xiv
Also Chia (1974) classifi ed “developmental patterns”, combining larval types (feeding vs.
non-feeding) and their “habitat” during development (pelagic, benthic, brooded, viviparous).
What are the prerequisites, causes and consequences of the emergence of different reproductive strategies and patterns? And what are the trends in the evolution of their key components: gametogenesis, fertilization and parental care? Finally, can we use data on sexual
reproduction for reconstructing stages in the evolutionary history of life, for instance, in specifying phylogenies and constructing evolutionary scenarios?
Since the main objective of zoology is the study of diversity, evolution and phylogenetic
relations among different animal groups, evolutionary studies of sexual reproduction would
appear to have a very important role. Traditionally, such information is widely applied when
reconstructing the historical past of organisms, since it concerns two key aspects of their existence: their structure and its replication. Changes in sexual reproduction are directly refl ected
in the evolutionary trajectories of the various groups. For instance, the transition from a longlived feeding larva to a short-lived non-feeding one, associated with changes in the mode of
oogenesis, should result in the isolation of distant populations, thus accelerating speciation
rates (Jablonski 1986, 2005; Jablonski and Lutz 1983). The origin of parental care certainly
resulted in better survival of progeny, and thus might have infl uenced the evolutionary success
of the animal group (Clutton -Brock 1991).
Investigations conducted within the framework of traditional morphological methodology
are usually confi ned to the comparative anatomy of reproductive systems, the results of this
kind of analysis being then applied to evolutionary and phylogenetic constructions. Numerous
studies also deal with the comparative morphology of gametes, the features of gametogenesis,
fertilization, and incubation and the structures responsible for them. However, the multi-sided
approach, integrating data from the various aspects of reproduction, is rare and the reviews on
reproduction in most invertebrate groups are often incomplete and fragmentary as well as lacking recent data. Besides, for obvious reasons, the evolution of sexual reproduction in most
groups is reconstructed mainly on the basis of information about living organisms.
The state of knowledge about sexual reproduction in marine invertebrates can be exemplifi ed by bryozoans (phylum Bryozoa Ehrenberg, 1831). An analysis of the literature shows that
over 230 articles and monographs published since the pioneering works of Ellis (1753, 1755)
and Pallas (1766) contain data on various aspects of sexual reproduction in more than 350 species of marine gymnolaemates (class Gymnolaemata Allmann, 1856). Notwithstanding, information adequate enough to allow a comprehensive picture of reproductive cycles can be found
in fewer than two dozen publications covering about 30 species (see Appendix I for the species
list and history of studies). As for the most abundant bryozoan order, Cheilostomata Busk,
1852, comprising more than 1,060 genera and 150 families (Gordon 2012), reproduction has
been studied in some detail in just 10 species representing 10 families. This is the factual basis
for the best review on sexual reproduction in the Bryozoa (published by Reed 1991). Can we
extrapolate these data to present an adequate picture for the whole phylum? Obviously we cannot. As a result, the evolution of sexual reproduction in bryozoans is hardly ever discussed in
the literature, even oogenetic changes appear to have played a crucial role in the emergence of
the lecithotrophic larva and possibly the consequent radiations of bryozoan clades (Taylor
1988; Ostrovsky 2009).
At present, researchers working with marine invertebrates tend to pay much more attention
to the study of larval types. Several explanations for this tendency may be proposed (discussed
in Strathmann 1978, 1986). Firstly, many structural features of planktotrophic larvae, being
highly conservative, have played a traditionally important role in evolutionary morphological
and phylogenetic reconstructions (e.g. Schneider 1869; Hatschek 1877, 1878, 1888–91;
Ostroumoff 1886a, b, c; Garstang 1951; Nielsen 1971, 1977, 1995, 1998, 2001, 2008, 2013;
Jägersten 1972; Zimmer 1973; Farmer 1977; Ivanova-Kazas 1986, 1995; Wray 1995a; Hall
and Wake 1999; Hickman 1999; Rouse 1999; Williamson 2001; Malakhov 2004).
Secondly, major differences in the dispersal of planktotrophic and lecithotrophic larvae have
formed the basis of zoogeographical studies as well as studies of genetic exchange between
Introduction
Also Chia (1974) classifi ed “developmental patterns”, combining larval types (feeding vs.
non-feeding) and their “habitat” during development (pelagic, benthic, brooded, viviparous).
What are the prerequisites, causes and consequences of the emergence of different reproductive strategies and patterns? And what are the trends in the evolution of their key components: gametogenesis, fertilization and parental care? Finally, can we use data on sexual
reproduction for reconstructing stages in the evolutionary history of life, for instance, in specifying phylogenies and constructing evolutionary scenarios?
Since the main objective of zoology is the study of diversity, evolution and phylogenetic
relations among different animal groups, evolutionary studies of sexual reproduction would
appear to have a very important role. Traditionally, such information is widely applied when
reconstructing the historical past of organisms, since it concerns two key aspects of their existence: their structure and its replication. Changes in sexual reproduction are directly refl ected
in the evolutionary trajectories of the various groups. For instance, the transition from a longlived feeding larva to a short-lived non-feeding one, associated with changes in the mode of
oogenesis, should result in the isolation of distant populations, thus accelerating speciation
rates (Jablonski 1986, 2005; Jablonski and Lutz 1983). The origin of parental care certainly
resulted in better survival of progeny, and thus might have infl uenced the evolutionary success
of the animal group (Clutton -Brock 1991).
Investigations conducted within the framework of traditional morphological methodology
are usually confi ned to the comparative anatomy of reproductive systems, the results of this
kind of analysis being then applied to evolutionary and phylogenetic constructions. Numerous
studies also deal with the comparative morphology of gametes, the features of gametogenesis,
fertilization, and incubation and the structures responsible for them. However, the multi-sided
approach, integrating data from the various aspects of reproduction, is rare and the reviews on
reproduction in most invertebrate groups are often incomplete and fragmentary as well as lacking recent data. Besides, for obvious reasons, the evolution of sexual reproduction in most
groups is reconstructed mainly on the basis of information about living organisms.
The state of knowledge about sexual reproduction in marine invertebrates can be exemplifi ed by bryozoans (phylum Bryozoa Ehrenberg, 1831). An analysis of the literature shows that
over 230 articles and monographs published since the pioneering works of Ellis (1753, 1755)
and Pallas (1766) contain data on various aspects of sexual reproduction in more than 350 species of marine gymnolaemates (class Gymnolaemata Allmann, 1856). Notwithstanding, information adequate enough to allow a comprehensive picture of reproductive cycles can be found
in fewer than two dozen publications covering about 30 species (see Appendix I for the species
list and history of studies). As for the most abundant bryozoan order, Cheilostomata Busk,
1852, comprising more than 1,060 genera and 150 families (Gordon 2012), reproduction has
been studied in some detail in just 10 species representing 10 families. This is the factual basis
for the best review on sexual reproduction in the Bryozoa (published by Reed 1991). Can we
extrapolate these data to present an adequate picture for the whole phylum? Obviously we cannot. As a result, the evolution of sexual reproduction in bryozoans is hardly ever discussed in
the literature, even oogenetic changes appear to have played a crucial role in the emergence of
the lecithotrophic larva and possibly the consequent radiations of bryozoan clades (Taylor
1988; Ostrovsky 2009).
At present, researchers working with marine invertebrates tend to pay much more attention
to the study of larval types. Several explanations for this tendency may be proposed (discussed
in Strathmann 1978, 1986). Firstly, many structural features of planktotrophic larvae, being
highly conservative, have played a traditionally important role in evolutionary morphological
and phylogenetic reconstructions (e.g. Schneider 1869; Hatschek 1877, 1878, 1888–91;
Ostroumoff 1886a, b, c; Garstang 1951; Nielsen 1971, 1977, 1995, 1998, 2001, 2008, 2013;
Jägersten 1972; Zimmer 1973; Farmer 1977; Ivanova-Kazas 1986, 1995; Wray 1995a; Hall
and Wake 1999; Hickman 1999; Rouse 1999; Williamson 2001; Malakhov 2004).
Secondly, major differences in the dispersal of planktotrophic and lecithotrophic larvae have
formed the basis of zoogeographical studies as well as studies of genetic exchange between
Introduction