171
5.3.2 Adaptations to Host Infection
Larvae of Unionida do not actively swim (Paling 1968). Irrespective of the fact if
they are host generalists or host specialists, unionoids have evolved diverse adaptations for parasitizing their hosts. These include: morphological modification of larvae (size, hooks, threads); physiological adaptations (e.g. ability to resist host
immune response and larvae probably facilitate encapsulation); various adult strategies for host infection (morphological and behavioral). These adaptations have recently been reviewed in detail by Barnhart et al. (2008) and Haag (2012).
Broadcasting the larvae implies releasing free larvae to the water with no strategy to attract the host (Barnhart et al. 2008; Fig. 5.3). This strategy is considered to
be the more primitive strategy by Haag (2012). It occurs in both the Margaritiferidae
and some Unionidae, and is associated with huge mortality of the offspring (Bauer
2001b; Haag 2013). Soler et al. (2018) observed, that Margaritifera auricularia
(kept in laboratory) in addition to massive broadcasting of free glochidia, releases
“small white masses composed of larval material, including glochidia”, These
masses remain close to the female and may act as a host attraction (conglutinates?),
or are a result of respiratory stress (Aldridge and McIvor 2003; Haag and Warren
2003). Haag (2012) noticed that bivalves with a broadcasting strategy sometimes
release larvae entangled in mucus strings or that larva have a larval thread, possibly
to facilitate being suspended in water and to increase the chances for attachment to
the host (Wood 1974b; found also in Hyriidae, Klunzinger et al. 2013). Margaritifera
margaritifera and Anodonta implicata, however, are known to enhance the chances
for host infection by synchronizing the timing of reproduction with the presence of
their host (Davenport and Warmuth 1965; Barnhart et al. 2008). Several other species of Anodonta can recognize the chemical cues when host fish are near and only
then release glochidia (Jokela and Palokangas 1993; Haag and Warren 2000; Bauer
2001b). Vicentini (2005) reported an unusual behavior of Unio crassus broadcasting
larvae by vigorously spurting out glochidia, causing the water to splash.
Hyriidae and many Unionidae, together with mentioned above Margaritiferidae
produce so-called conglutinates. Lefevre and Curtis (1912) who introduced this
term, defined conglutinates as masses of eggs (and later larvae) molded into the
shape of the watertubes in marsupia. These packages of larvae are formed in the
female’s marsupia and often mimic preferred food items of the host (Kat 1984;
Haag et al. 1995; Bauer 2001b; Walker et al. 2001; Barnhart et al. 2008; Haag
2013). They also prolong the time of survivorship for glochidia. Mussels that produce conglutinates often produce less glochidia than broadcasting ones (Barnhart
et al. 2008). These egg aggregations vary morphologically from irregular and fragile
that break during release (in Margaritiferidae), through leaf-like fragile and easily
breaking ones (in most conglutinate producing Unionidae), to the reinforced by
unfertilized, structural eggs that keep the shape of the aggregation (in tribes
Pleurobemini and Lampsilini, Unionidae). The latter conglutinates often mimic the
prey of the host (Barnhart et al. 2008).
5 Evolutionary History of Bivalves as Parasites
5.3.2 Adaptations to Host Infection
Larvae of Unionida do not actively swim (Paling 1968). Irrespective of the fact if
they are host generalists or host specialists, unionoids have evolved diverse adaptations for parasitizing their hosts. These include: morphological modification of larvae (size, hooks, threads); physiological adaptations (e.g. ability to resist host
immune response and larvae probably facilitate encapsulation); various adult strategies for host infection (morphological and behavioral). These adaptations have recently been reviewed in detail by Barnhart et al. (2008) and Haag (2012).
Broadcasting the larvae implies releasing free larvae to the water with no strategy to attract the host (Barnhart et al. 2008; Fig. 5.3). This strategy is considered to
be the more primitive strategy by Haag (2012). It occurs in both the Margaritiferidae
and some Unionidae, and is associated with huge mortality of the offspring (Bauer
2001b; Haag 2013). Soler et al. (2018) observed, that Margaritifera auricularia
(kept in laboratory) in addition to massive broadcasting of free glochidia, releases
“small white masses composed of larval material, including glochidia”, These
masses remain close to the female and may act as a host attraction (conglutinates?),
or are a result of respiratory stress (Aldridge and McIvor 2003; Haag and Warren
2003). Haag (2012) noticed that bivalves with a broadcasting strategy sometimes
release larvae entangled in mucus strings or that larva have a larval thread, possibly
to facilitate being suspended in water and to increase the chances for attachment to
the host (Wood 1974b; found also in Hyriidae, Klunzinger et al. 2013). Margaritifera
margaritifera and Anodonta implicata, however, are known to enhance the chances
for host infection by synchronizing the timing of reproduction with the presence of
their host (Davenport and Warmuth 1965; Barnhart et al. 2008). Several other species of Anodonta can recognize the chemical cues when host fish are near and only
then release glochidia (Jokela and Palokangas 1993; Haag and Warren 2000; Bauer
2001b). Vicentini (2005) reported an unusual behavior of Unio crassus broadcasting
larvae by vigorously spurting out glochidia, causing the water to splash.
Hyriidae and many Unionidae, together with mentioned above Margaritiferidae
produce so-called conglutinates. Lefevre and Curtis (1912) who introduced this
term, defined conglutinates as masses of eggs (and later larvae) molded into the
shape of the watertubes in marsupia. These packages of larvae are formed in the
female’s marsupia and often mimic preferred food items of the host (Kat 1984;
Haag et al. 1995; Bauer 2001b; Walker et al. 2001; Barnhart et al. 2008; Haag
2013). They also prolong the time of survivorship for glochidia. Mussels that produce conglutinates often produce less glochidia than broadcasting ones (Barnhart
et al. 2008). These egg aggregations vary morphologically from irregular and fragile
that break during release (in Margaritiferidae), through leaf-like fragile and easily
breaking ones (in most conglutinate producing Unionidae), to the reinforced by
unfertilized, structural eggs that keep the shape of the aggregation (in tribes
Pleurobemini and Lampsilini, Unionidae). The latter conglutinates often mimic the
prey of the host (Barnhart et al. 2008).
5 Evolutionary History of Bivalves as Parasites