22
the intraovarian zone now occupies not only the lower but
also the central part of the ovary. Ovulation of its predecessor
signals the transition to the growth and then vitellogenic
phase of the succeeding oocyte doublet (Fig. 1.5A, B ).
Ovary Structure and Oogenesis in Other Calloporids
The structure of the ovary and the character of oogenesis in
other calloporids (Figs. 1.7 , 1.8 and 1.9 ) correspond in general to the above description. Differences mostly concern the
position of the gonad, the number of oocyte doublets that are
formed and their size. Some of the fi ndings supplement the
above picture of ovarian function. For instance, in one of the
ovaries of Callopora craticula , two oocyte doublets of about
the same size were found. Cell size in the early vitellogenic
doublet (oocyte diameter 34 μm, nurse cell 30 × 25 μm) only
slightly exceeded the previtellogenic pair (oocyte 30 × 28 μm,
nurse cell 25 × 22 μm), implying the simultaneity or nearsynchronous development of two oocyte doublets in the
ovary at least up to the point of vitellogenesis.
Even while the leading oocyte doublet remains in the
ovary, younger previtellogenic doublets may grow without
vitellogenesis having started (Fig. 1.9A, C ). For instance, in
Tegella armifera the oldest previtellogenic oocyte may
achieve 55 × 42.5 μm prior to vitellogenesis. In T . unicornis
the largest previtellogenic oocyte found was 50 × 40 μm.
Thus, oocyte cell volume may increase as much as 91-fold
compared to the early oocyte. At the same time, following
ovulation of the leading doublet, oocytes may be considerably smaller at the onset of vitellogenesis. The smallest
vitellogenic oocyte found in this species was 40 × 37.5 μm,
its volume increasing only 58-fold by the beginning of
vitellogenesis. Thus, a necessary condition for the beginning
of vitellogenesis in a mature ovary is, besides fertilization,
ovulation of the leading doublet.
As with Callopora lineata , yolk granules were found not
only in the cytoplasm of the leading oocyte but also in the
nurse cell in Cauloramphus spinifer (Fig. 1.8C, D ). In most
oocytes of this species, granules of different size (from the
smallest to the largest) were arranged around the nucleus; in
some, granules were more or less evenly scattered throughout the cytoplasm. The same is true of the nurse cells, with
the distribution of granules in the cytoplasm of the sibling
pair being identical. Nurse cells can differ in their fate. In
some doublets they continue to grow, achieving a mean
diameter of 44.3 μm; in others their growth stops much earlier and their diameter does not exceed 30 μm.
In Corbulella maderensis the ovary was close to the polypide in all cases, pressed against the gut or the tentacle
sheath. At the same time, parts of the brown body were hard
against the ovary (as in Callopora lineata ). Insofar as the
zooidal cavity was relatively spacious, such close contact
between these structures can hardly be explained by the lack
of space. The association between the ovary and the polypide
may be a consequence of their mutual development (the
ovary, formed from the mesothelial cells of the polypide bud,
forever remains close to the latter) and their functioning
(transport of substances from the gut to the ovary via funicular cords). It is not known whether the products of polypide
resorption can be used for the needs of the ovary. Before the
new polypide begins to feed, the ovary should be sustained
by the transport of nutrients from neighbouring zooids. In
this case, it seems reasonable to suppose that the products of
resorption of the degenerated polypide might be used, but
further research is necessary to determine if this is the case.
1.2.4.2 Ovarian Structure and Oogenesis in
Other Cheilostomes with Reproductive
Pattern II
The structure of the ovary and the phenomenology of oogenesis in other studied cheilostomes with reproductive pattern
II are in general similar to those described above for
Calloporidae. Differences concern the position of the ovary
and the morphology and number of its cells, the number of
oogonia and oocyte and vitellogenic doublets, and their size
as well as the degree of enlargement of the female cells during their development (see Tables 1.1 , 1.3 , 1.4 , 1.5 , 1.6 , 1.7
and 1.8 ). The descriptions below provide a reasonably complete picture of reproductive pattern II within the order
Cheilostomata.
Structure and Functioning of the Ovary
The intraovarian zone is always confi ned to the site where
the gonad contacts the cystid wall (Figs. 1.10D , 1.11B, C ,
1.12C , 1.14C, D , 1.15A, B , 1.16C and 1.30A ). If the ovary is
suspended in the zooid cavity on funicular cords without
touching the wall, the intraovarian zone may be situated in
the lateral and, in rare cases, even the upper part of the gonad
(Fig. 1.13D ). In this case the position of the intraovarian
zone is determined by the site of its contact with the funicular
cords. Regardless of the location of the ovary, the vegetative
pole of the mature oocyte always adjoins the intraovarian
zone, whereas the animal pole is surrounded by the fl attened
follicle cells.
In Nematofl ustra fl agellata (Flustridae), the ovary is
located in the distal part of the maternal autozooid. In many
instances, most of the gonad was situated on the distal (transverse) cystid wall, whereas some peripheral areas were suspended in the zooid cavity on funicular cords. If the ovary
contains a vitellogenic doublet, the ovarian cells often form
a chalice-shaped structure. The foot of the “chalice” tapers
towards the base, while the bottom of the bowl envelops the
lower surface of the vitellogenic oocyte, which is in direct
contact with the intraovarian zone. The basal cells form a
complex three-dimensional network, fi lling the central part
of the ovarian “foot” together with the intercellular lacunae
(Fig. 1.12C ). The follicle cells surrounding the animal pole
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis
the intraovarian zone now occupies not only the lower but
also the central part of the ovary. Ovulation of its predecessor
signals the transition to the growth and then vitellogenic
phase of the succeeding oocyte doublet (Fig. 1.5A, B ).
Ovary Structure and Oogenesis in Other Calloporids
The structure of the ovary and the character of oogenesis in
other calloporids (Figs. 1.7 , 1.8 and 1.9 ) correspond in general to the above description. Differences mostly concern the
position of the gonad, the number of oocyte doublets that are
formed and their size. Some of the fi ndings supplement the
above picture of ovarian function. For instance, in one of the
ovaries of Callopora craticula , two oocyte doublets of about
the same size were found. Cell size in the early vitellogenic
doublet (oocyte diameter 34 μm, nurse cell 30 × 25 μm) only
slightly exceeded the previtellogenic pair (oocyte 30 × 28 μm,
nurse cell 25 × 22 μm), implying the simultaneity or nearsynchronous development of two oocyte doublets in the
ovary at least up to the point of vitellogenesis.
Even while the leading oocyte doublet remains in the
ovary, younger previtellogenic doublets may grow without
vitellogenesis having started (Fig. 1.9A, C ). For instance, in
Tegella armifera the oldest previtellogenic oocyte may
achieve 55 × 42.5 μm prior to vitellogenesis. In T . unicornis
the largest previtellogenic oocyte found was 50 × 40 μm.
Thus, oocyte cell volume may increase as much as 91-fold
compared to the early oocyte. At the same time, following
ovulation of the leading doublet, oocytes may be considerably smaller at the onset of vitellogenesis. The smallest
vitellogenic oocyte found in this species was 40 × 37.5 μm,
its volume increasing only 58-fold by the beginning of
vitellogenesis. Thus, a necessary condition for the beginning
of vitellogenesis in a mature ovary is, besides fertilization,
ovulation of the leading doublet.
As with Callopora lineata , yolk granules were found not
only in the cytoplasm of the leading oocyte but also in the
nurse cell in Cauloramphus spinifer (Fig. 1.8C, D ). In most
oocytes of this species, granules of different size (from the
smallest to the largest) were arranged around the nucleus; in
some, granules were more or less evenly scattered throughout the cytoplasm. The same is true of the nurse cells, with
the distribution of granules in the cytoplasm of the sibling
pair being identical. Nurse cells can differ in their fate. In
some doublets they continue to grow, achieving a mean
diameter of 44.3 μm; in others their growth stops much earlier and their diameter does not exceed 30 μm.
In Corbulella maderensis the ovary was close to the polypide in all cases, pressed against the gut or the tentacle
sheath. At the same time, parts of the brown body were hard
against the ovary (as in Callopora lineata ). Insofar as the
zooidal cavity was relatively spacious, such close contact
between these structures can hardly be explained by the lack
of space. The association between the ovary and the polypide
may be a consequence of their mutual development (the
ovary, formed from the mesothelial cells of the polypide bud,
forever remains close to the latter) and their functioning
(transport of substances from the gut to the ovary via funicular cords). It is not known whether the products of polypide
resorption can be used for the needs of the ovary. Before the
new polypide begins to feed, the ovary should be sustained
by the transport of nutrients from neighbouring zooids. In
this case, it seems reasonable to suppose that the products of
resorption of the degenerated polypide might be used, but
further research is necessary to determine if this is the case.
1.2.4.2 Ovarian Structure and Oogenesis in
Other Cheilostomes with Reproductive
Pattern II
The structure of the ovary and the phenomenology of oogenesis in other studied cheilostomes with reproductive pattern
II are in general similar to those described above for
Calloporidae. Differences concern the position of the ovary
and the morphology and number of its cells, the number of
oogonia and oocyte and vitellogenic doublets, and their size
as well as the degree of enlargement of the female cells during their development (see Tables 1.1 , 1.3 , 1.4 , 1.5 , 1.6 , 1.7
and 1.8 ). The descriptions below provide a reasonably complete picture of reproductive pattern II within the order
Cheilostomata.
Structure and Functioning of the Ovary
The intraovarian zone is always confi ned to the site where
the gonad contacts the cystid wall (Figs. 1.10D , 1.11B, C ,
1.12C , 1.14C, D , 1.15A, B , 1.16C and 1.30A ). If the ovary is
suspended in the zooid cavity on funicular cords without
touching the wall, the intraovarian zone may be situated in
the lateral and, in rare cases, even the upper part of the gonad
(Fig. 1.13D ). In this case the position of the intraovarian
zone is determined by the site of its contact with the funicular
cords. Regardless of the location of the ovary, the vegetative
pole of the mature oocyte always adjoins the intraovarian
zone, whereas the animal pole is surrounded by the fl attened
follicle cells.
In Nematofl ustra fl agellata (Flustridae), the ovary is
located in the distal part of the maternal autozooid. In many
instances, most of the gonad was situated on the distal (transverse) cystid wall, whereas some peripheral areas were suspended in the zooid cavity on funicular cords. If the ovary
contains a vitellogenic doublet, the ovarian cells often form
a chalice-shaped structure. The foot of the “chalice” tapers
towards the base, while the bottom of the bowl envelops the
lower surface of the vitellogenic oocyte, which is in direct
contact with the intraovarian zone. The basal cells form a
complex three-dimensional network, fi lling the central part
of the ovarian “foot” together with the intercellular lacunae
(Fig. 1.12C ). The follicle cells surrounding the animal pole
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis