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Class Phylactolaemata, a notably depauperate group of 74 extant species (Massard
and Geimer 2008), whilst myxosporeans exploit freshwater and marine oligochaetes
and polychaetes which comprise 3500 and 8000 extant species, respectively
(Ruppert et al. 2004). Coincident with myxosporean radiations in polychaete and
oligochaete radiation is residency in marine and freshwater environments, respectively (Fiala et al. 2015a). The relatively limited radiation of malacosporeans
(despite their apparent capacity to infect diverse fish families) suggests that invertebrate host diversity may primarily explain the contrasting diversification patterns of
malacosporeans and myxosporeans. Congruence in molecular phylogenies of some
myxosporeans and fish provides evidence for co-diversification with certain fish
(Holzer et al. 2018).
In the present day the highly speciose myxosporeans infect vertebrate host gills,
skin, fins, eyes, kidney, intestine, liver, gall bladder, nervous system, cartilage, muscles, swimbladder and gonad to produce infectious spores (Feist and Longshaw
2006; Molnár and Eszterbauer 2015) whilst the malacosporeans exploit fish kidney
for spore production. The acquisition of plasmodia suited for sporulation in organs
and tissues may have been critical for myxosporeans to exploit tissues (Fiala et al.
2015a). Phylogenetic analyses have provided evidence that patterns of ‘tissue tropism’ (site-preference) are lineage-specific and that the ability to exploit particular
host environments may support radiations in at least some myxosporeans (e.g.
Eszterbauer 2004; Holzer et al. 2004; Heiniger et al. 2013).
Several other traits have been identified as potential drivers of the spectacular
myxosporean radiation or as constraints on malacosporeans to undergo radiation
(Fiala et al. 2015a). These include: acquisition of hardened, environmentally resistant spore valves in myxosporeans; improved uptake of metabolites by syncytial
myxosporean stages (vs. epithelial malacosporean stages), and; incorporation of
additional vertebrate host groups (e.g. amphibians, waterfowl and shrews) by myxosporeans. Some non-fish hosts (e.g. shrews) are entirely terrestrial thus terrestrial
oligochaetes are highly likely to serve as primary hosts with trophic transmission
achieved when infected worms are ingested. This raises the spectre of a diversity of
fully terrestrialised myxozoans with life cycles involving vertebrate hosts that eat
earthworms.
A current hindrance to understanding the significance of host diversity and environments for myxozoan diversification is our considerable ignorance of parasite
diversity in general and of myxozoans in particular (Okamura et al. 2018). Even in
relatively well-sampled regions, novel myxozoan diversity is being detected by
eDNA sampling (Hartikainen et al. 2016). Particularly poorly sampled regions
include the deep sea, polar and tropical environments. Ongoing research suggests
that extrapolation of diversity in tropical fish may lead to predictions of myxozoan
species richness that rivals or exceeds that of their free-living cnidarian relatives.
For instance, the high host specificity of many ceratomyxids (Gunter et al. 2010;
Heiniger and Adlard 2013) suggests that Australia’s coral reef fish will be exploited
by over 1500 species of Ceratomyxa of which <1% have been described (Queensland
Museum Network 2010). Recall that the currently described 2596 myxozoans represent some 20% of cnidarian species diversity already. While new free-living
4 Evolution, Origins and Diversification of Parasitic Cnidarians
Class Phylactolaemata, a notably depauperate group of 74 extant species (Massard
and Geimer 2008), whilst myxosporeans exploit freshwater and marine oligochaetes
and polychaetes which comprise 3500 and 8000 extant species, respectively
(Ruppert et al. 2004). Coincident with myxosporean radiations in polychaete and
oligochaete radiation is residency in marine and freshwater environments, respectively (Fiala et al. 2015a). The relatively limited radiation of malacosporeans
(despite their apparent capacity to infect diverse fish families) suggests that invertebrate host diversity may primarily explain the contrasting diversification patterns of
malacosporeans and myxosporeans. Congruence in molecular phylogenies of some
myxosporeans and fish provides evidence for co-diversification with certain fish
(Holzer et al. 2018).
In the present day the highly speciose myxosporeans infect vertebrate host gills,
skin, fins, eyes, kidney, intestine, liver, gall bladder, nervous system, cartilage, muscles, swimbladder and gonad to produce infectious spores (Feist and Longshaw
2006; Molnár and Eszterbauer 2015) whilst the malacosporeans exploit fish kidney
for spore production. The acquisition of plasmodia suited for sporulation in organs
and tissues may have been critical for myxosporeans to exploit tissues (Fiala et al.
2015a). Phylogenetic analyses have provided evidence that patterns of ‘tissue tropism’ (site-preference) are lineage-specific and that the ability to exploit particular
host environments may support radiations in at least some myxosporeans (e.g.
Eszterbauer 2004; Holzer et al. 2004; Heiniger et al. 2013).
Several other traits have been identified as potential drivers of the spectacular
myxosporean radiation or as constraints on malacosporeans to undergo radiation
(Fiala et al. 2015a). These include: acquisition of hardened, environmentally resistant spore valves in myxosporeans; improved uptake of metabolites by syncytial
myxosporean stages (vs. epithelial malacosporean stages), and; incorporation of
additional vertebrate host groups (e.g. amphibians, waterfowl and shrews) by myxosporeans. Some non-fish hosts (e.g. shrews) are entirely terrestrial thus terrestrial
oligochaetes are highly likely to serve as primary hosts with trophic transmission
achieved when infected worms are ingested. This raises the spectre of a diversity of
fully terrestrialised myxozoans with life cycles involving vertebrate hosts that eat
earthworms.
A current hindrance to understanding the significance of host diversity and environments for myxozoan diversification is our considerable ignorance of parasite
diversity in general and of myxozoans in particular (Okamura et al. 2018). Even in
relatively well-sampled regions, novel myxozoan diversity is being detected by
eDNA sampling (Hartikainen et al. 2016). Particularly poorly sampled regions
include the deep sea, polar and tropical environments. Ongoing research suggests
that extrapolation of diversity in tropical fish may lead to predictions of myxozoan
species richness that rivals or exceeds that of their free-living cnidarian relatives.
For instance, the high host specificity of many ceratomyxids (Gunter et al. 2010;
Heiniger and Adlard 2013) suggests that Australia’s coral reef fish will be exploited
by over 1500 species of Ceratomyxa of which <1% have been described (Queensland
Museum Network 2010). Recall that the currently described 2596 myxozoans represent some 20% of cnidarian species diversity already. While new free-living
4 Evolution, Origins and Diversification of Parasitic Cnidarians