185
that glochidia encystment has a physiological effect on their host (e.g. higher energetic costs of movement; Slavík et al. 2017). Douda et al. (2017) reported reduction
in the body mass, condition factor and changes in several physiological parameters
measured in plasma of European chub (Squalius cephalus) infested with glochidia
of Sinanodonta woodiana. Filipsson et al. (2017) recently noticed, that juvenile
brown trout (Salmo trutta) infested with glochidia of Margaritifera margaritifera
had an increased metabolic rate and haematocrit values, compared with the control
group. This is possibly an effect of a compensatory response to the potential respiratory stress caused by the encapsulation of glochidia on its gills. Nevertheless, it is
believed that positive selection for fish to resist glochidial infection is weaker, than
strong positive selection of larvae, to develop resistance to its host (Haag 2012). To
survive, the glochidium must resist the host’s immune response (Rogers and Dimock
2003; Dodd et al. 2005, 2006; Watters 2007). Mussels that possess a more resistant
phenotype will be positively selected for (Graf 1997; Barnhart et al. 2008). High
levels of glochidial infestation may cause a severe load to fish gills, enough to kill
the fish host (Howerth and Keller 2006), but in nature overcrowding of glochidia on
host (which may primarily lead to increased larval mortality) is rare, due to temporal and spatial differences in glochidial distribution on their hosts (Hastie and Young
2001; Jansen et al. 2001; Blazek and Gelnar 2006). Nevertheless, lower levels of
infestation may change hosts’ physiology (e.g. by stress reaction) enough to
decrease glochidium chances for metamorphosis (Dubansky et al. 2011). Strayer
(2008) discussed epidemiological models of mussel- host dynamics to illuminate the
importance of the immune response of the fish and mussel competition for hosts. He
concluded that there must be a density limit to the number of glochidia that successfully metamorphose on an individual host.
5.5.3.4 Duration of Encapsulation and Metamorphosis
Larvae of most species do not grow during encapsulation (with the exceptions of
Margaritiferidae, Iridinidae, and a dozen of North American species listed by
Patterson (2018); Wächtler et al. 2001; Barnhart et al. 2008), however stable isotope
analysis proved they obtain nutrients from their host (Wächtler et al. 2001; Fritts
et al. 2013; Denic et al. 2015) possibly due to the close presence of fish blood capillaries, which are involved in formation of the cyst. The food is gathered from the
host by the dense microvilli of the larval mantle cells and the larval mantle also
produces storage granules (Wächtler et al. 2001). An additional source of nutrients
comes from the larval adductor and mantle remnant that are degenerated during
metamorphosis (Arey 1932b). Encapsulation lasts from days to months, often
depending on the glochidium size (Bauer 1994, 2001d; Wächtler et al. 2001), which
is species specific. Smaller glochidia grow during encapsulation. They are usually
unhooked, and need more time to complete metamorphosis when compared to
large, hooked ones (respectively 1300–1600 days compared to 240–544 days in
similar temperatures; Bauer 1994; Barnhart et al. 2008). Meaning they are exposed
to the host immune response for a longer time, so they must be closely adapted to
5 Evolutionary History of Bivalves as Parasites
that glochidia encystment has a physiological effect on their host (e.g. higher energetic costs of movement; Slavík et al. 2017). Douda et al. (2017) reported reduction
in the body mass, condition factor and changes in several physiological parameters
measured in plasma of European chub (Squalius cephalus) infested with glochidia
of Sinanodonta woodiana. Filipsson et al. (2017) recently noticed, that juvenile
brown trout (Salmo trutta) infested with glochidia of Margaritifera margaritifera
had an increased metabolic rate and haematocrit values, compared with the control
group. This is possibly an effect of a compensatory response to the potential respiratory stress caused by the encapsulation of glochidia on its gills. Nevertheless, it is
believed that positive selection for fish to resist glochidial infection is weaker, than
strong positive selection of larvae, to develop resistance to its host (Haag 2012). To
survive, the glochidium must resist the host’s immune response (Rogers and Dimock
2003; Dodd et al. 2005, 2006; Watters 2007). Mussels that possess a more resistant
phenotype will be positively selected for (Graf 1997; Barnhart et al. 2008). High
levels of glochidial infestation may cause a severe load to fish gills, enough to kill
the fish host (Howerth and Keller 2006), but in nature overcrowding of glochidia on
host (which may primarily lead to increased larval mortality) is rare, due to temporal and spatial differences in glochidial distribution on their hosts (Hastie and Young
2001; Jansen et al. 2001; Blazek and Gelnar 2006). Nevertheless, lower levels of
infestation may change hosts’ physiology (e.g. by stress reaction) enough to
decrease glochidium chances for metamorphosis (Dubansky et al. 2011). Strayer
(2008) discussed epidemiological models of mussel- host dynamics to illuminate the
importance of the immune response of the fish and mussel competition for hosts. He
concluded that there must be a density limit to the number of glochidia that successfully metamorphose on an individual host.
5.5.3.4 Duration of Encapsulation and Metamorphosis
Larvae of most species do not grow during encapsulation (with the exceptions of
Margaritiferidae, Iridinidae, and a dozen of North American species listed by
Patterson (2018); Wächtler et al. 2001; Barnhart et al. 2008), however stable isotope
analysis proved they obtain nutrients from their host (Wächtler et al. 2001; Fritts
et al. 2013; Denic et al. 2015) possibly due to the close presence of fish blood capillaries, which are involved in formation of the cyst. The food is gathered from the
host by the dense microvilli of the larval mantle cells and the larval mantle also
produces storage granules (Wächtler et al. 2001). An additional source of nutrients
comes from the larval adductor and mantle remnant that are degenerated during
metamorphosis (Arey 1932b). Encapsulation lasts from days to months, often
depending on the glochidium size (Bauer 1994, 2001d; Wächtler et al. 2001), which
is species specific. Smaller glochidia grow during encapsulation. They are usually
unhooked, and need more time to complete metamorphosis when compared to
large, hooked ones (respectively 1300–1600 days compared to 240–544 days in
similar temperatures; Bauer 1994; Barnhart et al. 2008). Meaning they are exposed
to the host immune response for a longer time, so they must be closely adapted to
5 Evolutionary History of Bivalves as Parasites