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A study of the sexuality and gonad development of the little neck clam, Paphia staminea Conrad Quayle, Daniel Branch 1938

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A Study of the Sexuality and Gonad Development of the l i t t l e neck clam., Paphia staminea Conrad by Daniel B. Quayle  A Thesis submitted for the Degree of Master of Arts i n the Department of Zoology  The University of B r i t i s h Columbia,  October,  I938.  Table of Contents. Introduction  1  Materials and Methods  ........... 2  Size and Age  ........... 4  Development of the Primary Gonad  ...........  Sexual Differentiation  ........... 7  Ovogenesis  ...........  Spermatogenesis  ........... 8  Seasonal Gonad Changes  7  9  Discussion  Summary  5  «c....».•..15  '  14  Bibliography  ...........15  Plates  .  B  ...16  A STODY OP THE SIXTTILIIT JHD GONAD DEVELOPMENT m THE LITTLE NECK CLAM, PAPHIA STAKHEA CONRAD  Introduction Sex i n molluscs has been studied by scientists since the end of the 1 7 t h . century, but practically a l l workers have used the genus Ostrea as the subject of their - studies. More recently, however, careful studies have been made of the sexuality i n species of various other genera of pelecypods, namely, Teredo, Venus, and Mya.  The object of choosing Paphia  staminea for this study was chiefly because of the interest being shown i n this species by the Fisheries Research Board of Canada, which has begun a program of research into the l i f e history and economics of this and several other species of clams.  Fraser and Smith  (I928)  have already done much of  the pioneer work on the ecology of this species and their studies have provided an excellent foundation for this and for future work. Sex i n pelecypods and gastropods seems to be i n a very unstable condition and sex changes often occur as a result of age or change i n environment. Some species undergo a prolonged and complex series of sexual changes while i n others the change i s reduced to a single reversal of sex.  Of the  former group Teredo and the native or Olympia oyster (0. lurida) are typical while Venus mercenaria i s an example of the latter group. Many of the species exhibit partial or complete protandry. The question of protandry i n this species along with the development of the primary gonad and gametogenesis were the main objects of this work. Acknowledgme nt s Part of the work was done i n the Department of Zoology at the University of British Columbia. The remainder was done as part of a program of Shellfish Investigation by the Fisheries Research Board of Canada.  The  writer wishes to express thanks to Dr.. W.A.  Clemens, Director of the  Pacific Biological Station, for permission to use in the thesis that part of the work financed by the Board. The subject was originally suggested by Dr. C.R. Elsey and his constant interest i n the work i s appreciated. Thanks are due Dr. T.L. Loosanoff of the U.S. Bureau of Fisheries and Dr. W.R.  Ooe of the Osborn Zoological Laboratory, Tale University, for  advice and literature. The help and advice given by Mr.  G.J. Spencer and Mrs. D.P. Watney  of the Department of Zoology i s deeply appreciated. Dr. A.H. Hutchinson and Miss M.A. Ashton of the Department of Botany also assisted. Mr, X.L. McHugh, Mr. W.M.  Cameron, Mr. J.K. Jacob, and Mr.  F. Neave assisted materially with discussion and criticism. Gratitude i s expressed.to Dr. C. McLean Eraserj Head of the Department of Zoology, for his constant encouragement, discussion, advice, and for many other favours.  Materials, and Methods Monthly samples of clams were collected from a small beach on the southern point of Woods Island i n Ladysmith Harbour, British Columbia, The; size of the sample of elams were rather small and for that reason some phases of the work, are somewhat incomplete.  However, more than 1,000 slide,  were prepared from nearly 800 elams. With the smaller individuals serial sections were made. Of the larger individuals only several sections were prepared.  I t must be pointed out that there i s a possibility of error i n  sectioning only a small piece of gonad, for different parts of i t may have  attained different degrees of development.  It i s often found that d i f f e r -  ent tubules i n the same section may have reached a different degree of development. Standard methods of histology were used. was used exclusively as fixative.  Bouin^s picro-acetic-formol  Except for one sample where Ehrlich''s  acid haematoxylin was tried, Haidenhain*s iron haematoxylin with f e r r i c alum mordant was used throughout. A saturated solution of picric acid was used for differentiation after iron haematoxylin i n several instances. Excellent differentiation was obtainedj the cytoplasm being l e f t quite colorless and the nuclei retaining the stain well.  I t i s d i f f i c u l t , however,  to get the correct end point due to the obscuring action of the yellow color from the picric acid. Tor the f i r s t few months the paraffin technique was used, with cedar o i l and xylol as clearing agents. Later the dioxan technique was tried, and i t was found t o be more satisfactory both from the standpoint of efficiency and time. The following i s an outline of the dioxan technique used in this work. 1. Fix i n Bouin's 6 to 12 hours. 2» Wash i n 50f alcohol 24 hours. 3.  Wash in ?0/. alcohol 24 hours.  4. Wash i n dioxan 24 hours. 5«  Wash i n dioxan 24 hours, (If time i s of consequence 6 hours-may be substituted for 24 hours, but the latter i s safer. Material may be stored i n dioxan.)  6. I n f i l t r a t e i n 1 / 2 dioxan and 1/2 paraffin one hour. 7. ? changes of paraffin at hourly intervals.  8. Imbed. (It was found that imbedding several pieces, of ' 'tissue i n a block at least two inches square, gave a better wax matrix with no bubbles such as i s the case when Imbedding each piece of tissue separately i n a small block) The following i s the procedure used with slides. 1. Dissolve off wax i n x y l o l . 2. 2 changes of dioxan. 3. Wash i n water.' 4. Mordant i n 2$ iron alum 12 hours. 5. Stain In iron haematoxylin 12 hours. 6. Differentiate i n if. iron alum. 7* Wash i n water. 8. 2 changes of dioxan. 9. Xylol. 10. Mount i n balsam. The small clams of which serial sections were to be made were fixed whole i n Bouin^s end the acid dissolved off the shell which otherwise i s d i f f i c u l t to dissect off without injuring the tissue.  Sections were cut  from 3 to 10 u. thick.  Size and Age Since Paphia spawns from early i n the year t i l l late in the summer, there w i l l be considerable variation i n the size of a group of clams considered as one year old.  It has been found that Paphia, i f spawned early,  may reach a size of .8 millimeters and probably more by the end of the same summer i n which i t was spawned. Others which have set late, are s t i l l very small when growth i s considered to cease. Thus clams spawned i n the summer  - 5 -  of 1938 may vary from 2 mm. up to 10 mm. i n the spring of 1939 just before growth i s considered to start.  For the purpose of this paper only sizes  w i l l be mentioned, for age has probably l i t t l e connection with time of maturity (Eraser and Smith,  1928).  At any rate the age after the time of  maturity i s of no importance i n this work. Development of the Primary Gonad The primary gonad begins as two primordia, one on each side of and immediately ventral to the pericardial cavity, i n the posterior part of the body. The same general position i s maintained throughout the development of the primary gonad and i t i s only after .sexual differentiation that the gonad extends to completely surround the foot and visceral mass of the clam. The f i r s t indications of the gonad are found i n individuals slightly less than two millimeters i n length. At this time they may" be only one or two months old i f spawned i n the same summer. On the other hand they may be six or eight months old i f spawned the previous summer. The primordia at this time consists of a tubular structure of germinal epithelium upon which i s l a i d a single layer of vacuolated f o l l i c l e cells.  The centre of  the tubule i s vacant. The primary gonad and kidney develop almost simultaneously and the ureters can be traced through the centre of each of the lobes to the approximate middle of the animal where they enter the suprabranchial chamber' on each side of the animal. Apparently the ureters serve as gonaducts.  The kidney duct i s differentiated by the compact layer of  c i l i a t e d epithelial c e l l s . In clams 3 millimeters i n length the gonad i s distributed vertically  between the upper and lower limits of the posterior adductor muscle. Horizontally i t i s distributed from the visceral ganglia, just i n front of the posterior adductor muscle, anterior about one third the length of the body. The lobes are joined at the antero-dorsal region of the gonad. Gradual extension of the gonad i s accomplished by the growth and spread of the germinal epithelium forward and ventrally. Meanwhile the interior of the gonad i s becoming more and more compact by the evaginatlon and growth of parts of the germinal epithelium to form what might be called f o l l i c l e c e l l "stalks", for the evagination of the epithelium carries with i t the layer of f o l l i c l e cells (plate i , f i g . 3 ) .  At the same time the  f o l l i c l e cells are multiplying and a number of layers are formed. Formation of profusely branching tubules i s f i n a l l y brought about by the growth and fusion of the f o l l i c l e c e l l "stalks".  This i s accomplished when the  animal reaches a length of between 1.5 centimeters and 3 centimeters} probably depending on environment, inherent tendencies and time of setting. It i s not u n t i l the complete formation of the tubules that sexual d i f f e r entiation into the male and female phases takes place. Thus a period of from two to three years i s required for the development of the primary gonad. The f o l l i c l e cells are purely nutritive i n function. The delicate c e l l wall encloses a thin layer of cytoplasm which i s often d i f f i c u l t to observe.  The central portion of the c e l l i s occupied by a large vacuole.  The comparatively small nucleus i s imbedded i n the peripheral cytoplasm. The cells differ much i n shape and reach a size of 30/a.  As gametogenesis  proceeds the f o l l i c l e cells are gradually replaced by the developing sex cells.  After spawning there i s a reformation of the f o l l i c l e cells i n the  gonad tubules,, although apparently never to the original extent.  '  Sexual Differentiation  , I t was soon found that this species i s not protandric  As soon as  the tubules are fully formed, indifferent gonia, the sex of which cannot be determined, arise from the germinal epithelium. Very soon, however, for the process i s almost continuous, they become differentiated into ovogonia and spermatogonia. The ovogonia are slightly larger than the spermatogonia, which measure, on the average, 8yu. The cellular structure end the coarsely granular cytoplasm of the ovogonia i s more clearly shown than that of the male c e l l .  The nucleus of the spermatogonium i s much larger i n proportion  and almost completely f i l l s the c e l l . The tubules become f u l l y formed when the animals reach a size of between 1.5 cm. and J.O cm. i n length, there being some variation.  Conse-  quently the size at maturity varies, the majority of the clams reaching that stage at a length of about 2.5 centimeters or slightly less, while others reach a length of 3«5 oentimeters before spawning.  This supports  the conclusions of Eraser and Smith ( 1 9 2 8 ) . Wo hermaphrodites were found. Ovogenesis After sexual differentiation or i n mid-winter after the hibernation period i n the case of the adult, the ovocytes begin to grow and enlarge u n t i l they are attached to the walls of the f o l l i c l e s only by a slender stalk. Vilhen they reach a size of between 55 / 1 . and 63 / 1 . they drop off into the lumen and are ready to be spawned. By the time the ova are ripe  and f i l l the extended tubules there are very few f o l l i c l e c e l l s remaining, having been used as nutriment for the developing ova. Besides the numerous ripe ova there are many small ovocytes and gonia s t i l l attached to the walls of the f o l l i c l e s .  These and the residual ova  are retained after the spawning act. Some of these gonia and ovocytes are the forerunners of the next crop of gametes. Spermatogenesis Due to the extremely small male reproductive cells of this species the exact details were not worked out. However, the process i n general can be outlined. After sexual differentiation, or i n January and February i n the case of mature clams, the spermatogonia l i n i n g the tubules grow and divide u n t i l a layer of spermatogonia several cells deep i s formed.  The primary  spermatogonia are usually found i n close contact with the germinal epithelium,, although some are found some distance away from i t . They measure approximately 8 /i. The nucleus almost f i l l s the c e l l and i s somewhat oval i n shape.  The chromatin, is/dispersed and frequently two nucleoli are  found. Often superimposed on the primary spermatogonia are gonia of the second order, whose round nuclei measure about 4 /i. i n diameter. Sometimes the secondary spermatogonia are found grouped or dispersed some distance i n towards the centre of the lumen. Indeed, this i s generally the ease with most of the male sex c e l l s .  They are seldom arranged i n the order of their  development, and may be found i n almost any position i n relation to each other. In some sections definite layers of sex cells i n regular order are found. This i s usually so i n the case of poorly nourished individuals where rather narrow columns of reproductive cells extend radially i n towards  the centre of -the.tubule from the germinal epithelium at various intervals along i t .  The spaces i n between the columns are f i l l e d with f o l l i c l e c e l l s .  The nucleolus of the secondary spermatogonium usually occurs close to the nuclear membrane. Further i n towards the centre of the tubule are groups of primary spermatocytes evolved by the division of the secondary spermatogonia.  The  primary spermatocytes are slightly larger than the secondary gonia. As mentioned above details of the transformation and division of the primary spermatocytes was not observed i n d e t a i l .  The secondary spermatocytes are  usually clustered together i n large numbers. Their nuclei measure 2 /i. i n diameter. The transformation of the secondary spermatocytes into spermatids takes place after the last division of the spermatocyte by the concentration of the chromatin of the nucleus into a solid, heavily staining mass. This then begins to elongate and assume the form of the mature spermatozo&n head.  The other details were not observed. The head of the  spermatozoan, 6 u. i n length, i s conical i n shape and slightly bent. Excepting a single layer around the germinal epithelium, the f o l l i c l e c e l l s are gradually crowded out and replaced by the sex cells which are f i n a l l y converted to spermatozoa, which align themselves i n various patterns, heads together and t a i l s together, i n the lumens of the spermary. Seasonal Gonad Changes The author hesitates to draw definite conclusions regarding this phase of the investigation because of the small number of clams studied. Naturally this phase of the investigation requires the examination of adult clams and as the primary purpose of the work dealt with immature individuals  and those just approaching maturity, the major emphasis was placed on these young stages to the neglect of the mature forms. Only 300 adult individuals were prepared histologically and i t i s from the study of this rather small sample the following interpretation i s made. Also the whole sample was taken from a single beach i n Ladysmith Harbour and consequently may not be representative of the typical sexual cycle of the species. However, the interpretation does f i t quite well the results of the plankton investigation and the study of the setting period i n Ladysmith Harbour.'  _  At any rate i t i s thought advisable to give an outline of the seasonal gonad changes as represented by the sample of 300 individuals.  I t i s hoped  to extend the investigation to include other areas and so form a representative sample from which more reliable conclusions can be drawn. September i s the last month of the year i n which spawning takes place so i t would seem logical to start a discussion of seasonal gonad changes at a point immediately after the completion of the sexual cycle. Females By the end of September the great majority o f the clams of this species have spawned or have partly spawned. Even with complete spawning there i s usually a considerable number of residual, ova l e f t i n the tubules of the ovary. In this discussion ova refers to ovarian eggs or more specifically, primary ovocytes. These residual ova may be extruded later; others undergo cytolysis. S t i l l others may be carried right through the winter and spawned the following summer with the new crop of eggs. This  latter event i s not of frequent occurrence however,, Usually along with the residual ripe ova are a considerable number of residual young ovocytes. After spawning the walls of the tubules, consisting of the fine germinal epithelium and connective tissue are wrinkled and collapsed, TriLth varying quantities of residual ova i n the lumens and young ovocytes and undeveloped gonia attached to the germinal epithelium. Soon after spawning proliferation of the f o l l i c l e cells begins. At the same time growth of new ovogonia i s started but ceases when the average size of 10 to 15 /x. i s reached. Growth, of the f o l l i c l e cells continues u n t i l the tubules are f u l l or nearly so.  This i s usually completed by December or January.  Beyond the short  growth period immediately after spawning, the ovogonia show l i t t l e activity t i l l late i n December, when, although water temperatures are low, they begin to.show active growth.  By February and early ^arch ovogenetic  activity i s at i t s peak. In January there i s a great deal of variation i n the progress of ovogenesis i n different individuals.  F o l l i c l e c e l l s partly  f i l l the lumens of the ovaries i n a l l females. Gonia of various sizes and order have appeared i n a l l while some contain ovocytes i n a l l stages. . The condition at the end of January i s l i t t l e different from that at the end of December. During February, however, ovogenesis proceeds rapidly and by early March the majority of animals have gonads in a ripened condition, with the lumens of the tubules distended with mature ova. During March the f i r s t indication of spawning i s found. These were partial spawners.  It seems that from histological evidence, the spawning  of an individual may last over a period of time such as i s the case of Yenus mercenaria (Loosanoff, 1937).  I t i s often found that large numbers  of Pacific oysters (0. gigas) may discharge only a part of their spawn,  although this i s not the general method. It may he similar i n the case of P. staminea. I t does seem, however, that the males expell the whole contents of the spermary at one time. By the beginning of September the majority, but not a l l , have spawned, for larvae i n considerable numbers are found at the end of September.  The  samples were not large enough to discover what happens to animals that spawn early i n the season. From the degree of gametogenesis discovered throughout the summer i t may be that a new crop of gamete i s formed and even spawned before the water temperature drops. Indeed Edmondson (1?23) claims two spawning periods i n a single summer. This i s so of the species Paphia philippinarum, a Japanese mollusc. Males The male reproductive cycle follows that of the female quite closely.. The recuperative period i s longer i n the male however, and they may go through the f a l l and part of the winter i n much the same state as they were immediately after spawning. Once spermatogenesis starts, however, i t proceeds rapidly and the tubules soon become f i l l e d with sex cells i n a l l stages of development. Immediately after spawning the tubules contain a number of spermatozoa held loosely i n the centre of the lumen. The germinal epithelium containing a few inert gonia i s collapsed. This appearance i s retained u n t i l January when proliferation of both f o l l i c l e cells and spermatogonia begins almost simultaneously. time.  The residual spermatozoa are maintained up to this  Their fate i s unknown. The f o l l i c l e cells increase rapidly and soon  f i l l most of the lumen. After the f i r s t crop of gametes have been produced  the f o l l i c l e cells do not seem able to completely f i l l the tubules. In poorly nourished individual groups spermatogonia appear at intervals along the germinal epithelium. Superimposed upon these are layers of secondary spermatogonia. Then above these again are spermatocytes of both orders and f i n a l l y spermatids undergoing spermeoteleosis. The centre of the lumens are f i l l e d with parallel rows or columns of spermatozoa with the spermheads ranged alongside each other, and the t a i l s intertwined i n the clear space between the columns or rows of sperm-heads. Well nourished i n d i v i duals show the tubules tightly packed and distended with reproductive c e l l s •in a l l stages of development and arranged more or less as indicated above. Spermatogenetic activity reaches i t s peak i n February and March a l though indications of active spermatogenesis i s found i n some individuals in each month of the year, especially during the summer. Spawning i n the male i s completed at one time. Morphologically mature sperms are found i n a proportion of the clams i n each month of the year. Whether or not they are physiologically mature can not be determined for the greater part of the year at least, due to the lack of mature ova. Discussion It i s interesting to find i n Paphia.staminea a gonad structurally similar to that of Mya. arenaria, a species i n a different family.. On the other hand i t i s also interesting to find t h a t ' Venus mercenaria, which belongs to the family Veneridae, as does Paphia, has a gonad struc-tuTQlly different from the latter species. The only way i n which the two seem to be a l l i e d i s . i n the development of- gametes during a time of the year when  the water temperatures are nearly at t h e i r lowest. - Mya arenaria (Family ffiyacidae), the sexuality of which was i n v e s t i gated" by Coe and Turner i n 1937, has the same branched tubular structure of the gonad, with the a l v e o l i f i l l e d with vacuolated f o l l i c l e c e l l s , as Paphia. Mya i s also generally unisexual w i t h no evidence o f prbtandry or change of sex. The gonad p r o l i f e r a t e s by means of a terminal syncytium f i l l e d w i t h large and small n u c l e i , which are destined t o become germ nuclei and f o l l i c l e c e l l nuclei respectively.  This syncytium pushes i t s  way v e n t r a l l y and a n t e r i o r l y among the v i s c e r a , leaving behind i t germ c e l l and f o l l i c l e c e l l n u c l e i on the walls of the tubules. No such structure or development was found i n Paphia, where germ c e l l s o r t h e i r nuclei do not become d i f f e r e n t i a t e d as/such t i l l the animals are about two or three years o l d . In-Mya and In the m j o r i t y o f pelecypods that have been -thoroughly studied, development o f the gonad usually takes place i n spring when the water temperatures begin to r i s e .  As has already been Indicated, gameto-  genesis i n Paphia occurs at a time when the water temperatures are nearly at their, lowest f o r the year.  This i s also true of Venus mercenaria, In  which the main period of egg and sperm production occurs i n the f a l l and early winter.  i  Both Paphia and Venus r e t a i n morphologically mature spermatozoa throughout the year.  Venus, however, shows p a r t i a l protandry and ex-  periences at least one change of sex. Summary 1.  The primary gonad o f the l i t t l e neck clam i s d i f f e r e n t from most  pelecyppds.„in, being composed of profusely branched tubules f i l l e d with vacuolated f o l l i c l e cells, whose function i s nutritive.  I t i s , however,  similar to that of Mya arenaria, a species of clam investigated by Coe and Turner (1937).  2. Growth of the primary gonad, which starts from two primordia, one on each side of .the body, i s by extension and growth of the germinal epithelium.  In the growth of the primary gonad of Mya arenaria, Coe, and  Turner found the process was carried out by means of a terminal syncytium composed of large and small nuclei, vjhich were destined to become germ c e l l nuclei and f o l l i c l e c e l l nuclei respectively. Ho such development was found i n Paphia. The growth of the gonad i n both species i s forward and ventral i n to the tissues surrounding the stomach and intestine. 3«  The gonia are not differentiated on the germinal epithelium u n t i l  the alveoli are fully formed. This takes place when the animals reach a size of between 1.5 and 3«0 centimetres. The indifferent gonia develop immediately into male or female cells, depending on the sex of the clam. The species i s generally unisexual; i t is not protandric and there i s evidently no change of sex. 4. The development of the gametes causes disintegration of the f o l l i c l e cells u n t i l they are completely replaced by sex c e l l s , , The f o l l i c l e c e l l s are reformed after spawning.5.  The spawning season i s a long one', lasting from early spring t i l l  early i n the f a l l . 6.  After spawning, i n the female there i s slight growth of the gonia,  but active ovogenesis does not begin t i l l December and January. with ripe ova are found i n February.  Females  In the male there i s very l i t t l e  - 16 -  spermatogenet'ic a c t i v i t y t i l l January and February, although some i n d i cation of spermatogenesis may be found i n some animals in each month of the year.  Peak a c t i v i t y i s reached in February and March.  mature sperms can be found throughout the year.  Morphologically  BIBLIOGRAPHY Amemiya, I., 1929. On the Sex-change i n the Japanese common oyster (Ostrea • gigas). Proc. Imp. Acad. Tokyo. 5: 284-286. Belding, David L., 1912. A report on the quahaug and oyster fisheries of • Massachusetts. Commonwealth of Mass.., State Printers, Boston, pp. 1-134. Burkenroad, M.D., 1931. Sex i n the Louisiana oyster, Ostrea virgin!ca. Science, v o l . 7 4 , pp. 7 1 - 7 2 . Coe, Wesley R., 1932. Development of the gonads and the sequence of the sexual phases i n the California oyster,(Ostrea lurida). B u l l . Scripps Inst. Oceanogr., Tech. Ser., 3: 1 1 9 » Coe, Wesley R., 1,936a. Sex ratios and sex changes i n mollusks. ' Mem. du Musee d'Hist. Wat. de Belgique. Deuxieme Ser. Fasc. 3 : 6 9 . Coe, Wesley. R., 1 9 3 6 b . Sequence of functional sexual phases i n Teredo. Biol. Bull., 7 1 : 122. Coe, Yfesley R., 1 9 3 6 c . Environment and sex In the viviparous oyster, Ostrea virginica. B i o l . B u l l . , 7 1 : 3 5 3 . Coe, Wesley R. and Harry J . Turner, Jr., I 9 3 8 . Development of the gonad and gametes i n the soft shell clam. (Mya arenaria). J. Morph., v o l . 6 2 , No. 1 , Jan. 1 . Edmondson, Chas. H. Edible MOllusca of the Oregon Coast. Occasional papers of the Berniee Pauahi Bishop Museum of Polynesian Ethnology and Natural History. Vol. VII, No. 9 , I 9 2 3 . pp. 179^-201. Honolulu. Eraser, C. McLean, and Gertrude M. Smith, 1928. Notes on the Ecology of the l i t t l e neck clam, Paphia staminea Conrad. Trans. Roy. Soc. Can. Ser. 3, v o l . XXIV, Sect. V., 1930. Loosanoff, Victor L.,  1936a.  Sexual phases i n the quohog. Science,  83:  287.  Loosanoff, Victor L., 1 9 3 6 b . Temperature and hibernation of the hard-shell clam (Venus mercenaria). Fish. Service B u l l . , No. 252: 4. Needier, Alfreda B., 1932. Sex reversal i n Ostrea virginica. Cont. Can. B i o l , and Fish. (Series A), 7 : 285. Needier, Alfreda B., 1 9 3 2 b . American Atlantic Oysters Change Their Sex. Prog. Kept. Atlantic B i o l . Sta. and Fish. Exp. Sta. No. 5: 3 , 4 . Orton, J.H., 1927. Observations and; experiments on the sex-change i n the European oyster ( 0 . edulis). I. Jour. Mar. B i o l . Ass'n. N.S..  14:  $67.  Houghley, T.C., 1933. The l i f e history of the Australian oyster (Ostrea commercialis). Proc. Linn. Soc. N.S.W., 38s 279.  Plate 1. Pig. 1. Gross section of young PapMa, 4 mm. long, snowing ths .. position and extent of the primary gonad. Int.-intestine, 1-lumen, f . o . - f o l l i c l e c e l l s , m-mantie, m.c.-mantie cavity, g~ganglion, h-heart, um-umbo. Fig. 2. Section of tho gonad of young clam 1.24 cm.in length. June. Shows the close connection between the kidney and the gonad. U-ureter, e. b . c. -suprabranchial chamber, g l . - g i l l , ot.~ • connective tissue, k-kidney. Other letters as i n Fig. 1. Fig. 3. Enlarged section of the gonad of Paphia 1.26 cm. i n length, showing the f o l l i c l e c e l l s on the stalks and on the germinal epithelium. F.c.s.-follicle c e l l stalk, n-nucleue. Fig. 4. Section of the gonad of clam 2 cm. i n length, showing the compact tubules separated by connective tissue and containing numerous f o l l i c l e c e l l s . Primary gem calls (p.g.c.) are . seen on the walls.  Plate 11.  Fig.l.  Individual tubules snowing differentiation of the primary germ c e l l s i n to female sex c e l l s , o.c.-ovocyte, pog.cprimary germ c e l l , f . c . - f o l l i c l e c e l l .  Fig.2.  A more advanced tubule than i n Fig* 1. showing the growing  Pig.3*-  Fig.4.  ovocytes-j  Section of the gonad of a male clara2 5 em. i n length* shows • the proliferation, of the gonia and the disintegration of the f o l l i c l e c e l l s . This i s tha f i r s t crop of gemotes produced by this clam. 0  Enlarged portion of the ovary showing- the ovocytes growing betwe  en the f o l l i c l e  cellso  Fig.5, Semi-diagramatic representation of spermatogenesis with the spermatozoa (spz.) i n the centre of the tubule, spt.-spermatid, spc.2„-secondary spermatocytes, spel-primary spermatocytes, spg.2.-secondary spermatogonia, spg.l.- primary spermatogonia*  Plata 111. Fig.l.  Sectiog. of the gonad of Paphia 1.26 cm. i n length,, showing the groth and solidification of the primary gonad by the formation and extension of the f o l l i c l e c e l l stalks and by  multiplicationof the f o l l i c l e c e l l s , f . c . - f o l l i c l e cells, f . c * s . - f o l l i c l e c e l l stalks, int.-intestine. X 100* Pig.2.  Primary gonad of a clam 2<,64 cm. i n length showing the structure of the completed tubules. There are a few gexm c a l l nuclei (p.g.c.) along the genainal epithelium. X 100.  Pig«.3«,  Sonad of adulr i aphia4ol6 cm. i n leagth i n December. Shows the small ovocytes of the new crop of gametes with the alv>  eoli partially f i l l e d with f o l l i c l e c e l l s . X 100. Fig.4.  Same as Plate 11., Fig.3,  ;Plata If, Fig.l.  Adult clam is August just after spawning and snowing the residual spermatozoa {spz.), debids, and the collapsed condition of the walls of ths tubules. X IOC.  Fig.2. Spa unary of a poorly nourished individual i n Karch. X 100. Fig.3.  Gonad of a male clam 3.0 em. i n length ahaowing nxaaerous sex cells in all phases of development, and the relatively few darkly staining spermatozoa in the centre of the tubule. S.c.-sex c e l l s , spz.-spermatozoa,  Fig.4. Spermary of a clam 5.38 cm, i n March. Spermatogenesis is almost complete i n this individual as shown by the very few developing sex cells and the great number of morphologically mature spermatozoa f i l l i n g most of the lumens • of the tubules. X 100.  Plate y, Fig.l.  Ovary of a clam 15,55 cm. in length i n August. "Partial spawning has just occurred and many relict ova ars left in the lumens. Some of the ovocyte's arc s t i l l attached to the walls. X ICO.  Fig.2.  Gonad of female Paphia in January. Shows the developing ovocytes and the lumens partly f i l l e d with f o l l i c l e c e l l s . X 100.  Fig.3.  Gonad of adult female i n November showing residual ova (r.oc.) Some of the new crop of goaia (ov.) and ovocytes (oe.) can be seen. The walls of the tubules are s t i l l somewhat collapsed, but ths new follicle cells have begun to proliferate . X 100. :  ixgo  4. Hipe ovary of adult Paphiashowing the numerous ripe ova tending ths walls of the follicles. Very few folloele cells are lest. Some of ths ovocytes are s t i l l quit© small, x 100.  PLATE  I.  PLATE  III.  PLATE  II.  PLATE  IV.  PLATE V.  

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