120
indirectly by the surrounding trophamnion whilst larvae develop to the adult stolon.
Further yolk incorporated during the inversion of germ layers just prior to egg
spawning supports several days of free-living existence after the stolon is released
and undergoes fragmentation in the environment.
The endo- and ectodermal nature of myxozoan epithelia remains unknown, but,
if homologous with that of Polypodium, the outer epithelium of malacosporean sacs
and worms and the wall of myxosporean pansporocysts in invertebrate hosts would
be endodermal (see also below discussion). In contrast, sporogonic stages developing in fish hosts (plasmodia of myxosporeans and pseudoplasmodia of malacosporeans) are characterised by development within an outer cell—an arrangement
similar to the trophamnion-enclosed larval stages of Polypodium (Table 4.2).
Endocnidozoans have repeatedly evolved stages that retain multiple copies of
nuclear content within a single cell. These include cell-in-cell complexes of myxozoans, binucleate stages of Polypodium, syncytial plasmodia of myxosporeans, and
polyploidy in the trophamnion stage of Polypodium (the latter inferred in an early
cytophotometry study (Raikova 1965)). All cell-in-cell myxozoan stages appear to
originate from engulfment of one cell by another forming cell complexes (Feist
et al. 2015). These have mistakenly been described as binucleate stages similar to
those of Polypodium (Raikova 1994, 2008; Holzer et al. 2018). However, Morris
(2012) showed that the cell membrane surrounding engulfed myxozoan cells
becomes indistinct (suggesting a binucleate condition), but this membrane reappears later in sporoplasm development. Binucleate cells in Polypodium are regarded
as long-lived haploid stages that have not subsequently divided after the second
meiotic division (i.e. cytokinesis is postponed; Raikova 2008).
4.4 Evolution and Life Cycles of Endocnidozoans
4.4.1 Preadaptations to Parasitism
Cnidarians possess several traits that may predispose them for endoparasitic lifestyles (Okamura et al. 2015b). Their diploblastic body plan is manifested by the
extensive development of external and internal epithelial layers across which
resource capture, uptake and excretion are performed. An inherent capacity for
uptake of dissolved organic material across these surfaces (e.g. Grover et al. 2008)
may particularly facilitate adopting endoparasitic lifestyles (Okamura et al. 2015b).
These combined cnidarian features are similarly likely to support the intimate relationships cnidarians have repeatedly evolved with endosymbionts (Kayal et al.
2018) based on nutrient exchange (e.g. in sea anemones, corals, green Hydra, and
stalked jellyfish).
Nematocysts are triggered to discharge filaments in response to specific chemical
or mechanical environmental stimuli. These filaments function variously in prey
capture or defense, achieved by penetration and injection of venoms or digestive
B. Okamura and A. Gruhl
indirectly by the surrounding trophamnion whilst larvae develop to the adult stolon.
Further yolk incorporated during the inversion of germ layers just prior to egg
spawning supports several days of free-living existence after the stolon is released
and undergoes fragmentation in the environment.
The endo- and ectodermal nature of myxozoan epithelia remains unknown, but,
if homologous with that of Polypodium, the outer epithelium of malacosporean sacs
and worms and the wall of myxosporean pansporocysts in invertebrate hosts would
be endodermal (see also below discussion). In contrast, sporogonic stages developing in fish hosts (plasmodia of myxosporeans and pseudoplasmodia of malacosporeans) are characterised by development within an outer cell—an arrangement
similar to the trophamnion-enclosed larval stages of Polypodium (Table 4.2).
Endocnidozoans have repeatedly evolved stages that retain multiple copies of
nuclear content within a single cell. These include cell-in-cell complexes of myxozoans, binucleate stages of Polypodium, syncytial plasmodia of myxosporeans, and
polyploidy in the trophamnion stage of Polypodium (the latter inferred in an early
cytophotometry study (Raikova 1965)). All cell-in-cell myxozoan stages appear to
originate from engulfment of one cell by another forming cell complexes (Feist
et al. 2015). These have mistakenly been described as binucleate stages similar to
those of Polypodium (Raikova 1994, 2008; Holzer et al. 2018). However, Morris
(2012) showed that the cell membrane surrounding engulfed myxozoan cells
becomes indistinct (suggesting a binucleate condition), but this membrane reappears later in sporoplasm development. Binucleate cells in Polypodium are regarded
as long-lived haploid stages that have not subsequently divided after the second
meiotic division (i.e. cytokinesis is postponed; Raikova 2008).
4.4 Evolution and Life Cycles of Endocnidozoans
4.4.1 Preadaptations to Parasitism
Cnidarians possess several traits that may predispose them for endoparasitic lifestyles (Okamura et al. 2015b). Their diploblastic body plan is manifested by the
extensive development of external and internal epithelial layers across which
resource capture, uptake and excretion are performed. An inherent capacity for
uptake of dissolved organic material across these surfaces (e.g. Grover et al. 2008)
may particularly facilitate adopting endoparasitic lifestyles (Okamura et al. 2015b).
These combined cnidarian features are similarly likely to support the intimate relationships cnidarians have repeatedly evolved with endosymbionts (Kayal et al.
2018) based on nutrient exchange (e.g. in sea anemones, corals, green Hydra, and
stalked jellyfish).
Nematocysts are triggered to discharge filaments in response to specific chemical
or mechanical environmental stimuli. These filaments function variously in prey
capture or defense, achieved by penetration and injection of venoms or digestive
B. Okamura and A. Gruhl