| Summary: | A phylogeny for the 18 families of Siphonostomatoida (Copepoda) parasitic on
vertebrates is presented which considers these taxa a monophyletic group evolved from
siphonostome associates of invertebrates. Discussion of the evolutionary biology of
these families is presented using this phylogeny as a foundation for comparison.
Siphonostomes typically attach at specific locations on their hosts. Although copepod
morphology can sometimes be used to explain realized niches, most copepod
distributions remain mysteriously confined. Distribution data suggest that the branchial
chambers were the first regions of the vertebrate body to be colonized, and that the
olfactory capsules of vertebrates may have been derived from some premandibular
branchial component which caused an evolutionary split in the copepod fauna infecting
the branchial chambers of noseless and jawless vertebrates. The general body surfaces of
vertebrates were probably colonized by taxa infecting the gills and olfactory capsules,
and perhaps was facilitated by a new type larva possessing a frontal filament. Adults of
these larvae appear to have developed two modes of extending this progress in
attachment security throughout adulthood. One mode involved new methods of
permanent attachment of mature females, while the second mode allowed both powerful
swimming and efficient suctoral attachment.
Reduction in the number of molts required to reach adulthood is exhibited by some
lineages, and seems to have been realized through amalgamation of free living nauplius
and/or parasitic copepodid stages. The first copepodid serves as the initial infective stage
throughout all lineages.
While most siphonostome taxa are monoxenous, at least some pennellids are
heteroxenous. Evolution of two host life cycles perhaps was facilitated by a highly
mobile young adult capable of infecting another host, and by the close ecological
association of the intermediate and definitive hosts.
Although not widespread, mesoparasitism has apparently evolved several times among
siphonostome taxa infecting vertebrates. Phylogenetic data illustrate that once a lineage
becomes mesoparasitic, reversal to ectoparasitism is uncommon.
Two siphonostome lineages have successfully invaded fresh water. This significant
ecological shift appears to have been facilitated by a number of morphological,
developmental, and ecological traits.
Preliminary studies suggest that siphonostomes have sometimes coevolved with their
vertebrate hosts while at other times they have colonized phylogenetically distant but
ecologically similar hosts. Overall, the speciation rate of these copepods seems to have
lagged behind that of potential host taxa. === Science, Faculty of === Zoology, Department of === Graduate
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