164
the host—the bivalve is a hermaphrodite, producing sperm as a young adult and
becomes a female when older (Bristow et al. 2010a). Although it has not been determined to be a parasite or a commensal (E. nhatrangensis had diatoms in their gills,
suggesting active filtration from the sea water), the example of Entovalva may indicate an evolutionary sequence of steps towards parasitism—one may suppose that
being a commensal enclosed in the host body may facilitate the transition to parasitic mode of life, involving feeding on the surrounding host tissues.
Li et al. (2016) and Goto et al. (2012) provided a molecular phylogeny of
Galeommatoidea indicating that many commensal lineages lack clade-specific host
fidelity; they suggest host shifts between distantly related taxa. Nevertheless the
same host species are known to be shared by some congeneric galeommatoidean
species, which suggests the possibility of within-host speciation in these lineages
(Mikkelsen and Bieler 1989; Goto et al. 2014, 2016, 2018).
5.1.3 Unionida
The freshwater pearly mussels, naiads, unionoids, members of the order Unionida,
are characterized by their unique life cycle. Not only do their larvae receive parental
care, developing in the females’ gills, but they also must parasitize aquatic vertebrates, most often fish, to complete their development and to metamorphose into
juvenile mussels (e.g. Kat 1984; Bauer 1994, 2001a; Graf and Cummings 2006;
Barnhart et al. 2008; Strayer 2008; Patterson 2018).
All Recent unionoids are restricted to freshwater (e.g. Bauer 2001a; Graf 2013).
They are relatively large (often over a dozen cm), long-lived (tens to a hundred
years) pearly mussels, spending their lives as semi-infaunal burrowers in streams
and lakes of all continents, except Antarctica (Bauer 2001a, c, d; Graf and Cummings
2007; Bogan 2008; Bogan and Roe 2008). Their fossil record can be confidently
traced back to the Triassic (Good 1998; Watters 2001; Skawina and Dzik 2011; Van
Damme et al. 2015), however today their diversity is estimated to be at least 840 (up
to 958) species and is declining worldwide (Graf and Cummings 2007, 2021; Bogan
and Roe 2008; Patterson et al. 2018). Adults are filter feeders through the activity of
the cilia present on their paired gills, which results in water currents and the transportation of mucus with trapped food particles to the mouth of the animal. Gills of
these mussels serve not only as a food-capturing organ (and gaseous exchange
organ)—unionoids brood their larvae in the females’ modified demibranchs—socalled marsupia (e.g. Lefevre and Curtis 1910a, Tankersley and Dimock 1992). It is
believed that this is an adaptation to freshwater environments, where all the lotic
habitats (flowing waters like streams or rivers; e.g. Pennak 1971) finish in marine
conditions. Larval brooding and elimination of the typical free- swimming veliger
prevent larvae from being transported to the sea (Graf and O’Foighil 2000; Wächtler
et al. 2001; Graf and Cummings 2006; Graf 2013; Haag 2013). Because unionoids
are rather large and heavy, their dispersal relies mostly on their minute larvae, which
parasitise fish. Larvae (most commonly glochidia) must come in contact with a host
soon after being released into the water otherwise they die (they usually survive
A. Skawina
the host—the bivalve is a hermaphrodite, producing sperm as a young adult and
becomes a female when older (Bristow et al. 2010a). Although it has not been determined to be a parasite or a commensal (E. nhatrangensis had diatoms in their gills,
suggesting active filtration from the sea water), the example of Entovalva may indicate an evolutionary sequence of steps towards parasitism—one may suppose that
being a commensal enclosed in the host body may facilitate the transition to parasitic mode of life, involving feeding on the surrounding host tissues.
Li et al. (2016) and Goto et al. (2012) provided a molecular phylogeny of
Galeommatoidea indicating that many commensal lineages lack clade-specific host
fidelity; they suggest host shifts between distantly related taxa. Nevertheless the
same host species are known to be shared by some congeneric galeommatoidean
species, which suggests the possibility of within-host speciation in these lineages
(Mikkelsen and Bieler 1989; Goto et al. 2014, 2016, 2018).
5.1.3 Unionida
The freshwater pearly mussels, naiads, unionoids, members of the order Unionida,
are characterized by their unique life cycle. Not only do their larvae receive parental
care, developing in the females’ gills, but they also must parasitize aquatic vertebrates, most often fish, to complete their development and to metamorphose into
juvenile mussels (e.g. Kat 1984; Bauer 1994, 2001a; Graf and Cummings 2006;
Barnhart et al. 2008; Strayer 2008; Patterson 2018).
All Recent unionoids are restricted to freshwater (e.g. Bauer 2001a; Graf 2013).
They are relatively large (often over a dozen cm), long-lived (tens to a hundred
years) pearly mussels, spending their lives as semi-infaunal burrowers in streams
and lakes of all continents, except Antarctica (Bauer 2001a, c, d; Graf and Cummings
2007; Bogan 2008; Bogan and Roe 2008). Their fossil record can be confidently
traced back to the Triassic (Good 1998; Watters 2001; Skawina and Dzik 2011; Van
Damme et al. 2015), however today their diversity is estimated to be at least 840 (up
to 958) species and is declining worldwide (Graf and Cummings 2007, 2021; Bogan
and Roe 2008; Patterson et al. 2018). Adults are filter feeders through the activity of
the cilia present on their paired gills, which results in water currents and the transportation of mucus with trapped food particles to the mouth of the animal. Gills of
these mussels serve not only as a food-capturing organ (and gaseous exchange
organ)—unionoids brood their larvae in the females’ modified demibranchs—socalled marsupia (e.g. Lefevre and Curtis 1910a, Tankersley and Dimock 1992). It is
believed that this is an adaptation to freshwater environments, where all the lotic
habitats (flowing waters like streams or rivers; e.g. Pennak 1971) finish in marine
conditions. Larval brooding and elimination of the typical free- swimming veliger
prevent larvae from being transported to the sea (Graf and O’Foighil 2000; Wächtler
et al. 2001; Graf and Cummings 2006; Graf 2013; Haag 2013). Because unionoids
are rather large and heavy, their dispersal relies mostly on their minute larvae, which
parasitise fish. Larvae (most commonly glochidia) must come in contact with a host
soon after being released into the water otherwise they die (they usually survive
A. Skawina