297
crucial for its recognition (Wey-Fabrizius et al. 2014; Supplementary Figure S1 to
Laumer et al. 2015; also Herlyn et al. 2003). Mitochondrial gene order additionally accords with a monophyletic origin of Seisonidea and Acanthocephala (Sielaff
et al. 2016). A grouping of Seisonidea and Acanthocephala (Pararotatoria) is further
in line with morphological data, whereby some evolutionary novelties refer to the
tegument, again. In particular, the already-mentioned infoldings of the distal plasma
membrane widen inside the tegument to larger caverns in seisonids and acanthocephalans. This characteristic has obviously undergone expansion in the stem line
of crown-Acanthocephala, towards the already mentioned lacunar system (Fig.
8.6c). The tegument of seisonids and acanthocephalans is further distinguished by
containing larger filament bundles (Ahlrichs 1997). The third evolutionary novelty
in support of monophyletic Seisonidea-Acanthocephala relates to ultrastructural
details of the spermatozoa again, which in both taxa have two rows of electron-dark
bodies that accompany the anterior portion of the sperm flagellum (Ahlrichs 1997,
1998). Furthermore, there is considerable similarity in the cytomorphology of spermatogenesis states between seisonids and acanthocephalans (Marchand and Mattei
1976; Ferraguti and Melone 1999). In contrast, there seems to be no morphological
feature that could represent an evolutionary novelty of a clade comprising acanthocephalans and bdelloids only (Ricci 1998), although such grouping re-occurs in part
of the molecular analyses (e.g., García-Varela and Nadler 2006). Summing up all
evidence, a plausible tree topology for Syndermata or Rotifera (inclusively
Acanthocephala) appears to be: (Monogononta, (Bdelloidea, (Seisonidea,
Acanthocephala))) (Fig. 8.7). This phylogenetic hypothesis provides the backbone
for the inference of a scenario for the evolution of the endoparasitic two-host cycle
in the stem line of crown-Acanthocephala in the next section.
8.7 Evolution of Acanthocephalan Endoparasitism:
A Conditional Hypothesis
Given the life styles of extant species, the last common ancestors (LCAs)
of crown- Gnathostomulida, crown-Micrognathozoa, crown-Monogononta, and
crown- Bdelloidea were most probably free-living (e.g., Near et al. 1998; Kristensen
and Funch 2000; Sterrer and Sørensen 2015). The same should apply to the LCAs
of crown-Gnathifera and crown-Rotifera/Syndermata. The LCA of crownHemirotifera, from which Bdelloidea, Seisonidea, and Acanthocephala evolved,
should also have been free-living. Lifestyles of monogonont, bdelloid and seisonid
LCAs may be regarded as semi-sessile (compare Ahlrichs and Riemann 2019) but
is herein referred to as free-living because the animals can easily detach from their
substrates. However, while the substrate of monogononts and bdelloids is variable,
the LCA of seisonids probably lived on mandibulates, possibly also from such
hosts, just as the extant species of the group are doing or are assumed to do (e.g.,
Sørensen et al. 2005; see also Fontaneto and de Smet 2015). Lastly, the endoparasitic two-host cycle of all extant Acanthocephala should be a heritage of their LCA.
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…
crucial for its recognition (Wey-Fabrizius et al. 2014; Supplementary Figure S1 to
Laumer et al. 2015; also Herlyn et al. 2003). Mitochondrial gene order additionally accords with a monophyletic origin of Seisonidea and Acanthocephala (Sielaff
et al. 2016). A grouping of Seisonidea and Acanthocephala (Pararotatoria) is further
in line with morphological data, whereby some evolutionary novelties refer to the
tegument, again. In particular, the already-mentioned infoldings of the distal plasma
membrane widen inside the tegument to larger caverns in seisonids and acanthocephalans. This characteristic has obviously undergone expansion in the stem line
of crown-Acanthocephala, towards the already mentioned lacunar system (Fig.
8.6c). The tegument of seisonids and acanthocephalans is further distinguished by
containing larger filament bundles (Ahlrichs 1997). The third evolutionary novelty
in support of monophyletic Seisonidea-Acanthocephala relates to ultrastructural
details of the spermatozoa again, which in both taxa have two rows of electron-dark
bodies that accompany the anterior portion of the sperm flagellum (Ahlrichs 1997,
1998). Furthermore, there is considerable similarity in the cytomorphology of spermatogenesis states between seisonids and acanthocephalans (Marchand and Mattei
1976; Ferraguti and Melone 1999). In contrast, there seems to be no morphological
feature that could represent an evolutionary novelty of a clade comprising acanthocephalans and bdelloids only (Ricci 1998), although such grouping re-occurs in part
of the molecular analyses (e.g., García-Varela and Nadler 2006). Summing up all
evidence, a plausible tree topology for Syndermata or Rotifera (inclusively
Acanthocephala) appears to be: (Monogononta, (Bdelloidea, (Seisonidea,
Acanthocephala))) (Fig. 8.7). This phylogenetic hypothesis provides the backbone
for the inference of a scenario for the evolution of the endoparasitic two-host cycle
in the stem line of crown-Acanthocephala in the next section.
8.7 Evolution of Acanthocephalan Endoparasitism:
A Conditional Hypothesis
Given the life styles of extant species, the last common ancestors (LCAs)
of crown- Gnathostomulida, crown-Micrognathozoa, crown-Monogononta, and
crown- Bdelloidea were most probably free-living (e.g., Near et al. 1998; Kristensen
and Funch 2000; Sterrer and Sørensen 2015). The same should apply to the LCAs
of crown-Gnathifera and crown-Rotifera/Syndermata. The LCA of crownHemirotifera, from which Bdelloidea, Seisonidea, and Acanthocephala evolved,
should also have been free-living. Lifestyles of monogonont, bdelloid and seisonid
LCAs may be regarded as semi-sessile (compare Ahlrichs and Riemann 2019) but
is herein referred to as free-living because the animals can easily detach from their
substrates. However, while the substrate of monogononts and bdelloids is variable,
the LCA of seisonids probably lived on mandibulates, possibly also from such
hosts, just as the extant species of the group are doing or are assumed to do (e.g.,
Sørensen et al. 2005; see also Fontaneto and de Smet 2015). Lastly, the endoparasitic two-host cycle of all extant Acanthocephala should be a heritage of their LCA.
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…