xxii
reduced the duration of the dispersal phase, which in planktotrophic cyphonautes larvae may
last 1–2 months, resulting in the isolation of distant populations. It is the disruption of genetic
exchange between populations that is considered as a direct cause of speciation (allopatric and
parapatric models) (Jablonski and Lutz 1983; Jablonski 1986; Poulin and Féral 1994; discussed in Havenhand 1995). Modern data support this scenario: bryozoan species with endotrophic larvae are much more genetically heterogeneous than those with planktotrophic larvae
that also have wider geographical range (Goldson et al. 2001; Porter et al. 2002; Watts and
Thorpe 2006).
However, as emphasized above, the emergence of a non-feeding larva is the result of dramatic changes in the maternal organism, namely, a shift in oogenesis. Transition from an exotrophic larva to an endotrophic one is based on an increase in the amount of energy input into
a single offspring with an accompanying decrease in the number of descendants, and this
means a change in reproductive strategy. Besides, all incubating Bryozoa, marine as well as
freshwater, have an endotrophic larva. Does this mean that the transition to a new larval type
in bryozoans was in some way associated with the origin of parental care?
So far the only well-substantiated and non-contradictory explanation of the Late Cretaceous
radiation of Cheilostomata appears to be the hypothesis suggested by Taylor (1988). While
agreeing with it in general, Gordon and Voigt (1996) nevertheless asked: could lecithotrophy,
once acquired, have sustained high speciation rates for so long? The above authors put forward
their own hypothesis, according to which the progressive evolution of cheilostome bryozoans
was based on the emergence of new types of protective skeletal structures, the frontal shields.
The evolution of non-feeding larvae and brooding is seen as a trigger of radiation, later sustained by the evolution of skeletal structures. Jablonski et al. (1997) posited that Taylor’s
hypothesis is contradicted by the fact that in cyclostome bryozoans (which usually coexist with
cheilostomes), the acquisition of gonozooids (and, possibly, of an endotrophic larva) in the
Late Triassic (Taylor and Michalik 1991) resulted only in moderate diversifi cation (see also
Taylor and Larwood 1990; Lidgard et al. 1993). At the same time, these authors stressed that
the available data were insuffi cient for any fi nal judgement. However, the fact that endotrophic
larvae and incubation are widespread in bryozoans indicates that these novelties might have
played a very important role in their evolution.
Parental care is a common phenomenon. In particular, invertebrates are known to have different variants of brooding (Porifera, Cnidaria, Annelida, Mollusca, Arthropoda, Kamptozoa,
Echinodermata, Brachiopoda, Phoronida, Pterobranchia), viviparity and matrotrophy (found
in representatives of more than twenty of the 34 known phyla) (Giese and Pierse 1974, 1975a,
b, 1977; Giese et al. 1979, 1987, 1991; Adiyodi and Adiyodi 1989, 1990; Levin and Bridges
1995; Batygina et al. 2006). Bryozoans are no exception: parental care is characteristic of most
representatives of the phylum. All cyclostomes (and, presumably, some others of the Paleozoic
stenolaemates) as well as the cheilostome family Epistomiidae are viviparous. All phylactolaemates and most gymnolaemates brood their offspring in specialized brood chambers. The
question is, how and under what circumstances did different modes of parental care evolve?
What were the evolutionary consequences of these innovations? Why and in what directions
was sexual reproduction within the order Cheilostomata and other bryozoan groups evolving,
and how did this infl uence the evolutionary fate of these epibiotic organisms?
About This Book
This monograph is the result of a long period of comparative-anatomical study of oogenesis,
fertilization, brooding and associated organs and structures in cheilostome bryozoans.
Altogether, 258 recent and fossil species from 148 genera and 66 families have been studied
using light and scanning electron microscopy (see Appendix II: Materials and Methods and
List of Taxa Studied). Comparative analysis of the data obtained made it possible to reconstruct the main stages and to reveal the major trends in the evolution of sexual reproduction in
Introduction
reduced the duration of the dispersal phase, which in planktotrophic cyphonautes larvae may
last 1–2 months, resulting in the isolation of distant populations. It is the disruption of genetic
exchange between populations that is considered as a direct cause of speciation (allopatric and
parapatric models) (Jablonski and Lutz 1983; Jablonski 1986; Poulin and Féral 1994; discussed in Havenhand 1995). Modern data support this scenario: bryozoan species with endotrophic larvae are much more genetically heterogeneous than those with planktotrophic larvae
that also have wider geographical range (Goldson et al. 2001; Porter et al. 2002; Watts and
Thorpe 2006).
However, as emphasized above, the emergence of a non-feeding larva is the result of dramatic changes in the maternal organism, namely, a shift in oogenesis. Transition from an exotrophic larva to an endotrophic one is based on an increase in the amount of energy input into
a single offspring with an accompanying decrease in the number of descendants, and this
means a change in reproductive strategy. Besides, all incubating Bryozoa, marine as well as
freshwater, have an endotrophic larva. Does this mean that the transition to a new larval type
in bryozoans was in some way associated with the origin of parental care?
So far the only well-substantiated and non-contradictory explanation of the Late Cretaceous
radiation of Cheilostomata appears to be the hypothesis suggested by Taylor (1988). While
agreeing with it in general, Gordon and Voigt (1996) nevertheless asked: could lecithotrophy,
once acquired, have sustained high speciation rates for so long? The above authors put forward
their own hypothesis, according to which the progressive evolution of cheilostome bryozoans
was based on the emergence of new types of protective skeletal structures, the frontal shields.
The evolution of non-feeding larvae and brooding is seen as a trigger of radiation, later sustained by the evolution of skeletal structures. Jablonski et al. (1997) posited that Taylor’s
hypothesis is contradicted by the fact that in cyclostome bryozoans (which usually coexist with
cheilostomes), the acquisition of gonozooids (and, possibly, of an endotrophic larva) in the
Late Triassic (Taylor and Michalik 1991) resulted only in moderate diversifi cation (see also
Taylor and Larwood 1990; Lidgard et al. 1993). At the same time, these authors stressed that
the available data were insuffi cient for any fi nal judgement. However, the fact that endotrophic
larvae and incubation are widespread in bryozoans indicates that these novelties might have
played a very important role in their evolution.
Parental care is a common phenomenon. In particular, invertebrates are known to have different variants of brooding (Porifera, Cnidaria, Annelida, Mollusca, Arthropoda, Kamptozoa,
Echinodermata, Brachiopoda, Phoronida, Pterobranchia), viviparity and matrotrophy (found
in representatives of more than twenty of the 34 known phyla) (Giese and Pierse 1974, 1975a,
b, 1977; Giese et al. 1979, 1987, 1991; Adiyodi and Adiyodi 1989, 1990; Levin and Bridges
1995; Batygina et al. 2006). Bryozoans are no exception: parental care is characteristic of most
representatives of the phylum. All cyclostomes (and, presumably, some others of the Paleozoic
stenolaemates) as well as the cheilostome family Epistomiidae are viviparous. All phylactolaemates and most gymnolaemates brood their offspring in specialized brood chambers. The
question is, how and under what circumstances did different modes of parental care evolve?
What were the evolutionary consequences of these innovations? Why and in what directions
was sexual reproduction within the order Cheilostomata and other bryozoan groups evolving,
and how did this infl uence the evolutionary fate of these epibiotic organisms?
About This Book
This monograph is the result of a long period of comparative-anatomical study of oogenesis,
fertilization, brooding and associated organs and structures in cheilostome bryozoans.
Altogether, 258 recent and fossil species from 148 genera and 66 families have been studied
using light and scanning electron microscopy (see Appendix II: Materials and Methods and
List of Taxa Studied). Comparative analysis of the data obtained made it possible to reconstruct the main stages and to reveal the major trends in the evolution of sexual reproduction in
Introduction