130
phylactolaemate host morphology when the development of spore-producing infectious stages results in larger zooids, malformed statoblasts and reduced statoblast
production. Recognition of such effects in fossil material would require good preservation, lack of post-preservation deformation, and comparative assessment of
many individuals. Several extinct bryozoan groups could be promising candidates
as hosts of ancient myxozoans.
A predisposing trait of phylactolaemates as myxozoan hosts is the relatively
large and confluent colony-wide coelomic cavity. The continuous action of cilia lining this coelomic cavity ensures that metabolites are distributed throughout the
colony. In addition the unobstructed coelomic space allows malacosporean sacs and
myxoworms to move freely amongst the zooids of a colony, with the circulation of
sacs being assisted by ciliary beating. Cyclostome and gymnolaemate zooids, in
contrast, are separated by walls, which have communication pores that are either
very small (cyclostomes) or filled with tissue (gymnolaemates), thus preventing
myxozoan stages from migrating between zooids. Furthermore, nutrient transfer in
cyclostomes and gymnolaemates is achieved largely via tissue connections (the
funicular system)—leaving the coelomic fluids with relatively low concentrations
of nutrients. The Paleozoic stenolaemate Corynotrypida, however, completely
lacked interzooidal walls. In addition, some representatives of other Paleozoic
stenolaemate groups (Cystoporata, Esthonioporata, Trepostomata and
Cryptostomata) possessed zooids with larger communication pores while others
(the free-walled stenolaemates) lacked calcified exterior walls and thus are assumed
have had confluent hypostegal coelomic cavities (Boardman 1998; Ernst and
Schäfer 2006). These ancient bryozoan taxa could, thus potentially have served as
malacosporean or stem myxozoan hosts.
4.5.6 Other Potential Ancient Invertebrate Hosts
Discoveries of myxosporeans in octopus (a species of Kudoa; Yokoyama and
Masuda 2001) and as hyperparasites in three monogenean species (Myxidium or
Myxidium-like species) and in two digenean species (Fabespora vermicola and
Fabespora sp.; Freeman and Shinn 2011) suggest that a greater range of invertebrate
hosts may be routinely used. Notably Kudoa, Myxidium and Fabespora are all
derived lineages within the Myxosporea (Fiala 2006; Freeman and Shinn 2011).
These host findings are, however, not straightforward. Thus, at least in some cases
monogenean ‘hosts’ could be incidentally exploited by myxozoans infecting fish
and Kudoa infection has only been observed in a single octopus. It is conceivable
that the latter could have developed if infection was transmitted from infected fish
prey (perhaps if injury was sustained in catching and subduing the fish). In addition,
the spores that develop in both octopus and the platyhelminth species are typical
myxospores (Freeman and Shinn 2011). This implies that these invertebrates may
either be used as alternative hosts to fish or have been adopted as novel hosts via
B. Okamura and A. Gruhl
phylactolaemate host morphology when the development of spore-producing infectious stages results in larger zooids, malformed statoblasts and reduced statoblast
production. Recognition of such effects in fossil material would require good preservation, lack of post-preservation deformation, and comparative assessment of
many individuals. Several extinct bryozoan groups could be promising candidates
as hosts of ancient myxozoans.
A predisposing trait of phylactolaemates as myxozoan hosts is the relatively
large and confluent colony-wide coelomic cavity. The continuous action of cilia lining this coelomic cavity ensures that metabolites are distributed throughout the
colony. In addition the unobstructed coelomic space allows malacosporean sacs and
myxoworms to move freely amongst the zooids of a colony, with the circulation of
sacs being assisted by ciliary beating. Cyclostome and gymnolaemate zooids, in
contrast, are separated by walls, which have communication pores that are either
very small (cyclostomes) or filled with tissue (gymnolaemates), thus preventing
myxozoan stages from migrating between zooids. Furthermore, nutrient transfer in
cyclostomes and gymnolaemates is achieved largely via tissue connections (the
funicular system)—leaving the coelomic fluids with relatively low concentrations
of nutrients. The Paleozoic stenolaemate Corynotrypida, however, completely
lacked interzooidal walls. In addition, some representatives of other Paleozoic
stenolaemate groups (Cystoporata, Esthonioporata, Trepostomata and
Cryptostomata) possessed zooids with larger communication pores while others
(the free-walled stenolaemates) lacked calcified exterior walls and thus are assumed
have had confluent hypostegal coelomic cavities (Boardman 1998; Ernst and
Schäfer 2006). These ancient bryozoan taxa could, thus potentially have served as
malacosporean or stem myxozoan hosts.
4.5.6 Other Potential Ancient Invertebrate Hosts
Discoveries of myxosporeans in octopus (a species of Kudoa; Yokoyama and
Masuda 2001) and as hyperparasites in three monogenean species (Myxidium or
Myxidium-like species) and in two digenean species (Fabespora vermicola and
Fabespora sp.; Freeman and Shinn 2011) suggest that a greater range of invertebrate
hosts may be routinely used. Notably Kudoa, Myxidium and Fabespora are all
derived lineages within the Myxosporea (Fiala 2006; Freeman and Shinn 2011).
These host findings are, however, not straightforward. Thus, at least in some cases
monogenean ‘hosts’ could be incidentally exploited by myxozoans infecting fish
and Kudoa infection has only been observed in a single octopus. It is conceivable
that the latter could have developed if infection was transmitted from infected fish
prey (perhaps if injury was sustained in catching and subduing the fish). In addition,
the spores that develop in both octopus and the platyhelminth species are typical
myxospores (Freeman and Shinn 2011). This implies that these invertebrates may
either be used as alternative hosts to fish or have been adopted as novel hosts via
B. Okamura and A. Gruhl