A Review of Oocyte Development in
Fishes with Special Reference to
Pacific Hake (Merluccius productus)
R.P. Foucher and R.J. Beamish
Pacific Biologicall Station
Research and Resource Services
Nanaimo, British Columbia V9R 5K6
December 1977
Fisheries & Marine Service
Technical "R eport No. 755
.
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Fisheries and Marine Service
Technical Report 75 5
December 1977
A REVIEW OF OOCYTE DEVELOPMENT IN FISHES WITH SPECIAL REFERE NC E
TO PACIFIC HAKE (MERLUCCIUS PRODUCTUS)
by
R. P. Foucher and R. J. Beami s h
Pacific Biological Station
Fisheries and Marine Servic e
Research and Resource Service s
Nanaimo, British Columbia v9R 5K6
- ii -
(c)
Minister of Supply and Services Canad a 1977
Cat . no. Fs 97-6/750
ISSN 0 701 -7 62 6
- iii -
ABSTRACT
Foucher, Ro P., and R. J. Beamish. 1977. A review of oocyte development in
fishes with special reference to Pacific hake (Merluccius productus).
Fish. Mar. Servo Tech. Rep. 755: 16 p.
Much of the literature relating to reproductive development of
female fishes was reviewed to examine the methods of oocyte development.
There appeared to be considerable confusion concerning the possible mechanisms
of formation of each year's supply of oocytes. Three theories could be
separated from the literature: (1) the annual division of oogonia by mitosis
to increase the numbers of potential oocytes (still diploid); (2) t r ansformation
of follicle cells within the ovigerous lamellae into oocytesj (3) the full
complement of oocytes is present at maturity.
The literature contains few references to meiosis which i s necessary
for the production of haploid gametes. Meiosis appears to begin early in
oocyte development and then is halted while the oocyte goes through its major
growth phases. The later maturation stages occur when the oocyt e is almost
fully grown and ready for ovulation. Of specific interest was the oocy te development in true hakes (Merluccius sp.) and especially the Pacific hake, tl. productus.
Fish in this genus, because of their commercial importance around the worl~have
been studied extensively. Some true hakes, it has been found, are capable of
multiple spawnings in a season. The relatively poorly studied Pacific hake,
however, seems capable of only a single spawning in a year and has a great
variation in the number of advanced oocytes produced. This raises the question
of possible environmental influences on fecundity. A subsequent paper will
report the results of a study based on the present review attempting to explain
the origin and fate of the small y olked oocytes remaining a f ter spawn ing, the
source of each year's supply of oocytes and relations to environmental fa c t ors
and recruitment.
Key words :
Merluccius productus, e gg s, ovaries, meiosis, f ecundity , recruitment.
RESUME
Foucher, R. P. , and R. J. Beamish. 1977. A review of oocy te development i n
fishes with specia l reference to Pacific hake (Merluccius productus).
Fi sh. Mar. Servo Tech . Rep. 755 : 16 p .
Les auteurs ont consult~ un g rand nombre de travaux sur I e d~veloppem u nt
reproducteur des poissons femelles, traitant de l a maturation des ovocytes.
lIs
ont co n s t ate qu'il existait une confusion co nsiderable lorsqu'il s'agissait
d ' exp liquer la provenance ann ue l le des ovocytes . Trois hypotheses etaient
e xp osee s d ans l es p u blica t ions :
(1)
Une mitose an n uelle des ovogonies
a u gmen ter ait le n o mbre d ' ovocytes en puissance (encore
l'etat diplofd e) ;
(2) Le s fo 11 ic u1 es d es o v igeres se tra n sformeraient en ovocytes; (3) 1 'ef[ ec tif
des ov o cytes serai t co mp l et
1a maturite .
a
a
- iv -
11 existe peu de documentation sur la meiose, pourtant essentiell e
l'ovogenese. On suppose qu'elle debute des le debut du developpement de
1 'ovocyte et qu'elle est interrompue au moment ou celui - ci entrepreRd ses
principales phases d'accroissement. La maturation definitive se produit
lorsque 1 'ovocyte s'est completement transforme pour l'ovulation. La maturati on
de 1 'ovocyte du merlu (Merluccius sp.), et notamment du merlu du Pacifique
(~. productus) presente un interet particulier.
Comme ils ont un e grande
importance c ommerciale dans le monde entier, les poissons de ce genre ont
fait l'objet de maintes etudes. 11 a ete etabli que certains merlus fraient
plusieurs fois dans la saison . Quant au merlu du Pacifique, il ne sembl e etr e
c apable de frayer qu'une seule fois par annee . En outre, chez la f emell e ,
Ie nombre d'ovocytes a terme varie considerablement. 11 se rait donc possible
qu e des facteurs du milieu influent sur la fecondite. Un rapport ulterieur
comminiquera les resultats d'une etude fondee sur Ie present compte rendu e n
vue d'expliquer l'origine et le devenir des petits ovocytes vitell ins apres I e
frai, la provenance annuelle des ovocytes, ainsi que Ie role du mi l i eu sur leur
re generation.
a
Mots cles:
Merluccius productus, oeu f s, ovaires, meiose, fecondile, recrutement.
INTRODUCTION
This study was initiated as part of an examination of the biology
of Pacific hake (Merluccius productus) in the Strait of Georgia and off the
west coast of Vancouver Island. Many female Pacific hake examined immediately
prior to and following spawning had ovaries that appeared to have retained a
variety of sizes of oocytes. It did appear that only a percent of the oocytes
in the ovary matured and were released to be fertilized and that this proportion varied considerably. The possibility that the number of oocytes released
to be fertilized might be dependent on SOme factor other than size and age of
the female and thus affect the recruitment of young into a stock, stimulated
an examination of the developmental stages of the ovary and oocytes of Pacific
hake.
This report summarizes many of the published concepts but it is not
to be considered an extensive review of world literature. A second report
describing the mechanism of oocyte development in Pacific hake and discussing
the implications of the method of oocyte production with respect to recruitment
will be prepared upon completion of the study.
In beginning this study we were amazed at the relatively poor understanding of oocyte development in fishes. Conflicting statements with respect
to mechanisms of oocyte formation appear to remain unsolved in the literature
and only a few species seem to have been examined in detail. Much of the
terminology in this field may be somewhat confusing owing particularly to its
frequent misuse. This problem of terminology and the complexity of the whole
process necessitates the inclusion of a brief general overview of oocyte
development, the main purpose of which is to clarify the meanings of the terms
used.
SUMMARY OF OOCYTE DEVELOPMENT
The process leading to the production of eggs in fishes (Fig. 1)
is quite complex and contains many areas of uncertainty. It begins with
oogonia, the cells originally derived from special primordial germ cells
formed early in embryonic development which~ in a very young ovary~ divide
many times to produce the vast numbers of young oocy tes present in a typical
immature ovary. The youngest oocytes are still diploid and may be referred
to as primary oocytes. The first stage of meiosis, the pairing of homologous
chromosomes (synapsis)~ occurs early and then meiosis ceases while the oocyte
grows (with a pause at the reserve fund size) to nearly its full size at which
the first meiotic division is completed~ resulting in a polar body and a
secondary oocyte. During this growth period the oocyte becomes covered by a
follicle similar to that found in mammalian ovaries which is composed of a
layer of granulosa cells involved in the nutrition of the growing oocyte and
the deposition of yolk~ and a connective tissue layer, the theca externa~
by which it is connected to the ovigerous lamellae. At ovulation the oocyte
bursts from the follicle and comes to lie within the lumen of the ovary, the
actual spawning being a separate event in which the ovulated oocytes are
released into the water. The secondary oocyte then undergoes the second
- 3
Sources of each
years sto ck of
Lifelong supply of
certain fol licle
cells
division of
residual oogonia
pre-synaptic
oocytes (afte r
ma t urit y)
THEORY 3
THEORY 2
Transformation of
TIlEOR Y 1
Annual mit otic
{
00gon1a present
at maturit y
oogonia
trans formed
foll ide cells
oogonia
\~-----;::
'Y"--------------------~)
different ia tion
1
pre-synaptic oocy te
1
pa iring of chromosomes - beginning of meiosis
synaptic oocyte ( 2 n )
1
1
separation of chromatids
early post-synaptic oocy te
addi tion of cytoplasm and oil globules
reserve-fund primary oocy te (may be retained
within reserve fund for use in later seasons)
!
growth and addition of follicle and yolk
yolked primary oocyte with follicle
~
secondary oocyte
/\
completion of first meiotic division
first polar body
secondary oocyte
with follicle
I
second
~c division
ovulation
empty follicle
"\.
\.
second polar
/\
body
third polar
body
R
Resorption
In a few fish, for
unknown reasons no
oocytes are released
and the whol e crop
of cells is resorbed
Spawning
I
(Fertilizat ion may
occur either before
or after the second
meiotic division)
""
disintegration
~
cells resorbed
hypertr ophy
post ovulatory corpora-lutes
Extra- ovarian
second
~c division
fourth polar body
(possible endocrine function)
A
Intra-ovarian
fertilization
fertilized secondary oocyte
hapl oid ovum
seco nd meiotic division
fertllbation
fertili zed hapl oid egg-cell
1
fourth polar body
{usi"n
zygote
Fig. 1. The stages of egg production in fishes including theories on
the source of oocytes which join the oocyte reserve fund each year.
(The reserve fund is a size-class of oocytes from which a proportion
are removed in each season.)
- 5 -
division of meiosis to produce an ovum which, by definition, is an unfertilized
haploid egg cell. Since fertilization (but not nuclear fusion) may occur in
one of the two oocyte stages, in some species an ovum does not exist and this
term, strictly speaking, should not be used.
The word egg is also a much
overused and thus poorly defined term. In general usage in this field
it refers to the female reproductive cells but with no reference to the
stage of development and thus includes everything from oogonia to fertilized
free-floating ·zygotes and later stages.
The empty follicle resulting at ovulation may simply disintegrate ,
its cells may be transformed into young oocytes, or it can change into a glandular structure of probable endocrine function similar to those found in higher
vertebrates and known as post-ovulatory corpora 1utea. Individual oocytes may
also be resorbed before ovulation and form pre-ovulatory corpora 1utea or
corpora atretica. There is also a possibility that oocytes take more than one
season to grow to maturity from the reserve fund stage in some fishes.
EARLY DEVELOPMENT AND ORIGINS OF OOCYTES
The development of the ovary and or1g1n of the primordial germ cells
of vertebrates is discussed by Brambe11 (1956) and Ba1insky (1975). The gonads
and the primordial germ cells are distinguishable very early in development.
These cells are formed in the entoderm and are distinguishable from the
surrounding cells by their much larger size and large nucleus with one or more
nucleoli (Brambe11 1956). The germ cells migrate from their point of origin
to the forming genital ridges in the embryo. Brambe11 (1956) discusses the
possible means of migration. Once they reach the genital ridges the
undifferentiated germ cells presumably transform to oogonia during the period
of sexual differentiation although there is some controversy on their fate
(Brambe11 1956 and Hann 1927). More detailed descriptions of aspects of the
primordial germ cells can be found in Ba1insky (1975) and Brambe11 (1956) .
It is developments from this pOint on that we are mostly concerned with in
this report.
Hickling (1935) uses the term oogonia to refer to cells still
capable of division to form new oogonia. "The end products of oogonial
divisions no longer divide, but are the starting point of oocyte growth or
maturation and are better themselves referred to as oocytes"{Hick1ing 1935).
He also includes within this term the follicle cells which, as he pOints out,
are sister cells of the oocytes and which Wheeler (1924) considers to be
capable of metamorphosis back into oocytes. In early immature ovaries nests
of oogonia and frequent mitotic figures indicate that rapid division is
occurring (Hann 1927).
The first indication of maturing in oocytes is a clumping of the
chromatin at one side of the nucleus resulting in cells in synapsis, marking
the onset of meiosis. Such cells are commonly visible in an immature ovary.
Following synapsis the chromatin threads scatter through the nucleus which
begins to increase in size (Hann 1927). The cell is now regarded as a primary
oocyte and still has a diploid number of chromosomes. Cells in synapsis are
commonly mentioned in descriptions of ovary development but such descriptions
- 6 -
do not elaborate how the synaptic stage fits into the process of
necessary for the production of haploid gametes.
meiosis~
GAMETE FORMATION BY MEIOSIS
Synapsis corresponds to the zygotene stage of the prophase of the
first division of meiosis (Table 1). During this stage the homologous
chromosomes come together to form the bivalents. In the next stage of the
first meiotic prophase (pachytene) the bivalents are duplicated forming the
tetrads. The tetrads then separate (diplotene) and the final stage~ diakinesis~
is then delayed until after the oocyte has grown to its final size (Balinsky
1975).
Therefore~ throughout the stages in which its major growth occurs~
the oocyte is in the late prophase of the first meiotic division and is still
diploid. During the growth of the oocyte its chromosomes take on the "lampbrush" configuration in which loops of the chromosomes are extended into the
nucleoplasm to facilitate the synthesis of messenger RNA (Balinsky 1975).
In the further maturation of the oocyte which proceeds at the
diakinesis stage of the first meiotic division the nuclear membrane breaks
down resulting in a mixing of the nucleoplasm and the cytoplasm. (It has been
found for SOme animals that this must occur before the egg cytoplasm can
respond to the spermatozoon.) A process of coiling and contraction results in
short~ thick chromosomes which are carried to the periphery of the oocyte.
Here the chromatids separate producing two sets of chromosomes~ one of which
enters the first polar body. The first polar body~ containing very little
cytoplasm~ separates from the oocyte which retains virtually all of the
cytoplasm. The resulting secondary oocyte then undergoes a second division
resulting in a second polar body and a fully mature~ haploid ovum. Meanwhile~
the first polar body may divide to produce two cells resulting in a total of
three polar bodies and one ovum (Balinsky 1975).
Yamamoto (1956)~ in his study on the formation of fish eggs~ found
a migratory nucleus stage in which the nucleus moved to one end of the egg
(animal pole) opposite the accumulation of yolk (at the vegetal pole) prior
to the nuclear membrane breaking down. The nuclear membrane disappears in
the pre-maturation stage with the diffused nuclear elements forming a
hyaloplasmic mass at the animal pole. At the maturation stage there are
unfused yolk globules within the hyaloplasm (cytoplasm which is hyaline in
appearance) near the animal pole and a continuous mass of fused yolk occupying
the opposite pole. A layer of hyaloplasm accumulates and covers the yolk mass
while the first meiotic division is occurring. The eggs are extruded from the
follicle at this point and remain in the lumen of the ovary (Yamamoto 1956).
No mention is made of the second meiotic division~ which results finally in a
ripe ovum although ova (unfertilized haploid gametes) may never be formed since
fertilization may occur in the secondary oocyte stage (Abercrombie et al. 1951).
- 7 -
Table 1. Summary of the characteristics and changes of oocytes of fishes
during meiosis.
Oocyte
Meiotic event
Prophase of first division
Leptotene
- pre-synaptic oocyte (diploid) - chromosomes
appear as a tangled mass of very slender
threads.
Zygotene
- synaptic oocyte - pairing (synapsis) of
homologous chromosomes resulting in formation
of biva1ents.
Pachytene
- biva1ents shorten and thicken and duplicate to
form tetrads.
Diplotene
- pairs of chromatids from homologous chromosomes
begin to separate from each other.
Meiosis halted
temporarily
- growth of post-synaptic oocytes including pause
in growth at reserve fund size - lamp-brush
configuration of chromosomes and production of
m-RNA.
Diakinesis
Metaphase and later stages
of first meiotic division
Second meiotic division
- chromosomes become short and thick and move
to periphery of the nucleus, nucleus migrates
to animal pole, nucleoli disappear, nuclear
membrane breaks down and a division spindle is
formed.
- produces first polar body and secondary oocyte
which retains all of the cytoplasm of the
primary oocyte.
produces second polar body and a haploid ripe
ovum.
- 8 -
YEARLY OOCYTE FORMATION
At all stages from late immature onwards and throughout the year
a "reserve fund II of oocytes is present in the ovaries of many species of fish.
The "reserve fund" consists of oocytes that have grown from the youngest
stage up to a size of 50-150 ~ (range varies from one species to another) at
which pOint they seem to cease development for a period before further growth
resumes in the season they are shed. A "reserve fund" class of oocytes has
been noted in many species including the European hake, tl. mer1uccius
(Hickling 1935), the mottled sculpin, cottus bairdii (Hann 1927), the
Atlantic herring, C1upea harengus (Bowers and Holliday 1961), and a species
of flounder, Liopsetta obscura (Yamamoto 1956). It is absent, however, in
species that spawn only once in their lifetime (e.g. sa1monids).
Hickling and Rutenberg (1936) examined the relation between oocyte
diameters in advanced ovaries and the length of the spawning season for
several species of fish. In those which have a short and well-defined
spawning period they found a very distinct separation between the size of the
maturing oocytes and that of the reserve fund stock from which they were
derived. The European hake (tl. mer1uccius) that they studied, which has a
long and indefinite spawning period, was found to have no such sharp separation in size between the general oocyte stock and the maturing oocytes, the
withdrawal from the reserve fund being a more continuous process. However,
we have found this not to be the case for the Pacific hake, tl. productus,
in which each year prior to spawning, a proportion of the reserve fund
oocytes begins to increase rapidly in size. Rann (1927) found this proportion
to be about 10% for the first spawning of cottus bairdii. Thompson (1914)
found, in the halibut, a reserve supply of oocytes of about 100 ~ which
numbered about 5 times as many as were spawned in a single season.
"A problem of great biological interest is the origin of the large
numbers of young oocytes that appear in the spent ovary. Few young oocytes
are to be seen in sections of ovaries containing eggs in an advanced stage of
maturation, yet, when these eggs are shed, the ovigerous lamellae at once
become full of young oocytes" (Hickling 1930). There are various explanations
for the occurrence of this event in fishes. The annual mitotic division of
oogonia and the transformation of the cells of empty follicles into oocytes as
well as the theory that their sudden appearance is an illusion are the means
proposed. The problem of oocyte production is controversial and can be
summarized into the following three theories. However, many authors express
uncertainty in their findings or doubt in the statements of other workers and
no one method of regenerating young oocytes has been established as the most
probable method used by most fishes.
THEORY 1 - OOCYTES PRODUCED BY ANNUAL DIVISION OF OOGONIA
Rann (1927) found that oogonial division and early maturation takes
place after the first breeding season of Cottus and that "each year a portion
of the oogonia differentiate into oocytes and increase the reserve supply and ,
at the same time, a portion of the reserve supply enters the secondary growth
- 9 -
phase and develops during the year into mature ova" (Hann 1927). Matthews and
Marshall (1956) stated that the period of oogonial division occurring shortly
after the breeding season as found by Hann for Cottus is also true of other
species. Braekevelt and McMillan (1967) said that "each succesive year's
batch of eggs appears to arise from the mitotic division of residual oogonia
which remain from year to year in the ovigerous lamellae." They saw mitotic
figures in these oogonia and their results for the brook stickleback are thus
similar to those of Hann.
These observations suggest that the mitotic division of oogonia and
their development into oocytes is the means by which the reserve fund of
oocytes is replenished after each spawning; however, there are also opposing
views. Wheeler (1924) found no trace of mitosis in the oogonia after maturity.
Hickling (1935) after finding no signs of mitosis in the mature ovary of the
hake at any stage said that "many workers have searched carefully for signs
of mitosis in spent ovaries, by which the new crop of young oocytes might be
regenerated, but without success" (Hickling 1935).
THEORY 2 - OOCYTES OF FOLLICULAR ORIGIN
Wheeler (1924), working on the dab pleuronectes limanda suggested
that the young oocytes which grow up to replenish the depleted reserve fund
are supplied each year by the transformation of certain cells remaining from
the empty follicles of the previous year's spawning. The follicular layer
surrounding the mature oocytes contains large, sharply staining nuclei and
after ovulation and the collapse of the follicle "it seems fairly certain
that a few of these cells develop into oocytes," The follicle cells are
sister cells of the oocytes, both being formed from germ cells produced in
the first 2 yr of life (Wheeler 1924),
Matthews and Marshall (1956) state that Bullough observed the
production of new oogonia in the minnow (Phoximus laevis) but was uncertain
of their mode of origin as he never saw them in division, He suggested that
new oogonia are supplied by direct growth of the follicle cells remaining from
the previous year's spawning. Yamamoto (1956) also supports Wheeler's view
and states that earlier workers claiming signs of mitotic figures in spent
ovaries of fishes have presented inadequate evidence. He found some large,
intensely staining nuclei together with the elongated follicle cell nuclei in
the empty follicles. "In some follicles these cells with sharply staining
nuclei are regarded as the youngest oocytes" (Yamamoto 1956),
THEORY 3 - THE "APPARENT" REGENERATION OF OOCYTES
Hickling (1930) supports Wheeler's theory of a follicular or~g~n
of oocytes as "possible and probable" but claims that there is a sufficient
number of undeveloped oogonia and young oocytes present in the ovary at all
times to account for the total number of cells added to the reserve fund
each year, thus supplying the adult female with oocytes throughout its
- 10 reproductive lifespan. According to Hickling (1935), "it can safely be said
that oogonial division is completed, in the hake, by the 4th yr of life
(when 3 yr old) and the young ovary then contains only vast numbers of
oocytes." The eggs which are ripening in a spawning cycle increase several
hundredfold in volume, and as a result, the remaining young oocytes are
spread out through a larger volume and appear greatly reduced in numbers.
"The sudden reappearance of young oocytes in the spent ovary, after their
apparent scarcity in the ripe ovary, is an illusion, consequent upon the
collapse of the ovigerous lamellae after the ripe eggs have been spawned or
resorbed" and thus "there is no need to look for a great formation of new
oocytes at the close of a spawning season" (Hickling 1930).
GROWTH AND DEVELOPMENT OF OOCYTES OF HAKE (MERLUCCIUS SP.)
As previously discussed, it was the purpose of this review to
examine oocyte development in fishes in general and more specifically in
hake of the genus Merluccius. In hake the majority of the oocyte's growth
takes place between the synaptic stage of the young oocyte and the commencement of the later stages of maturation which occur in the ripe ovary. The
histological changes of the growing oocyte and its associated follicular
structures can be described for hake and related to a classification of the
corresponding macroscopic characters of the ovary (Table Z) which is currently
in use by us.
The early immature ovary (11) contains large numbers of oogonia
and oocytes in the resting or net-like stage in nests beneath the surface of
the ovigerous lamellae (Hickling 1935). These numerous lamellae which are
folds of the ovarian wall each have a lining of germinal epithelium and a
rich vascular system to supply the follicles.
As maturation of the pre-synaptic oocyte commence~ a single
nucleolus appears, and the chromosomes beco.me more conspicuous with the cell
itself increasing considerably in size and attaining a thin envelope of
cytoplasm. Synaptic oocytes are very obvious due to their concentration of
short, thick chromosomes. post-synaptic oocytes soon attain a number of
nucleoli arranged around the periphery of the nucleus and quickly add
cytoplasm which becomes densely basophilic. A ring of oil globules appears
in the cytoplasm near the nucleus as the oocyte enters the reserve-fund
class (Hickling 1935)0
Each season a number of oocytes from this reserve supply commence
further development in preparation for spawning. The follicular layer
becomes apparent around these oocytes as they enlarge and sink into the
connective tissue of the ovigerous lamellae (Rl stage of macroscopic
classification). The cells do not stain as darkly due to their cytoplasm
being spread over the surfaces of numerous vacuoles (Hickling 1930). An
oocyte of about ZOO ~ in diameter has a well developed follicle and is
acquiring yolk droplets (3-6 ~ in diameter) and a chorion or oocyte cell
membrane (about 3 ~ thick).
- 11 Table 2. Macroscopic characters of different stages in the development of the
ovaries of the Pacific hake (Merluccius productus).
Stage
Macroscopic characteristics
Immature 1 (II)
- virgin; ovary small, light pink and semi-transparent;
no oocytes.
Immature 2 (12)
- same as above but some yolk less oocytes visible.
Riping 1 (Rl)
- ovary starting to enlarge up to 1/4 of volume of
body cavity and light yellow in colour; oocytes with
yolk and opaque; blood vessels on ovary pronounced.
Riping 2 (R2)
- ovary filling more than 1/3 of body cavity and
yellow; oocytes with yolk and opaque; blood vessels
pronounced.
Ripe (R)
- ovary filling 1/2 to 1/3 body cavity, yellow.
1 Ripe (lR)
- ovary fills 1/2 to 1/3 body cavity; translucent
yellow; less than 1/2 the oocytes are translucent.
2 Ripe (2R)
- ovary fills 2/3 body cavity, translucent yellow;
more than 1/2 the oocytes are translucent.
Runn~ng
ripe (RR)
translucent oocytes flow from vent with slight
pressure; ovaries almost fill body cavity; oocytes
loose in translucent ovary.
Spent (S)
- ovary bloodshot, purple in colour and flaccid; fills
about 1/3 body cavity; some translucent oocytes may
remain.
Resorbing (Resb)
- fish has not spawned; ovaries large, about 1/2 of
body cavity, soft and flaccid; oocytes are large
and watery.
Recovering (Rec)
- ovaries not flaccid or bloodshot, moderately firm,
filling less than 1/2 body cavity and returning to
pre-ripening size; oocytes small.
Resting (Rest)
- ovaries less than 1/4 body cavity, moderately firm
with white sheen to surface, not bloodshot.
- 12 The oocytes increase in size to about 600 ~ at a point equivalent
to the ripe stage described above. At this point the chorion is 12-14 ~
thick with fine radial striations; there is a very well-developed granulosa
layer and large yolk granules are beginning to form. The larger oocytes have
a more or less central oil vacuole with a diameter 1/4 to 1/2 that of the cell.
Shortly prior (lR and 2R stages) to their extrusion, the oocytes of
hake suddenly increase in volume by 3 to 5 times, apparently due to the
intake of fluid through the ovarian follicles. This uptake of fluids dilutes
the opaque contents of the cell making it transparent and greatly distending
the ovary (Hickling 1930). The eggs of the Pacific hake (~. productus)
average 1.12 mm in diameter with a single oil globule of just under 1/3 mm
(Hart 1973).
Variations in the size of oocytes in a very ripe ovary have been
noted for many hake species with some appearing to be so insufficiently
developed as to be unviable at the time of spawning. The causes for and
implications of these observations have yet to be explained.
Two modes in the sizes of yolked oocytes might suggest that hake
could spawn twice in 1 yr or that the smaller mode of oocytes might be
retained for the following season.
Ciechomski (1967) examining Patagonian hake (~. merluccius hubbsi)
in Argentine waters found a very long spawning season with mature specimens
present throughout most of the year and suggested that the hake could spawn
twice in a season under the right conditions. Hickling (1930) found hake
(~. merluccius) with ripe ovaries in every month except December although
the main spawning period is from April to July. Christiansen and Cousseau
(1971) also mention the very favourable conditions present for the hake in
Argentina (~. merluccius hubbsi) which could favour a second spawning and
point out this difference with the North Pacific~. productus which has a
comparatively short season. MacGregor (1966) noted that Pacific hake appear
to be very thin and in poor condition after spawning due to gonad development
exerting a considerable drain on body resources. "The short spawning season,
the relationship between eggs per gram of fish and percentage of yolked eggs
in an advanced stage of development, and the poor condition of the fish coupled
with the considerable volume of yolk material that would be needed to develop
a second spawning batch, indicate that the hake (~. productus) spawns once a
year" (MacGregor 1966).
In the Strait of Georgia in mid-March of 1975, 70% of a ge 4 and
older female Pacific hake were in the Rl or R2 maturity states and 4% were
ripe but not showing signs of oocyte release (Cass et ale 1977). In a cruise
starting in mid-June of the same year most of the females appeared to have
s pawned shortly before capture (Beamish et ale 1976) . Thus it appears that
the spawning season for hake in the Strait of Georgia is relatively short
lasting for no more than the 3 mo from mid-March to mid-June.
MacGregor (1966) looked at advanced ovaries of hake (~. productus
and found a great range in the proportion of small and advanced yolked
oocytes. Since he found such a large variation it would seem reasonable that
some spent fish would be found with large numbers of oocytes remaining and
others would be almost entirely spent.
- 13 Christiansen (1971) examined ovaries of hake (M. merluccius hubbsi)
at different stages throughout the year and found spent ovaries that were
totally spawned and others that had a great number of remaining yolked oocytes.
We have examined ovaries of Pacific hake from the Strait of Georgia
collected at most times of the year and at all stages of development and have
found considerable variability in the apparent number or percentage of oocytes
shed at spawning and in the size of the ovary relative to the adult. Some
ovaries have also been observed after the population had spawned which did not
release their contents and were full of large, watery oocytes (resorption
stage) in early stages of resorption. "The mechanics of ovarian development
of hake in the Strait of Georgia are still in question" (Beamish et ale 1976).
Our current but as yet incomplete investigations into the problem
of differential growth rates of Pacific hake oocytes appear to indicate two
modes of sizes of yolked oocytes separating from each other out of the single
mode present before the R2 stage and continuing until spawning.
A possible explanation for this phenomenon is that, at the beginning of each spawning season, the number of oocytes which commence development
from the "reserve-fund" size is very likely in excess of the number which the
fish will be able to bring to maturity but is enough so that if conditions are
very good in a season a sufficient number will have started development so that
a large crop can be produced. In the event that conditions such as food supply,
temperature or other physical factors are not very favourabl~ the growth of
some maturing oocytes may be slowed resulting in a proportion of them not being
ready for spawning.
In most fishes no large oocytes remain after spawning, however, the
common occurrence in Pacific hake of several modes of sizes of remaining
oocytes raises the question of their possible role, if any, after spawning.
By careful examination of post-spawned stages of ovaries that have retained
smaller oocytes it should be possible to follow their further development for
a follOWing year or their resorption. Other authors have made observations on
remaining unspawned oocytes of hake but little has been done on the Pacific
hake in particular.
MacGregor (1966) concluded that the remaLnLng small yolked oocytes
of Pacific hake are undoubtedly resorbed. Hoar (1955) points out that the
"resorption of ripening ova is a COmmon feature in teleosts" and that
"occasionally the whole batch of eggs may be resorbed" but the controlling
mechanisms for these events are not understood. Hickling (1930) observed
many oocytes being resorbed in ovaries of M. merluccius and regarded these
as ones which presumably ripened too late for spawning.
Both the pre- and post-ovulatory resorption of oocytes may be
related to endocrine functions. An oocyte can be transformed prior to
ovulation into a structure similar to the corpora lutea of mammals and may
be a site of estrogen formation although this is a subject of "lively debate"
(Hoar 1965). This takes place through the resorption by phagocytosis of the
oocyte and a rapid amitotic increase in the number of granulosa cells. Thecal
cells and blood vessels grow into the resulting structure which shows evidence
of hormone secretion (Hoar 1955). post-ovulatory corpora lutea may also
occur but these are Simpler, being derived directly from the hypertrophied
- 14 -
cells of empty follicles. The empty follicles more often rapidly disappear
with no evidence of hypertrophy (Hoar 1955).
Unspawned oocytes are resorbed in two stages. While still enclosed
by the follicle the oocytes cell membrane and nuclear membrane disappear, the
nuclear and cytoplasmic structure breaks dawu and the yolk is scattered in
large droplets throughout the cytoplasm. Finally, follicle cells transformed
into phagocytes invade the oocyte and effect its complete breakdown (Hickling
1930).
DISCUSSION
Since the true hake (Merluccius sp.) are of such worldwide
importance to commercial fisheries, a knowledge of the mechanisms of their
oocyte production, its relation to fecundit~ and possible effects on recruitment may be important to the mamagement of their fishery.
In particular we were interested in factors that might control the
fecundity of Pacific hake. Before concluding a study of such factors it was
apparent that the process of oocyte development would have to be described.
This review shows that, as with most biological studies, the mechanisms of
oocyte development in fishes are poorly understood. By reviewing the literature
we were not able to determine how or if new oocytes were formed once hake or any
other fish matured. We were able to determine that some true hake species have
multiple spawnings each year and most true hake species do not shed all the
oocytes in the ovary during spawning. We remain uncertain concerning the fate
of the oocytes of the various mode sizes found in the ovary after spawning.
Some studies clearly showed that the largest oocytes were resorbed; however,
the fate of the intermediate sizes of oocytes was uncertain. If these intermediate size oocytes that are common in some hake such as the Pacific hake are
not resorbe~ then it appears that oocyte development from the reserve fund size
requires at least 2 yr.
If individual fecundity is important to the size of a stock and if
fecundity is controlled either direc t ly or indirectly to some extent by
environmental factors such as temperatur~ then it is important to relate such
factors to the total period of development and not just to the conditions
present during the spawning year as is usually done in studies attempting to
estimate year-class strength. It has been reported that strong year-classes
exist for the Pacific hake (Dark 1975) and MacGregor (1966) has found
considerable variation in the number of advanced oocytes produced regardless
of size. This suggests that the number of advanced oocytes produced is not
entirely proportional to body size but is also related, at some pOint, to
environmental factors.
This first report summarizes relevant literature. A second report
describing oocyte production in Pacific hake will be produced upon completion
of the study.
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