Histological Studies on Albacore (Thunnus alalunga)

Gonads frorn the Eastern Pacific'

Br J. M. Penr,o2Pacific Biological Stati.on" Narwimo, B.C.

ABSTRACTSections of testes and ovaries f.rom 44 ffsh showed maturing individuals in age-groups

V and VI, but none in IV. There was no evidence of previous spawning.

INTRODUCTION

Pentlo (1954) identiffed modes in the length frequency of samples of albacorecatches taken in the Eastern Paciffc with age-groups III to VI by graphic analysisand by counting rings on vertebral centra. His age designations are used here.Further information on the life history can be obtained by an histological ex-amination of the gonads.

Some information is available in the literature concerning the seasonalchanges in the gouads of a number of species. Hann (1927) deals with those ofCottus and ]ames (1946) with the bluegill and largemouth bass.

DEVELOPMENT OF THE GONADS

Gonads from a series of 44 fish of ages IV, V and VI (Table VII, p. 56)were collected, ffxed in Bouin's fluid, sectioned and stained by routine haema-toxylin and eosin techniques. Sections selected from anterior, medial andposterior segments of each gonad were studied.

The gonad in both sexes is suspended in the posterior part of the body cavityby its mesentry and is associated with a fat body (Godsil and Byers, 1944).

Tnn Mals GoxeoThe male gonad is suspended by the mesorchium and its products are

collected by a series of small ducts leading to a large gonoduct (Fig. L-4). Thehomology of these ducts is at present uncertain but, on a functional basis, theywill subsequently be referred to as vasa efierentia and vas deferens respectively.

In each specimen examined an antero-posterior gradation in maturity couldbe observed. In all age-groups mature sperm could be observed.

In age-group IV (62.00 cm., Fig. S-7) spermatozoa could be seen in theseminiferous tubules only. In the anterior portion of the gonad spermato-gonial divisions only were noted.

In age-group V (70.5 cm., Fig. 8) spermatozoa were present in seminiferoustubules, vasa efferentia and vas deferens. In some specimens, however (74.0cm.), spermatozoa could be found only in the seminiferous tubules.

lReceived for publication July 19, 1954.2Present ad&ess: 104 Union Blvd., Kitchener, Ont.

J. Frsn. Rrs. Bo, ClNene, 12(1), 1955.Printed in Canada.

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In age-group VI spermatozoa were found in tubules and ducts in all casesexamined (Fig. 9-10).

From these observations it would seem that ffsh in age-group VI (78.S cm.)and some individuals in age-group V were in a condition approaching spawn-ing.

Trm Oveny

The female gonad is suspended in the body cavity by a mesovarium. It is ahollow structure and its lumen connects directly posteriad to a ttrick-walledgonoduc! subsequently referred to as an oviduct. Numerous ovigerous lamellaeproject into the lumen.

As in the testis, development of the gametes shows an antero-posteriorgradation.

- In all age-groups examined oocytes were present in various stages of de-velopment.

In age-group IV (61.0 cm., Fig. ff-15) oocytes varying in size fuom2.5pto52p in diameter are conspicuous. These contain some yolk. The nuclei are wellmarked and contain some basophilic aggregates, possibly nucleoU.

_ fn age-groyp V (69.f cm., Fig. 16-17) oocytes showing a size range of J:04pto 180p were observed. These possessed sheaths of follicular cells and were en-veloped in a zona pellucida. The amount of yolk present had increased and the

PLATE I

Figures 1, 2, I and 4 are photomicrographs of sections of successive segments from an-terior to postedor of-a testis from a specimen 70.5 cm, in length (age-group V). Hematoxylinand eosin. Bv, blood vessel; M, mesorchium; Psd, primary spermatic duct;-Spa, spermatozoa;Spd. Spermatids; Ssd, secondary spermatic duct; Sg seminiferous tubule; V, villur,

Frc. I Cross section of a segment showing a vas efierens duct containing spermia (top offfgure) opening into the vas deferens. Note vas deferens fflled with sp-ermatozoa, Thick-ness of section, 7p; magniffcation, 100y,

Frc, 2 Cross-section of a segment showing the vas deferens and the attachment of mesor--chium to testis. Note gerir cells near tie testis periphery are in earlier stages of growth

and maturation than those more interior, also that the walls of the vas deferens duct areslightly folded. 7p; LO}X.

Frc' 3 cross-section of a segment showing large secondary sperm ducts near periphery oftestis as well as tubules containing spJrmatigenic cells in'latter stages of dr"iopni"tt.Note folded Iining of the vas deferens with numerous blood vesseli in each fold. 7p;100x.

Frc.4 Cross-section of tle sixth segment (posterior segment) showing oval vas deferenscontaining many spermia. Note the large vasa efierentia fflled with spermia at thetop of the figure. fti; 100y.

Frc.5 cross-section of a mid-segment of a testis from a specimen 62.0 cm. in length (age-g-roup IV) showing numerous tubules containing spermatogenic cells in severalltages-ofdevelopment. Note that no spenn are evident in-&e vas defereru. 8p; 100X.

Frc' 6 -Anterior tiP of testis from 70.5 cm. specimen (age-group V) showing mature spermand spermatids. Note flagella bn spermia. 8p; 4b0X.

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accumulation of vacuoles in the cytoplasm suggested the presence of fat drop-lets.

In age-group VI (84.9 cm., Fig. 18) the ovary showed a similar degree ofdevelopment to that of ffsh in age-group V, but the ooqytes, on the basis ofcytoplasmic inclusions, appeared to be less mature.

The observations recorded above indicate that, as in the case of the malegonad, fish in age-groups V and VI were in a condition approaching spawning.

DISCUSSIONThe histological studies indicate that the British Columbia albacore ftshery

is dependent on immature ffsh. No fully ripe gonads nor any atretic follicleswere observed.

As noted above, individuals of age-group V are approaching maturity andsome of them may be expected to spawn during the current season. Other mem-bers of age-group V, however, are less mature and it is believed delay approach-ing maturity until the following season when they will have entered age-groupVI as estimated on the length of the individuals sampled. No individuals of age-groups older than those of group VI were taken and it is believed that individualsdo not re-enter the fishery after spawning.

ACKNOWLEDGMENTS

The author is grateful for the assistance of his colleague, Mrs. K. Herlin-veaux, in preparing sections, and for the criticism and general help of Dr. P.Ford of the Department of Zoology, University of British Columbia.

PLATE II

Figures 7, 8, 9 and 10 are photomicrographs of sections of testes from several specimens.Hematoxylin and eosin. Bv, blood vessel; M, mesorchium; Psd, primary spermatic duct; Sf,spernatozoa flagella; Spa, spermatozoa; Spm. spermatogonium; St, seminiferous tubule; V,villus.

Figures 11 and 12 are first of a series of 5 photomicrographs of sections from successiveparts from anterior to posterior of an ovary of a specimen 61.0 cm. in length (age-group IV).Hematoxylin and eosin. Ol, ovary lumen; Ot, oocyte; P, peritoneum.

Frc. 7 Spermia in a testis section taken from a specimen 62.0 cm. in length (age-group IV).8p; 1000X.

Frc. 8 Cross-section of mid-segment of a testis taken from a specirnen 74.0 cm. in length(age-group V) showing tubules containing germ cells in early stages of development.Some tubules contain spermia. 8p; I00X.

Frc, 9 Cross-section of mid-segment of a testis from a specimen 78.8 cm. in length (age-group VI) showing many tubules containing spermia, spermatids, and primary germcells, The vas efierens in the upper left corner of the figure is fflled witl spermia.

Frc. 10 Higher magnfication of section in Figure 9 showing spermatogenic cells in severalstages of development. Note groups of spermatozoa with fagella visible. 7p; 450X.

Frc, 11 Cross-section of a segment (anterior tip) showing a few ovigerous lamellae andoogonia in early stages of development. 7p; 100X.

Frc. 12 Cross-section of a segment. Oocytes in the growing stage are located near peripheryof each lamella and the central lumen of the ovary is lined with epithelial cells, Theperitoneum covering the ovary is evident. 8p; 100X.

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REFERENCES

Goosrr-, H. C., exo R. D. Bvrns. Ig44. A systematic study of the Paciffc tttnas' Cahfornia

Di.aision Fish and Game, Fish Bull., No. 60' 13f pp'

HaNN, H. W. 1927. The history of the germ cells oi-Couus bairili' Girard. I' Morphology'

42: 427498.

Jaurs, M. F. 1946. Histology of gonadal changes in the bluegill, Lepomis mactochirus Pra-"

ffrr"rq.r", and the largemiuth b"ass, I/uro sahnoid'es Lac6pdde' l' Morphology' 79: 63-91'

PLATE III

Figures 13, 14 and 15 are photomicrographs of three sections from further successive

,"g-"rri, from anterior to posteriior of an oiary from a sPecimen 6l'0.cm. in length (age-

grlup IV). Figures fO andi7 are photomicrogiaphs of mid-ovarian sections from a specimen

6g.0 cm. in length 1ug"_gro,rp v) iaptured in"August, 1g51. Figure 18 is a microphotograph

of a mid-ovarjan section'froio "'rp"ii*"n

84.9 clm. in length-(age-group -VI) "1Ptq".q t:

September, 1g50. Hematorylin and eosin. Ct, _connective tissue; Cy, cytoPlasm; Fc, follicle

celis; N, nucleus; No, basophilic aggregates; Od, oviduct; Osl' ovigerous lamella; Ot' oocyte;

P, peritoneum; Zp, zona Pellucida.

Frc. 13 Cross-section of a segment (medial) showing greater numbers of ovigerous lamellae

and a corresponding incre'ase in the number of growing oocytes.' 8p; 100X'

Frc. 14 Cross-section of a segment with many ovigerous lamellae and growing oocytes'

7p,; 100X.Frc. 15 Cross-section of a posterior segment shows the oviduct. Surrounding tissue is dense

collagen with an outer peritoneal sheath' 8p; I00x'

Frc. 16 Numerous growing^oocytes and one oocyte in an advanced stage-of growth' Note the

decreased thickness of connective tissue surrounding ova and the distinct follicule sur-

rounding the large ovum. 7P; 100X'

Frc. 17 EnLrged ,"ftion of nig*" 16 showing the characteristics of an ovum' Note the oval- --

,rrr"l"n, *iih l"rop}ritic agg"regates. Two la-yers of follicle cells surrgund the egg' A zona

f"rr""ia" may be ibr"r,r"i"irrt""rior from the follicular epithelium. 7p; 400x.

Frc. 18 Thin strands of "orrr,""ti,,"

tissue and growing ova-of various sizes may be observed'

8p ; l00X '

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