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dedifferentiated organism (ypsigon) (Glenner et al. 2008). This morphology indicates that later stage facetotectans are parasitic, most likely endoparasitic. Yet, it
still remains totally unclear which organisms could be the host for facetotectans.
Ascothoracida is the next group branching off in Euthecostraca. Adults live on
echinoderms or cnidarians, possess a distinct piercing mouth cone and have therefore been interpreted as parasites, but also as commensalistic forms (e.g., Grygier
and Salvat 1984; Grygier and Høeg 2005).
Cirripedia is the sister group to Ascothoracida. Within Cirripedia, Acrothoracica
is sistergroup to the remaining forms. Acrothoracicans interact with other organisms
as they bore into different calcareous substrates, including corals, molluscan shells,
and crinoidan skeletons (e.g., Petriconi 1971; Tomlinson 1987). Hence, they are at
least commensalistic, possibly even parasitic.
The remaining cirripedians form two distinct groups, Thoracica and Rhizocephala.
Rhizocephalans are obligate parasites as adults and heavily affect their host.
Their adult body is a mycel-like root-system inside another crustacean (most famous
in crabs, but also in various other groups of crustaceans including many types of
decapodans, but also barnacles). Only their gonads, called externae, are still differentiated true organs (e.g., Glenner and Høeg 2002). Rhizocephalans often castrate
their host and also seem to influence its behaviour. Interestingly, they are often
associated with their non-parasitic relatives (O’Brien and Van Wyk 1985). Longerliving rhizocephalans influence the moulting frequency of their hosts, i.e. they slow
it down (e.g., O’Brien and Van Wyk 1985). In this way, barnacles have longer time
to attach and prosper on the back of an infested crab. Barnacle-bearing crabs are
therefore interesting candidates for being checked for rhizocephalan infestations.
Also, the exact role of barnacles, or better non-parasitic lineages of Cirripedia,
should be evaluated carefully. There are modern forms of lepadomorphan cirripedians that are fully parasitic on sharks, apparently having evolved many traits similar
to rhizocephalans convergently (Rees et al. 2014). Furthermore, also “normal” barnacles sitting on other organisms need to be seen in a differentiated view. In most
cases, the barnacles may affect the organisms not recognisably. Yet, if there are too
many attached to one organism they will indeed have a negative effect on their host
by causing additional energy costs, while they benefit from a stable substrate that is
even able to move to more beneficial environments. In such a case, we can employ
an ecological point of view: the barnacle benefits from this interaction, while the
host has costs. Based on this calculation the barnacles could indeed be considered
as facultative parasites.
10.5.2 Phylogenetic Inference of Appearance and Molecular
Estimations of Early Evolution
As pointed out above, the position of Thecostraca within Eucrustacea can still not
easily be resolved.
J. T. Haug et al.
dedifferentiated organism (ypsigon) (Glenner et al. 2008). This morphology indicates that later stage facetotectans are parasitic, most likely endoparasitic. Yet, it
still remains totally unclear which organisms could be the host for facetotectans.
Ascothoracida is the next group branching off in Euthecostraca. Adults live on
echinoderms or cnidarians, possess a distinct piercing mouth cone and have therefore been interpreted as parasites, but also as commensalistic forms (e.g., Grygier
and Salvat 1984; Grygier and Høeg 2005).
Cirripedia is the sister group to Ascothoracida. Within Cirripedia, Acrothoracica
is sistergroup to the remaining forms. Acrothoracicans interact with other organisms
as they bore into different calcareous substrates, including corals, molluscan shells,
and crinoidan skeletons (e.g., Petriconi 1971; Tomlinson 1987). Hence, they are at
least commensalistic, possibly even parasitic.
The remaining cirripedians form two distinct groups, Thoracica and Rhizocephala.
Rhizocephalans are obligate parasites as adults and heavily affect their host.
Their adult body is a mycel-like root-system inside another crustacean (most famous
in crabs, but also in various other groups of crustaceans including many types of
decapodans, but also barnacles). Only their gonads, called externae, are still differentiated true organs (e.g., Glenner and Høeg 2002). Rhizocephalans often castrate
their host and also seem to influence its behaviour. Interestingly, they are often
associated with their non-parasitic relatives (O’Brien and Van Wyk 1985). Longerliving rhizocephalans influence the moulting frequency of their hosts, i.e. they slow
it down (e.g., O’Brien and Van Wyk 1985). In this way, barnacles have longer time
to attach and prosper on the back of an infested crab. Barnacle-bearing crabs are
therefore interesting candidates for being checked for rhizocephalan infestations.
Also, the exact role of barnacles, or better non-parasitic lineages of Cirripedia,
should be evaluated carefully. There are modern forms of lepadomorphan cirripedians that are fully parasitic on sharks, apparently having evolved many traits similar
to rhizocephalans convergently (Rees et al. 2014). Furthermore, also “normal” barnacles sitting on other organisms need to be seen in a differentiated view. In most
cases, the barnacles may affect the organisms not recognisably. Yet, if there are too
many attached to one organism they will indeed have a negative effect on their host
by causing additional energy costs, while they benefit from a stable substrate that is
even able to move to more beneficial environments. In such a case, we can employ
an ecological point of view: the barnacle benefits from this interaction, while the
host has costs. Based on this calculation the barnacles could indeed be considered
as facultative parasites.
10.5.2 Phylogenetic Inference of Appearance and Molecular
Estimations of Early Evolution
As pointed out above, the position of Thecostraca within Eucrustacea can still not
easily be resolved.
J. T. Haug et al.