127
Carboniferous boundary. Further teleost diversification occurred in the Triassic,
including transitions from marine to freshwater and back. As a result the majority of
recent marine actinopterygians derive from freshwater ancestors (Carrete Vega and
Wiens 2012). These radiations are of potential significance, especially for the diversification of myxosporeans, which show a distinct subdivision into freshwater and
marine clades (Fiala et al. 2015a, b; Holzer et al. 2018). Cophylogenetic analyses
that take these habitat changes into account may better resolve myxozoan radiations
and could additionally serve to explain switches of invertebrate hosts. Moreover,
node calibrations for fish radiations following invasions of freshwater or marine
habitats could be directly transferred to the corresponding parasite groups. For
example, the hypothesis could be examined that the marine myxosporean clade
diverged from a freshwater clade that infected oligochaetes following the first transition of teleosts to the marine environment.
Myxozoans infect a range of organs and tissues of their vertebrate hosts causing
diverse disease symptoms. The site of sporogony usually demonstrates the greatest
pathology and is often quite specific (Molnár and Eszterbauer 2015). The fossilization potential of myxozoan stages in lumina and soft tissues is probably very low.
Fish immune responses can, however, lead to encapsulation of parasites by dense
connective or cartilaginous tissues, which may have a higher probability of preservation. Furthermore, a common reaction against myxozoan infection is the formation of granulomata with melanomacrophage centres—dense accumulations of
immune cells containing melanin (Sitjà-Bobadilla et al. 2015; Steinel and Bolnick
2017), a substance which has been characterised from well-preserved fossil vertebrates (Colleary et al. 2015). Cysts occurring in gills, skin or muscle tissue are the
most common form of tissue alteration and may be recognisable in fossils with
exceptional soft-tissue preservation, but recognition of spores (see above) within
these structures is crucial to unequivocally link to myxozoan infection. Myxozoan
infection symptoms most likely to be preserved in the fossil record would be
expected to result from species that infect cartilage and cause skeletal deformation.
This phenomenon is best studied in salmonid whirling disease caused by Myxobolus
cerebralis, which infects cartilage prior to ossification leading to malformations of
spine, skull, jaw or fin rays (Sarker et al. 2015).
Due to their high phosphate content and rapid fossilization potential, vertebrate
faeces could provide a further means of detecting myxozoans via analyses of coprolite contents. Coprolites can sometimes offer exceptionally well-preserved soft tissue remains comparable to those of a Konservat-Lagerstätte (Qvarnström et al.
2016). Accordingly, coprolites have been found to contain parasite remains (Poinar
and Boucot 2006; Hunt et al. 2012; Dentzien-Dias et al. 2013; Hugot et al. 2014;
Brachaniec et al. 2015; Dentzien-Dias et al. 2018; Chin 2021) along with preserved
hair, feathers, muscles, bones, chitinous exoskeletons, bacteria, and fungi. Coprolites
could therefore have a high potential to yield myxozoan spores. These spores may
derive from species infecting sites where spores are released into the digestive tract
(e.g. infections in the bile ducts, gall bladder or intestine), or they could derive from
infected prey. The latter possibility is supported by the detection of myxozoan DNA
in faeces collected from fish-eating birds (cormorants) (Briscoe et al. unpublished
4 Evolution, Origins and Diversification of Parasitic Cnidarians
Carboniferous boundary. Further teleost diversification occurred in the Triassic,
including transitions from marine to freshwater and back. As a result the majority of
recent marine actinopterygians derive from freshwater ancestors (Carrete Vega and
Wiens 2012). These radiations are of potential significance, especially for the diversification of myxosporeans, which show a distinct subdivision into freshwater and
marine clades (Fiala et al. 2015a, b; Holzer et al. 2018). Cophylogenetic analyses
that take these habitat changes into account may better resolve myxozoan radiations
and could additionally serve to explain switches of invertebrate hosts. Moreover,
node calibrations for fish radiations following invasions of freshwater or marine
habitats could be directly transferred to the corresponding parasite groups. For
example, the hypothesis could be examined that the marine myxosporean clade
diverged from a freshwater clade that infected oligochaetes following the first transition of teleosts to the marine environment.
Myxozoans infect a range of organs and tissues of their vertebrate hosts causing
diverse disease symptoms. The site of sporogony usually demonstrates the greatest
pathology and is often quite specific (Molnár and Eszterbauer 2015). The fossilization potential of myxozoan stages in lumina and soft tissues is probably very low.
Fish immune responses can, however, lead to encapsulation of parasites by dense
connective or cartilaginous tissues, which may have a higher probability of preservation. Furthermore, a common reaction against myxozoan infection is the formation of granulomata with melanomacrophage centres—dense accumulations of
immune cells containing melanin (Sitjà-Bobadilla et al. 2015; Steinel and Bolnick
2017), a substance which has been characterised from well-preserved fossil vertebrates (Colleary et al. 2015). Cysts occurring in gills, skin or muscle tissue are the
most common form of tissue alteration and may be recognisable in fossils with
exceptional soft-tissue preservation, but recognition of spores (see above) within
these structures is crucial to unequivocally link to myxozoan infection. Myxozoan
infection symptoms most likely to be preserved in the fossil record would be
expected to result from species that infect cartilage and cause skeletal deformation.
This phenomenon is best studied in salmonid whirling disease caused by Myxobolus
cerebralis, which infects cartilage prior to ossification leading to malformations of
spine, skull, jaw or fin rays (Sarker et al. 2015).
Due to their high phosphate content and rapid fossilization potential, vertebrate
faeces could provide a further means of detecting myxozoans via analyses of coprolite contents. Coprolites can sometimes offer exceptionally well-preserved soft tissue remains comparable to those of a Konservat-Lagerstätte (Qvarnström et al.
2016). Accordingly, coprolites have been found to contain parasite remains (Poinar
and Boucot 2006; Hunt et al. 2012; Dentzien-Dias et al. 2013; Hugot et al. 2014;
Brachaniec et al. 2015; Dentzien-Dias et al. 2018; Chin 2021) along with preserved
hair, feathers, muscles, bones, chitinous exoskeletons, bacteria, and fungi. Coprolites
could therefore have a high potential to yield myxozoan spores. These spores may
derive from species infecting sites where spores are released into the digestive tract
(e.g. infections in the bile ducts, gall bladder or intestine), or they could derive from
infected prey. The latter possibility is supported by the detection of myxozoan DNA
in faeces collected from fish-eating birds (cormorants) (Briscoe et al. unpublished
4 Evolution, Origins and Diversification of Parasitic Cnidarians