131
host switching. There is no evidence so far of a further range of invertebrate hosts
associated with actinospore- or malacospore-producing stages of myxozoans.
Annelids and phylactolaemate bryozoans share traits that might predispose them
to be parasitised. These include collecting small particles for ingestion by screening
relatively large environmental samples (via suspension or deposit feeding) and large
body cavities in which parasites can develop. Other invertebrate groups with such
traits (for example bivalves, echinoderms or phoronids), might act or have acted as
hosts but have yet to be detected. Infection of fish may also have predisposed myxosporean hyperparasitism of platyhelminths enabling diversification in host use
over time.
4.5.7 Summary of Origins and Ancient Hosts
Due to the complete lack of fossils, we have no means of calibrating the age of the
last common ancestor of endocnidozoans. We can, however, constrain the divergence of the endocnidozoan stem lineage by using both the maximum age estimate
for crown Cnidaria and the minimum age estimate for Medusozoa (505 Ma)
(Fig. 4.2). Endocnidozoan characters, however, could have evolved at any point
along the endocnidozoan stem lineage—between their divergence from the rest of
the Cnidaria and the presence of the last common ancestor of all living endocnidozoans. Thus we cannot currently assess the traits of endocnidozoan stem lineage
members, including whether they were parasitic or which hosts they parasitised.
Molecular clock estimates so far undertaken propose ages of 651 Ma (601–700 Ma)
for Endocnidozoa and 588 Ma (540–642 Ma) for Myxozoa (Holzer et al. 2018) and
divergence of Malacosporea and Myxosporea at 540 ± 73 Ma (Kodádková et al.
2015). Although the crown groups of the main hosts of Recent endocnidozoans are
mostly estimated to be slightly younger than endocnidozoans (see above) it is
entirely possible that stem group members acted as hosts.
4.6 Inferring Endocnidozoan Origins and Acquisition
of Early Hosts
The lack of a fossil record and the possibility of host switching over time may
highly constrain our understanding of endocnidozoan origins and what hosts were
acquired when. Any insights, however flawed they may be, must be gained by evaluating data on extant taxa. Here we consider in general how hosts may be acquired
and, by extension, host acquisition by endocnidozoans. We then go on to examine
more closely some of the pitfalls of inferring parasite origins and patterns of host
use over time on the basis of molecular clock and cophylogenetic analyses.
4 Evolution, Origins and Diversification of Parasitic Cnidarians
host switching. There is no evidence so far of a further range of invertebrate hosts
associated with actinospore- or malacospore-producing stages of myxozoans.
Annelids and phylactolaemate bryozoans share traits that might predispose them
to be parasitised. These include collecting small particles for ingestion by screening
relatively large environmental samples (via suspension or deposit feeding) and large
body cavities in which parasites can develop. Other invertebrate groups with such
traits (for example bivalves, echinoderms or phoronids), might act or have acted as
hosts but have yet to be detected. Infection of fish may also have predisposed myxosporean hyperparasitism of platyhelminths enabling diversification in host use
over time.
4.5.7 Summary of Origins and Ancient Hosts
Due to the complete lack of fossils, we have no means of calibrating the age of the
last common ancestor of endocnidozoans. We can, however, constrain the divergence of the endocnidozoan stem lineage by using both the maximum age estimate
for crown Cnidaria and the minimum age estimate for Medusozoa (505 Ma)
(Fig. 4.2). Endocnidozoan characters, however, could have evolved at any point
along the endocnidozoan stem lineage—between their divergence from the rest of
the Cnidaria and the presence of the last common ancestor of all living endocnidozoans. Thus we cannot currently assess the traits of endocnidozoan stem lineage
members, including whether they were parasitic or which hosts they parasitised.
Molecular clock estimates so far undertaken propose ages of 651 Ma (601–700 Ma)
for Endocnidozoa and 588 Ma (540–642 Ma) for Myxozoa (Holzer et al. 2018) and
divergence of Malacosporea and Myxosporea at 540 ± 73 Ma (Kodádková et al.
2015). Although the crown groups of the main hosts of Recent endocnidozoans are
mostly estimated to be slightly younger than endocnidozoans (see above) it is
entirely possible that stem group members acted as hosts.
4.6 Inferring Endocnidozoan Origins and Acquisition
of Early Hosts
The lack of a fossil record and the possibility of host switching over time may
highly constrain our understanding of endocnidozoan origins and what hosts were
acquired when. Any insights, however flawed they may be, must be gained by evaluating data on extant taxa. Here we consider in general how hosts may be acquired
and, by extension, host acquisition by endocnidozoans. We then go on to examine
more closely some of the pitfalls of inferring parasite origins and patterns of host
use over time on the basis of molecular clock and cophylogenetic analyses.
4 Evolution, Origins and Diversification of Parasitic Cnidarians