142
cnidarians species can also be expected (Appeltans et al. 2012) the hidden nature of
endoparasites is likely to make myxozoans particularly undersampled. Further
impediments to understanding myxozoan diversification include: limited study of
invertebrates as parasite hosts; a research focus on economically important parasites; inapparency of many myxozoan infections; and difficulties in sampling and
identification (including low infection prevalences, lack of expertise, and requirement of microscopy and molecular sequence data) (Okamura et al. 2018).
Finally, it is clear that endocnidozoans are characterised by rapid rates of molecular evolution. How this trait may relate to diversification remains unclear. We predict that this issue will begin to be resolved with the development and analysis of
new transcriptome, genome and proteome datasets with larger taxonomic coverage.
4.8 Conclusions
Cnidarians have evolved parasitism on multiple occasions but only in the Myxozoa
has this entailed substantial radiation. There is evidence that myxozoan diversification is linked with host diversity and it is likely that various cnidarian traits served
as preadaptations for assuming parasitic lifestyles. However, our chapter raises
more questions than answers. Was parasitism adopted as an ancient lifestyle in the
cnidarians when the Endocnidozoa first appeared in the Ediacaran or was there a
substantial period when early endocnidozoans were free-living? Does undersampling and our current lack of knowledge of endocnidozoan diversity and host use
greatly compromise insights on host groups and co-phylogenetic analyses? To what
extent might host switching preclude inferences about ancient hosts and how have
mass extinctions driven patterns of endocnidozoan diversity over time? The holy
grail of gaining insights into endocnidozoan stem lineages may begin to be addressed
if relevant data can be gathered. Such data include fossil evidence that would
strengthen molecular clock analyses. Thus, systematic investigation of host pathologies and attempts to retrieve remains of myxozoan spores in palynological samples
or from coprolites could be fruitful, particularly from sites with exceptional softtissue preservation. Investigations of myxozoan spore taphonomy would be complementary and informative for such research. Meanwhile, more convincing
cophylogenetic analyses require better understanding of myxozoan diversity and
better resolution of host phylogenies. This could be gained by systematic sampling
of further potential host groups, eDNA sampling (especially from severely undersampled habitats), screening of existing archival databases, and incorporation of
more comprehensive data in molecular phylogenies of host groups. We expect that
advances will be made on at least some of these fronts and could thus help to reveal
how endocnidozoans have evolved and diversified to become one of the major cnidarian clades in the present day. We also expect that further work will provide a
better understanding of the origins and diversification of the other more limited
radiations of parasitic cnidarians.
B. Okamura and A. Gruhl
cnidarians species can also be expected (Appeltans et al. 2012) the hidden nature of
endoparasites is likely to make myxozoans particularly undersampled. Further
impediments to understanding myxozoan diversification include: limited study of
invertebrates as parasite hosts; a research focus on economically important parasites; inapparency of many myxozoan infections; and difficulties in sampling and
identification (including low infection prevalences, lack of expertise, and requirement of microscopy and molecular sequence data) (Okamura et al. 2018).
Finally, it is clear that endocnidozoans are characterised by rapid rates of molecular evolution. How this trait may relate to diversification remains unclear. We predict that this issue will begin to be resolved with the development and analysis of
new transcriptome, genome and proteome datasets with larger taxonomic coverage.
4.8 Conclusions
Cnidarians have evolved parasitism on multiple occasions but only in the Myxozoa
has this entailed substantial radiation. There is evidence that myxozoan diversification is linked with host diversity and it is likely that various cnidarian traits served
as preadaptations for assuming parasitic lifestyles. However, our chapter raises
more questions than answers. Was parasitism adopted as an ancient lifestyle in the
cnidarians when the Endocnidozoa first appeared in the Ediacaran or was there a
substantial period when early endocnidozoans were free-living? Does undersampling and our current lack of knowledge of endocnidozoan diversity and host use
greatly compromise insights on host groups and co-phylogenetic analyses? To what
extent might host switching preclude inferences about ancient hosts and how have
mass extinctions driven patterns of endocnidozoan diversity over time? The holy
grail of gaining insights into endocnidozoan stem lineages may begin to be addressed
if relevant data can be gathered. Such data include fossil evidence that would
strengthen molecular clock analyses. Thus, systematic investigation of host pathologies and attempts to retrieve remains of myxozoan spores in palynological samples
or from coprolites could be fruitful, particularly from sites with exceptional softtissue preservation. Investigations of myxozoan spore taphonomy would be complementary and informative for such research. Meanwhile, more convincing
cophylogenetic analyses require better understanding of myxozoan diversity and
better resolution of host phylogenies. This could be gained by systematic sampling
of further potential host groups, eDNA sampling (especially from severely undersampled habitats), screening of existing archival databases, and incorporation of
more comprehensive data in molecular phylogenies of host groups. We expect that
advances will be made on at least some of these fronts and could thus help to reveal
how endocnidozoans have evolved and diversified to become one of the major cnidarian clades in the present day. We also expect that further work will provide a
better understanding of the origins and diversification of the other more limited
radiations of parasitic cnidarians.
B. Okamura and A. Gruhl