109
© The Author(s) 2021
K. De Baets, J. W. Huntley (eds.), The Evolution and Fossil Record of Parasitism,
Topics in Geobiology 49, https://doi.org/10.1007/978-3-030-42484-8_4
Chapter 4
Evolution, Origins and Diversification
of Parasitic Cnidarians
Beth Okamura and Alexander Gruhl
Abstract Parasitism has evolved in cnidarians on multiple occasions but only one
clade—the Myxozoa—has undergone substantial radiation. We briefly review
minor parasitic clades that exploit pelagic hosts and then focus on the comparative
biology and evolution of the highly speciose Myxozoa and its monotypic sister
taxon, Polypodium hydriforme, which collectively form the Endocnidozoa.
Cnidarian features that may have facilitated the evolution of endoparasitism are
highlighted before considering endocnidozoan origins, life cycle evolution and
potential early hosts. We review the fossil evidence and evaluate existing inferences
based on molecular clock and cophylogenetic analyses. Finally, we consider patterns of adaptation and diversification and stress how poor sampling might preclude
adequate understanding of endocnidozoan diversity.
Keywords Myxozoa · Polypodium · Adaptations to parasitism · Life-cycle
evolution · Cnidarian origins · Fossil record · Host acquisition · Molecular clock
analysis · Co-phylogenetic analysis · Unknown diversity
4.1 Introduction
Cnidarians are generally regarded as a phylum of predatory free-living animals that
occur as benthic polyps and pelagic medusae in the world’s oceans. They include
some of the most iconic residents of marine environments, such as corals, sea anemones and jellyfish. Cnidarians are characterised by relatively simple body-plans,
formed entirely from two tissue layers (the ectoderm and endoderm), and by their
stinging cells or nematocytes. Nematocytes are unique to Cnidaria and function
primarily for prey capture and defense. Phylogenetic analyses identify cnidarians as
B. Okamura ()
Department of Life Sciences, Natural History Museum, London, UK
e-mail: b.okamura@nhm.ac.uk
A. Gruhl
Department of Symbiosis, Max Planck Institute for Marine Microbiology, Bremen, Germany
© The Author(s) 2021
K. De Baets, J. W. Huntley (eds.), The Evolution and Fossil Record of Parasitism,
Topics in Geobiology 49, https://doi.org/10.1007/978-3-030-42484-8_4
Chapter 4
Evolution, Origins and Diversification
of Parasitic Cnidarians
Beth Okamura and Alexander Gruhl
Abstract Parasitism has evolved in cnidarians on multiple occasions but only one
clade—the Myxozoa—has undergone substantial radiation. We briefly review
minor parasitic clades that exploit pelagic hosts and then focus on the comparative
biology and evolution of the highly speciose Myxozoa and its monotypic sister
taxon, Polypodium hydriforme, which collectively form the Endocnidozoa.
Cnidarian features that may have facilitated the evolution of endoparasitism are
highlighted before considering endocnidozoan origins, life cycle evolution and
potential early hosts. We review the fossil evidence and evaluate existing inferences
based on molecular clock and cophylogenetic analyses. Finally, we consider patterns of adaptation and diversification and stress how poor sampling might preclude
adequate understanding of endocnidozoan diversity.
Keywords Myxozoa · Polypodium · Adaptations to parasitism · Life-cycle
evolution · Cnidarian origins · Fossil record · Host acquisition · Molecular clock
analysis · Co-phylogenetic analysis · Unknown diversity
4.1 Introduction
Cnidarians are generally regarded as a phylum of predatory free-living animals that
occur as benthic polyps and pelagic medusae in the world’s oceans. They include
some of the most iconic residents of marine environments, such as corals, sea anemones and jellyfish. Cnidarians are characterised by relatively simple body-plans,
formed entirely from two tissue layers (the ectoderm and endoderm), and by their
stinging cells or nematocytes. Nematocytes are unique to Cnidaria and function
primarily for prey capture and defense. Phylogenetic analyses identify cnidarians as
B. Okamura ()
Department of Life Sciences, Natural History Museum, London, UK
e-mail: b.okamura@nhm.ac.uk
A. Gruhl
Department of Symbiosis, Max Planck Institute for Marine Microbiology, Bremen, Germany