190
The parasitic association could be established with the fish that co-occurred in
freshwaters and which possibly were common and lived close to the bottom of
ancient lakes or rivers (e.g. durophagous fish). In the context of this close proximity
(possibly including predation), the phoretic relationship was possibly established,
which enhanced the dispersal abilities of the bivalve associates (e.g. via larval
mucous threads, which presumably were tangled on fish, but could also serve as
anti-sinking floating web or dragline). Then a firmer attachment was acquired, by
clamping the bivalve larval valves on the fish tissue, which started a wound reaction
in fish tissue and resulted in quick larval encapsulation due to host epithelium cell
migration. Further development of mechanisms, which allowed resistance to the
host immunity, strengthened this association and opened the possibility of nutrition
on fish tissues. On the one hand, larval feeding on host tissues enabled reducing the
size of larvae, which may lower the cost of females producing a huge amount of
small larvae (which could cause high infestation of the host increasing the probability of survival and dispersal of the offspring). On the other hand, the small size of
larvae requires a longer timespan spent in the environment of fish tissues in order to
gather nutrition for growth to a size that ensures safe metamorphosis and thus being
exposed to immune responses for a longer time than larger larvae. In consequence a
small larva must have developed a close adaptation to the host species, which in
addition to their small size might hamper their fossil preservation potential.
Little direct fossil evidence is known to constrain the transition from free-living
to a parasitic lifestyle in bivalves from both parasitic groups (Galeommatoidea,
Unionida). Based on knowledge on the reasons or consequences of establishing a
parasitic relationship from living species and indirect fossil record, at least two
major evolutionary steps towards parasitism can be described: close contact with
the host body, followed by the enclosing of the bivalve within the host’s tissues.
Detailed considerations about the mechanisms and scenario of such relations originating still need more direct data.
Acknowledgments I am grateful to the Editors, Kenneth De Baets and John Warren Huntley for
their immense effort in editing and reviewing this chapter. I am thankful to John M. Pfeiffer for the
kind discussions about evolution of larva types in Unionida, to Andrzej Kołodziejczyk who proofread the draft, to Maria Cristina Dreher Mansur for sharing her observations on lasidium morphology and to Marcello Guimares Simões for sharing his knowledge on bivalves from Crato Formation.
I am grateful to the University of Chicago Press for granting the permission to use material from
the publication of Rogers-Lowery and Dimock (2006) as the model for a new illustration in
this work.
References
Aldridge DC, Horne DC (1998) Fossil glochidia (Bivalvia, Unionidae): identification and value in
palaeoenvironmental reconstructions. J Micropalaeontol 17:179–182
Aldridge DC, McIvor AL (2003) Gill evacuation and release of glochidia by Unio pictorum and
Unio tumidus (Bivalvia: Unionidae) under thermal and hypoxic stress. J Molluscan Stud
69:55–59
A. Skawina
The parasitic association could be established with the fish that co-occurred in
freshwaters and which possibly were common and lived close to the bottom of
ancient lakes or rivers (e.g. durophagous fish). In the context of this close proximity
(possibly including predation), the phoretic relationship was possibly established,
which enhanced the dispersal abilities of the bivalve associates (e.g. via larval
mucous threads, which presumably were tangled on fish, but could also serve as
anti-sinking floating web or dragline). Then a firmer attachment was acquired, by
clamping the bivalve larval valves on the fish tissue, which started a wound reaction
in fish tissue and resulted in quick larval encapsulation due to host epithelium cell
migration. Further development of mechanisms, which allowed resistance to the
host immunity, strengthened this association and opened the possibility of nutrition
on fish tissues. On the one hand, larval feeding on host tissues enabled reducing the
size of larvae, which may lower the cost of females producing a huge amount of
small larvae (which could cause high infestation of the host increasing the probability of survival and dispersal of the offspring). On the other hand, the small size of
larvae requires a longer timespan spent in the environment of fish tissues in order to
gather nutrition for growth to a size that ensures safe metamorphosis and thus being
exposed to immune responses for a longer time than larger larvae. In consequence a
small larva must have developed a close adaptation to the host species, which in
addition to their small size might hamper their fossil preservation potential.
Little direct fossil evidence is known to constrain the transition from free-living
to a parasitic lifestyle in bivalves from both parasitic groups (Galeommatoidea,
Unionida). Based on knowledge on the reasons or consequences of establishing a
parasitic relationship from living species and indirect fossil record, at least two
major evolutionary steps towards parasitism can be described: close contact with
the host body, followed by the enclosing of the bivalve within the host’s tissues.
Detailed considerations about the mechanisms and scenario of such relations originating still need more direct data.
Acknowledgments I am grateful to the Editors, Kenneth De Baets and John Warren Huntley for
their immense effort in editing and reviewing this chapter. I am thankful to John M. Pfeiffer for the
kind discussions about evolution of larva types in Unionida, to Andrzej Kołodziejczyk who proofread the draft, to Maria Cristina Dreher Mansur for sharing her observations on lasidium morphology and to Marcello Guimares Simões for sharing his knowledge on bivalves from Crato Formation.
I am grateful to the University of Chicago Press for granting the permission to use material from
the publication of Rogers-Lowery and Dimock (2006) as the model for a new illustration in
this work.
References
Aldridge DC, Horne DC (1998) Fossil glochidia (Bivalvia, Unionidae): identification and value in
palaeoenvironmental reconstructions. J Micropalaeontol 17:179–182
Aldridge DC, McIvor AL (2003) Gill evacuation and release of glochidia by Unio pictorum and
Unio tumidus (Bivalvia: Unionidae) under thermal and hypoxic stress. J Molluscan Stud
69:55–59
A. Skawina