117
develop in the lumen of organs (e.g. bile duct, urinary bladder). The most common
infection sites include gills, skin, fins, eyes, kidney, intestine, liver and gall bladder,
nervous system, cartilage, musculature, and swim bladder (Molnár and Eszterbauer
2015). Myxosporean plasmodia and pseudoplasmodia are generally tiny, but some
are mm to cm in dimensions. Annelid hosts of myxosporeans are mostly unknown—
infections so far have been detected in 7 (Alexander et al. 2015) of approximately
120 families (Fauchald and Rouse 1997; Erséus 2005). Fish are widely exploited by
myxosporeans with infections reported in many families of both cartilaginous and
bony fishes.
Myxozoans are causative agents of several diseases impacting aquaculture and
wild fish populations. These include whirling disease, proliferative kidney disease
and enteronecrosis in salmonids and pharyngeal myxosporidiosis in carp (Feist and
Longshaw 2006; Jones et al. 2015).
4.3.2 Comparative Development and Body Plans
There are noteworthy similarities and differences in morphology and life history
stages between Polypodium and myxozoans consistent with the retention of a freeliving stage in the life cycle of the former and the entirely endoparasitic life cycle of
the latter (Table 4.2). In particular, myxozoans have lost many organs (e.g. no recognisable gut or gonads) and certain tissues (e.g. nerves, gonad) although proper
epithelia characterise the early diverging malacosporeans. Polypodium and myxoworms both develop independent, sub-epidermal muscle systems (Raikova et al.
2007; Gruhl and Okamura 2012) in contrast to the typical cnidarian epitheliomuscular cells. Cilia, centrioles, and cnidocils are absent in myxozoans but present in
Polypodium. Pinpointing the cells that act as gametes requires observing meiosis
and the brief and transitory process of fusion and there are conflicting interpretations of these events in myxozoans (reviewed in Feist et al. 2015; Okamura et al.
2015b). Fusion may occur within spores that develop in myxozoan invertebrate
hosts (resulting in self-fertilisation) or in fish hosts after cells released from spores
invade and proliferate within fish (also potentially involving self-fertilisation). In
Polypodium fusion has been inferred to happen at some time after fish hosts are
invaded and to involve self-fertilisation of ‘blastomeres’ that develop within a nurse
cell (the trophamnion) which becomes polyploid (Raikova 2008). This trophamnionblastomere complex arises from the original binucleate cells that invade fish hosts.
The complex is initiated by engulfment of one nucleus in the binucleate cell, resulting in a cell-in-cell organisation (Raikova 1994). It is apparent that fish hosts of both
Polypodium and myxozoans support early stages of development. In addition, both
Polypodium and myxozoans have similar nematocysts (organelles within nematocytes and referred to as polar capsules in myxozoans) that are used for attachment
and are characterised by hollow tubes and an absence of spines. Such nematocysts,
called ‘atrichous isorhizae’ are also found in free-living cnidarians (Okamura et al.
2015b). Polypodium additionally possesses nematocysts that penetrate prey.
4 Evolution, Origins and Diversification of Parasitic Cnidarians
develop in the lumen of organs (e.g. bile duct, urinary bladder). The most common
infection sites include gills, skin, fins, eyes, kidney, intestine, liver and gall bladder,
nervous system, cartilage, musculature, and swim bladder (Molnár and Eszterbauer
2015). Myxosporean plasmodia and pseudoplasmodia are generally tiny, but some
are mm to cm in dimensions. Annelid hosts of myxosporeans are mostly unknown—
infections so far have been detected in 7 (Alexander et al. 2015) of approximately
120 families (Fauchald and Rouse 1997; Erséus 2005). Fish are widely exploited by
myxosporeans with infections reported in many families of both cartilaginous and
bony fishes.
Myxozoans are causative agents of several diseases impacting aquaculture and
wild fish populations. These include whirling disease, proliferative kidney disease
and enteronecrosis in salmonids and pharyngeal myxosporidiosis in carp (Feist and
Longshaw 2006; Jones et al. 2015).
4.3.2 Comparative Development and Body Plans
There are noteworthy similarities and differences in morphology and life history
stages between Polypodium and myxozoans consistent with the retention of a freeliving stage in the life cycle of the former and the entirely endoparasitic life cycle of
the latter (Table 4.2). In particular, myxozoans have lost many organs (e.g. no recognisable gut or gonads) and certain tissues (e.g. nerves, gonad) although proper
epithelia characterise the early diverging malacosporeans. Polypodium and myxoworms both develop independent, sub-epidermal muscle systems (Raikova et al.
2007; Gruhl and Okamura 2012) in contrast to the typical cnidarian epitheliomuscular cells. Cilia, centrioles, and cnidocils are absent in myxozoans but present in
Polypodium. Pinpointing the cells that act as gametes requires observing meiosis
and the brief and transitory process of fusion and there are conflicting interpretations of these events in myxozoans (reviewed in Feist et al. 2015; Okamura et al.
2015b). Fusion may occur within spores that develop in myxozoan invertebrate
hosts (resulting in self-fertilisation) or in fish hosts after cells released from spores
invade and proliferate within fish (also potentially involving self-fertilisation). In
Polypodium fusion has been inferred to happen at some time after fish hosts are
invaded and to involve self-fertilisation of ‘blastomeres’ that develop within a nurse
cell (the trophamnion) which becomes polyploid (Raikova 2008). This trophamnionblastomere complex arises from the original binucleate cells that invade fish hosts.
The complex is initiated by engulfment of one nucleus in the binucleate cell, resulting in a cell-in-cell organisation (Raikova 1994). It is apparent that fish hosts of both
Polypodium and myxozoans support early stages of development. In addition, both
Polypodium and myxozoans have similar nematocysts (organelles within nematocytes and referred to as polar capsules in myxozoans) that are used for attachment
and are characterised by hollow tubes and an absence of spines. Such nematocysts,
called ‘atrichous isorhizae’ are also found in free-living cnidarians (Okamura et al.
2015b). Polypodium additionally possesses nematocysts that penetrate prey.
4 Evolution, Origins and Diversification of Parasitic Cnidarians