41
1.2.5.1 Ovary Structure and Oogenesis
in Cheilostomes with Reproductive
Pattern III
The most important difference between the ovaries of species with reproductive pattern II and those with pattern III is
that the latter have far fewer cells. Further, an intraovarian
zone was not found in some species with pattern III examined by light microscopy. For instance, the central part of
the gonad in Bugula spp. contains the oocyte doublet(s),
being surrounded from above and laterally by a few oval or
fl at follicle cells (Fig. 1.18C, F ). The basal part of the ovary
is represented by a loose mass of oval or irregular cells (presumably including basal ones) underlying the doublet(s).
The lacunae of the intraovarian space are often indiscernible. To compare, Dyrynda and King ( 1983 ) described the
follicle cells of Bugula fl abellata as being differentiated into
a continuous layer of squamous cells and a small cone of
columnar cells at the onset of vitellogenesis. In this species
an ovary with a doublet containing a mature oligolecithal
oocyte was found only once (Fig. 1.18D ), with all other ovaries seen in sections containing one or two (in one instance,
three) small previtellogenic doublets (Fig. 1.18C ). It should
be added that Dyrynda and King ( 1983 ) described the
mature oocyte in this species as telolecithal (e.g. macrolecithal), corresponding to their published photograph (Plate
IIIg), while Reed ( 1991 , p. 134) characterized it as “small
mesolecithal”. The reasons for such differences are presently unclear, but may be associated with interpopulation
variability.
The structure of the ovary in those colonies of B . neritina
that were studied (Fig. 1.18F ) was identical to that in B . fl abellata . If more than one doublet was present in the gonad,
the latter was represented by a corresponding number of follicles, connected but spaced apart.
This type of ovary structure is also characteristic of Mollia
multijuncta , Cellaria fi stulosa (Fig. 1.19A ) and Adeonella
calveti . In contrast, the wall of the ovary containing a vitellogenic oocyte doublet in Gregarinidra serrata is more similar to that in the Calloporidae, being represented by oval or,
rarely, cubic cells. As a rule, fl attened cells form the follicle
roof, but are sometimes also found in the lower part of the
ovary (Fig. 1.17 inset). The narrow lacunae of the intraovarian zone were detected between the cells of the leading
oocyte doublet or between early oocytes. In both cases basal
cells of the intraovarian zone were found. If the ovary contained small previtellogenic doublets, its structure was similar to that in Bugula , whose ovaries consist of a few small
cells with a barely discernible intraovarian zone.
Finally, ovaries consisting of a few cells (as in bugulids),
but structurally similar to the ovaries of bryozoans with
reproductive pattern II (as in calloporids), were characteristic of the catenicellid Pterocella scutella (Fig. 1.24 inset) and
the urceoliporids Reciprocus regalis and Urceolipora nana
(Fig. 1.23A ).
The size of oogonia and oocytes in early doublets in species with reproductive pattern III is almost identical in species with pattern II (see Tables 1.3 and 1.4 ). The number of
oocyte doublets in the ovaries of most of the species studied
did not as a rule exceed two. In Bugula fl abellata the ovary
could contain up to three doublets and, in Gregarinidra serrata , up to six.
Data on the size of oocytes and nurse cells at the beginning of vitellogenesis were obtained only for Cellaria fi stulosa . In this species vitellogenesis starts when the oocyte is
45 × 25 μm in size and its nurse cell is 25 × 12 μm.
Mature oocytes (about to ovulate or ovulated) are oligoor mesolecithal (Figs. 1.17 inset, 1.18D and 1.24 inset).
Their size ranged from a minimum of 54 × 45 μm in
Reciprocus regalis to a maximum of 87.5 μm in Gregarinidra
serrata , comparable to cheilostomes with pattern I and to
some others with pattern II. Overall, mature oocytes in bryozoans with reproductive pattern III are much smaller than in
bryozoans with pattern II. The co-occurrence of more than
one vitellogenic doublet in the ovary was not recorded in any
species with pattern III.
Compared to bryozoans with reproductive pattern II,
oocyte volume increases by two orders of magnitude in species with pattern III. The minimum calculated enlargement
was 182-fold in Cellaria fi stulosa and the maximum was
578.7-fold in Gregarinidra serrata . In C . fi stulosa , oocyte
volume increased 9.3-fold during vitellogenesis.
My data on embryo enlargement in Bugula fl abellata
(6.3-fold) compare well with the 7.1-fold increase reported
for this species by Dyrynda and King ( 1983 ) (see Table 1.8 ).
Embryo enlargement during brooding ranged from a maximum of 257.7-fold in Reciprocus regalis and 310-fold in
Bugula neritina to a minimum of 4.9-fold in C . fi stulosa . The
signifi cant difference in these values indicates that matrotrophy may play a different role in embryonic development in
different species (see Sect. 3.3 ).
1.2.5.2 The Embryophore and Associated
Structures
In the great majority of gymnolaemate bryozoans the incubation cavity of the brood chamber is isolated from the
external medium (see Fig. 1 in Introduction). In cheilostomes the entrance to the calcifi ed protective ovicell is
normally plugged either by the contractile ooecial vesicle
or by the fl exible area of the distal wall of the maternal
zooid, sometimes with the aid of the zooidal operculum
(see Figs. 2.3 , 2.5 , 2.6a , 2.6b (A–D), 2.7a (A–C, F–I),
2.7b (A, B, F) and 2.8A–D, F ). Both the ooecial vesicle and
the distal wall are non- skeletal, whereas most of the ovicell
brood cavity is surrounded by a rigid calcifi ed wall
1.2 Reproductive Patterns of Bryozoa
1.2.5.1 Ovary Structure and Oogenesis
in Cheilostomes with Reproductive
Pattern III
The most important difference between the ovaries of species with reproductive pattern II and those with pattern III is
that the latter have far fewer cells. Further, an intraovarian
zone was not found in some species with pattern III examined by light microscopy. For instance, the central part of
the gonad in Bugula spp. contains the oocyte doublet(s),
being surrounded from above and laterally by a few oval or
fl at follicle cells (Fig. 1.18C, F ). The basal part of the ovary
is represented by a loose mass of oval or irregular cells (presumably including basal ones) underlying the doublet(s).
The lacunae of the intraovarian space are often indiscernible. To compare, Dyrynda and King ( 1983 ) described the
follicle cells of Bugula fl abellata as being differentiated into
a continuous layer of squamous cells and a small cone of
columnar cells at the onset of vitellogenesis. In this species
an ovary with a doublet containing a mature oligolecithal
oocyte was found only once (Fig. 1.18D ), with all other ovaries seen in sections containing one or two (in one instance,
three) small previtellogenic doublets (Fig. 1.18C ). It should
be added that Dyrynda and King ( 1983 ) described the
mature oocyte in this species as telolecithal (e.g. macrolecithal), corresponding to their published photograph (Plate
IIIg), while Reed ( 1991 , p. 134) characterized it as “small
mesolecithal”. The reasons for such differences are presently unclear, but may be associated with interpopulation
variability.
The structure of the ovary in those colonies of B . neritina
that were studied (Fig. 1.18F ) was identical to that in B . fl abellata . If more than one doublet was present in the gonad,
the latter was represented by a corresponding number of follicles, connected but spaced apart.
This type of ovary structure is also characteristic of Mollia
multijuncta , Cellaria fi stulosa (Fig. 1.19A ) and Adeonella
calveti . In contrast, the wall of the ovary containing a vitellogenic oocyte doublet in Gregarinidra serrata is more similar to that in the Calloporidae, being represented by oval or,
rarely, cubic cells. As a rule, fl attened cells form the follicle
roof, but are sometimes also found in the lower part of the
ovary (Fig. 1.17 inset). The narrow lacunae of the intraovarian zone were detected between the cells of the leading
oocyte doublet or between early oocytes. In both cases basal
cells of the intraovarian zone were found. If the ovary contained small previtellogenic doublets, its structure was similar to that in Bugula , whose ovaries consist of a few small
cells with a barely discernible intraovarian zone.
Finally, ovaries consisting of a few cells (as in bugulids),
but structurally similar to the ovaries of bryozoans with
reproductive pattern II (as in calloporids), were characteristic of the catenicellid Pterocella scutella (Fig. 1.24 inset) and
the urceoliporids Reciprocus regalis and Urceolipora nana
(Fig. 1.23A ).
The size of oogonia and oocytes in early doublets in species with reproductive pattern III is almost identical in species with pattern II (see Tables 1.3 and 1.4 ). The number of
oocyte doublets in the ovaries of most of the species studied
did not as a rule exceed two. In Bugula fl abellata the ovary
could contain up to three doublets and, in Gregarinidra serrata , up to six.
Data on the size of oocytes and nurse cells at the beginning of vitellogenesis were obtained only for Cellaria fi stulosa . In this species vitellogenesis starts when the oocyte is
45 × 25 μm in size and its nurse cell is 25 × 12 μm.
Mature oocytes (about to ovulate or ovulated) are oligoor mesolecithal (Figs. 1.17 inset, 1.18D and 1.24 inset).
Their size ranged from a minimum of 54 × 45 μm in
Reciprocus regalis to a maximum of 87.5 μm in Gregarinidra
serrata , comparable to cheilostomes with pattern I and to
some others with pattern II. Overall, mature oocytes in bryozoans with reproductive pattern III are much smaller than in
bryozoans with pattern II. The co-occurrence of more than
one vitellogenic doublet in the ovary was not recorded in any
species with pattern III.
Compared to bryozoans with reproductive pattern II,
oocyte volume increases by two orders of magnitude in species with pattern III. The minimum calculated enlargement
was 182-fold in Cellaria fi stulosa and the maximum was
578.7-fold in Gregarinidra serrata . In C . fi stulosa , oocyte
volume increased 9.3-fold during vitellogenesis.
My data on embryo enlargement in Bugula fl abellata
(6.3-fold) compare well with the 7.1-fold increase reported
for this species by Dyrynda and King ( 1983 ) (see Table 1.8 ).
Embryo enlargement during brooding ranged from a maximum of 257.7-fold in Reciprocus regalis and 310-fold in
Bugula neritina to a minimum of 4.9-fold in C . fi stulosa . The
signifi cant difference in these values indicates that matrotrophy may play a different role in embryonic development in
different species (see Sect. 3.3 ).
1.2.5.2 The Embryophore and Associated
Structures
In the great majority of gymnolaemate bryozoans the incubation cavity of the brood chamber is isolated from the
external medium (see Fig. 1 in Introduction). In cheilostomes the entrance to the calcifi ed protective ovicell is
normally plugged either by the contractile ooecial vesicle
or by the fl exible area of the distal wall of the maternal
zooid, sometimes with the aid of the zooidal operculum
(see Figs. 2.3 , 2.5 , 2.6a , 2.6b (A–D), 2.7a (A–C, F–I),
2.7b (A, B, F) and 2.8A–D, F ). Both the ooecial vesicle and
the distal wall are non- skeletal, whereas most of the ovicell
brood cavity is surrounded by a rigid calcifi ed wall
1.2 Reproductive Patterns of Bryozoa