DESCRIPTION OF THE SUPERCLASS MYXINOMORPHI (NELSON 2006)
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SUPERCLASS MYXINOMORPHI LINKS
| Myxinomorphi (mix-I-no-MOR-fi) is derived from two Greek roots that mean slime or mucus forms [mucus = μύξα (myxa) + forms = μορφή (morphe)]. This name was adopted by Nelson (2006) as a modification of Myxiniformes (Berg 1940). |
| INTRODUCTION TO THE MYXINOMORPHI The name of the group comes from the ability of living taxa to exude copious amounts of slime when disturbed. This habit has earned them the name of hagfish. Typically, they live in burrows on the ocean floor where they feed on carrion and invertebrates. They also tend to enter an opening in a fish and eat it from the inside out, because of which Linnaeus (described in Janvier 2007a) designated them as intestinal parasites. The mouth is surrounded by four pairs of tentacles (Figure 1) and contains bony plates covered with keratin toothlets. Though jawless, they can grasp prey by protruding the mouth, which allows the toothlets to hold by a pinching action. Then, they apply leverage by creating a knot around the head with their bodies to provide a base against which the mouth can pull (Figure 2). Glover et al. (2011) also report that hagfish can acquire dissolved organics through their skin and gills. |
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| FIGURE 1. A hagfish striking a typical pose on the ocean floor. Note the ring of tentacles around the mouth. Image from: http://oceanexplorer.noaa.gov | FIGURE 2. A hagfish in the knotting posture. Image from: http:// www.zoology.ubc.ca/labs/biomaterials/overhand.jpg | FIGURE 4. An illustration of Myllokunmingia, the oldest known fish. Image by Giant Blue Anteater and in the Public Domain |
FIGURE 3. A cladogram based on Benton (2005), shows the agnathan taxa as a paraphyletic group. Myllokunmingida and Mixini, considered classes here, are sisters within a clade, the Myxinomorphi, that is sister to the Petromyzontomorphi (lampreys). Together, the lampreys and hagfishes form the Cyclostomata.
Myllokunmingiida is in red because it has no extant members.
| The myxinomorphs resemble the Petromyzontomorphi (lampreys) which also have eel-like bodies with no jaws, no body armor, or paired fins. This similarity, recognized by Durmeril (1806), caused them to be grouped in a single class called Cyclostomata (the round mouths). However, what appeared to be fundamental differences between lampreys and hagfish, caused them to be placed in two different basal groups: the Myxinimorphs and the Vertebrata within a phylum called the Craniata (Janvier 2008a). Major physiological differences included control of heart function, whose atrium and ventricle are separated, by means of the endocrine system rather than by the nervous system. Aside from the separation of the atrium and ventricle, they differ according to other major anatomical structures: the structures of their inner ears, their eyes, the ways that they move water into the pharynx and the gill pouches. Most importantly, hagfish have no vertebrae, although they do have crania. Thus, hagfish were considered to be basal craniates, but not vertebrates. The assumed
paraphyletic
(text with tooltip)
A paraphyletic group is a collection of taxa that is not natural or monophyletic. That is the taxa do not share apomorphic characters but are joined by shared primitive (synpleisiomorphic) characters.
relationship between the lampreys and the hagfishes [see Benton (2005) and Janvier (2008a)] seemed to be the norm and was manifest in the Craniate Hypothesis (all animals with crania are not vertebrates, see above). The view of hagfish-lamprey paraphyly was not universally accepted, though. Molecular work tended to support the monophyly of the cytostomes (lampreys+hagfish) in opposition to the growing support for their separation based on morphology. For example, Stock and Whitt (1992, using rRNA), Kuraku et al. (1999, using nuclear DNA-coded genes), Delarbre et al. (2002, using mitochondrial DNA nucleotide sequence analyses, nuclear gene nucleotide sequence analyses, and amino acid sequence analyses), Furlong and Holland (2002, using 18S rDNA and mitochondrial genes and protein-coding nuclear DNA), Mallatt and Sullivan (2002, using 28S and 18S rDNA), and Heimberg et al. (2010, using microRNAs) all supported a robust monophyletic cyclostome clade that was sister to the living gnathostomes. Since then, molecular work consistently has supported a monophyletic cyclostome clade. Resolution to the conflict between the anatomists and geneticists regarding this question finally came by careful studies of hagfish development. First reported by Ota et al. (2007), hagfish development appeared to be similar in major ways to other vertebrate animals. Ota et al. (2011) identified structures in the developing hagfish that were incipient vertebrae; thus, the fundamental anatomical separation of the hagfish from the other vertebrates vanished. Subsequent studies, especially Oisi et al. (2013) showed that the details of the embryology of hagfish and lampreys, especially of the pituitary and nasal organs, are very similar even though the adult forms are quite different. The arguments from evo-devo in this question caused Janvier (2013), a longtime advocate of the Craniate hypothesis, to declare, “the anatomical features that make present-day hagfish seem more primitive than lampreys are the result of a subsequent loss of features in hagfish during evolution.” Extinct taxa from the lower Cambrian have features that ally them with the hagfish. Detailed fossils of Myllokunmingia (Shu et al. 1999, Figure 4) are interpreted to have gill pouches like the hagfish. In addition, they also have square cartilaginous structures along the notochord which might have been the precursors to vertebrae |
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Berg, L. S. 1940. Classification of Fishes, both Recent and Fossil. Travaux de l’Institute Zoologique de l’Academie des Sciences de l’URSS 5: 1-517. [in Russian with English translation 1947]. Delarbre, C., C. Gallut, V. Barriel, P. Janvier, and G. Gachelin. 2002. Complete mitochondrial DNA of the hagfish, Eptatretus burgeri: the comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly. Molecular Phylogenetics and Evolution. 22: 184–192. Dumeril, A. M. C. 1806. Zoologie analytique methode naturelle de classification des animaux, rendue plus facile a l’aide de tableaux synoptiques. Allais, Paris. pp. i-xxxiii, 1-334. Furlong, R. F. and P. W. H. Holland. 2002. Bayesian phylogenetic analysis supports monophyly of Ambulacraria and of cyclostomes. Zoological Science. 19: 593-599. Glover, C. N., C. Bucking, and C. M. Wood. 2011. Adaptations to in situ feeding: novel nutrient acquisition pathways in an ancient vertebrate. Proceedings of the Royal Society. B. 278: 3096-3101. Heimberg, A., R. Cowper-Sal-lari, M. Semon, P. C. J. Donoghue, and K. J. Peterson. 2010. microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate. Proceedings of the National Academy of Sciences, USA. 107(45): 19379-19383. Janvier, P. 2007a. Born again hagfishes. Nature. 446:622-623. Janvier, P. 2008a. Early jawless vertebrates and cyclostome origins. Zoological Science. 25: 1045-1056. Janvier, P. 2013. led by the nose. Nature. 493. 169-170. Kuraku, S., D. Hoshiyama, K. Katoh, H. Suga, T. Miyata. 1999. Monophyly of lampreys and hagfishes supported by nuclear DNA-coded genes. Journal of Molecular Evolution. 49: 729-735. Mallatt, J. and J. Sullivan. 1998. 28S and 18S rDNA sequences support the monophyly of lampreys and hagfishes. Molecular Biology and Evolution. 15(12): 1706-1718. Monastersky, R. 1999. Waking up to the dawn of vertebrates. Science News. 158(19): 292. Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York. Oisi, Y., K. G. Ota, S. Kuraku, S. Fugimoto, and S. Kuratani. 2013. Craniofacial development of hagfishes and the evolution of vertebrates. Nature, 493: 175-180. Ota, K. G., S. Fugimoto, Y. Oisi, and S. Kuratani. 2011. Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish. Nature Communications. 2:373 doi:10.1038/ncomms1355 Ota, K. G., S. Kuraku, and S. Kuratani. 2007. Hagfish embryology with reference to the evolution of the neural crest. Nature. 446: 672-675. Stock, D. W. and G. S. Whitt. 1992. Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group. Science. 257: 787-789. Shu, D. G., H. L. Luo, S. Conway Morris, X. L. Zhang, S. X. Hu, L. Chen, J. Han, M. Zhu, Y. Li, and L. Z. Chen. 1999. Lower Cambrian vertebrates from south China. Nature. 402: 42-46. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 02/03/2018 |
