189
were also reported from Mesozoic freshwaters like hybodontid sharks (Hybodus
spp.) and neopterygians (e.g. Lepidotes [Lepistosteiformes]; Kear and Godthelp
2008; Bermúdez-Rochas 2009; Bermudez-Rochas et al. 2013; Van Damme et al.
2015; Cavin 2017) whose diet in combination with close contact with early
unionoids could have facilitated the establishment of parasite-host associations.
Other freshwater fish, e.g. non-durophagous neopterygians like Pliodetes
(Lepistosteiformes), or Semionotus (Semionotiformes) as well as freshwater members of ancient Palaeonisciformes also co-occurred with early unionoids (Dzik and
Sulej 2007; Lucas and Tanner 2007; Cavin 2017; Gibson 2018), thus might also have
served as their early associates.
5.6 Conclusions
Many bivalves live in associations with the other organisms, but only members of
the marine superfamily Galeommatoidea and larvae of the freshwater order
Unionida are known to live in facultative (Galeommatoidea) or obligate (Unionida)
parasitising relationships. In both cases, but for different reasons, these relationships require close association with the host body followed by covering the bivalve
by the host tissues.
In the case of galeommatoideans, it is proposed, that the close association with
their hosts evolved from the stage of a free living ancestor, through a burrow-wall
commensal, to a commensal that settles on a precise area of a host body, sometimes
being covered by the soft tissues of the host, or settling inside the gill chambers of
the host (depending on both the bivalve and host species). This may be corroborated
by recent findings of interpreted accidental larval settlement within the body cavity
of the host-crab in case of Kurtiella pedroana, leading to feeding and presumably
reproduction attempt in the crab’s hemocoel, or living in the body and presumably
parasitising the holothurian by Entovalva nhatrangensis.
Their freshwater habitat, parasitising larvae, and maternal care characterize
unionoids. Because of their large adult size and limited mobility, their dispersal
abilities rely on their minute larvae (most commonly the glochidium) and their close
association with fish hosts. To enhance the larval survival chances, females brood
them in their own modified gills (interpreted as protection against the chemical
parameters of freshwaters, ensuring calcium for the larval shell, and a protection
against water currents, which are accompanied by mantle morphology and behavior
that lure potential host fish, differently developed by different lineages). Unionoids
appeared in Pangean freshwaters by the Late Triassic. At this time gills of the
bivalves were not yet modified (which does not exclude the possibility of maternal
brooding), but presumably in the Middle Jurassic and surely by the Cretaceous they
were, allowing larval incubation. Middle Jurassic unionoids of modern lineages
probably already inhabited freshwaters on Laurasia (Unionidae, later
Margaritiferidae) and Gondwana (Hyriidae, Etherioidea). Thus, at the latest during the Middle Jurassic, one may suppose the presence of both characters—derived
gills with marsupia and parasitic larvae for dispersion in their last common ancestor.
5 Evolutionary History of Bivalves as Parasites
were also reported from Mesozoic freshwaters like hybodontid sharks (Hybodus
spp.) and neopterygians (e.g. Lepidotes [Lepistosteiformes]; Kear and Godthelp
2008; Bermúdez-Rochas 2009; Bermudez-Rochas et al. 2013; Van Damme et al.
2015; Cavin 2017) whose diet in combination with close contact with early
unionoids could have facilitated the establishment of parasite-host associations.
Other freshwater fish, e.g. non-durophagous neopterygians like Pliodetes
(Lepistosteiformes), or Semionotus (Semionotiformes) as well as freshwater members of ancient Palaeonisciformes also co-occurred with early unionoids (Dzik and
Sulej 2007; Lucas and Tanner 2007; Cavin 2017; Gibson 2018), thus might also have
served as their early associates.
5.6 Conclusions
Many bivalves live in associations with the other organisms, but only members of
the marine superfamily Galeommatoidea and larvae of the freshwater order
Unionida are known to live in facultative (Galeommatoidea) or obligate (Unionida)
parasitising relationships. In both cases, but for different reasons, these relationships require close association with the host body followed by covering the bivalve
by the host tissues.
In the case of galeommatoideans, it is proposed, that the close association with
their hosts evolved from the stage of a free living ancestor, through a burrow-wall
commensal, to a commensal that settles on a precise area of a host body, sometimes
being covered by the soft tissues of the host, or settling inside the gill chambers of
the host (depending on both the bivalve and host species). This may be corroborated
by recent findings of interpreted accidental larval settlement within the body cavity
of the host-crab in case of Kurtiella pedroana, leading to feeding and presumably
reproduction attempt in the crab’s hemocoel, or living in the body and presumably
parasitising the holothurian by Entovalva nhatrangensis.
Their freshwater habitat, parasitising larvae, and maternal care characterize
unionoids. Because of their large adult size and limited mobility, their dispersal
abilities rely on their minute larvae (most commonly the glochidium) and their close
association with fish hosts. To enhance the larval survival chances, females brood
them in their own modified gills (interpreted as protection against the chemical
parameters of freshwaters, ensuring calcium for the larval shell, and a protection
against water currents, which are accompanied by mantle morphology and behavior
that lure potential host fish, differently developed by different lineages). Unionoids
appeared in Pangean freshwaters by the Late Triassic. At this time gills of the
bivalves were not yet modified (which does not exclude the possibility of maternal
brooding), but presumably in the Middle Jurassic and surely by the Cretaceous they
were, allowing larval incubation. Middle Jurassic unionoids of modern lineages
probably already inhabited freshwaters on Laurasia (Unionidae, later
Margaritiferidae) and Gondwana (Hyriidae, Etherioidea). Thus, at the latest during the Middle Jurassic, one may suppose the presence of both characters—derived
gills with marsupia and parasitic larvae for dispersion in their last common ancestor.
5 Evolutionary History of Bivalves as Parasites