312
and Hageman ( 1981 , 1983 ) in the cheilostome Membranipora
serrilamella . The tentacle sheath then everts, exposing the
egg to the ambient water, as Reed thought, for fertilisation.
Being retracted afterwards, the egg is surrounded by the
fertilization envelope. Similar behaviour was described by
Joliet ( 1877a ) in Walkeria uva (as Valkeria cuscuta ) and
Bowerbankia imbricata . However, Temkin ( 1996 ) showed
intracoelomic fertilisation in B . gracilis .
In a similar study by Owrid and Ryland ( 1991 ), the main
features of gonado- and gametogenesis in the ctenostome
Alcyonidium hirsutum were revealed. This species is a protandrous hermaphrodite at both colonial and zooidal levels.
Spermatogenic tissue differentiates before the development
(in the new zooid) or regeneration (in the existed zooid) of
the polypide from the peritoneum of the proximal part of the
cystid wall. Primary oogonia originate in the bud of the
newly developing polypide [presumably from its peritoneum], and from the gut peritoneum in the replacement polypide. Thus, gonads develop each time before or during the
formation of the new functioning polypide. Peritoneal cells
proliferate to form the follicle cover surrounding young
oocytes. Where the ovary contacts the gut, special cells with
tongue-like parts protruding into the caecal lumen were
found. These cells were recorded fi rst by Chrétien ( 1958 )
who studied Alcyonidium diaphanum (see above). Owrid
and Ryland suggested that they could play a nutritive role.
Finally, each of several growing oocytes is enveloped by its
own follicle, which becomes two-layered, and the ovary
appears to be no longer in contact with the caecum [possibly
suspended on funicular strands instead]. Increase in the size
of the ovary during vitellogenesis is accompanied by degeneration of the polypide; it disappears when the ovary attains
its full size. At the end of vitellogenesis the follicle layer
becomes very thin and a new small polypide without tentacles is developed. A similar process was described by Joliet
( 1877a ) in Walkeria uva (as Valkeria cuscuta ) (see above).
Mature eggs ovulate and are transferred to the “polypide
sac” [modifi ed tentacle sheath] via the coelomopore. The
authors believed that they are fertilized prior to or just after
oviposition. Some 4–11 larvae are brooded simultaneously.
Cadman and Ryland ( 1996 ) studied reproduction in
Alcyonidium mytili . They showed that the ovary develops on
the funicular cord and confi rmed the presence of an intertentacular organ that forms in the existing lophophore.
Four reviews on bryozoan sexual reproduction appeared
during the 1990s. Nielsen ( 1990 ) published a short chapter in
which he stressed that, in addition to the three basic reproductive patterns known in Bryozoa, there are “a large number of intermediate types” (p. 185).
Reed’s ( 1991 ) review, despite the inevitable inaccuracies
associated with large gaps in our knowledge, is the most
complete compilation of this topic at present. In addition
to the descriptions and examples presented, Reed widely
discussed and interpreted the bryozoan data, comparing
them with other invertebrate groups. In particular, he
arranged the scanty data on the origin of the germ cells in
Bryozoa in a logical system of facts and suggestions, creating the modern view of this topic. It should be noted that
Reed often included non-published results of observations
and studies made by other authors. Of special interest are the
data from the Hageman’s ( 1983 ) Ph.D. dissertation; except
for one short note (Hageman 1981 ), he never published the
results of his studies on the cheilostome Membranipora
serrilamella . Brief account of bryozoan sexual reproduction
was included in the book chapter of Mukai et al. ( 1997 ).
A detailed review of fertilization in hermaphrodite colonial invertebrates was published by Ryland and Bishop
( 1993 ). Inter alia, they mentioned some of the fi ndings in the
unpublished Ph.D. dissertation of Temkin ( 1991 ), who
achieved bringing to reproduction isolated colonies of
Membranipora sp. Cancino et al. ( 1991 ), on the other hand,
did not achieve this in Membranipora isabelleana , which
failed to complete oogenesis in isolation (sexual zooids are
markedly protandrodrous in this species – sperm and mature
oocytes were never observed to occur simultaneously in the
coelom – however zooids with either sperm or oocytes cooccurred in the same colonies). Temkin’s results accorded
with those of Maturo ( 1991a ). Six gymnolaemate species
produced larvae in his experiments when grown from single
ancestrulae in isolation. In spite of this, it was concluded that
cross- fertilisation is usually a rule among Bryozoa, and selfing, if at all existing, might be used in an “emergency” situation. Precocious insemination and the ability to store sperm
(Hughes et al. 2002b ) mean that it is important to carefully
isolate colonies grown from ancestrulae early in their development in future experiments.
The excellent experimental work on bryozoan fertilization conducted by Temkin ( 1994 , 1996 ) resulted in a reconsideration of some generally accepted opinions. It was shown
that fertilization is internal in gymnolaemate bryozoans,
either intracoelomic or intraovarian. In Membranipora membranacea , spermatozeugmata are pushed through the tentacle lumen by an undulating movement of the midpiece region
and spawned tail-fi rst via the terminal pores of the two distomedial tentacles into the exhalant current created by the
colony. Temkin suggested that this should increase the chances
for sperm to be removed from the colony (thus preventing
intracolonial self-fertilization), for which purpose the tips of
the distomedial tentacles bend towards the exhalant current.
[Silén ( 1966 ) thought that releasing sperm through the tentacle tips would position them beyond the feeding currents of
the parent zooid.] Being entrapped by the feeding currents of
another lophophore, sperm attaches to the tentacles and performs undulating movements (although sperm are sometimes
ingested, rejected with food particles or ensnared in the
tentacles). Those spermatozeugmata that are attached near the
Appendices
and Hageman ( 1981 , 1983 ) in the cheilostome Membranipora
serrilamella . The tentacle sheath then everts, exposing the
egg to the ambient water, as Reed thought, for fertilisation.
Being retracted afterwards, the egg is surrounded by the
fertilization envelope. Similar behaviour was described by
Joliet ( 1877a ) in Walkeria uva (as Valkeria cuscuta ) and
Bowerbankia imbricata . However, Temkin ( 1996 ) showed
intracoelomic fertilisation in B . gracilis .
In a similar study by Owrid and Ryland ( 1991 ), the main
features of gonado- and gametogenesis in the ctenostome
Alcyonidium hirsutum were revealed. This species is a protandrous hermaphrodite at both colonial and zooidal levels.
Spermatogenic tissue differentiates before the development
(in the new zooid) or regeneration (in the existed zooid) of
the polypide from the peritoneum of the proximal part of the
cystid wall. Primary oogonia originate in the bud of the
newly developing polypide [presumably from its peritoneum], and from the gut peritoneum in the replacement polypide. Thus, gonads develop each time before or during the
formation of the new functioning polypide. Peritoneal cells
proliferate to form the follicle cover surrounding young
oocytes. Where the ovary contacts the gut, special cells with
tongue-like parts protruding into the caecal lumen were
found. These cells were recorded fi rst by Chrétien ( 1958 )
who studied Alcyonidium diaphanum (see above). Owrid
and Ryland suggested that they could play a nutritive role.
Finally, each of several growing oocytes is enveloped by its
own follicle, which becomes two-layered, and the ovary
appears to be no longer in contact with the caecum [possibly
suspended on funicular strands instead]. Increase in the size
of the ovary during vitellogenesis is accompanied by degeneration of the polypide; it disappears when the ovary attains
its full size. At the end of vitellogenesis the follicle layer
becomes very thin and a new small polypide without tentacles is developed. A similar process was described by Joliet
( 1877a ) in Walkeria uva (as Valkeria cuscuta ) (see above).
Mature eggs ovulate and are transferred to the “polypide
sac” [modifi ed tentacle sheath] via the coelomopore. The
authors believed that they are fertilized prior to or just after
oviposition. Some 4–11 larvae are brooded simultaneously.
Cadman and Ryland ( 1996 ) studied reproduction in
Alcyonidium mytili . They showed that the ovary develops on
the funicular cord and confi rmed the presence of an intertentacular organ that forms in the existing lophophore.
Four reviews on bryozoan sexual reproduction appeared
during the 1990s. Nielsen ( 1990 ) published a short chapter in
which he stressed that, in addition to the three basic reproductive patterns known in Bryozoa, there are “a large number of intermediate types” (p. 185).
Reed’s ( 1991 ) review, despite the inevitable inaccuracies
associated with large gaps in our knowledge, is the most
complete compilation of this topic at present. In addition
to the descriptions and examples presented, Reed widely
discussed and interpreted the bryozoan data, comparing
them with other invertebrate groups. In particular, he
arranged the scanty data on the origin of the germ cells in
Bryozoa in a logical system of facts and suggestions, creating the modern view of this topic. It should be noted that
Reed often included non-published results of observations
and studies made by other authors. Of special interest are the
data from the Hageman’s ( 1983 ) Ph.D. dissertation; except
for one short note (Hageman 1981 ), he never published the
results of his studies on the cheilostome Membranipora
serrilamella . Brief account of bryozoan sexual reproduction
was included in the book chapter of Mukai et al. ( 1997 ).
A detailed review of fertilization in hermaphrodite colonial invertebrates was published by Ryland and Bishop
( 1993 ). Inter alia, they mentioned some of the fi ndings in the
unpublished Ph.D. dissertation of Temkin ( 1991 ), who
achieved bringing to reproduction isolated colonies of
Membranipora sp. Cancino et al. ( 1991 ), on the other hand,
did not achieve this in Membranipora isabelleana , which
failed to complete oogenesis in isolation (sexual zooids are
markedly protandrodrous in this species – sperm and mature
oocytes were never observed to occur simultaneously in the
coelom – however zooids with either sperm or oocytes cooccurred in the same colonies). Temkin’s results accorded
with those of Maturo ( 1991a ). Six gymnolaemate species
produced larvae in his experiments when grown from single
ancestrulae in isolation. In spite of this, it was concluded that
cross- fertilisation is usually a rule among Bryozoa, and selfing, if at all existing, might be used in an “emergency” situation. Precocious insemination and the ability to store sperm
(Hughes et al. 2002b ) mean that it is important to carefully
isolate colonies grown from ancestrulae early in their development in future experiments.
The excellent experimental work on bryozoan fertilization conducted by Temkin ( 1994 , 1996 ) resulted in a reconsideration of some generally accepted opinions. It was shown
that fertilization is internal in gymnolaemate bryozoans,
either intracoelomic or intraovarian. In Membranipora membranacea , spermatozeugmata are pushed through the tentacle lumen by an undulating movement of the midpiece region
and spawned tail-fi rst via the terminal pores of the two distomedial tentacles into the exhalant current created by the
colony. Temkin suggested that this should increase the chances
for sperm to be removed from the colony (thus preventing
intracolonial self-fertilization), for which purpose the tips of
the distomedial tentacles bend towards the exhalant current.
[Silén ( 1966 ) thought that releasing sperm through the tentacle tips would position them beyond the feeding currents of
the parent zooid.] Being entrapped by the feeding currents of
another lophophore, sperm attaches to the tentacles and performs undulating movements (although sperm are sometimes
ingested, rejected with food particles or ensnared in the
tentacles). Those spermatozeugmata that are attached near the
Appendices