129
et al. 1991; Bomfleur et al. 2012; De Baets et al. 2021b) suggesting a late Paleozoic
origin of this group. Most modern polychaete groups first appear in the Carboniferous,
but some are already known from the Devonian (Parry et al. 2014).
Few studies have investigated myxozoan infections in annelids (e.g. El-Matbouli
and Hoffmann 1998; Bartholomew et al. 1997). Pathological effects on annelid
hosts include tissue damage, reduced fecundity and hypertrophic growth (Elwell
et al. 2009; Alexander et al. 2015)—as occurs in infected phylactolaemate bryozoans (see below). In general, myxozoan stages are unlikely to be preserved in fossil
annelids, however, soft-body features are clearly recognisable in several exceptional
preservations (e.g. Briggs and Bartels 2010; Timm et al. 2016). A particularly stunning example is the preservation of spermatozoa—structures comparable in size to
myxozoan spores—from Eocene clitellate cocoons (Bomfleur et al. 2015).
Considering the vast diversity of annelids, it is clear that only a minor proportion of
recent annelid taxa is currently known to serve as hosts of myxozoans. This could
argue for a late acquisition of this host group, but might also reflect serious undersampling, as definitive hosts of the nearly 2200 myxosporean species described from
fish are yet to be resolved (Eszterbauer et al. 2015).
4.5.5 Bryozoan Origins and Fossil Record
Recent Bryozoa comprise the taxa Phylactolaemata, Stenolaemata and
Gymnolaemata. Phylactolaemata is considered sister to the other two groups
(Waeschenbach et al. 2012) and includes <100 described species (Massard and
Geimer 2008). In contrast to the vast majority of bryozoans, phylactolaemates are
uncalcified and occur exclusively in fresh water. Relationships amongst phylactolaemates are mostly unresolved in molecular phylogenetic analyses (Waeschenbach
et al. 2012; Hartikainen et al. 2013b). The oldest bryozoan fossils occur in the lowest Ordovician (Xia et al. 2007). The diversity of the calcified stenolaemates present
at that time was already high (six major groups, of which only the cyclostomes
survived the later Permian and Triassic extinctions), suggesting an earlier Cambrian
radiation. However, neither calcified nor soft-bodied bryozoans have been found so
far in the Cambrian (Taylor and Waeschenbach 2015). The lack of bryozoan soft
body fossils hampers reconstruction of the origin and history of the uncalcified
Phylactolaemata. Stem members of this group theoretically should have co-occurred
with stenolaemates and ctenostomatous gymnolaemates. The oldest chitinous statoblasts (asexual propagules and resting buds of phylactolaemates) are found in the
Permian (Vinogradow 1996). Statoblasts are interpreted as clear adaptations to fresh
water, indicating that this lifestyle had evolved by then. Earlier stem phylactolaemates are likely to have been marine. Other recent marine bryozoan groups (the
cheilostomes and cyclostomes) diversified in the mid-Mesozoic (Taylor and
Waeschenbach 2015).
So far phylactolaemates are the only known bryozoan hosts of myxozoans.
Hartikainen et al. (2013a) have shown that malacosporeans can affect
4 Evolution, Origins and Diversification of Parasitic Cnidarians
et al. 1991; Bomfleur et al. 2012; De Baets et al. 2021b) suggesting a late Paleozoic
origin of this group. Most modern polychaete groups first appear in the Carboniferous,
but some are already known from the Devonian (Parry et al. 2014).
Few studies have investigated myxozoan infections in annelids (e.g. El-Matbouli
and Hoffmann 1998; Bartholomew et al. 1997). Pathological effects on annelid
hosts include tissue damage, reduced fecundity and hypertrophic growth (Elwell
et al. 2009; Alexander et al. 2015)—as occurs in infected phylactolaemate bryozoans (see below). In general, myxozoan stages are unlikely to be preserved in fossil
annelids, however, soft-body features are clearly recognisable in several exceptional
preservations (e.g. Briggs and Bartels 2010; Timm et al. 2016). A particularly stunning example is the preservation of spermatozoa—structures comparable in size to
myxozoan spores—from Eocene clitellate cocoons (Bomfleur et al. 2015).
Considering the vast diversity of annelids, it is clear that only a minor proportion of
recent annelid taxa is currently known to serve as hosts of myxozoans. This could
argue for a late acquisition of this host group, but might also reflect serious undersampling, as definitive hosts of the nearly 2200 myxosporean species described from
fish are yet to be resolved (Eszterbauer et al. 2015).
4.5.5 Bryozoan Origins and Fossil Record
Recent Bryozoa comprise the taxa Phylactolaemata, Stenolaemata and
Gymnolaemata. Phylactolaemata is considered sister to the other two groups
(Waeschenbach et al. 2012) and includes <100 described species (Massard and
Geimer 2008). In contrast to the vast majority of bryozoans, phylactolaemates are
uncalcified and occur exclusively in fresh water. Relationships amongst phylactolaemates are mostly unresolved in molecular phylogenetic analyses (Waeschenbach
et al. 2012; Hartikainen et al. 2013b). The oldest bryozoan fossils occur in the lowest Ordovician (Xia et al. 2007). The diversity of the calcified stenolaemates present
at that time was already high (six major groups, of which only the cyclostomes
survived the later Permian and Triassic extinctions), suggesting an earlier Cambrian
radiation. However, neither calcified nor soft-bodied bryozoans have been found so
far in the Cambrian (Taylor and Waeschenbach 2015). The lack of bryozoan soft
body fossils hampers reconstruction of the origin and history of the uncalcified
Phylactolaemata. Stem members of this group theoretically should have co-occurred
with stenolaemates and ctenostomatous gymnolaemates. The oldest chitinous statoblasts (asexual propagules and resting buds of phylactolaemates) are found in the
Permian (Vinogradow 1996). Statoblasts are interpreted as clear adaptations to fresh
water, indicating that this lifestyle had evolved by then. Earlier stem phylactolaemates are likely to have been marine. Other recent marine bryozoan groups (the
cheilostomes and cyclostomes) diversified in the mid-Mesozoic (Taylor and
Waeschenbach 2015).
So far phylactolaemates are the only known bryozoan hosts of myxozoans.
Hartikainen et al. (2013a) have shown that malacosporeans can affect
4 Evolution, Origins and Diversification of Parasitic Cnidarians