186
the host species. They commonly parasitize one, or a few closely related, fish species while mussel species with large glochidia (and a shorter time of encapsulation) are host generalists—commonly successfully developing on over a dozen host
fish species (Bauer 1994, 2001d; Hastie and Young 2001; Barnhart et al. 2008;
Strayer 2008). The duration of encapsulation is variable within species and among
species of mussels (O’Brien and Box 1999; O’Dee and Watters 2000; McLeod et al.
2017), as well as between hosts (Yeager and Saylor 1995; Haag and Warren 1997;
O’Brien and Box 1999; Taeubert et al. 2013). It is suggested that the most significant external factor is the water temperature, which influences both the metabolism
of the mussel and the fish and leads to slower metamorphosis if it decreases (Bauer
1994; Jansen et al. 2001; Van Snik Gray et al. 2009; Taeubert et al. 2013, 2014).
Metamorphosis during encapsulation includes two main stages: (1) disintegration of larval adductor muscle and formation of a mushroom-like body by the larval
mantle cells; (2) de novo development of two post-larval adductor muscles; formation of stomach, digestive gland and intestine; formation of large ciliated foot with
foot retractor; larval mantle cells replaced by post-larval mantle cells; formation of
nervous system and rudimentary gills; and finally the disappearance of the mushroom-like body (Waller and Mitchell 1989; Wächtler et al. 2001; Fisher and
Dimock 2002).
During encapsulation fish are dispersing the glochidia, possibly to new places
(Sepkoski and Rex 1974; Graf 1997; Graf and O’Foighil 2000; Wächtler et al. 2001;
Strayer 2008). If metamorphosis is completed, juveniles actively break the wall of cyst
(which become thinner in this time; Waller and Mitchell 1989) and drop off as freeliving mussels (Karna and Millemann 1978; Jansen et al. 2001). Juveniles migrate into
sediments (where they feed on sediment pore water by employing a pedal feeding
strategy), until they emerge at a size of about 1 cm as filter feeders (Yeager et al. 1994;
Bauer 2001c; Schwalb and Push 2007; Schartum et al. 2017; Araujo et al. 2018).
5.5.3.5 In Search of the Host of Early Unionids
Irrespective if unionoids used fish as their hosts or just in a phoretic relationship,
between Late Triassic and Middle Jurassic, they must have developed this relationship with the fishes they could encounter. Today unionoids usually parasitize small,
young fishes of the most common species in their environments. Related mussel
species often parasitize related fish species (Strayer 2008). The global geographical
distribution of mussel families (Graf and Cummings 2007; Bogan 2008; Bogan and
Roe 2008; Lopes-Lima et al. 2017) is today correlated with the distribution of their
likely fish hosts’ families (Berra 2001). Today unionoid diversity is primarily related
to the fish diversity, not to the drainage area (Watters 1992; Modesto et al. 2018).
Nevertheless it appears that the mussel populations are not just controlled by the
presence of their host, as geographic ranges of mussel species are smaller than that
of their hosts (Strayer 1983, 2008; Bauer et al. 1991; Watters 1992). Today the
most common host species belong to the modern families of teleostei (class:
Actinopterygii) like Percidae, Cottidae, Cyprinidae, Centrarchidae, and Salmonidae
A. Skawina
the host species. They commonly parasitize one, or a few closely related, fish species while mussel species with large glochidia (and a shorter time of encapsulation) are host generalists—commonly successfully developing on over a dozen host
fish species (Bauer 1994, 2001d; Hastie and Young 2001; Barnhart et al. 2008;
Strayer 2008). The duration of encapsulation is variable within species and among
species of mussels (O’Brien and Box 1999; O’Dee and Watters 2000; McLeod et al.
2017), as well as between hosts (Yeager and Saylor 1995; Haag and Warren 1997;
O’Brien and Box 1999; Taeubert et al. 2013). It is suggested that the most significant external factor is the water temperature, which influences both the metabolism
of the mussel and the fish and leads to slower metamorphosis if it decreases (Bauer
1994; Jansen et al. 2001; Van Snik Gray et al. 2009; Taeubert et al. 2013, 2014).
Metamorphosis during encapsulation includes two main stages: (1) disintegration of larval adductor muscle and formation of a mushroom-like body by the larval
mantle cells; (2) de novo development of two post-larval adductor muscles; formation of stomach, digestive gland and intestine; formation of large ciliated foot with
foot retractor; larval mantle cells replaced by post-larval mantle cells; formation of
nervous system and rudimentary gills; and finally the disappearance of the mushroom-like body (Waller and Mitchell 1989; Wächtler et al. 2001; Fisher and
Dimock 2002).
During encapsulation fish are dispersing the glochidia, possibly to new places
(Sepkoski and Rex 1974; Graf 1997; Graf and O’Foighil 2000; Wächtler et al. 2001;
Strayer 2008). If metamorphosis is completed, juveniles actively break the wall of cyst
(which become thinner in this time; Waller and Mitchell 1989) and drop off as freeliving mussels (Karna and Millemann 1978; Jansen et al. 2001). Juveniles migrate into
sediments (where they feed on sediment pore water by employing a pedal feeding
strategy), until they emerge at a size of about 1 cm as filter feeders (Yeager et al. 1994;
Bauer 2001c; Schwalb and Push 2007; Schartum et al. 2017; Araujo et al. 2018).
5.5.3.5 In Search of the Host of Early Unionids
Irrespective if unionoids used fish as their hosts or just in a phoretic relationship,
between Late Triassic and Middle Jurassic, they must have developed this relationship with the fishes they could encounter. Today unionoids usually parasitize small,
young fishes of the most common species in their environments. Related mussel
species often parasitize related fish species (Strayer 2008). The global geographical
distribution of mussel families (Graf and Cummings 2007; Bogan 2008; Bogan and
Roe 2008; Lopes-Lima et al. 2017) is today correlated with the distribution of their
likely fish hosts’ families (Berra 2001). Today unionoid diversity is primarily related
to the fish diversity, not to the drainage area (Watters 1992; Modesto et al. 2018).
Nevertheless it appears that the mussel populations are not just controlled by the
presence of their host, as geographic ranges of mussel species are smaller than that
of their hosts (Strayer 1983, 2008; Bauer et al. 1991; Watters 1992). Today the
most common host species belong to the modern families of teleostei (class:
Actinopterygii) like Percidae, Cottidae, Cyprinidae, Centrarchidae, and Salmonidae
A. Skawina