Postdoctoral Associate, Institute for Computational Biomedicine, Weill Cornell Medicine, New York, New York
This month's featured taxon is Myxozoa: a bizarre, poorly understood group of microscopic, obligate parasites. Members of this taxon are typically found parasitizing teleost fish and annelid worms, though they have been observed in a wide spectrum of hosts including amphibians, birds, bryozoans, cephalopods, reptiles, shrews, and waterfowl. These parasites are globally distributed in marine and freshwater aquatic environments (though some are exclusively terrestrial), and have been found in nearly all tissue and organ types. Myxozoa is an extremely diverse group not only in distribution but in species richness, comprising over 2,200 described species distributed among over 60 genera (Lom and Dyková, 2006) – which likely represents a small fraction of the total diversity, with some estimates of 16,000 species in the Neotropics alone (Naldonia et al., 2011).
Although most myxozoan infections are innocuous, some species are well known pathogens that cause fatal diseases that can have significant economic impact, particularly on fish farms (Kent et al., 2001). One especially nasty example is Tetracapsuloides bryosalmonae, the causative agent of Proliferative Kidney Disease, which can wipe out 90% of infected salmonid populations, and even caused authorities to shut down a 183-mile stretch of Yellowstone River last summer (Young, 2016).
Each individual myxozoan has a fantastically complex life cycle that involves radical physiological transformations. Upon penetration of a host, an individual amoeboid-like reproductive body will undergo complex rounds of cellular fusion and division, before ultimately producing a reproductive spore that will eventually emerge from its host into the water column in search of its next host. These spores exhibit a stunningly diverse array of morphologies, including spherical, fusiform, pyriform, floral, round, ovoid, flattened, elongated, with or without caudal appendages, and all variations exhibit a wide variety of variation in orientation and number of constituent parts. The image gives just a taste of the incredibly morphological diversity of this taxon. In rather dramatic fashion, these spores harbor a complex organelle known as a polar capsule, which contains a coiled up filament that, upon stimulation, will rapidly evert from the capsule like the finger of a glove. The sticky filament flies through the water and latches onto the integument of the target animal like a grappling hook, allowing the spore to wriggle its way into its next host and beginning the parasitic cycle anew.
But perhaps the most impressive thing about Myxozoa is its position within the tree of life. These microscopic, morphologically simplistic parasites are members of the phylum Cnidaria, the lineage containing animals such as jellyfish, sea anemones, and corals. Indeed, myxozoans are extremely divergent, incredibly reduced, highly derived evolutionary cousins of these commonly known creatures. And this relationship of myxozoans to its cnidarian allies renders the group one of the most dramatically degenerate parasitic radiations known to biology. Myxozoans have neither tentacles, nor gastrovascular cavities, nor even tissue layers – and yet, they are cnidarians, by virtue of their polar capsules, which are homologous to cnidocytes (the stinging organelles only found within Cnidaria).
To put it into perspective: the size difference between an individual myxozoan spore and the common moon jelly is equivalent to the size difference between a human and Mt. Everest. Quite the difference.
And yet, this evolutionary relationship was not understood for nearly two hundred years. After first discovery in the early 19th century, myxozoans were categorized as various protistan lineages. Upon their confirmation as cnidarians not much more than 20 years ago (Siddall et al., 1995), biologists realized that myxozoans are not only incredibly derived cnidarians, but that they are the smallest and perhaps simplest animals in existence. Having lost nearly all diagnostic features known to animals (cellular structures such as centrioles and cilia), myxozoans stretch the very limit of what we understand to be "animals". It is only fitting that we save for the end of the year a taxon that stretches the limits of our biological imagination.
Kent, M.L., K.B. Andree, J.L. Bartholomew, M. El-Matbouli, S.S. Desser, R.H. Devlin, S.W. Feist, P.P. Hedrick, R.W. Hoffmann, J. Khattra, S.L. Hallett, R.J.G. Lester, M. Longshaw, O. Palenzeula, M.E. Siddall ME & C.X. Xiao. 2001. Recent advances in our knowledge of the Myxozoa. J Eukaryot Microbiol 48: 395–413.
Lom, J., & I. Dykova. 2006. Myxozoan genera: definition and notes on taxonomy, life-cycle terminology and pathogenic species. Folia Parasitology 53: 1-36. London, pp. 115–154.
Naldonia, J., S. Aranab, A.A.M. Maiac, M.R.M. Silvac, M.M. Carrieroc, P.S. Ceccarellid, L.E.R. Tavarese & E.A. Adrianof. 2011. Host–parasite–environment relationship, morphology and molecular analyses of Henneguya eirasi n. sp. parasite of two wild Pseudoplatystoma spp. in Pantanal Wetland, Brazil. Veterinary Parasitology 177: 247–255.
Siddall, M.E., D.S. Martin, D. Bridge, S.S. Desser & D.K. Cone. 1995. The demise of a phylum of protists: phylogeny of Myxozoa and other parasitic Cnidaria. Journal of Parasitology 81: 961–967.
Young, Ed. 2016. A Tiny Jellyfish Relative Just Shut Down Yellowstone River. The Atlantic: https://www.theatlantic.com/science/archive/2016/08/the-parasite-that-just-shut-down-a-montana-river-has-an-unbelievable-origin/496817/ .