122
2015). These features may have aided precursors of parasitic cnidarians initially to
survive within hosts. For example, cryptic dormant stages of myxozoans are viable
and present in dormant propagules produced by invertebrate hosts (freshwater bryozoans) (Abd-Elfattah et al. 2014). Such capacities of regeneration and transdifferentiation may have facilitated the evolution of novel stages during transition to
parasitism.
4.4.2 Life Cycle Speculations
Sexual reproduction, evidenced by meiosis, occurs when myxozoans develop in
their invertebrate hosts. This suggests that trophic stages that develop in invertebrates are extremely morphologically simplified stages that may be equivalent to the
free-living, sexual stages of Polypodium (Okamura et al. 2015b). Although the typical medusa umbrella is lacking, the free-living stage of Polypodium has been
inferred to be a modified medusa (Raikova 1994; Raikova and Raikova 2016). This
inference is primarily based on the assumption that sexual reproduction invariably
is expressed in the medusa stage. The developing stolon enclosed within fish eggs
therefore is proposed to correspond to a polyp stage. However, molecular phylogenetic placement and life cycle plasticity arguments (see above) would equally suggest that the free-living stage could be equivalent to a polyp stage. Regardless of the
unresolved nature of Polypodium’s free-living stage (medusa vs. polyp), it is
achieved by larval development in fish eggs, albeit this is delayed until fish reach
sexual maturity. In contrast, myxozoans incorporate a second life cycle phase that
develops in vertebrate intermediate hosts. The lack of a biphasic life cycle in
Polypodium would suggest this could have been absent in the common ancestor of
Polypodium and Myxozoa and thus that myxozoan stages in vertebrate hosts
are novel.
4.5 Origins and Fossil Records of Endocnidozoa and Their
Recognised Major Host Groups
The fossil record of endoparasites is poor for many reasons (De Baets and Littlewood
2015; De Baets et al. 2021a; Leung 2017, 2021). Although endocnidozoan parasites
are diverse and abundant and have probably existed on earth for hundreds of millions of years, there is so far neither a direct nor indirect fossil record for this group.
The fossilization potential of within-host life-cycle stages of endocnidozoans is
arguably low. Such stages are soft, microscopic and most often reside within soft
host tissues which themselves are only rarely preserved. However, mature myxozoan spores, especially myxospores (myxosporean spores produced in vertebrate
hosts) can withstand adverse conditions like desiccation, extreme temperatures,
B. Okamura and A. Gruhl
2015). These features may have aided precursors of parasitic cnidarians initially to
survive within hosts. For example, cryptic dormant stages of myxozoans are viable
and present in dormant propagules produced by invertebrate hosts (freshwater bryozoans) (Abd-Elfattah et al. 2014). Such capacities of regeneration and transdifferentiation may have facilitated the evolution of novel stages during transition to
parasitism.
4.4.2 Life Cycle Speculations
Sexual reproduction, evidenced by meiosis, occurs when myxozoans develop in
their invertebrate hosts. This suggests that trophic stages that develop in invertebrates are extremely morphologically simplified stages that may be equivalent to the
free-living, sexual stages of Polypodium (Okamura et al. 2015b). Although the typical medusa umbrella is lacking, the free-living stage of Polypodium has been
inferred to be a modified medusa (Raikova 1994; Raikova and Raikova 2016). This
inference is primarily based on the assumption that sexual reproduction invariably
is expressed in the medusa stage. The developing stolon enclosed within fish eggs
therefore is proposed to correspond to a polyp stage. However, molecular phylogenetic placement and life cycle plasticity arguments (see above) would equally suggest that the free-living stage could be equivalent to a polyp stage. Regardless of the
unresolved nature of Polypodium’s free-living stage (medusa vs. polyp), it is
achieved by larval development in fish eggs, albeit this is delayed until fish reach
sexual maturity. In contrast, myxozoans incorporate a second life cycle phase that
develops in vertebrate intermediate hosts. The lack of a biphasic life cycle in
Polypodium would suggest this could have been absent in the common ancestor of
Polypodium and Myxozoa and thus that myxozoan stages in vertebrate hosts
are novel.
4.5 Origins and Fossil Records of Endocnidozoa and Their
Recognised Major Host Groups
The fossil record of endoparasites is poor for many reasons (De Baets and Littlewood
2015; De Baets et al. 2021a; Leung 2017, 2021). Although endocnidozoan parasites
are diverse and abundant and have probably existed on earth for hundreds of millions of years, there is so far neither a direct nor indirect fossil record for this group.
The fossilization potential of within-host life-cycle stages of endocnidozoans is
arguably low. Such stages are soft, microscopic and most often reside within soft
host tissues which themselves are only rarely preserved. However, mature myxozoan spores, especially myxospores (myxosporean spores produced in vertebrate
hosts) can withstand adverse conditions like desiccation, extreme temperatures,
B. Okamura and A. Gruhl