295
again might be homologous to the grasping spines in arrow worms (Chaetognatha; Shu
et al. 2017), and, indeed, arrow worms currently appear to be close relatives of
Gnathifera (Fig. 8.7; Fröbius and Funch 2016). Alternatively, arrow worms may
branch off within Gnathifera, then as a sister to a clade comprised of micrognathozoans, rotifers and acanthocephalans (Marlétaz et al. 2019). In any case, the naming
after jaw-like elements (ἡ γνάθος) is a recurrent theme in the kinship circle of
Gnathifera. Yet, the eponymous differentiations obviously got lost in the stem line
of crown-Acanthocephala, along with the reduction of the alimentary tract (e.g.,
Conway Morris and Crompton 1982)—which has the noteworthy consequence that
acanthocephalans are jaw-less members of a clade, which is named after jaw-like
solid-parts (Gnathifera). To top it all, these jaw-less members of Gnathifera parasitize jawed arthropods (Mandibulata) and jawed vertebrates (Gnathostomata).
Inside Gnathifera, the already-mentioned rotifers or wheel animals represent the
next phylogenetic relatives of acanthocephalans amongst the extant species
(Fig. 8.7). Such a relationship was already expected by von Haffner (1950) and others (Rieger and Tyler 1995; see also Conway Morris and Crompton 1982), and in
the meantime gained support from molecular analyses (e.g., Mark Welch 2000;
Herlyn et al. 2003; Struck et al. 2014). Inspired by the presumed evolutionary novelty of a syncytial organization of the tegument, the name Syndermata was introduced for the Rotifera-Acanthocephala clade (Ahlrichs 1997). However, while
some authors use the newly introduced name, others preferentially regard acanthocephalans as highly derived rotifers (e.g., Mark Welch 2000; García-Varela and
Nadler 2006). In fact, the naming of the taxon heated some authors in a surprising
way although there is wide agreement on the decisive point, i.e., the monophyletic
origin of Acanthocephala and the three traditional rotifer taxa Monogononta,
Bdelloidea, and Seisonidea (also Seisonacea, Seisonidae) (e.g., Nielsen 2012).
Apart from its syncytial organization, the tegument is special in all members of the
Acanthocephala-Rotifera group, by having an intra-syncytial lamina—a fact mentioned above with respect to the preservation potential of the acanthocephalan tegument. Another evolutionary novelty of the Acanthocephala-Rotifera group seems to
be that the distal plasma membrane of the tegument shapes crypt-like infoldings
(Ahlrichs 1997; Near 2002). These infoldings increase the surface of the tegument,
which should be of special relevance for nutrient uptake via surface in gut-less acanthocephalans (Graeber and Storch 1978). Members of the Rotifera- Acanthocephala
clade are also specific with respect to sperm ultrastructure, as far as spermatozoa are
produced at all (only females are known for bdelloid rotifers). In particular, the
flagellum inserts at the anterior pole of the sperm head, instead of at its rear end
(Ahlrichs 1997; Ferraguti and Melone 1999).
Molecular studies suggest that Rotifera in the traditional understanding
(Monogononta, Bdelloidea, Seisonidea) represents a paraphyletic assemblage. In
particular, bdelloids appear to be closer related to acanthocephalans than to monogononts (e.g., Near et al. 1998; Near 2002; García-Varela and Nadler 2006; Witek
et al. 2008). Even closer related to acanthocephalans could be seisonids (Fig. 8.7).
A seisonid-acanthocephalan sister-group relationship gains support from part of the
molecular studies, whereby the choice of the substitution model seems to be
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…
again might be homologous to the grasping spines in arrow worms (Chaetognatha; Shu
et al. 2017), and, indeed, arrow worms currently appear to be close relatives of
Gnathifera (Fig. 8.7; Fröbius and Funch 2016). Alternatively, arrow worms may
branch off within Gnathifera, then as a sister to a clade comprised of micrognathozoans, rotifers and acanthocephalans (Marlétaz et al. 2019). In any case, the naming
after jaw-like elements (ἡ γνάθος) is a recurrent theme in the kinship circle of
Gnathifera. Yet, the eponymous differentiations obviously got lost in the stem line
of crown-Acanthocephala, along with the reduction of the alimentary tract (e.g.,
Conway Morris and Crompton 1982)—which has the noteworthy consequence that
acanthocephalans are jaw-less members of a clade, which is named after jaw-like
solid-parts (Gnathifera). To top it all, these jaw-less members of Gnathifera parasitize jawed arthropods (Mandibulata) and jawed vertebrates (Gnathostomata).
Inside Gnathifera, the already-mentioned rotifers or wheel animals represent the
next phylogenetic relatives of acanthocephalans amongst the extant species
(Fig. 8.7). Such a relationship was already expected by von Haffner (1950) and others (Rieger and Tyler 1995; see also Conway Morris and Crompton 1982), and in
the meantime gained support from molecular analyses (e.g., Mark Welch 2000;
Herlyn et al. 2003; Struck et al. 2014). Inspired by the presumed evolutionary novelty of a syncytial organization of the tegument, the name Syndermata was introduced for the Rotifera-Acanthocephala clade (Ahlrichs 1997). However, while
some authors use the newly introduced name, others preferentially regard acanthocephalans as highly derived rotifers (e.g., Mark Welch 2000; García-Varela and
Nadler 2006). In fact, the naming of the taxon heated some authors in a surprising
way although there is wide agreement on the decisive point, i.e., the monophyletic
origin of Acanthocephala and the three traditional rotifer taxa Monogononta,
Bdelloidea, and Seisonidea (also Seisonacea, Seisonidae) (e.g., Nielsen 2012).
Apart from its syncytial organization, the tegument is special in all members of the
Acanthocephala-Rotifera group, by having an intra-syncytial lamina—a fact mentioned above with respect to the preservation potential of the acanthocephalan tegument. Another evolutionary novelty of the Acanthocephala-Rotifera group seems to
be that the distal plasma membrane of the tegument shapes crypt-like infoldings
(Ahlrichs 1997; Near 2002). These infoldings increase the surface of the tegument,
which should be of special relevance for nutrient uptake via surface in gut-less acanthocephalans (Graeber and Storch 1978). Members of the Rotifera- Acanthocephala
clade are also specific with respect to sperm ultrastructure, as far as spermatozoa are
produced at all (only females are known for bdelloid rotifers). In particular, the
flagellum inserts at the anterior pole of the sperm head, instead of at its rear end
(Ahlrichs 1997; Ferraguti and Melone 1999).
Molecular studies suggest that Rotifera in the traditional understanding
(Monogononta, Bdelloidea, Seisonidea) represents a paraphyletic assemblage. In
particular, bdelloids appear to be closer related to acanthocephalans than to monogononts (e.g., Near et al. 1998; Near 2002; García-Varela and Nadler 2006; Witek
et al. 2008). Even closer related to acanthocephalans could be seisonids (Fig. 8.7).
A seisonid-acanthocephalan sister-group relationship gains support from part of the
molecular studies, whereby the choice of the substitution model seems to be
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…