114
non- endocnidozoan lineages of cnidarians that incorporate parasitic stages in their
life history therefore appear to have evolved fairly recently and to have undergone
modest to minimal radiation.
4.3 The Endocnidozoa
As the name implies, the Endocnidozoa is comprised of endoparasitic cnidarians.
This recently recognised clade incorporates the sister taxa Polypodium hydriforme
(henceforth referred to as Polypodium) and the diverse Myxozoa (Collins 2009;
Chang et al. 2015; Kayal et al. 2018). For a considerable time period long-branch
attraction obscured phylogenetic placement of both Myxozoa and Polypodium in
molecular phylogenetic analyses (Zrzavý and Hypša 2003; Foox and Siddall 2015;
Okamura and Gruhl 2015). However, phylogenomic (Chang et al. 2015; Kayal et al.
2018) and some morphological (e.g. Siddall et al. 1995; and see below) evidence
currently places these as sister taxa comprising the Endocnidozoa, which itself is
sister to the Medusozoa (Fig. 4.1).
4.3.1 General Biology
Polypodium’s one-host life cycle (Fig. 4.1c) includes a free-living adult phase and
parasitic larval stages in acipenseriform fish (sturgeon and paddlefish). Myxozoans
have complex parasitic life cycles and require both invertebrate and vertebrate hosts
for development (Fig. 4.1a, b). Invertebrate hosts include freshwater (phylactolaemate) bryozoans (exploited by the Malacosporea) and marine and freshwater oligochaetes and polychaetes (exploited by the Myxosporea) (Fiala et al. 2015a).
Myxosporean infections (including spore production) reported in octopus
(Yokoyama and Masuda 2001) and in a monogenean infecting fish (a case of hyperparasitism) (Freeman and Shinn 2011) suggest that other invertebrate hosts may at
least occasionally be exploited. By far the greatest number of recognised vertebrate
hosts of myxozoans are fish (including representatives of both subclasses of primitive cartilaginous fishes and a broad range of derived bony fish; Lom and Dyková
2006; Kodádková et al. 2015), but myxozoans also infect reptiles (turtles and tortoises), waterfowl (ducks), small mammals (shrews and probably moles) (Lom and
Dyková 2006; Hallett et al. 2015) and all orders of amphibians (Hartigan et al. 2016).
The free-living Polypodium stage emerges from spawned eggs of acipenseriform
fish as chains or stolons of budded but connected tentaculate individuals. The stolons fragment into individual buds that take up benthic life, actively feeding and
undergoing growth and fission during summer months (Fig. 4.1). Reproductively
mature individuals produce a specialised multicellular stage derived from gonadal
tissue (Raikova 1994, 2008) that enables infection following direct contact with
larval fish. Post-invasion infection dynamics are unknown until fish become
B. Okamura and A. Gruhl
non- endocnidozoan lineages of cnidarians that incorporate parasitic stages in their
life history therefore appear to have evolved fairly recently and to have undergone
modest to minimal radiation.
4.3 The Endocnidozoa
As the name implies, the Endocnidozoa is comprised of endoparasitic cnidarians.
This recently recognised clade incorporates the sister taxa Polypodium hydriforme
(henceforth referred to as Polypodium) and the diverse Myxozoa (Collins 2009;
Chang et al. 2015; Kayal et al. 2018). For a considerable time period long-branch
attraction obscured phylogenetic placement of both Myxozoa and Polypodium in
molecular phylogenetic analyses (Zrzavý and Hypša 2003; Foox and Siddall 2015;
Okamura and Gruhl 2015). However, phylogenomic (Chang et al. 2015; Kayal et al.
2018) and some morphological (e.g. Siddall et al. 1995; and see below) evidence
currently places these as sister taxa comprising the Endocnidozoa, which itself is
sister to the Medusozoa (Fig. 4.1).
4.3.1 General Biology
Polypodium’s one-host life cycle (Fig. 4.1c) includes a free-living adult phase and
parasitic larval stages in acipenseriform fish (sturgeon and paddlefish). Myxozoans
have complex parasitic life cycles and require both invertebrate and vertebrate hosts
for development (Fig. 4.1a, b). Invertebrate hosts include freshwater (phylactolaemate) bryozoans (exploited by the Malacosporea) and marine and freshwater oligochaetes and polychaetes (exploited by the Myxosporea) (Fiala et al. 2015a).
Myxosporean infections (including spore production) reported in octopus
(Yokoyama and Masuda 2001) and in a monogenean infecting fish (a case of hyperparasitism) (Freeman and Shinn 2011) suggest that other invertebrate hosts may at
least occasionally be exploited. By far the greatest number of recognised vertebrate
hosts of myxozoans are fish (including representatives of both subclasses of primitive cartilaginous fishes and a broad range of derived bony fish; Lom and Dyková
2006; Kodádková et al. 2015), but myxozoans also infect reptiles (turtles and tortoises), waterfowl (ducks), small mammals (shrews and probably moles) (Lom and
Dyková 2006; Hallett et al. 2015) and all orders of amphibians (Hartigan et al. 2016).
The free-living Polypodium stage emerges from spawned eggs of acipenseriform
fish as chains or stolons of budded but connected tentaculate individuals. The stolons fragment into individual buds that take up benthic life, actively feeding and
undergoing growth and fission during summer months (Fig. 4.1). Reproductively
mature individuals produce a specialised multicellular stage derived from gonadal
tissue (Raikova 1994, 2008) that enables infection following direct contact with
larval fish. Post-invasion infection dynamics are unknown until fish become
B. Okamura and A. Gruhl