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enzymes, attachment, and entanglement. Nematocysts have been co-opted in endoparasitic cnidarians. For example, the filaments discharged from myxozoan polar
capsules anchor transmission stages (spores) to host surfaces to enable the initiation
of infection (Kallert et al. 2015). Cnidarians secrete, either via nematocysts or standard secretory pathways, a wide variety of compounds such as toxins and antimicrobial peptides as well as cytolytic, proteolytic and other digestive enzymes
(Balasubramanian et al. 2012; Dunlap et al. 2013). These substances have a high
potential of becoming co-opted for host-parasite interactions such as host invasion,
tissue modification or defence against host immune system elements. Furthermore,
soft-bodied cnidarians have evolved specific epithelial defence mechanisms manifested in a high structural and molecular diversity of the glycocalyx, the apical
membrane surface layer (Bosch 2016). The glycocalyx forms the outer layer of
myxozoan spores, but also covers endoparasitic stages possibly functioning in
immune evasion or other host-parasite interactions (Gruhl and Okamura 2015).
Cnidarians are characterised by a high degree of asexual reproduction in the
form of e.g. fission, fragmentation and budding (Fautin 2002). The ability to proliferate asexually is a hallmark of many parasites, enabling them to exploit host
resources, to produce new stages, to overwhelm host immune responses, and to
facilitate horizontal transmission (Poulin 2007). Examples of such asexual replication in endoparasitic cnidarian stages include evidence for fission (Fig. 3.2c, d; in
Okamura et al. 2015a) and budding (Okamura 1996; McGurk et al. 2006) in sacforming myxozoans in the body cavity of bryozoan hosts, and budding of medusae
from polyp-like endoparasitic narcomedusans within the body cavity of polychaete
hosts (Bentlage et al. 2018). Parasitic polyp stages also bud off medusae in the ring
canal system of parental medusae (Bigelow 1909). However, the proliferation of
early stages of endocnidozoan infections and of parasitic larval narcomedusans is
achieved by binucleate cells and cell-within-cell stages (Bigelow 1909; Feist et al.
2015) that, as far as we are aware, have no ready homology with cell complexes in
free-living cnidarians.
There is clearly substantial plasticity in cnidarian life histories and the ability to
develop novel stages (Cartwright and Nawrocki 2010; Okamura et al. 2015b). For
example, medusa stages have been lost in many hydrozoans, including in Hydra,
with progenesis enabling sexual reproduction (Boero et al. 1992) and some medusozoans incorporate parasitic stages inferred to be specialised polyps (Table 4.1).
Novel stages of cnidarians can undergo dormancy, regeneration and transdifferentiation (reviewed in Lai and Aboobaker 2017)—processes that are intimately related
to asexual reproduction. For example, planulae or polyps shrink and persist as cysts
and dormant hydrorhizae with low metabolic costs during stressful periods. When
favourable conditions return, cell proliferation and morphogenesis ensue (Boero
et al. 1992). Polyp stages of the hydrozoan genus, Turritopsis, can be reformed from
regressed tissues of sexual stages (medusa) via reverse ontogeny—a process mediated by proliferation of interstitial stem cells and cell transdifferentiation (Piraino
et al. 2004). Medusa buds of the hydrozoan, Sarsia tubulosa, transform back to
polyp buds when exposed to different temperatures (Werner 1963). This phenomenon could be more common in cnidarians than previously anticipated (e.g. He et al.
4 Evolution, Origins and Diversification of Parasitic Cnidarians
enzymes, attachment, and entanglement. Nematocysts have been co-opted in endoparasitic cnidarians. For example, the filaments discharged from myxozoan polar
capsules anchor transmission stages (spores) to host surfaces to enable the initiation
of infection (Kallert et al. 2015). Cnidarians secrete, either via nematocysts or standard secretory pathways, a wide variety of compounds such as toxins and antimicrobial peptides as well as cytolytic, proteolytic and other digestive enzymes
(Balasubramanian et al. 2012; Dunlap et al. 2013). These substances have a high
potential of becoming co-opted for host-parasite interactions such as host invasion,
tissue modification or defence against host immune system elements. Furthermore,
soft-bodied cnidarians have evolved specific epithelial defence mechanisms manifested in a high structural and molecular diversity of the glycocalyx, the apical
membrane surface layer (Bosch 2016). The glycocalyx forms the outer layer of
myxozoan spores, but also covers endoparasitic stages possibly functioning in
immune evasion or other host-parasite interactions (Gruhl and Okamura 2015).
Cnidarians are characterised by a high degree of asexual reproduction in the
form of e.g. fission, fragmentation and budding (Fautin 2002). The ability to proliferate asexually is a hallmark of many parasites, enabling them to exploit host
resources, to produce new stages, to overwhelm host immune responses, and to
facilitate horizontal transmission (Poulin 2007). Examples of such asexual replication in endoparasitic cnidarian stages include evidence for fission (Fig. 3.2c, d; in
Okamura et al. 2015a) and budding (Okamura 1996; McGurk et al. 2006) in sacforming myxozoans in the body cavity of bryozoan hosts, and budding of medusae
from polyp-like endoparasitic narcomedusans within the body cavity of polychaete
hosts (Bentlage et al. 2018). Parasitic polyp stages also bud off medusae in the ring
canal system of parental medusae (Bigelow 1909). However, the proliferation of
early stages of endocnidozoan infections and of parasitic larval narcomedusans is
achieved by binucleate cells and cell-within-cell stages (Bigelow 1909; Feist et al.
2015) that, as far as we are aware, have no ready homology with cell complexes in
free-living cnidarians.
There is clearly substantial plasticity in cnidarian life histories and the ability to
develop novel stages (Cartwright and Nawrocki 2010; Okamura et al. 2015b). For
example, medusa stages have been lost in many hydrozoans, including in Hydra,
with progenesis enabling sexual reproduction (Boero et al. 1992) and some medusozoans incorporate parasitic stages inferred to be specialised polyps (Table 4.1).
Novel stages of cnidarians can undergo dormancy, regeneration and transdifferentiation (reviewed in Lai and Aboobaker 2017)—processes that are intimately related
to asexual reproduction. For example, planulae or polyps shrink and persist as cysts
and dormant hydrorhizae with low metabolic costs during stressful periods. When
favourable conditions return, cell proliferation and morphogenesis ensue (Boero
et al. 1992). Polyp stages of the hydrozoan genus, Turritopsis, can be reformed from
regressed tissues of sexual stages (medusa) via reverse ontogeny—a process mediated by proliferation of interstitial stem cells and cell transdifferentiation (Piraino
et al. 2004). Medusa buds of the hydrozoan, Sarsia tubulosa, transform back to
polyp buds when exposed to different temperatures (Werner 1963). This phenomenon could be more common in cnidarians than previously anticipated (e.g. He et al.
4 Evolution, Origins and Diversification of Parasitic Cnidarians