237
Continuous egg production possibly explains the fact that
Temkin did not notice polypide recycling in M . membranacea . To sum up, in malacostegines, one fertile zooid during
the reproductive season may produce several tens and even
hundreds of small zygotes about 50–60 μm in diameter that
further develop into planktotrophic larvae.
The situation is very similar in ctenostome broadcasters
in which the number of ovarian and ovulated oocytes
(25–91 μm in diameter) in one zooid at a given time varies
from 6–10 to up to 60 in different species (6–30 in
Hypophorella expansa , about 20 in Victorella pavida ,
Alcyonidium albidum , A . mytili and A . nodosum , from 9–15
to 45 in Farrella repens , up to 60 in Аlcyonidium sp. and A .
fl abelliforme ) (van Beneden 1844 ; Joliet 1877 ; Ehlers
1876 ; Prouho 1892 ; Calvet 1900 ; Marcus 1926a ; Braem
1951 ; Cadman and Ryland 1996 ; Temkin 1996 ; Ryland
2001 ; see also Sect. 3.4.4 and Table 3.2 ). In A . condylocinereum , A . epispiculum and A . cellarioides up to 15 ovarian
oocytes are seen in single section plane (Porter and
Hayward 2004 ). Exceptions are A . hydrocoalitum and
Victorella pseudoarachnidia in which zooids with seven
(Porter 2004 ) and four (Jebram and Everitt 1982 ) ovarian
oocytes were subsequently illustrated. However, the number of eggs is not mentioned in the texts, so this information
has to be checked. Noticeably, in some of the broadcasting
ctenostomes mentioned ( H . expansa , V . pavida , F . repens ,
A . albidum and Аlcyonidium sp.), ovulated oocytes have an
irregular shape, similar to that in broadcasting electrid
cheilostomes.
Thus, both cheilostome and ctenostome broadcasters
show a range in oocyte production, with maximal numbers
of 50–60 eggs and minimal numbers not exceeding 10 per
zooid at a given time. Egg diameter in both instances is
mostly less than 100 μm (always in ctenostomes).
The number of oocytes produced by most gymnolaemate
brooders is usually less and their diameter is larger than in
broadcasters although the correlation is not strict. For instance,
among 22 species of Ctenostomata for which such data are
available in the literature, 11 species produce 10 eggs or less
per zooid (mostly 3–5) and their diameter varies from 70 to
370 μm (see Table 3.2 ). Five species produce 11–16 eggs of
90–340 μm. Six species produce 20 oocytes ( Tanganella
muelleri , Potsiella erecta , see Braem 1951 ; Smith et al. 2003 )
or more: 60 in Triticella fl ava (Ström 1969 ), about 40 in
Paludicella articulata (according to the illustration of Allman
1856 ), about 90 in Nolella dilatata (depicted by Calvet 1900 ),
and more than a 100 in Labiostomella gisleni (see Silén 1944 ),
and oocyte diameter here ranges from 65 to 160 μm in diameter. It should be noted here that the latter species was initially
described as a “protocheilostome” but later was accommodated among ancient ctenostomes (Todd 2000 ). Its method of
brooding, inferred from Silén’s ( 1944 ) anatomical sections,
supports such placement.
Those species that produce maximal numbers of oocytes
have the smallest eggs (65 μm in T . fl ava , 70 μm in L . gisleni )
and those producing the largest eggs (200–350 μm in
Bowerbankia gracilis and 370 μm in Alcyonidium disciforme ) form just 1–4 of them. On the other hand, there are
species with an egg diameter of 70 μm that produce 5–6
oocytes ( Panolicella nutans ), and others with an egg diameter of 110 μm ( Paludicella articulata ) and 160 μm ( Pottsiella
erecta ) that respectively produce up to 20 and more than 40.
Thus, in half the ctenostome brooders their oocytes are
larger than in broadcasters (more than 100 μm) although
their numbers can be either small (2–3) or large (up to 20). In
the remainder of the brooding species mature oocyte diameter is comparable with that in broadcasters and egg number
varies from 4–5 to 100 (Table 3.2 ). Since the duration of
embryogenesis in ctenostomes is, like in cheilostomes, 1.5–2
weeks on average (Reed 1988 , 1991 ), the total number of
eggs formed by an ovary throughout the reproductive period
should potentially vary from several tens to hundreds.
However, it should be stressed that, except for Triticella fl ava
which can simultaneously brood up to 20 embryos (Ström
1969 ), in all of these cases the number of ovarian oocytes is
much greater than the number of incubated embryos, and
thus oogenesis is excessive.
To sum up, there is a large overlap in the number and size
of oocytes between ctenostome brooders and broadcasters,
perhaps indicating an evolutionary connection between these
reproductive patterns. Despite acquired embryonic incubation, some brooding species still produce a large number of
ovarian eggs (comparable with or even exceeding that in
broadcasters) most of which will never be brooded,
however.
In brooding cheilostomes with reproductive patterns II
and IV, 1–3 oocyte doublets (or 2–6 oocytes including nurse
cells) are usually simultaneously present in the ovary (75%
of all species studied). From 7 to 12 oocyte doublets were
found in the ovaries of cribrimorphs, a paraphyletic clade
(probably not monophyletic) with plesiomorphic features.
Margaretta barbata , a more advanced form, is the only species to have up to 25 oocyte doublets in the ovary simultaneously, which is comparable to the number of oocytes in
broadcasting bryozoans. As only one embryo at a time is
incubated in the peristomial ovicells of Margaretta (see
Waters 1907 ), the reason for such a large number of oocytes
remains obscure, similar to the situation in the abovementioned ctenostome brooders.
Thus, the productivity of the maternal zooid is limited by
the carrying capacity of the brood chamber (Silén 1945 ).
There are, in fact, a few cheilostome species (genera
Scruparia ,
Tendra ,
Thalamoporella ,
Macropora ,
Monoporella ) in which several embryos occur in the brood
cavity at the same time, contrary to most cheilostome
brooders. It is possible, therefore, that the total number of
3.1 Modifi cation of Oogenesis and Its Evolutionary Consequences
Continuous egg production possibly explains the fact that
Temkin did not notice polypide recycling in M . membranacea . To sum up, in malacostegines, one fertile zooid during
the reproductive season may produce several tens and even
hundreds of small zygotes about 50–60 μm in diameter that
further develop into planktotrophic larvae.
The situation is very similar in ctenostome broadcasters
in which the number of ovarian and ovulated oocytes
(25–91 μm in diameter) in one zooid at a given time varies
from 6–10 to up to 60 in different species (6–30 in
Hypophorella expansa , about 20 in Victorella pavida ,
Alcyonidium albidum , A . mytili and A . nodosum , from 9–15
to 45 in Farrella repens , up to 60 in Аlcyonidium sp. and A .
fl abelliforme ) (van Beneden 1844 ; Joliet 1877 ; Ehlers
1876 ; Prouho 1892 ; Calvet 1900 ; Marcus 1926a ; Braem
1951 ; Cadman and Ryland 1996 ; Temkin 1996 ; Ryland
2001 ; see also Sect. 3.4.4 and Table 3.2 ). In A . condylocinereum , A . epispiculum and A . cellarioides up to 15 ovarian
oocytes are seen in single section plane (Porter and
Hayward 2004 ). Exceptions are A . hydrocoalitum and
Victorella pseudoarachnidia in which zooids with seven
(Porter 2004 ) and four (Jebram and Everitt 1982 ) ovarian
oocytes were subsequently illustrated. However, the number of eggs is not mentioned in the texts, so this information
has to be checked. Noticeably, in some of the broadcasting
ctenostomes mentioned ( H . expansa , V . pavida , F . repens ,
A . albidum and Аlcyonidium sp.), ovulated oocytes have an
irregular shape, similar to that in broadcasting electrid
cheilostomes.
Thus, both cheilostome and ctenostome broadcasters
show a range in oocyte production, with maximal numbers
of 50–60 eggs and minimal numbers not exceeding 10 per
zooid at a given time. Egg diameter in both instances is
mostly less than 100 μm (always in ctenostomes).
The number of oocytes produced by most gymnolaemate
brooders is usually less and their diameter is larger than in
broadcasters although the correlation is not strict. For instance,
among 22 species of Ctenostomata for which such data are
available in the literature, 11 species produce 10 eggs or less
per zooid (mostly 3–5) and their diameter varies from 70 to
370 μm (see Table 3.2 ). Five species produce 11–16 eggs of
90–340 μm. Six species produce 20 oocytes ( Tanganella
muelleri , Potsiella erecta , see Braem 1951 ; Smith et al. 2003 )
or more: 60 in Triticella fl ava (Ström 1969 ), about 40 in
Paludicella articulata (according to the illustration of Allman
1856 ), about 90 in Nolella dilatata (depicted by Calvet 1900 ),
and more than a 100 in Labiostomella gisleni (see Silén 1944 ),
and oocyte diameter here ranges from 65 to 160 μm in diameter. It should be noted here that the latter species was initially
described as a “protocheilostome” but later was accommodated among ancient ctenostomes (Todd 2000 ). Its method of
brooding, inferred from Silén’s ( 1944 ) anatomical sections,
supports such placement.
Those species that produce maximal numbers of oocytes
have the smallest eggs (65 μm in T . fl ava , 70 μm in L . gisleni )
and those producing the largest eggs (200–350 μm in
Bowerbankia gracilis and 370 μm in Alcyonidium disciforme ) form just 1–4 of them. On the other hand, there are
species with an egg diameter of 70 μm that produce 5–6
oocytes ( Panolicella nutans ), and others with an egg diameter of 110 μm ( Paludicella articulata ) and 160 μm ( Pottsiella
erecta ) that respectively produce up to 20 and more than 40.
Thus, in half the ctenostome brooders their oocytes are
larger than in broadcasters (more than 100 μm) although
their numbers can be either small (2–3) or large (up to 20). In
the remainder of the brooding species mature oocyte diameter is comparable with that in broadcasters and egg number
varies from 4–5 to 100 (Table 3.2 ). Since the duration of
embryogenesis in ctenostomes is, like in cheilostomes, 1.5–2
weeks on average (Reed 1988 , 1991 ), the total number of
eggs formed by an ovary throughout the reproductive period
should potentially vary from several tens to hundreds.
However, it should be stressed that, except for Triticella fl ava
which can simultaneously brood up to 20 embryos (Ström
1969 ), in all of these cases the number of ovarian oocytes is
much greater than the number of incubated embryos, and
thus oogenesis is excessive.
To sum up, there is a large overlap in the number and size
of oocytes between ctenostome brooders and broadcasters,
perhaps indicating an evolutionary connection between these
reproductive patterns. Despite acquired embryonic incubation, some brooding species still produce a large number of
ovarian eggs (comparable with or even exceeding that in
broadcasters) most of which will never be brooded,
however.
In brooding cheilostomes with reproductive patterns II
and IV, 1–3 oocyte doublets (or 2–6 oocytes including nurse
cells) are usually simultaneously present in the ovary (75%
of all species studied). From 7 to 12 oocyte doublets were
found in the ovaries of cribrimorphs, a paraphyletic clade
(probably not monophyletic) with plesiomorphic features.
Margaretta barbata , a more advanced form, is the only species to have up to 25 oocyte doublets in the ovary simultaneously, which is comparable to the number of oocytes in
broadcasting bryozoans. As only one embryo at a time is
incubated in the peristomial ovicells of Margaretta (see
Waters 1907 ), the reason for such a large number of oocytes
remains obscure, similar to the situation in the abovementioned ctenostome brooders.
Thus, the productivity of the maternal zooid is limited by
the carrying capacity of the brood chamber (Silén 1945 ).
There are, in fact, a few cheilostome species (genera
Scruparia ,
Tendra ,
Thalamoporella ,
Macropora ,
Monoporella ) in which several embryos occur in the brood
cavity at the same time, contrary to most cheilostome
brooders. It is possible, therefore, that the total number of
3.1 Modifi cation of Oogenesis and Its Evolutionary Consequences