140
spaces, such as body cavities, blood vessels and between or within cells (Okamura
et al. 2015b). Simplification along with miniaturisation has enabled myxozoans to
converge on patterns of host exploitation similar to those of protists (Okamura et al.
2015a), involving extensive multiplication as unicellular forms within hosts. In
myxozoans these forms comprise invasive cells and cell-in-cell complexes that multiply within hosts whilst journeying to target infection sites, as well as sacs, myxoworms, pansporocysts, pseudoplasmodia and plasmodia that support spore
development. Free-living cnidarians produce relatively few and large eggs and
numerous tiny sperm (anisogamy). Even though there is some confusion over which
myxozoan cells are gametes, it is clear that both Polypodium and myxozoans are
isogamous (Raikova 2008; Okamura et al. 2015b). This may be adaptive for parasitism if self-fertilisation is undertaken because finding a mate is difficult within hosts
(Okamura et al. 2015b). Other adaptations to parasitism in Polypodium include
inversion of germ layers and a polyploid trophamnion (Raikova 1994) functioning
as a nurse cell during larval development and the multinucleate plasmodia that support sporogony in myxosporeans.
Parasites, however, may also evolve innovations involving complex and specialised designs. Examples include arrays of hooks and spines for host attachment in
the scolex of cestodes and the proboscis of acanthocephalans, and the outer syncytial tegument specialised for absorption, secretion and protection in trematodes and
cestodes (Ruppert et al. 2004). Multicellular spores of myxozoans and the modified
detachable gonads (gonophores) of Polypodium are both specialised endocnidozoan
innovations for transmission using anchoring nematocysts (Ibragimov and Raikova
2004). In addition, myxosporean spores display further morphological and structural innovations in the form of inflatable caudal processes in actinospores and hardening of the outer wall of myxospores. Caudal processes enable actinospores to be
carried away from the benthic habitats of worm hosts, presumably facilitating transmission to fish hosts. Hardening confers environmental resistance and hence spore
longevity, presumably facilitating eventual ingestion of myxospores by worms.
Myxospores also notably vary in form with some morphotypes arising from convergence. The drivers and functional significance of variation in myxospore morphology remain obscure. Similar actinospore morphotypes are produced in species that
develop different myxospore morphotypes suggesting that myxospores have undergone greater morphological diversification than actinospores (Fiala et al. 2015a).
However, it must be stressed that actinospores are very poorly sampled (see below).
4.7.2 Patterns of Diversification
A meta-analysis study has revealed that parasite species richness is strongly correlated with that of their hosts—richer host clades harbour richer parasite assemblages (Kamiya et al. 2014). This pattern is reflected in the invertebrate host
diversities of the relatively species-poor malacosporeans and the highly speciose
myxosporeans. Malacosporeans exploit freshwater bryozoans belonging to the
B. Okamura and A. Gruhl
spaces, such as body cavities, blood vessels and between or within cells (Okamura
et al. 2015b). Simplification along with miniaturisation has enabled myxozoans to
converge on patterns of host exploitation similar to those of protists (Okamura et al.
2015a), involving extensive multiplication as unicellular forms within hosts. In
myxozoans these forms comprise invasive cells and cell-in-cell complexes that multiply within hosts whilst journeying to target infection sites, as well as sacs, myxoworms, pansporocysts, pseudoplasmodia and plasmodia that support spore
development. Free-living cnidarians produce relatively few and large eggs and
numerous tiny sperm (anisogamy). Even though there is some confusion over which
myxozoan cells are gametes, it is clear that both Polypodium and myxozoans are
isogamous (Raikova 2008; Okamura et al. 2015b). This may be adaptive for parasitism if self-fertilisation is undertaken because finding a mate is difficult within hosts
(Okamura et al. 2015b). Other adaptations to parasitism in Polypodium include
inversion of germ layers and a polyploid trophamnion (Raikova 1994) functioning
as a nurse cell during larval development and the multinucleate plasmodia that support sporogony in myxosporeans.
Parasites, however, may also evolve innovations involving complex and specialised designs. Examples include arrays of hooks and spines for host attachment in
the scolex of cestodes and the proboscis of acanthocephalans, and the outer syncytial tegument specialised for absorption, secretion and protection in trematodes and
cestodes (Ruppert et al. 2004). Multicellular spores of myxozoans and the modified
detachable gonads (gonophores) of Polypodium are both specialised endocnidozoan
innovations for transmission using anchoring nematocysts (Ibragimov and Raikova
2004). In addition, myxosporean spores display further morphological and structural innovations in the form of inflatable caudal processes in actinospores and hardening of the outer wall of myxospores. Caudal processes enable actinospores to be
carried away from the benthic habitats of worm hosts, presumably facilitating transmission to fish hosts. Hardening confers environmental resistance and hence spore
longevity, presumably facilitating eventual ingestion of myxospores by worms.
Myxospores also notably vary in form with some morphotypes arising from convergence. The drivers and functional significance of variation in myxospore morphology remain obscure. Similar actinospore morphotypes are produced in species that
develop different myxospore morphotypes suggesting that myxospores have undergone greater morphological diversification than actinospores (Fiala et al. 2015a).
However, it must be stressed that actinospores are very poorly sampled (see below).
4.7.2 Patterns of Diversification
A meta-analysis study has revealed that parasite species richness is strongly correlated with that of their hosts—richer host clades harbour richer parasite assemblages (Kamiya et al. 2014). This pattern is reflected in the invertebrate host
diversities of the relatively species-poor malacosporeans and the highly speciose
myxosporeans. Malacosporeans exploit freshwater bryozoans belonging to the
B. Okamura and A. Gruhl