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Abstract NEW INSIGHTS INTO CNIDARIAN EVOLUTION Allen Collins Being conspicuous and sometimes painful inhabitants of most marine
environments – living in, on, and above substrates as polyps,
medusae, and a variety of other life-cycle stages – cnidarians
have been documented for hundreds of years. Cnidaria has modest
species richness, with some 10,000-12,000 known species, but a remarkable
level of disparity in form, ranging from minute creeping worms,
to massive corals and jellyfish, to complex swimming siphonophore
colonies. Recent efforts by a number of different research groups,
including those involved with the Cnidarian Tree of Life project,
are compiling massive amounts of molecular sequence data that are
greatly refining our understanding of the phylogenetic history of
Cnidaria at all taxonomic levels. This is an exciting time for invertebrate
zoologists because this robust phylogenetic hypothesis allows us
to use the comparative method to resolve numerous long-debated questions
about cnidarian evolution. We can infer the relative timing of when
various groups either gained or lost particular life-cycle stages.
For instance, the ancestral cnidarian was very likely a solitary
benthic polyp, as we can pinpoint the origin of the pelagic jellyfish
stage within Medusozoa, one of the two main branches of Cnidaria.
In addition, a number of obscure forms that have defied classification
can now readily be referred to given taxa, allowing one to form
hypotheses about how these unusual species originated. Among these
odd forms are Tetraplatia and Otohydra. Tetraplatia
is a free-swimming, worm-shaped animal with swimming flaps, whereas
Otohydra is a minute tentacled form that lives between
sand grains. Both appear to have evolved from medusae that spend
their entire lives in the open ocean. Finally, the parasitic myxozoans,
which have variously been classified as a phylum of either unicellular
protists or derived bilaterian animals, have recently been shown
to have an origin within Cnidaria.
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