh rea~h

the alimentary e&nal. That. they may .playaconsiderable role in the reeding of lamellibranehsis indicated by: the experimeht8 ...Qf ZQB~ andLandon (1 937}, and·ZoBelI and Fe1th&m· (1938)1with theC&liforIUa mUM, ·Which wasIed known

amounts of red coccus and a spore-fonningLllocillus. Within 3 hours the mussel removedabout 200 million bacteria per 1 ml. of water.The microorganisms were actually ingested andafter 6 hours disappeared from the digestivetract. In 9 months the mussels which were fedred co(',cus gained an average of 12.4 percent, thebacillus fed animals gained 9.7 percent, and thefasting mussels, kept as controls, lost about 6.8percent. These experiments suggest an explana­tion of the observations by Kincaid (1938) thatoysters kept for several months in water withnothing to feed on except bacteria appeared to benormal and even increa.sed their glycogen content.Kincaid's experiment should, of course, be re­peated and the question of the role of bacteriashould be adequa.tely studied before a conclusioncan be made of their significance in the feeding ofoysters and other bivalves.

By feeding the oyster known concentrations ofcoliform bacteria, Galteoff and Arcisz (1954)found that 15 minutes after the start of additionof the culture the two oysters retained from 21 to49 percent of &cAeri8ehia. coli available in seawater. The accumulation of bacteria eoon reachedthe point at which no more microorganisms wereretained and the efHuent leaving the oysterscontained more E. coli than the surroundingwater. Retention and elimination of microorgan­isms are probably associated with the secretiona.nd discharge of mucus by the gill epithelium.These results confirm the previous observationsby Galtsoff (1928) that over 50 percent of thebacteria p888 through the gills and that only afraction of their total number is retained.

The organisms found in the stomach of theoyster reflect the composition of plankton andnannoplankton present in the surrounding water.Selection is made primarily by the size and shapeof food particles, although the ability of theoyster to discriminate between two suspensionsof microorganisms of different 0010rs but of thesame size W'&8 suggested by LooBa.noff's experi­ments (19-4:9). A more· d&t&iled study should bemade, however, before the discriminating abilityof the oyster is confirmed.

There·are several weaknesses COJJltn()n to all the6tOOieson the feeding of oysters. The conclusionsllt'e bI8ed on examinations of the contente of thestoma.eh and compositional feces without givingproper'·ooD8i~t~. 'to the. nutritive nlue ofdiiferimt'ronM andt~ir dige&tibility. The.8imple

test of feeding the oysters inert materials such ascarmine powder, carbonmdum, clay, pulverizedwilliamite, and colloidal carbon would show thatthese undigestible materials, if fed gradually andnot in excessive quantities, are swallowed and passthrough the digestive tract. The fluorescentmineral williamite, which I used extensively in mystudies, is particularly suitable for this purposebecause it permits easy detection of the mostminute granules of the mineral inside the intestinaltract or in the feces when illuminated by ultra­violet light. The fact that some of the micro­organisms found in the stoma.ch are not destroyedand can be recaptured alive in the feces has beenknown for a long time. The dinoflagellateProroeenfrum micans was seen by Blegvad (1914,p. 47) to pass unharmed. Living Chiarella andNitzschia ~ri'Um given to C. uirginica in largequantities can be recovered alive from the fecesand recultured (Loosanoff and Engle, 1947). Instudies of the effect of feeding oysters in the labora­tory I frequently used a light suspension ofFleishmann's yeast, and observed that such alarge number of yeast cells passed undigested thatthe feces acquired a milky color. Thus, thepresence of an organism or its remnants in thealimentary tract in itself is not a proof that it isbeing used by the oyster as food and that it hasnutritive value. Neither the enumeration of theorganisms found in the stomach nor the deter­mination of their volume gives satisfactory quanti­tative data. It is at present impossible to judgewhether, for instance, one cell of Cf}sci'TWdi8C'U8equals or differs in nutritive value from a singlecell of Pleuro3igma, Skekfffnema, Nituchia, orother forms. Information is lacking about thecaloric value and chemical composition of variousforms and, therefore, it is impossible to determinethe number tbat should satisfy the energy re­quirements of the oyster.

Through trial and error oyster growers knowthat certain grounds in their possession are partic­ularly suitable either for the growth or forfattening and conditioning of oysters for market.Sometimes a great difference in the productivecapacity of grounds may be found in the twoareas loca.ted. a short distance apart. In anecological survey of the bottom it is relativelyeasy to detect conditions which are UDBUitable forgrowth. It is, however, imposaible at present toevaluate the potential productivity on the bottom

234

because of the inadequacy of our knowledge of thenutrition of the oyster.

ARTIFICIAL FEEDING

SO far only a few experiments on artificialfeeding reported in the literature were suooeaefulin producing an increase in the weight of theoysters. As a rule oysters kept in the laboratoryshow lack of nutrition and die sooner or later.Better results may be obtained by keeping themin large outdoor tanks adequately supplied withsea water which has not been stored for anylength of time. Experiments by Martin (1927a,1927b) in feeding oysters with pure cultures ofplankton forms resulted in very poor growth.Sparck (1926), experimenting with Zostera as apotential food for the European oyster, emphasizedthe fact that oysters "may thrive, increase insize and even spawn in very small limited watervolumes without any renewal of water worthmentioning." Such conditions occur in the Nor­wegian oyster basins and in the French "parks"which, however, must contain "some sourceproducing nourishment in sufficient quality andquantity." This material presumably may derivefrom the organic detritus. He also reportsthat in his experiments the "development ofbacteria did not. seem in any way to hurt theoyster, rather the opposite."

A unique experiment, unfortunately not wellknown to biologists, was made by Gavard (1927,quoted from Korringa, 1949) in Algiers. He fedthe oysters an artificial detritus prepared froma.nimal and plant material and obtained an increaseof 15 kg. per 1,000 oysters per season. Korringastates that these results demonstrate the abilityof the oysters to grow without using living organ­isms as food. Without access to Gavard'soriginal paper it is impossible to judge if thedetritus was directly consumed by the oystersas food or whether it stimulated the growth ofbacteria a.nd nannoplankton.

Artificial enrichment of sea. water by addingcommercial fertilizers at one time seemed to bea simple answer to the problem of providing in­creased food supply to the oyster. To teet theidea a series of experiments was conducted in theBureau of CommercialFisheries Biological Labora­tory at Milford,Conn.., wbieh ret!lul~ in theinteresting discovery that an ei:CEl8l!live OOtlOentra­tion of microorganisms· (CAlM''' .SP'l Nitzschiaclo8Uri~m, P1'(ll'~ ~,.FAtglena,

viridis) adve~ aBectathefeec1i:Dg of:oystem

J'$H. :ANfJ..~.QlWICE

(Loosanoff and Engle, 1947). A large-scale"natural" experiment along the same line tookplace in Great South Bay where unbalanced fer­tilizatioil of sea water by manure from the duckfarms located along the banks of the bay boostedsuch reproduction of GhloreUa-like organisms thatthe heretofore prosperous shellfish industry of thebay suffered a serious setback (Redfield, 1952).

Nelson (1934) made a series of tests of severalsubstances as artificial foods for oysters. Heused corn starch, ground alfalfa, soybean meal,and ground meat of the king crab. It is not clearin hiB report if the criterion of results was theweight of the oyster meat. Nelson states thatonly with corn starch "was any success obtained."The details of these experiments have not beendisclosed.

In spite of doubtful results the artificial feedingof oysters appears to be a definite possibliity whichshould be carefully investigated. Since oystersare able to absorb glucose dissolved in sea water(Yonge, 1928), it seems desirable to explore morethoroughly this method of feeding. Furthermore,the diet of the oyster and the nutritive value ofdifferent diatoms and flagellates should be investi­gated together with the methods of their cultiva­tion. It is reasonable to expect that certain formsricher in protein, may be more useful for obtainingbetter growth of oysters; others, richer in car­bohydrates, may prove more valuable for theirfattening. Research along these lines offersmany interesting possibilities that may proveuseful in the artificial culture of oysters.

BIBLIOGRAPHY

ALLEN, WILLIAM RAY.

1921. 8tudie6 on the biology of freshwater mUBBels.Experimental studies of the food relations of certainUnionidae. Biological Bulletin, vol. 40, No.4,pp. 210-241. .

AWATI, P. R., and H. 8. RAt.1931. ·o,tTlla clU)ullata (the Bombay oyster). The

Indian Zoological Memoirs on Indian AnimalTypes, III. Methodist Publishing House, Luck­now, India, 10i pp.

BARKOrs, TdoDOJlE.1~90. 1£ stylet eriatallin dee lamellibranches.

Revue Biologique du Nord de m Fran1le, A:!1neeJ889. No.4, pp. 124-141; Ann6e 1889, No. 5,pp.161-169; Annoo 1889. No.7, pp. 2e3-271; Allnee1890, No.6, pp. 200-226; Aalloo 1S9O, No. 8, pp.299-311; Annee 1890, No.9J pp.~1__357.

BmtxELJ;Y, GrElL.1923. On the oryata.Wne atyle JIS a P:Ot'JI'ible r~e~

in the llllaerobici· I't'J8piraUoi) of ~. marine

mollusks. Journal of Experimental ZoOlogy. vol.37, No.5. pp. 477-488.

1933. The o:ridase and dehydrogenase systelD8 of thecrystalline style of Mollusca. Biochemical Journalvol. 2i, part II, pp. 1357-1365.

1935. The chemical composition of the crystallinestyle and of the gastric shield: with BOme new obser­vations on the oocurrenoe of the style oxidase.Biological Bulletin, vol. 68, No.1, PP. 107-114.

BLJCGVAD, H.1914. Food and conditions of nourishment among

the communities of invertebrate animalB found onor in the sea bottom in Danish waters. Report of theDanish Biologioal Station to the Board of Agricul­ture, 22. pp. 41-78.

CHESTNUT, A. F.1949. Some studies of the digestive enzymes of the

oyster (06treD. virginica). Journal of the ElishaMitchell Scientific Society, voL 65, No.2, pp.215-216.

COLLIER, ALBERT, SAIIMY RAY, and WATNIl MAGNITSK:Y.

1950. A preliminary note on naturally occurringorganic substances in sea water affecting the feedingof oysters. Scienoe, vol. 111, No. 2816, pp.151-152.

DAHMEN, PETIlR.

1923. Anatomie von Ol/treD. cAilemill Philippi. Jen­aieche Zeitschrift fiir Naturwiasenschaft, Bank 52,pp. 575--626.

DEAN, DAVID.

1958. New property of the crystalline style ofCra3306tTeD. lIirginica. Science, vol. 128. No. 3328,p.837.

DOLLEY, CHAIlLES S.1896. The planktonolait, a centrifugal apparatus for

the volumetric estimation of the food-eupply ofoysters and other aquatic animals. Proceedingsof the Academy of Natural Scienees of Philadelphia,1896, part II. vol. 48, pp. 276-289.

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