179
maternal calcium concretions inside their gills, while spending their lives in a habitat with low availability of environmental calcium (Silverman et al. 1985, 1987a, b;
Richard et al. 1991; Machado and Lopes-Lima 2011; Hinzmann et al. 2015). As far
as the advantages of maternal care in unionoids seems to be resolved, there are still
open questions such as when did unionoids evolve eulamellibranch gills allowing
them to incubate larvae in interlamellar spaces, and independently—from when
onwards did their larvae became parasites?
5.5.2 Parasitic Larvae
All the extant members of the Unionida have parasitic larvae (glochidium or lasidium—sometimes with haustorium; e.g. Kat 1984; Wächtler et al. 2001; Graf and
Cummings 2006; Pfeiffer et al. 2019; Fig. 5.3), brooded in the female gills—in
marsupium, until they are released to parasitize their host. It is not clear when this
type of larvae and their parasitism arose. The oldest fossil glochidia are found as
late as the Miocene (Gross and Piller 2018), Pleistocene (Brodniewicz 1968), and
some younger deposits (Aldridge and Horne 1998). If the behavior of the extant
mussels has been similar in the past (Boucot and Poinar 2010; Nagler and Haug
2015), one may expect finding fossils with glochidia while encapsulated on fish
gills, skin or fins. This could provide direct fossil evidence of the evolution of this
relationship. The preservation of fossilized soft tissues of fishes (most often muscles, but including the gills or skin) is not rare in several so-called KonservatLagerstätte (Wilby and Briggs 1997). The detailed preservation of soft tissue of fish
in marine Jurassic Solnhofen Lagerstätte (Germany) allowed identification of
pathologies—skin lesions (Petit and Khalloufi 2012); such lesions are also recognizable even at the cellular level on mineralized fish from the marine Jurassic Osteno
Lagerstätte (Italy; Pinna 1985). Unfortunately freshwater sites like this are exceptional (e.g. review by Cavin 2017), but in brackish deposits of the Romualdo
Member of the Santana Formation of Brazil (Cretaceous) three-dimensionally preserved fishes (most commonly Rhacolepis sp.) with mineralized soft tissues occur
(e.g. Martill 1988; Martill and Harper 1990; Wilby and Martill 1992; Maldanis et al.
2016; Osés et al. 2017). On the gills of Cladocyclus sp. from this Formation tiny
fossil parasitic copepods are preserved (Cressey and Patterson 1973; Cressey and
Boxshall 1989). Bruno and Hessel (2006) report, that Cladocyclus sp. could be
tolerant to variations in salinity and thus could be contaminated with marine parasites as copepods, thus this does not exclude possibility of carrying glochidia as
well. Glochidia of extant Glebula rotundata (Lampsilini, Unionidae) were found on
two estuarine fish species (Trinectes maculatus and Anchoa mitchilli; Parker et al.
1984). Although there is no morphological evidence of glochidiosis in mineralized
gills of Rhacolepis sp. or Cladocyclus sp., as well as to my knowledge no unionoid
fossils were discovered from this formation, the resolution of preservation together
with the freshwater depositional environment are promising (Martill and Harper
1990). Additionally, in the underlying lacustrine deposits of the Crato Formation,
5 Evolutionary History of Bivalves as Parasites
maternal calcium concretions inside their gills, while spending their lives in a habitat with low availability of environmental calcium (Silverman et al. 1985, 1987a, b;
Richard et al. 1991; Machado and Lopes-Lima 2011; Hinzmann et al. 2015). As far
as the advantages of maternal care in unionoids seems to be resolved, there are still
open questions such as when did unionoids evolve eulamellibranch gills allowing
them to incubate larvae in interlamellar spaces, and independently—from when
onwards did their larvae became parasites?
5.5.2 Parasitic Larvae
All the extant members of the Unionida have parasitic larvae (glochidium or lasidium—sometimes with haustorium; e.g. Kat 1984; Wächtler et al. 2001; Graf and
Cummings 2006; Pfeiffer et al. 2019; Fig. 5.3), brooded in the female gills—in
marsupium, until they are released to parasitize their host. It is not clear when this
type of larvae and their parasitism arose. The oldest fossil glochidia are found as
late as the Miocene (Gross and Piller 2018), Pleistocene (Brodniewicz 1968), and
some younger deposits (Aldridge and Horne 1998). If the behavior of the extant
mussels has been similar in the past (Boucot and Poinar 2010; Nagler and Haug
2015), one may expect finding fossils with glochidia while encapsulated on fish
gills, skin or fins. This could provide direct fossil evidence of the evolution of this
relationship. The preservation of fossilized soft tissues of fishes (most often muscles, but including the gills or skin) is not rare in several so-called KonservatLagerstätte (Wilby and Briggs 1997). The detailed preservation of soft tissue of fish
in marine Jurassic Solnhofen Lagerstätte (Germany) allowed identification of
pathologies—skin lesions (Petit and Khalloufi 2012); such lesions are also recognizable even at the cellular level on mineralized fish from the marine Jurassic Osteno
Lagerstätte (Italy; Pinna 1985). Unfortunately freshwater sites like this are exceptional (e.g. review by Cavin 2017), but in brackish deposits of the Romualdo
Member of the Santana Formation of Brazil (Cretaceous) three-dimensionally preserved fishes (most commonly Rhacolepis sp.) with mineralized soft tissues occur
(e.g. Martill 1988; Martill and Harper 1990; Wilby and Martill 1992; Maldanis et al.
2016; Osés et al. 2017). On the gills of Cladocyclus sp. from this Formation tiny
fossil parasitic copepods are preserved (Cressey and Patterson 1973; Cressey and
Boxshall 1989). Bruno and Hessel (2006) report, that Cladocyclus sp. could be
tolerant to variations in salinity and thus could be contaminated with marine parasites as copepods, thus this does not exclude possibility of carrying glochidia as
well. Glochidia of extant Glebula rotundata (Lampsilini, Unionidae) were found on
two estuarine fish species (Trinectes maculatus and Anchoa mitchilli; Parker et al.
1984). Although there is no morphological evidence of glochidiosis in mineralized
gills of Rhacolepis sp. or Cladocyclus sp., as well as to my knowledge no unionoid
fossils were discovered from this formation, the resolution of preservation together
with the freshwater depositional environment are promising (Martill and Harper
1990). Additionally, in the underlying lacustrine deposits of the Crato Formation,
5 Evolutionary History of Bivalves as Parasites