6
female zooids are correlated with spawning and brooding,
and may involve the polypide, the cystid or both (Reed 1991 ;
Ostrovsky 2009 ).
Gonadogenesis and gametogenesis depend on the cycles
of degeneration and regeneration of polypides, on the age
and size of the colony and on environmental conditions.
The ancestrula (see Fig. 1.20C ) and several of the fi rstbudded (generations of) autozooids are sterile. Fertile
(ovary- bearing) zooids are predominantly found more in
the peripheral/distal part/area of the colony, sometimes
arranged in compact groups. In brooding species such
zooids can be easily identifi ed by the presence of embryos
visible through the semitransparent walls of skeletal
brood chambers (ovicells) or the frontal walls of the zooids
(in species with internal brooding). The distal/peripheral
location of fertile zooids appears to be associated with stages
of maturation of the colonies. Gonads develop in later zooidal generation, and, in species with seasonal reproduction,
the closer to the peak of reproductive season a colony is
established, the earlier it starts reproduction (see Ostrovsky
1998 ). In cheilostomes with frontal budding, fertile zooids
may be found in any part of the colony except the most
central/proximal region.
In some instances sexual zooids are formed only in certain places, which points to a high level of colonial integration. For instance, in free-living colonies of Selenaria
maculata (Selenariidae) male and female zooids are always
formed at or near the periphery of the colony (Chimonides
and Cook 1981 ). The position of male zooids around the
edge of the colony may be explained by the necessity of
sperm distribution, perhaps most effective in the peripheral
zone in relation to centrifugal exhalant water currents (see
Cook and Chimonides 1978 ) [although in bryozoans with
conical-discoidal colonies the main exhalant water fl ow is
apical (Cook 1977 ) and its suitability for sperm removal is
less obvious]. In contrast, female zooids are placed subperipherally, which may prevent them from receiving sperm
from the same colony (see also below). In the free-living
Cupuladriidae, which is similar to Selenariidae in overall
colonial morphology, the positioning of brooding zooids
with embryos in the central part of the colony correlates with
common peripheral colony fragmentation, and species with
colonies less likely to fragment preferentially brood at the
colony periphery (O’Dea et al. 2010 ).
It should be emphasized that seasonal observations are
essential for ascertaining the dynamics of sexual structure
(composition) of bryozoan colonies, since male and female
gonads may be absent or yet to develop at the time of sampling. For instance, if the colony under study consists of sterile and female zooids, it may be potentially represented by
any combination of four zooidal types (hermaphroditic or
gonochoristic, protandrous or protogynous), the observed
state indicating only that it is currently at the female phase of
the reproductive cycle. Differently aged colonies within the
same population may have different sexual structure.
My data and an analysis of the literature show that there
are four variants of sexual structure to be found in
Cheilostomata. The colony may consist of (1) sterile and hermaphroditic zooids, (2) sterile and gonochoristic (male and
female) zooids, (3) sterile, male and hermaphrodite zooids,
and (4) sterile, hermaphrodite and female zooids (Ostrovsky
et al. 2008 ; Ostrovsky 2009 ). As a rule, congeneric species
have the same sexual structure. Nevertheless, different intrageneric variants have been noted too: 1 and 4 in Callopora , 1
and 2 in Bugula and Celleporella , 2 and 4 in Steginoporella ,
2 and 3 in Schizomavella . No bryozoan family has yet been
found to have more than two variants of colonial sexual
structure (see Table 1.1 ).
The location of male, female and hermaphrodite zooids
within a colony varies. Thus, colonies consisting of sterile,
male and hermaphrodite zooids (variant 3) are generally
characterized by the proximal position of male zooids in relation to hermaphrodite ones. Similarly, male zooids are located
more proximally than females in most species with variant 2
of sexual structure. Nevertheless, in fi ve species male zooids
were found to be situated more distally than hermaphrodites
( Smittina concinna , Hippoporina propinqua ) or females
( Myriapora truncata , Eminoecia carsonae , Urceolipora nana ).
In only a single instance were male zooids located both more
proximally and more distally than female ones ( Emballotheca
quadrata ). In Mucropetraliella ellerii and Reciprocus regalis
male zooids were found between hermaphrodites.
Among the cheilostomes I have studied anatomically,
colonies consisting of sterile and hermaphrodite zooids
(variant 1) are commonest (at least 29 species, see Table 1.1 ).
Judging from the literature, it is this variant that is characteristic of most species (40) for which the sexual structure of
colonies has been described (altogether about 50, see
Appendix I). This contradicts the statement made by Reed
( 1991 ) that Gymnolaemata are predominantly colonial
hermaphrodites with gonochoristic zooids. Nevertheless,
both of these opinions call for scrutiny, since (1) for most of
the species studied, seasonal observations were not made,
(2) in the colonies of the same species at various localities,
the time of appearance of gonads and the duration of their
functioning can be different, and (3) the character of sexual
differentiation of zooids sometimes changes depending on the
season and the age of the colony.
For instance, three cheilostome species were described
as having colonies consisting of sterile, male, female and
hermaphrodite zooids, the latter being common ( Tendra
zostericola ) or rare ( Chartella membranaceotruncata ,
“Carbasea” indivisa ). This phenomenon was fi rst described
by Repiachoff ( 1875 ) in T . zostericola , though this researcher
was not sure if the male and female zooids that he observed
were truly gonochoristic or resulted from non-simultaneous
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis
female zooids are correlated with spawning and brooding,
and may involve the polypide, the cystid or both (Reed 1991 ;
Ostrovsky 2009 ).
Gonadogenesis and gametogenesis depend on the cycles
of degeneration and regeneration of polypides, on the age
and size of the colony and on environmental conditions.
The ancestrula (see Fig. 1.20C ) and several of the fi rstbudded (generations of) autozooids are sterile. Fertile
(ovary- bearing) zooids are predominantly found more in
the peripheral/distal part/area of the colony, sometimes
arranged in compact groups. In brooding species such
zooids can be easily identifi ed by the presence of embryos
visible through the semitransparent walls of skeletal
brood chambers (ovicells) or the frontal walls of the zooids
(in species with internal brooding). The distal/peripheral
location of fertile zooids appears to be associated with stages
of maturation of the colonies. Gonads develop in later zooidal generation, and, in species with seasonal reproduction,
the closer to the peak of reproductive season a colony is
established, the earlier it starts reproduction (see Ostrovsky
1998 ). In cheilostomes with frontal budding, fertile zooids
may be found in any part of the colony except the most
central/proximal region.
In some instances sexual zooids are formed only in certain places, which points to a high level of colonial integration. For instance, in free-living colonies of Selenaria
maculata (Selenariidae) male and female zooids are always
formed at or near the periphery of the colony (Chimonides
and Cook 1981 ). The position of male zooids around the
edge of the colony may be explained by the necessity of
sperm distribution, perhaps most effective in the peripheral
zone in relation to centrifugal exhalant water currents (see
Cook and Chimonides 1978 ) [although in bryozoans with
conical-discoidal colonies the main exhalant water fl ow is
apical (Cook 1977 ) and its suitability for sperm removal is
less obvious]. In contrast, female zooids are placed subperipherally, which may prevent them from receiving sperm
from the same colony (see also below). In the free-living
Cupuladriidae, which is similar to Selenariidae in overall
colonial morphology, the positioning of brooding zooids
with embryos in the central part of the colony correlates with
common peripheral colony fragmentation, and species with
colonies less likely to fragment preferentially brood at the
colony periphery (O’Dea et al. 2010 ).
It should be emphasized that seasonal observations are
essential for ascertaining the dynamics of sexual structure
(composition) of bryozoan colonies, since male and female
gonads may be absent or yet to develop at the time of sampling. For instance, if the colony under study consists of sterile and female zooids, it may be potentially represented by
any combination of four zooidal types (hermaphroditic or
gonochoristic, protandrous or protogynous), the observed
state indicating only that it is currently at the female phase of
the reproductive cycle. Differently aged colonies within the
same population may have different sexual structure.
My data and an analysis of the literature show that there
are four variants of sexual structure to be found in
Cheilostomata. The colony may consist of (1) sterile and hermaphroditic zooids, (2) sterile and gonochoristic (male and
female) zooids, (3) sterile, male and hermaphrodite zooids,
and (4) sterile, hermaphrodite and female zooids (Ostrovsky
et al. 2008 ; Ostrovsky 2009 ). As a rule, congeneric species
have the same sexual structure. Nevertheless, different intrageneric variants have been noted too: 1 and 4 in Callopora , 1
and 2 in Bugula and Celleporella , 2 and 4 in Steginoporella ,
2 and 3 in Schizomavella . No bryozoan family has yet been
found to have more than two variants of colonial sexual
structure (see Table 1.1 ).
The location of male, female and hermaphrodite zooids
within a colony varies. Thus, colonies consisting of sterile,
male and hermaphrodite zooids (variant 3) are generally
characterized by the proximal position of male zooids in relation to hermaphrodite ones. Similarly, male zooids are located
more proximally than females in most species with variant 2
of sexual structure. Nevertheless, in fi ve species male zooids
were found to be situated more distally than hermaphrodites
( Smittina concinna , Hippoporina propinqua ) or females
( Myriapora truncata , Eminoecia carsonae , Urceolipora nana ).
In only a single instance were male zooids located both more
proximally and more distally than female ones ( Emballotheca
quadrata ). In Mucropetraliella ellerii and Reciprocus regalis
male zooids were found between hermaphrodites.
Among the cheilostomes I have studied anatomically,
colonies consisting of sterile and hermaphrodite zooids
(variant 1) are commonest (at least 29 species, see Table 1.1 ).
Judging from the literature, it is this variant that is characteristic of most species (40) for which the sexual structure of
colonies has been described (altogether about 50, see
Appendix I). This contradicts the statement made by Reed
( 1991 ) that Gymnolaemata are predominantly colonial
hermaphrodites with gonochoristic zooids. Nevertheless,
both of these opinions call for scrutiny, since (1) for most of
the species studied, seasonal observations were not made,
(2) in the colonies of the same species at various localities,
the time of appearance of gonads and the duration of their
functioning can be different, and (3) the character of sexual
differentiation of zooids sometimes changes depending on the
season and the age of the colony.
For instance, three cheilostome species were described
as having colonies consisting of sterile, male, female and
hermaphrodite zooids, the latter being common ( Tendra
zostericola ) or rare ( Chartella membranaceotruncata ,
“Carbasea” indivisa ). This phenomenon was fi rst described
by Repiachoff ( 1875 ) in T . zostericola , though this researcher
was not sure if the male and female zooids that he observed
were truly gonochoristic or resulted from non-simultaneous
1 Reproductive Patterns of Gymnolaemate Bryozoa: General Overview and Comparative Analysis