116
reproductively mature, at which stage the development of Polypodium within fish
eggs has been characterised (Raikova 1994). Larvae and budding stolons inside the
eggs have inverted germ layers, an inner ectoderm and outer endoderm—a condition that is reversed prior to emerging from fish eggs. Stolons are liberated from
eggs in the oviducts of spawning fish (Raikova 2002). Polypodium infections have
been recorded in 78% of mature female sterlet in the Volga and Kama Rivers and up
to 100% of eggs per female can apparently be infected (Raikova 1994)—a cause for
concern given potential impacts on caviar production. The lineage is currently
regarded as monotypic although sequence divergence in housekeeping genes has
recently been revealed between North American and Russian isolates (Hartigan
et al. unpublished data).
Myxozoans exploit invertebrate and vertebrate hosts that act as definitive and
intermediate hosts, respectively (Fig. 4.1). Multicellular spores released from hosts
into the environment to achieve transmission are non-feeding and metabolically
inactive. The early-diverging major myxozoan clades, the Malacosporea and
Myxosporea, differ in invertebrate host use and morphological complexity (Fiala
et al. 2015a; Gruhl 2015). In their freshwater bryozoan hosts, malacosporean sporogonic (spore-producing) stages develop in the host coelomic cavity as sacs
(~300–700 μm in diameter) and ‘myxoworms’ (up to ~3 mm in length) that exhibit
clearly recognisable epithelial layers and muscle systems (the latter only in myxoworms) (Feist et al. 2015; Gruhl 2015). Multicellular spores that achieve transmission to fish are produced within the hollow spaces of sacs and myxoworms. In
contrast, malacosporean sporogonic stages that develop in fish kidney (called pseudoplasmodia) are comprised of a single cell (the so-called primary cell) within
which multicellular spores infectious to bryozoans are produced. Malacosporeans
exploit a broad range of freshwater bryozoans (Hartikainen et al. 2014) and infections have so far been detected in fish hosts in the families Salmonidae, Cyprinidae
and Percidae (Grabner and El-Matbouli 2010; Bartošová-Sojková et al. 2014;
Naldoni et al. 2019), however some fish in these families may be accidental hosts
because spore development has not always been demonstrated.
Myxosporean sporogonic stages in annelids, called pansporocysts, are comparable to malacosporean sacs, but have an outer lining that is made up of only eight
cells (El-Matbouli and Hoffmann 1998). These cells are extremely thin and show
hardly any epithelial characteristics. Pansporocysts are very small (10–100 μm
range) and develop in the epidermis, gut epithelium, and coelomic cavities of annelid hosts (Lom and Dyková 1997; Gruhl 2015). Myxosporean sporogonic stages in
vertebrate hosts are either plasmodia (unicellular, multinucleate forms) or pseudoplasmodia (unicellular, uninucleate forms) within which multicellular spores
develop. Spores produced by malacosporeans in invertebrate and vertebrate hosts
(malacospores and fish malacospores, respectively) are morphologically similar and
short-lived (e.g. <24 h; de Kinkelin et al. 2002). Spores produced by myxosporeans
(myxospores produced in fish hosts and actinospores produced in annelid hosts)
show variable morphologies some of which can be useful taxonomically (Atkinson
et al. 2015; Fiala et al. 2015b). Histozoic myxosporeans develop in small intercellular spaces within epithelia or connective tissues, whereas coelozoic species
B. Okamura and A. Gruhl
reproductively mature, at which stage the development of Polypodium within fish
eggs has been characterised (Raikova 1994). Larvae and budding stolons inside the
eggs have inverted germ layers, an inner ectoderm and outer endoderm—a condition that is reversed prior to emerging from fish eggs. Stolons are liberated from
eggs in the oviducts of spawning fish (Raikova 2002). Polypodium infections have
been recorded in 78% of mature female sterlet in the Volga and Kama Rivers and up
to 100% of eggs per female can apparently be infected (Raikova 1994)—a cause for
concern given potential impacts on caviar production. The lineage is currently
regarded as monotypic although sequence divergence in housekeeping genes has
recently been revealed between North American and Russian isolates (Hartigan
et al. unpublished data).
Myxozoans exploit invertebrate and vertebrate hosts that act as definitive and
intermediate hosts, respectively (Fig. 4.1). Multicellular spores released from hosts
into the environment to achieve transmission are non-feeding and metabolically
inactive. The early-diverging major myxozoan clades, the Malacosporea and
Myxosporea, differ in invertebrate host use and morphological complexity (Fiala
et al. 2015a; Gruhl 2015). In their freshwater bryozoan hosts, malacosporean sporogonic (spore-producing) stages develop in the host coelomic cavity as sacs
(~300–700 μm in diameter) and ‘myxoworms’ (up to ~3 mm in length) that exhibit
clearly recognisable epithelial layers and muscle systems (the latter only in myxoworms) (Feist et al. 2015; Gruhl 2015). Multicellular spores that achieve transmission to fish are produced within the hollow spaces of sacs and myxoworms. In
contrast, malacosporean sporogonic stages that develop in fish kidney (called pseudoplasmodia) are comprised of a single cell (the so-called primary cell) within
which multicellular spores infectious to bryozoans are produced. Malacosporeans
exploit a broad range of freshwater bryozoans (Hartikainen et al. 2014) and infections have so far been detected in fish hosts in the families Salmonidae, Cyprinidae
and Percidae (Grabner and El-Matbouli 2010; Bartošová-Sojková et al. 2014;
Naldoni et al. 2019), however some fish in these families may be accidental hosts
because spore development has not always been demonstrated.
Myxosporean sporogonic stages in annelids, called pansporocysts, are comparable to malacosporean sacs, but have an outer lining that is made up of only eight
cells (El-Matbouli and Hoffmann 1998). These cells are extremely thin and show
hardly any epithelial characteristics. Pansporocysts are very small (10–100 μm
range) and develop in the epidermis, gut epithelium, and coelomic cavities of annelid hosts (Lom and Dyková 1997; Gruhl 2015). Myxosporean sporogonic stages in
vertebrate hosts are either plasmodia (unicellular, multinucleate forms) or pseudoplasmodia (unicellular, uninucleate forms) within which multicellular spores
develop. Spores produced by malacosporeans in invertebrate and vertebrate hosts
(malacospores and fish malacospores, respectively) are morphologically similar and
short-lived (e.g. <24 h; de Kinkelin et al. 2002). Spores produced by myxosporeans
(myxospores produced in fish hosts and actinospores produced in annelid hosts)
show variable morphologies some of which can be useful taxonomically (Atkinson
et al. 2015; Fiala et al. 2015b). Histozoic myxosporeans develop in small intercellular spaces within epithelia or connective tissues, whereas coelozoic species
B. Okamura and A. Gruhl