308
Pergens’s ( 1889 ) observations on congeneric F. malusii , in
which oviposition was described as being accompanied by
strong compression of the egg, occurring when the polypide
degenerates. Additionally, Nielsen found distal zooids with
two ovicells (their formation being induced by maternal zooids
from two different colonies), confi rming the suggestion of
Silén that the oncet of oogenesis triggers ovicell formation.
Jebram and Everitt ( 1982 ) corroborated Braem’s ( 1951 )
results on reproduction in the ctenostomes Bulbella
abscondita and Victorella . Like V . pavida , non- brooding V .
pseudoarachnidia also has an intertentacular organ. A coelomopore and the brooding of up to six embryos in body-wall
invaginations were also described in Tanganella appendiculata , similar to the situation in Victorella muelleri .
Dyrynda and coworkers successfully combined anatomical, ultrastructural and ecological research, studying bryozoan sexual reproduction. Following Marcus ( 1941b ),
intracoelomic brooding was discovered in the cheilostome
Epistomia bursaria (Dyrynda 1981 ; Dyrynda and King
1982 ). Its colonies are hermaphrodite with gonochoristic
zooids, the females being larger and rarer than males. There
is only one polypide generation in each zooid, and the polypide in male zooids persists until the zooid is between fi ve and
ten zooid pairs from the growing tip of the colony, whereas
the female polypide degenerates at a distance of only 2–4
zooid pairs. Each female produces only one larva. A TEMstudy was undertaken to investigate both spermato- and
oogenesis. The testis develops proximally on the axial funicular cord and all nutrients for spermatogenesis are probably
developed intrazooidally, since it ceases when the polypide
degenerates. The ovary is also associated with the funicular
cord, but, in contrast with the testis, it becomes established in
the distomedial corner and its nutrient supply is intrazooidal
during oogenesis and mainly extrazooidal during embryogenesis, although the by-products of polypide cycling may
perhaps be used. A single small alecithal oocyte matures by
the onset of polypide degeneration. It is surrounded by “follicle” or “nurse” cells that are presumably of germ origin,
suggested by the presence of cytoplasmic bridges between
the oocyte and the “follicle” cells. In the inferred initial syncytial cluster of germ cells, the central one may differentiate
into the oocyte, whereas the rest may become the “follicle”
(“nurse” cells in their plate 2 caption). Fertilization is intraovarian, since cleavage of the embryo begins inside the “follicle”. Extraembryonic nutrition is obvious – the embryo
increases about 1,000-fold in volume and the axial funicular
cord hypertrophies during this time. Additionally, if the polypides of the neighbouring zooids degenerate, the embryo
fails to develop. Self-fertilization was assumed – isolated
colonies are able to produce embryos. Finally it was suggested that the combination of endocoelomic brooding,
larval viviparity and one polypide generation per zooid may
be a primitive feature from which polypide cycling and
extracoelomic brooding may have evolved (see Chap. 1 for
the critical analysis of this hypothesis).
Dyrynda ( 1981 ) also gave a brief description of the
reproductive cycle of Chartella papyracea , noting the formation of an “oocyte nurse cell syncytial duplet” (p. 78).
Dyrynda and Ryland ( 1982 ) published an excellent paper
comparing the contrasting reproductive strategies and life
histories of brooding non-placental C . papyracea and placental Bugula fl abellata in detail. Colonial and zooidal
sexual changes were described in relation to seasonality
and polypide cycling. It was suggested that the interrelationship between polypide and sexual cycling controls
nutrient budgeting for sexual and other processes within
zooids and the colony. Sexes are separate in C . papyracea ,
with male zooids appearing fi rst. Fronds, however, are hermaphrodite since females develop at the end of the fi rst
reproductive season. The switch from the male to female
zooid production coincides with the summer peak of water
temperature. Nutrient- intensive stages of gametogenesis
(late spermatogenesis and late vitellogenesis) take place
only in the presence of a feeding polypide in a zooid,
except in the fi rst polypide generation that never produces
gonads. Male germ cells are formed on the proximal transverse wall, and the testis develops with spermatogenesis
progressing on the proximal and lateral walls during the
life of the next polypide. As soon as the second polypide
degenerates, the testis regresses, but the male cycle recommences as the next polypide nears completion, and it is
repeated for each polypide generation; it is not known if
the new testis is established or the old one is renovated. A
cluster of female germ cells is said to develop “in the coelom [of the female zooid], opposite the fi rst polypide bud”
(p. 248) where it is suspended by funicular cords as seen in
their fi gure 6A. It should be mentioned that Grant ( 1827 )
and Vigelius ( 1884a , b ) wrote that the ovary develops on
the zooid wall in species of Flustridae (see above). In contrast, it seems that the ovary of C . papyracea develops in
association with the polypide bud, then migrates towards
the middle of the lateral cystid wall to establish an ovary
and the ovicell is formed at the same time. Oocytes develop
in doublets, which, in contrast with the description of
Marcus ( 1941a ), was stated to be the consequence of
arrested cytokinesis, not cell fusion (see also Dyrynda and
King 1983 ). The polypide starts to feed when the fi rst doublet appears. Previtellogenic growth takes place through
the rest of the polypide cycle. Vitellogenesis commences
during polypide cycling, speeding up when the next polypide starts to feed. This polypide transfers the ovulated egg
to the ovicell as described by Gerwerzhagen ( 1913 ) and
Silén ( 1945 ). Following oviposition, the polypide degenerates and the ovarian cycle is repeated with each subsequent
polypide cycle. If the oocyte is not ovulated before the polypide degenerates, it will be transferred by the next polypide.
Appendices
Pergens’s ( 1889 ) observations on congeneric F. malusii , in
which oviposition was described as being accompanied by
strong compression of the egg, occurring when the polypide
degenerates. Additionally, Nielsen found distal zooids with
two ovicells (their formation being induced by maternal zooids
from two different colonies), confi rming the suggestion of
Silén that the oncet of oogenesis triggers ovicell formation.
Jebram and Everitt ( 1982 ) corroborated Braem’s ( 1951 )
results on reproduction in the ctenostomes Bulbella
abscondita and Victorella . Like V . pavida , non- brooding V .
pseudoarachnidia also has an intertentacular organ. A coelomopore and the brooding of up to six embryos in body-wall
invaginations were also described in Tanganella appendiculata , similar to the situation in Victorella muelleri .
Dyrynda and coworkers successfully combined anatomical, ultrastructural and ecological research, studying bryozoan sexual reproduction. Following Marcus ( 1941b ),
intracoelomic brooding was discovered in the cheilostome
Epistomia bursaria (Dyrynda 1981 ; Dyrynda and King
1982 ). Its colonies are hermaphrodite with gonochoristic
zooids, the females being larger and rarer than males. There
is only one polypide generation in each zooid, and the polypide in male zooids persists until the zooid is between fi ve and
ten zooid pairs from the growing tip of the colony, whereas
the female polypide degenerates at a distance of only 2–4
zooid pairs. Each female produces only one larva. A TEMstudy was undertaken to investigate both spermato- and
oogenesis. The testis develops proximally on the axial funicular cord and all nutrients for spermatogenesis are probably
developed intrazooidally, since it ceases when the polypide
degenerates. The ovary is also associated with the funicular
cord, but, in contrast with the testis, it becomes established in
the distomedial corner and its nutrient supply is intrazooidal
during oogenesis and mainly extrazooidal during embryogenesis, although the by-products of polypide cycling may
perhaps be used. A single small alecithal oocyte matures by
the onset of polypide degeneration. It is surrounded by “follicle” or “nurse” cells that are presumably of germ origin,
suggested by the presence of cytoplasmic bridges between
the oocyte and the “follicle” cells. In the inferred initial syncytial cluster of germ cells, the central one may differentiate
into the oocyte, whereas the rest may become the “follicle”
(“nurse” cells in their plate 2 caption). Fertilization is intraovarian, since cleavage of the embryo begins inside the “follicle”. Extraembryonic nutrition is obvious – the embryo
increases about 1,000-fold in volume and the axial funicular
cord hypertrophies during this time. Additionally, if the polypides of the neighbouring zooids degenerate, the embryo
fails to develop. Self-fertilization was assumed – isolated
colonies are able to produce embryos. Finally it was suggested that the combination of endocoelomic brooding,
larval viviparity and one polypide generation per zooid may
be a primitive feature from which polypide cycling and
extracoelomic brooding may have evolved (see Chap. 1 for
the critical analysis of this hypothesis).
Dyrynda ( 1981 ) also gave a brief description of the
reproductive cycle of Chartella papyracea , noting the formation of an “oocyte nurse cell syncytial duplet” (p. 78).
Dyrynda and Ryland ( 1982 ) published an excellent paper
comparing the contrasting reproductive strategies and life
histories of brooding non-placental C . papyracea and placental Bugula fl abellata in detail. Colonial and zooidal
sexual changes were described in relation to seasonality
and polypide cycling. It was suggested that the interrelationship between polypide and sexual cycling controls
nutrient budgeting for sexual and other processes within
zooids and the colony. Sexes are separate in C . papyracea ,
with male zooids appearing fi rst. Fronds, however, are hermaphrodite since females develop at the end of the fi rst
reproductive season. The switch from the male to female
zooid production coincides with the summer peak of water
temperature. Nutrient- intensive stages of gametogenesis
(late spermatogenesis and late vitellogenesis) take place
only in the presence of a feeding polypide in a zooid,
except in the fi rst polypide generation that never produces
gonads. Male germ cells are formed on the proximal transverse wall, and the testis develops with spermatogenesis
progressing on the proximal and lateral walls during the
life of the next polypide. As soon as the second polypide
degenerates, the testis regresses, but the male cycle recommences as the next polypide nears completion, and it is
repeated for each polypide generation; it is not known if
the new testis is established or the old one is renovated. A
cluster of female germ cells is said to develop “in the coelom [of the female zooid], opposite the fi rst polypide bud”
(p. 248) where it is suspended by funicular cords as seen in
their fi gure 6A. It should be mentioned that Grant ( 1827 )
and Vigelius ( 1884a , b ) wrote that the ovary develops on
the zooid wall in species of Flustridae (see above). In contrast, it seems that the ovary of C . papyracea develops in
association with the polypide bud, then migrates towards
the middle of the lateral cystid wall to establish an ovary
and the ovicell is formed at the same time. Oocytes develop
in doublets, which, in contrast with the description of
Marcus ( 1941a ), was stated to be the consequence of
arrested cytokinesis, not cell fusion (see also Dyrynda and
King 1983 ). The polypide starts to feed when the fi rst doublet appears. Previtellogenic growth takes place through
the rest of the polypide cycle. Vitellogenesis commences
during polypide cycling, speeding up when the next polypide starts to feed. This polypide transfers the ovulated egg
to the ovicell as described by Gerwerzhagen ( 1913 ) and
Silén ( 1945 ). Following oviposition, the polypide degenerates and the ovarian cycle is repeated with each subsequent
polypide cycle. If the oocyte is not ovulated before the polypide degenerates, it will be transferred by the next polypide.
Appendices