74
the cystid wall (always associated with the funicular
cord(s)). Thus, it seems that both variants are possible, but
this requires additional study.
1.4.3 Ovarian Structure and Functioning
Ovarian structure has been poorly known in both
Ctenostomata and Cheilostomata until now. In cheilostomes
there are actually two cell groups (not including oogonia and
oocytes) constituting the ovary – peripheral (building the
ovarian wall) and subovarian (basal), that strongly differ in
morphology and presumably also function. These groups are
easily recognizable in many instances, and yet the majority
of researchers, although depicting basal cells, mentioned the
cells of the ovarian wall only. Ovarian ultrastructure is
described in only a few species and all but one of the descriptions are rather superfi cial. The function of ovary cells is
poorly understood. Some evidence exists for synthesis or
transport activities, or both, during vitellogenesis. However,
the number of species studied is so small that we have only
the most general idea about these processes. TEM studies are
urgently needed to create an integral picture of ovary structure and function.
1.4.4 Origin of Ovary Cells
Cells forming the ovary wall are described as originating
either from the peritoneum (most of the cases described) or
from a germ-cell cluster. In the latter case, the central cells
of the cluster differentiate into oocytes whereas the peripheral cells form the follicle (stated for Bugula simplex by
Calvet ( 1900 ) and suggested for Epystomia bursaria by
Dyrynda and King ( 1982 )). In Nolella dilatata , ovary cells
are neither mentioned nor depicted (Calvet 1900 ). In fact,
all three latter variants are poorly documented and should be
restudied. Differentiation of the basal cells of a subovarian
space is also obscure.
1.4.4.1 Timing of Sperm-Egg Fusion in Brooding
Ctenostomata
As to differences between ctenostome and cheilostome
fertilization, it seems that there is no early syngamy in
ctenostomes. In brooding Bowerbankia gracilis , syngamy
occurs before the breakdown of the germinal vesicle. Only
one late- stage ovarian oocyte per gonad contains a sperm
nucleus, and it was suggested that the rupture of the follicle cell layer might expose the oocyte to sperm (Temkin
1996 ). In Alcyonidium sp. and Nolella stipata sperm penetrates oocytes while they are still growing (Marcus
1938a ), but it is not clear from the description exactly
when. In contrast, the spermatozoid fuses with late-stage
ovarian oocytes following collapse of the nuclear membrane at or near ovulation in broadcasting Alcyonidium sp.
(Temkin 1996 ). Thus, it is still unclear what happens in
ctenostome brooders. Additional questions are associated
with the fact that the polypide degenerates during vitellogenesis in some taxa, prohibiting fertilization during polypide cycling. Sperm “capture” should then occur while the
polypide is still functioning and result in rather early
fertilization.
1.4.5 Placental Brooding
Reed ( 1991 ) suggested that EEN has evolved independently
numerous times within Bryozoa, but the mechanism and
structure of the embryophore have not been studied in most
matrotrophic species. Apart from three cheilostomes
(Woollacott and Zimmer 1975 ; Hughes 1987 ; Moosbrugger
et al. 2012 ), the fi rst ultrastructural confi rmation of EEN was
obtained only recently for one ctenostome (Ostrovsky and
Schwaha 2011 ). Cyclostomata and Phylactolaemata are
unstudied in this respect. In fact, the same holds true for the
most aspects of their sexual reproduction.
1.4.6 Origin of the Intertentacular Organ
Future studies in this area should focus on exploring the
hypothesis on ITO evolution presented above. Specifi cally it
would be useful to obtain more data on both brooding and
non-brooding gymnolaemates with uniserial colonies.
Further observations on the formation of the ITO and spawning in multiserial broadcasters, the presence of the SNP in
non-fertile zooids, anatomical research on any structural differences in the ITO between Ctenostomata and Cheilostomata,
as well as presence of the SNP in Cyclostomata, would be
particularly useful.
1.4.7 Dynamics of Colonial and Zooidal
Sexual Structure and Life Cycles
Most available data on the sexual structure of bryozoan colonies refl ects the colony state at the time of collection.
However, the sexual condition of colonies and zooids integrates short- and long-term external and internal processes
and states, including polypide recycling and colony longevity.
More comprehensive studies are needed to determine sexual
dynamics based on seasonal observations. Such studies would
have the additional benefi t of clarifying bryozoan life-cycles,
which are still poorly known .
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis
the cystid wall (always associated with the funicular
cord(s)). Thus, it seems that both variants are possible, but
this requires additional study.
1.4.3 Ovarian Structure and Functioning
Ovarian structure has been poorly known in both
Ctenostomata and Cheilostomata until now. In cheilostomes
there are actually two cell groups (not including oogonia and
oocytes) constituting the ovary – peripheral (building the
ovarian wall) and subovarian (basal), that strongly differ in
morphology and presumably also function. These groups are
easily recognizable in many instances, and yet the majority
of researchers, although depicting basal cells, mentioned the
cells of the ovarian wall only. Ovarian ultrastructure is
described in only a few species and all but one of the descriptions are rather superfi cial. The function of ovary cells is
poorly understood. Some evidence exists for synthesis or
transport activities, or both, during vitellogenesis. However,
the number of species studied is so small that we have only
the most general idea about these processes. TEM studies are
urgently needed to create an integral picture of ovary structure and function.
1.4.4 Origin of Ovary Cells
Cells forming the ovary wall are described as originating
either from the peritoneum (most of the cases described) or
from a germ-cell cluster. In the latter case, the central cells
of the cluster differentiate into oocytes whereas the peripheral cells form the follicle (stated for Bugula simplex by
Calvet ( 1900 ) and suggested for Epystomia bursaria by
Dyrynda and King ( 1982 )). In Nolella dilatata , ovary cells
are neither mentioned nor depicted (Calvet 1900 ). In fact,
all three latter variants are poorly documented and should be
restudied. Differentiation of the basal cells of a subovarian
space is also obscure.
1.4.4.1 Timing of Sperm-Egg Fusion in Brooding
Ctenostomata
As to differences between ctenostome and cheilostome
fertilization, it seems that there is no early syngamy in
ctenostomes. In brooding Bowerbankia gracilis , syngamy
occurs before the breakdown of the germinal vesicle. Only
one late- stage ovarian oocyte per gonad contains a sperm
nucleus, and it was suggested that the rupture of the follicle cell layer might expose the oocyte to sperm (Temkin
1996 ). In Alcyonidium sp. and Nolella stipata sperm penetrates oocytes while they are still growing (Marcus
1938a ), but it is not clear from the description exactly
when. In contrast, the spermatozoid fuses with late-stage
ovarian oocytes following collapse of the nuclear membrane at or near ovulation in broadcasting Alcyonidium sp.
(Temkin 1996 ). Thus, it is still unclear what happens in
ctenostome brooders. Additional questions are associated
with the fact that the polypide degenerates during vitellogenesis in some taxa, prohibiting fertilization during polypide cycling. Sperm “capture” should then occur while the
polypide is still functioning and result in rather early
fertilization.
1.4.5 Placental Brooding
Reed ( 1991 ) suggested that EEN has evolved independently
numerous times within Bryozoa, but the mechanism and
structure of the embryophore have not been studied in most
matrotrophic species. Apart from three cheilostomes
(Woollacott and Zimmer 1975 ; Hughes 1987 ; Moosbrugger
et al. 2012 ), the fi rst ultrastructural confi rmation of EEN was
obtained only recently for one ctenostome (Ostrovsky and
Schwaha 2011 ). Cyclostomata and Phylactolaemata are
unstudied in this respect. In fact, the same holds true for the
most aspects of their sexual reproduction.
1.4.6 Origin of the Intertentacular Organ
Future studies in this area should focus on exploring the
hypothesis on ITO evolution presented above. Specifi cally it
would be useful to obtain more data on both brooding and
non-brooding gymnolaemates with uniserial colonies.
Further observations on the formation of the ITO and spawning in multiserial broadcasters, the presence of the SNP in
non-fertile zooids, anatomical research on any structural differences in the ITO between Ctenostomata and Cheilostomata,
as well as presence of the SNP in Cyclostomata, would be
particularly useful.
1.4.7 Dynamics of Colonial and Zooidal
Sexual Structure and Life Cycles
Most available data on the sexual structure of bryozoan colonies refl ects the colony state at the time of collection.
However, the sexual condition of colonies and zooids integrates short- and long-term external and internal processes
and states, including polypide recycling and colony longevity.
More comprehensive studies are needed to determine sexual
dynamics based on seasonal observations. Such studies would
have the additional benefi t of clarifying bryozoan life-cycles,
which are still poorly known .
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis