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character polarisation and character transformation order (Donoghue et al. 1989; Rust
2007; Edgecombe and Legg 2013; Legg et al. 2013; Leung 2017). Even more modern
day methods such as molecular clocks have to rely on calibration by fossils in order to
provide accurate and reliable divergence data for specific groups (Warnock and
Engelstädter 2021). In short, fossils provide additional and direct information that
can, for logical reasons, never be gathered by looking at modern forms exclusively.
Besides this strong advantage, considering fossils when it comes to lifestyle
questions comes with a difficulty: We cannot directly observe the life habits of a
once living organism, but we have to infer it. Inferring the life strategy of fossil
parasites can be based on four different types of observations (Nagler and Haug
2015): association reflecting a direct interaction (of host and parasite, both fossilised in contact with each other), indirect pathological changes (of the host), indirect
functional morphological inference (of structures of the putative parasite), indirect
phylogenetic inference (concerning the position of the putative parasite). The latter
can include forms which have particular parasitic life stages which have so far only
been inferred from presence of non-parasitic adults.
Crustaceans are quite good candidates for a ‘fossil parasites’ approach due to the
fact that representatives are relatively well preserved in rocks and amber due to their
relatively large size and the specifics of the arthropod cuticle with a comparably
high fossilization potential and thus often exceptional preservation (e.g., Briggs
1999; Gupta et al. 2006; Pohl et al. 2010; Edgecombe and Legg 2013). As we will
see in some groups it may prove to be more difficult to identify a fossil parasite as
such as some forms have a rather reduced morphology, as for example in parasitic
copepods with a sack-like body form. Also some modern day parasitic crustaceans
display a low abundance in natural populations (Rohde 2005). Still many parasitic
crustaceans are comparably large in relation to their hosts and to other parasitic
organisms (for example, parasitic isopods; Brusca 1981; Bunkley-Williams and
Williams 1998) and thus, also their fossils have a comparably high preservation
potential and can be integrated into an evolutionary reconstruction.
In the following, we discuss examples of parasites in various crustacean groups,
namely Amphipoda (beach hoppers, scuds, whale lice), Isopoda (pill bugs, wood
lice, sea slaters), Copepoda (copepods), Thecostraca (barnacles and their relatives),
and Branchiura (fish lice). All these should have the potential to actually yield fossil
representatives and we discuss which of them indeed do and which do not and
speculate why. Furthermore, we will also discuss Pentastomida (tongue worms), a
group which has by some authors been interpreted as an ingroup of Eucrustacea (see
discussion below). We discuss the forms of parasitism in the widest sense, including
parasitism sensu stricto (“traditional” type of parasitism) as well as different more
exotic-appearing types such as parasitic castrators or “micropredators”. We cannot
provide a strict categorisation as there can be no strict threshold criteria, hence all
categories of parasitism should be seen as flexible concepts. Developing a less
ambiguous categorisation of parasitism types will be an enterprise for future
research. Such an endeavour might benefit from also studying functional morphology of putative parasitic representatives or their close relatives (Nagler and
Haug 2016).
10 Evolutionary History of Crustaceans as Parasites
character polarisation and character transformation order (Donoghue et al. 1989; Rust
2007; Edgecombe and Legg 2013; Legg et al. 2013; Leung 2017). Even more modern
day methods such as molecular clocks have to rely on calibration by fossils in order to
provide accurate and reliable divergence data for specific groups (Warnock and
Engelstädter 2021). In short, fossils provide additional and direct information that
can, for logical reasons, never be gathered by looking at modern forms exclusively.
Besides this strong advantage, considering fossils when it comes to lifestyle
questions comes with a difficulty: We cannot directly observe the life habits of a
once living organism, but we have to infer it. Inferring the life strategy of fossil
parasites can be based on four different types of observations (Nagler and Haug
2015): association reflecting a direct interaction (of host and parasite, both fossilised in contact with each other), indirect pathological changes (of the host), indirect
functional morphological inference (of structures of the putative parasite), indirect
phylogenetic inference (concerning the position of the putative parasite). The latter
can include forms which have particular parasitic life stages which have so far only
been inferred from presence of non-parasitic adults.
Crustaceans are quite good candidates for a ‘fossil parasites’ approach due to the
fact that representatives are relatively well preserved in rocks and amber due to their
relatively large size and the specifics of the arthropod cuticle with a comparably
high fossilization potential and thus often exceptional preservation (e.g., Briggs
1999; Gupta et al. 2006; Pohl et al. 2010; Edgecombe and Legg 2013). As we will
see in some groups it may prove to be more difficult to identify a fossil parasite as
such as some forms have a rather reduced morphology, as for example in parasitic
copepods with a sack-like body form. Also some modern day parasitic crustaceans
display a low abundance in natural populations (Rohde 2005). Still many parasitic
crustaceans are comparably large in relation to their hosts and to other parasitic
organisms (for example, parasitic isopods; Brusca 1981; Bunkley-Williams and
Williams 1998) and thus, also their fossils have a comparably high preservation
potential and can be integrated into an evolutionary reconstruction.
In the following, we discuss examples of parasites in various crustacean groups,
namely Amphipoda (beach hoppers, scuds, whale lice), Isopoda (pill bugs, wood
lice, sea slaters), Copepoda (copepods), Thecostraca (barnacles and their relatives),
and Branchiura (fish lice). All these should have the potential to actually yield fossil
representatives and we discuss which of them indeed do and which do not and
speculate why. Furthermore, we will also discuss Pentastomida (tongue worms), a
group which has by some authors been interpreted as an ingroup of Eucrustacea (see
discussion below). We discuss the forms of parasitism in the widest sense, including
parasitism sensu stricto (“traditional” type of parasitism) as well as different more
exotic-appearing types such as parasitic castrators or “micropredators”. We cannot
provide a strict categorisation as there can be no strict threshold criteria, hence all
categories of parasitism should be seen as flexible concepts. Developing a less
ambiguous categorisation of parasitism types will be an enterprise for future
research. Such an endeavour might benefit from also studying functional morphology of putative parasitic representatives or their close relatives (Nagler and
Haug 2016).
10 Evolutionary History of Crustaceans as Parasites