115
A. Ostrovsky, Evolution of Sexual Reproduction in Marine Invertebrates: Example of gymnolaemate bryozoans,
DOI 10.1007/978-94-007-7146-8_2, © Springer Science+Business Media Dordrecht 2013
2.1
History of Studies of Cheilostome
Brood Chambers
Cheilostome bryozoans possess a broad range of methods
for embryonic incubation. Embryos are brooded in the external membranous sacs, skeletal (calcifi ed) chambers and
internal brood sacs formed by non-calcifi ed zooidal walls, or
develop intracoelomically in viviparous species. In some
instances extraembryonic nutrition (EEN) has evolved.
Most cheilostomes temporarily house their offspring
in skeletal chambers called ovicells. The presence or
absence of ovicells, and their morphology, are important
characters in cheilostome taxonomy. There are several
morphological types, the commonest being hyperstomial
ovicells that often look like prominent hemispherical
bubbles or helmets on the colony surface. Basically, the
hyperstomial ovicell consists of (1) a double-walled, calcifi ed protective fold (ooecium) with a coelomic cavity
between the two walls, (2) a non- calcifi ed part of the distal
wall of the egg-producing maternal autozooid, and (3) the
brood cavity between these two components (see Fig. 1 in
Introduction, Figs. 2.1 , 2.3 , and 2.5 ).
Ovicells were fi rst described by Ellis ( 1753 , 1755 ) who
suggested that they were snail-like “neritae,” formed from
the “polypes,” able to detach from a branch (to drop, fi x to
the substratum, and give rise to a new animal) or to lay eggs
(see also Ellis and Solander 1786 ) (Fig. 2.2 ). Following
Linnaeus ( 1758 ), Pallas ( 1766 , p. 36) opined that these
“bulla[e], galeae” [helmet-like bubbles, i.e. ovicells] might
be ovaria. He speculated that both ovicells and avicularia
might serve for fertilization and sometimes called them
“Nectariums” (see also Ostrovsky 2008a , and Appendix I for
details and discussion).
Later authors followed Linnaeus and Pallas, calling
ovicells “corps vesiculaires”, “corps globuleux” (Lamouroux
1816 ), “vesicules gemmifères”, “capsules gemmifères”
(Milne Edwards 1836 ), “vesiculae gemmifèrae” (de Lamarck
1836 ), “ovary-capsules” (Reid 1845 ), “calcareous capsules”
(Johnston 1847 ), “ovarian capsules” (Landsborough 1852 ),
and considering them as ovaries. This concept came to be
Abstract
This chapter focuses on brood-chamber structure and evolution in different cheilostome
lineages. Following a review of the history of studies on brooding in the order Cheilostomata,
different variants of brood- chamber structure and development are described, most for the
fi rst time. Their classifi cation is developed and the terminology involved has been clarifi ed.
The data obtained show that cheilostome brooding evolved independently several times
from modifi ed mural spines, kenozooids, outgrowths of the zooid wall and fertilization
envelopes. Accordingly, suborder Flustrina as currently conceived is considered polyphyletic.
Major trends in the evolution of skeletal brood chambers (ovicells) are reconstructed using
living and fossil taxa. The early evolution of conventional ovicells included curvature of the
most proximal mural spines, their fl attening, and reduction in number as well as loss of
joints and fusion. Further changes were intimately connected with the evolution of complex
frontal zooidal shields.
Keywords
Brood chambers • Diversity • Frontal shields • Independent evolution • Kenozooids •
Ovicells • Spines
Cheilostome Brood Chambers:
Structure, Formation, Evolution
2
A. Ostrovsky, Evolution of Sexual Reproduction in Marine Invertebrates: Example of gymnolaemate bryozoans,
DOI 10.1007/978-94-007-7146-8_2, © Springer Science+Business Media Dordrecht 2013
2.1
History of Studies of Cheilostome
Brood Chambers
Cheilostome bryozoans possess a broad range of methods
for embryonic incubation. Embryos are brooded in the external membranous sacs, skeletal (calcifi ed) chambers and
internal brood sacs formed by non-calcifi ed zooidal walls, or
develop intracoelomically in viviparous species. In some
instances extraembryonic nutrition (EEN) has evolved.
Most cheilostomes temporarily house their offspring
in skeletal chambers called ovicells. The presence or
absence of ovicells, and their morphology, are important
characters in cheilostome taxonomy. There are several
morphological types, the commonest being hyperstomial
ovicells that often look like prominent hemispherical
bubbles or helmets on the colony surface. Basically, the
hyperstomial ovicell consists of (1) a double-walled, calcifi ed protective fold (ooecium) with a coelomic cavity
between the two walls, (2) a non- calcifi ed part of the distal
wall of the egg-producing maternal autozooid, and (3) the
brood cavity between these two components (see Fig. 1 in
Introduction, Figs. 2.1 , 2.3 , and 2.5 ).
Ovicells were fi rst described by Ellis ( 1753 , 1755 ) who
suggested that they were snail-like “neritae,” formed from
the “polypes,” able to detach from a branch (to drop, fi x to
the substratum, and give rise to a new animal) or to lay eggs
(see also Ellis and Solander 1786 ) (Fig. 2.2 ). Following
Linnaeus ( 1758 ), Pallas ( 1766 , p. 36) opined that these
“bulla[e], galeae” [helmet-like bubbles, i.e. ovicells] might
be ovaria. He speculated that both ovicells and avicularia
might serve for fertilization and sometimes called them
“Nectariums” (see also Ostrovsky 2008a , and Appendix I for
details and discussion).
Later authors followed Linnaeus and Pallas, calling
ovicells “corps vesiculaires”, “corps globuleux” (Lamouroux
1816 ), “vesicules gemmifères”, “capsules gemmifères”
(Milne Edwards 1836 ), “vesiculae gemmifèrae” (de Lamarck
1836 ), “ovary-capsules” (Reid 1845 ), “calcareous capsules”
(Johnston 1847 ), “ovarian capsules” (Landsborough 1852 ),
and considering them as ovaries. This concept came to be
Abstract
This chapter focuses on brood-chamber structure and evolution in different cheilostome
lineages. Following a review of the history of studies on brooding in the order Cheilostomata,
different variants of brood- chamber structure and development are described, most for the
fi rst time. Their classifi cation is developed and the terminology involved has been clarifi ed.
The data obtained show that cheilostome brooding evolved independently several times
from modifi ed mural spines, kenozooids, outgrowths of the zooid wall and fertilization
envelopes. Accordingly, suborder Flustrina as currently conceived is considered polyphyletic.
Major trends in the evolution of skeletal brood chambers (ovicells) are reconstructed using
living and fossil taxa. The early evolution of conventional ovicells included curvature of the
most proximal mural spines, their fl attening, and reduction in number as well as loss of
joints and fusion. Further changes were intimately connected with the evolution of complex
frontal zooidal shields.
Keywords
Brood chambers • Diversity • Frontal shields • Independent evolution • Kenozooids •
Ovicells • Spines
Cheilostome Brood Chambers:
Structure, Formation, Evolution
2