xv
populations (e.g. Thorson 1950; Mileikovsky 1971; Sheltema 1971; Jablonski 1986; Strathmann
1986; Kasyanov 1989; Poulin and Féral 1994, 1996; McEdward 1995; Levin 2006).
Thirdly, the transition from planktotrophy to lecithotrophy, which occurred repeatedly in
the history of different groups of marine invertebrates, has enabled studies of the evolutionary
ecology of larval types and reconstructions of the evolution of life cycles (e.g. Vance 1973;
Smith and Fretwell 1974; Strathmann 1977, 1985, 1993, 2007; Christiansen and Fenchel 1979;
Kasyanov 1989; Havenhand 1995; Nielsen 1998; Hall and Wake 1999; Pechenik 1999;
Hickman 1999; Peterson 2005).
Fourthly, the presence of different larval types within the same taxon affords an opportunity
to study the molecular basis of the emergence and further evolution of the new larval types as
well as developmental changes accompanying this process (Strathmann 1978; Sinervo and
McEdward 1988; Byrne and Barker 1991; Wray and Raff 1991; Wray 1995b; Byrne 1995;
Byrne and Cerra 1996; Raff 1996).
On the whole, most researchers have focused their attention not on the causes but on the
consequences of the transition to a new larval type or else on the adaptive costs and benefi ts of
the retention of larval types under changed environmental conditions (see McEdward 1995).
The fact that the emergence of new larval types is caused by changes in reproductive processes
in the maternal organism, which is also subject to external infl uences, is generally left without
comment.
In my opinion, the situation calls for a synoptic approach, with all the important components of sexual reproduction such as gametogenesis, fertilization, incubation of embryos and
development of larvae being studied together in a holistic evolutionary dynamic. Especially
promising in this regard are clades including both living taxa with different reproductive patterns and fossil taxa with identifi able reproductive characters. Comparison of reproductive
strategies and the corresponding patterns, the analysis of their distribution within clades and
information about the time of their origin allow us to formulate ideas about the directions and
stages of the evolution of sexual reproduction. This information may then be used for reconstruction of the evolutionary history and phylogenetic relationships of these groups.
This approach seems to hold much promise. For instance, successful attempts have recently
been made to use data on the distribution of planktotrophy, lecithotrophy and parental care for
reconstructions of the evolution and phylogeny of echinoderms, in particular, sea stars and sea
urchins (e.g. Wray 1996; Smith 1997; Jeffery 1997; Byrne 2006). Owing to the extensive fossil
record of Echinoidea, this kind of analysis was able to embrace both Recent and fossil species
and turned out to be very fruitful, confi rming previous phylogenetic relationships constructed
on the basis of morphological (skeletal) characters.
Bryozoa are another promising model for such research. With their high diversity of reproductive patterns and larval types, as well as their extensive fossil record, they are in fact ideally
suited for the application of the synoptic approach mentioned above.
Phylogenetic Relationships of the Phylum Bryozoa
Bryozoans (=Ectoprocta Nitsche, 1869) had been traditionally assigned, together with phoronids and brachiopods, to the group Tentaculata (Hatschek 1888–91; Marcus 1958; IvanovaKazas 1977; Hadorn and Wehner 1978; Westheide and Rieger 2007). Later, this name was
superseded by its synonym Lophophorata (Hyman 1959; Emig 1982, 1984; Willmer 1990;
Brusca and Brusca 2003; Malakhov 2004). The validity of Lophophorata as a monophyletic
group and its position amongst the Metazoa remains ambiguous (Willmer 1990; Nielsen 2001,
2002a; Dewel et al. 2002; Ruppert et al. 2004; Valentine 2004). On the basis of comparative
embryological and morphological data, most zoologists considered lophophorates as protostomes (Marcus 1958; Hyman 1959; Beklemishev 1969; Hadorn and Wehner 1978; Remane
et al. 1989; Malakhov 2004; Nielsen 2012), whereas some assigned them, either altogether or
in part, to Deuterostomia (Zimmer 1973; Meglitsch and Schram 1991; Eernisse et al. 1992;
Introduction
populations (e.g. Thorson 1950; Mileikovsky 1971; Sheltema 1971; Jablonski 1986; Strathmann
1986; Kasyanov 1989; Poulin and Féral 1994, 1996; McEdward 1995; Levin 2006).
Thirdly, the transition from planktotrophy to lecithotrophy, which occurred repeatedly in
the history of different groups of marine invertebrates, has enabled studies of the evolutionary
ecology of larval types and reconstructions of the evolution of life cycles (e.g. Vance 1973;
Smith and Fretwell 1974; Strathmann 1977, 1985, 1993, 2007; Christiansen and Fenchel 1979;
Kasyanov 1989; Havenhand 1995; Nielsen 1998; Hall and Wake 1999; Pechenik 1999;
Hickman 1999; Peterson 2005).
Fourthly, the presence of different larval types within the same taxon affords an opportunity
to study the molecular basis of the emergence and further evolution of the new larval types as
well as developmental changes accompanying this process (Strathmann 1978; Sinervo and
McEdward 1988; Byrne and Barker 1991; Wray and Raff 1991; Wray 1995b; Byrne 1995;
Byrne and Cerra 1996; Raff 1996).
On the whole, most researchers have focused their attention not on the causes but on the
consequences of the transition to a new larval type or else on the adaptive costs and benefi ts of
the retention of larval types under changed environmental conditions (see McEdward 1995).
The fact that the emergence of new larval types is caused by changes in reproductive processes
in the maternal organism, which is also subject to external infl uences, is generally left without
comment.
In my opinion, the situation calls for a synoptic approach, with all the important components of sexual reproduction such as gametogenesis, fertilization, incubation of embryos and
development of larvae being studied together in a holistic evolutionary dynamic. Especially
promising in this regard are clades including both living taxa with different reproductive patterns and fossil taxa with identifi able reproductive characters. Comparison of reproductive
strategies and the corresponding patterns, the analysis of their distribution within clades and
information about the time of their origin allow us to formulate ideas about the directions and
stages of the evolution of sexual reproduction. This information may then be used for reconstruction of the evolutionary history and phylogenetic relationships of these groups.
This approach seems to hold much promise. For instance, successful attempts have recently
been made to use data on the distribution of planktotrophy, lecithotrophy and parental care for
reconstructions of the evolution and phylogeny of echinoderms, in particular, sea stars and sea
urchins (e.g. Wray 1996; Smith 1997; Jeffery 1997; Byrne 2006). Owing to the extensive fossil
record of Echinoidea, this kind of analysis was able to embrace both Recent and fossil species
and turned out to be very fruitful, confi rming previous phylogenetic relationships constructed
on the basis of morphological (skeletal) characters.
Bryozoa are another promising model for such research. With their high diversity of reproductive patterns and larval types, as well as their extensive fossil record, they are in fact ideally
suited for the application of the synoptic approach mentioned above.
Phylogenetic Relationships of the Phylum Bryozoa
Bryozoans (=Ectoprocta Nitsche, 1869) had been traditionally assigned, together with phoronids and brachiopods, to the group Tentaculata (Hatschek 1888–91; Marcus 1958; IvanovaKazas 1977; Hadorn and Wehner 1978; Westheide and Rieger 2007). Later, this name was
superseded by its synonym Lophophorata (Hyman 1959; Emig 1982, 1984; Willmer 1990;
Brusca and Brusca 2003; Malakhov 2004). The validity of Lophophorata as a monophyletic
group and its position amongst the Metazoa remains ambiguous (Willmer 1990; Nielsen 2001,
2002a; Dewel et al. 2002; Ruppert et al. 2004; Valentine 2004). On the basis of comparative
embryological and morphological data, most zoologists considered lophophorates as protostomes (Marcus 1958; Hyman 1959; Beklemishev 1969; Hadorn and Wehner 1978; Remane
et al. 1989; Malakhov 2004; Nielsen 2012), whereas some assigned them, either altogether or
in part, to Deuterostomia (Zimmer 1973; Meglitsch and Schram 1991; Eernisse et al. 1992;
Introduction