The Biology of Hagfishes the Biology of Hagfishes

THE BIOLOGY OF HAGFISHES THE BIOLOGY OF HAGFISHES

J0RGEN M0RUP J0RGENSEN, JENS PETER LOMHOLT, ROY E. WEBER and HANSMALTE

Department of Zoophysiology Institute of Biological Sciences University of Aarhus Denmark

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8 Printed on acid-free text paper, manufactured in accordance with ANSIINISO Z39.48-1992 (Permanence of Paper). CONTENTS

List of contributors Vll

Editors' preface xii

Introduction: Early hagfish research Ragnar Fiinge xiii

Part One Evolution, Taxonomy and Ecology 1 1 Relationships of living and fossil hagfishes David Bardack 3 2 Conodonts: a sister group to hagfishes? Richard]. Aldridge and Philip c.]. Donoghue 15 3 Hagfish systematics Bo Fernholm 33 4 Asian hagfishes and their fisheries biology Yoshiharu Honma 45 5 The ecology of hagfishes Frederic H. Martini 57

Part Two Development and Pathology 79 6 Chromatin diminution and chromosome elimination in hagfishes Sei-ichi Kohno, Souichirou Kubota and Yasuharu Nakai 81 7 The tumour pathology of Myxine glutinosa Sture Falkmer 101

Part Three The Integument and Associated Glands 107 8 Hagfish skin and slime glands Robert H. Spitzer and Elizabeth A. Koch 109 9 The dermis Ulrich Welsch, Simone Buehl and Rainer Erlinger 133

Part Four Supporting Tissues 143 10 The notochord Ulrich Welsch, Akira Chiba and Yoshiharu Honma 145 11 Hagfish cartilage Glenda M. Wright, Fred W. Keeley and M. Edwin DeMont 160

Part Five The Muscular System 171 12 The skeletal muscle fibre types of Myxine glutinosa Per R. Flood 173

Part Six The Respiratory System 203 13 The gills of hagfishes Helmut Bartels 205 14 Ventilation and gas exchange Hans Malte and Jens Peter Lomholt 223

Part Seven The Cardiovascular System 235 15 Cardiovascular function in hag fishes Malcolm E. Forster 237 16 The sinus system of hagfishes -lymphatic or secondary circulatory system? Jens Peter Lomholt and Frida Franko-Dossar 259 17 Dermal capillaries Ulrich Welsch and Ian C. Potter 273 vi Contents Part Eight The blood and Immune System 285 18 Hagfish blood cells and their formation Ragnar Fiinge 287 19 Volume regulation in red blood cells Niels Dohn and Hans Malte 300 20 Transport of bicarbonate, other ions and substrates across the red blood cell membrane of hagfishes Thomas Peters and Gerolf Gros 307 21 Hagfish haemoglobins Angela Fago and Roy E. Weber 321 22 The hagfish immune system Robert L. Raison and Nicholas J. dos Remedios 334

Part Nine The Uro-genital System 345 23 The hagfish kidney as a model to study renal physiology and toxicology Luder M. Fels, Sabine Kastner and Hilmar Stolte 347 24 An analysis of the function of the glomeruli of the hagfish mesonephric kidney Jay A. Riegel 364 25 Gonads and reproduction in hagfishes Robert A. Patzner 378

Part Ten The Endocrine System 397 26 The endocrine system of hag fishes Michael C. Thorndyke and Sture Falkmer 399 27 The control of catecholamine secretion in hagfishes Nicholas J. Bernier and Steve F. Perry 413

Part Eleven The Nervous System 429 28 Ontogeny of the head and nervous system of myxinoids Helmut Wicht and Udo Tusch 431 29 The central nervous system of hagfishes Mark Ronan and R. Glenn Northcutt 451 30 The autonomic nervous system and chromaffin tissue in hagfishes Stefan Nilsson and Susanne Holmgren 478

Part Twelve The Sensory Organs 495 31 Skin sensory organs in the Atlantic hagfish Myxine glutinosa Monika von During and Karl H. Andres 497 32 Cutaneous exteroreceptors and their innervation in hagfishes Christopher B. Braun and R. Glenn Northcutt 510 33 The olfactory system of hagfishes Kjell B. DelVing 531 34 The eyes of hagfishes N. Adam Locket and JfJrgen MfJrup JfJrgensen 539 35 Structure of the hagfish inner ear JfJrgen MfJrup JfJrgensen 555 36 Physiology of the inner ear Alistair R. McVean 562 LIST OF CONTRIBUTORS

Richard J. Aldridge Christopher B. Braun Department of Geology Parmly Hearing Institute University of Leicester Loyola University Chicago University Road 6525 N. Sheridan Rd Leicester LE1 7RH Chicago IL 60626 United Kingdom USA E-mail: [email protected] E-mail: [email protected]

Karl H. Andres Simone Buehl Abt. fur Neuroanatomie Anatomische Anstalt, Lehrstuhl II Institut fur Anatomie Pettenkoferstrasse 11 Ruhr-Universitat Bochum Ludwig-Maximilians-Universitat Munchen Universitatsstrasse 150 D-80336 D-44780 Bochum Deutschland (Germany) Deutschland (Germany) Akira Chiba E-mail: [email protected] Department of Histology David Bardack Nippon Dental University Department of Biological 1-8 Hamaura-Cho Sciences (M/C 066) Niigata, 951 University of Illinois at Chicago Japan 845 West Taylor Street M. Edwin DeMont Chicago, Illinois 60607-7060 Department of Biology USA St Francis Xavier University E-mail: [email protected] Antigonish, Nova Scotia B2G 2W5 Canada Helmut Bartels E-mail: [email protected] Anatomische Anstalt Universitat Munchen Niels Dohn Pettenkoferstrasse 11 Department of Zoophysiology D-80336 Munchen Institute of Biological Sciences Deutschland (Germany) University of Aarhus Fax: +498951604857 Universitetsparken, building 131 E-mail: [email protected] DK-8000 Aarhus C Danmark (Denmark) Nicholas J. Bernier Department of Biology Philip c.J. Donoghue University of Ottawa School of Earth Sciences 30 Marie Curie University of Birmingham PO Box 450 STN A Edgbaston Ottawa ON K1N 6N5 Birmingham B15 2TT Canada United Kingdom E-mail: [email protected] E-mail: [email protected] viii List of contributors Monika von During Luder M. Fels Abt. fur Neuroanatomie Abteilung fur Nephrologie Institut fur Anatomie Arbeitsbereich Experimentelle Nephrologie Ruhr-Universitat Bochum Medizinische Hochschule Hannover Universitatsstrasse 150 MA6/162 Carl-Neuberg-Strassel 0-44780 Bochum 0-30625 Hannover Deutschland (Germany) Deutschland (Germany) Fax: +49 234 709 4457 Fax: +49 511 5323 780 E-mail: [email protected]• bochum-de BoFemholm Sektionen for Vertebratzoologi Naturhistoriska Riksmuseet Kjell B. Daving Box 50007 A vdeling for generell fysiologi 5-104 05 Stockholm Biologisk Institut Sverige (Sweden) Universitetet i Oslo E-mail: VE-Bo@NRMSE PO Box 1051 N-0316 Oslo Norge (Norway) Per R. Flood Fax: +472285 4664 Zoologisk Institut E-mail: [email protected] Allegaten 41 N-5007 Bergen Norge (Norway) Rainer Erlinger E-mail: [email protected] Anatomische Anstalt, Lehrstuhl II Ludwig-Maximilians-Universitat Munchen Malcolm E. Forster Pettenkoferstrasse 11 Department of Zoology 0-80336 M unchen University of Canterbury Deutschland (Germany) Private Bag 4800 Christchurch New Zealand Angela Fago E-mail: [email protected] Department of Zoophysiology Institute of Biological Sciences University of Aarhus Frida Franko-Dossar Universitetsparken, building 131 Department of Zoophysiology DK-8000 Aarhus C Institute of Biological Sciences Danmark (Denmark) University of Aarhus E-mail: [email protected] Universitetsparken, building 131 DK-8000 Aarhus C Danmark (Denmark) Sture Falkmer Institute of Morphology and Pathology Regionsykehuset i Trondheim Ragnar Fange N-7006 Trondheim Storangsgatan 24 Norge (Norway) 5-413 19 Goteborg E-mail: [email protected] Sverige (Sweden) List of contributors ix Gerolf Gros Elizabeth A. Koch Abteilung Vegetative Physiologie Department of Biological Chemistry Zentrum Physiologie The Chicago Medical School Medizinische Hochschule Hannover Finch University of Health Sciences Postfach 61 01 80 3333 Green Bay Road 0-30623 Hannover North Chicago, Illinois 60064-3095 Deutschland (Germany) USA Fax: +49 511 532 2938 E-mail: [email protected] E-mail: [email protected] Susanne Holmgren, Dr Zoofysiologiska A vdelningen Sei-ichi Kohno Zoologiska Institutionen Department of Biology Goteborgs Universitet Faculty of Science Medicinaregatan 18 Toho University S-413 90 Goteborg Miyama 2-2-1, Funabashi, Sverige (Sweden) Chiba 274 E-mail: [email protected] Japan E-mail: [email protected] Yoshiharu Honma, Dr 3460-55 Inarimachi Niigata, 951 Souichirou Kubota Japan Department of Biology Jorgen Morup Jorgensen Faculty of Science Department of Zoophysiology Toho University Institute of Biological Sciences Miyama 2-2-1, Funabashi, University of Aarhus Chiba 274 Universitetsparken, building 131 Japan DK-8000 Aarhus C Danmark (Denmark) E-mail: [email protected] N. Adam Locket Department of Anatomy and Histology Sabine Kastner University of Adelaide Abteilung fur Nephrologie South Australia 5005 Arbeitsbereich Experimentelle Nephrologie Australia Medizinische Hochschule Hannover E-mail: Carl-Neuberg-Strasse 1 [email protected] 0-30625 Hannover Deutschland (Germany) Fax: +49 511 5323 780 Jens Peter Lomholt Fred W. Keeley Department of Zoophysiology Division of Cardiovascular Research Institute of Biological Sciences Research Institute University of Aarhus Hospital for Sick Children Universitetsparken, building 131 Toronto, Ontario M5G lX8 DK-8000 Aarhus C Canada Danmark (Denmark) E-mail: [email protected] Fax: +4586194186 x List of contributors Hans MaIte Robert A. Patzner Department of Zoophysiology Zoologisches Institut Institute of Biological Sciences Universitat Salzburg University of Aarhus Hellbrunnerstrasse 34 Universitetsparken, building 131 A-5020 Salzburg DK-8000 Aarhus C Osterreich (Austria) Danmark (Denmark) E-mail: [email protected] E-mail: [email protected] Steve F. Perry Frederic H. Martini Department of Biology School of Ocean and Earth Sciences and University of Ottawa Technology 30 Marie Curie University of Hawaii PO Box 450 Stn A 5071 Hana Hwy Ottawa Haiku, Hawai 96708 Ontario 6N5 USA Canada E-mail: [email protected] Alistair R. McVean Thomas Peters School of Biological Sciences Abteilung Vegetative Physiologie University of London Zentrum Physiologie Royal Holloway Medizinische Hochschule Hannover Egham, Surrey TW20 OEX Postfach 61 01 80 United Kingdom D-30623 Hannover E-mail: [email protected] Deutschland (Germany) Yasuharu Nakai Fax: +49 511 532 2938 Safety Research Department E-mail: [email protected] Pharmaceuticals Development Research Laboratories Ian C. Potter Teijin Limited School of Environmental and Life Sciences Asahigaoka 4-3-2 Murdoch University Hino, Tokyo, 191 Murdoch Japan Western Australia, 6150 Australia Stefan Nilsson Zoofysiologiska A vdelningen Robert L. Raison Zoologiska Institutionen Department of Cell and Molecular Biology Goteborgs Universitet University of Technology, Sydney Medicinaregatan 18 PO Box 123 Broadway S-413 90 Goteborg New South Wales 2007 Sverige (Sweden) Australia E-mail: [email protected] E-mail: [email protected] R. Glenn Northcutt Department of Neurosciences, 0201 Nicholas J. dos Remedios University of California, San Diego Department of Cell and Molecular Biology 9500 Gilman Drive University of Technology, Sydney La Jolla, California 92093-0201 PO Box 123 Broadway USA New South Wales 2007 Fax: +16195345622 Australia List of contributors xi Jay A. Riegel UdoTusch Department of Zoology Klinikum der Johann Wolfgang Goethe• University of Cambridge Universihit Downing Street Dr. Senckenbergische Anatomie Cambridge CB2 3EJ Theodor-Stern-Kai 7 United Kingdom 0-60590 Frankfurt/Main Fax: +44 223 336 676 Deutschland (Germany)

Mark Ronan Physiology Section RoyE. Weber School of Medicine Department of Zoo physiology Indiana University Institute of Biological Sciences Meyers Hall 263 University of Aarhus Bloomington, Indiana 47405-4201 Universitetsparken, building 131 USA DK-8000 Aarhus C Fax: +1 812 855 4436 Danmark (Denmark) E-mail: [email protected] Robert H. Spitzer Ulrich Welsch Department of Biological Chemistry Lehrstuhl Anatomie II The Chicago Medical School Anatomische Anstalt Finch University of Health Sciences Ludwig-Maximilians-Universihit Munchen 3333 Green Bay Road Pettenkoferstrasse 11 North Chicago, Illinois 60064-3095 0-80336 Munchen USA Deutschland (Germany) E-mail: [email protected] Fax: +498951604897 E-mail: [email protected] Hilmar Stolte Abteilung fUr Nephrologie Helmut Wicht Arbeitsbereich Experimentelle Klinikum der Johann Wolfgang Goethe• Nephrologie Universitat Medizinische Hochschule Hannover Dr. Senckenbergische Anatomie Carl-N euberg-Strassel Theodor-Stern-Kai 7 0-30625 Hannover 0-60590 Frankfurt am Main Deutschland (Germany) Deutschland (Germany) E-mail: [email protected] E-mail: [email protected] Glenda M. Wright Michael C. Thorndyke Department of Anatomy and Physiology School of Biological Sciences Atlantic Veterinary College University of London University of Prince Edward Island Royal Holloway 550 University Avenue Egham, Surrey TW20 OEX Charlottetown, Prince Edward Island United Kingdom Canada CIA 4P3 E-mail: [email protected] E-mail: [email protected] EDITORS' PREFACE

The hagfishes comprise a uniform group of some 60 species inhabiting the cool or deep parts of the oceans of both hemispheres. They are considered the most primitive representatives of the group of craniate chordates, which - apart from the hagfishes that show no traces of verte• brae - includes all vertebrate animals. Consequently the hagfishes have played and still playa central role in discussions concerning the evolution of the vertebrates. Although most of the focus on hagfishes may be the result of their being primitive, it should not be forgotten that, at the same time, they are specialized animals with a unique way of life that is interesting in its own right. It is now more than 30 years since a comprehensive treatise on hagfishes was published. The Biology of Myxine, edited by Alf Brodal and Ragnar Fange (Universitetsforlaget, Oslo, 1963), provided a wealth of information on the biology of hagfishes, and over the years remained a major source of information and inspiration to students of hagfishes. The three decades since the publication of that volume have witnessed a surge of new infor• mation and insight in traditional as well as in newly developed areas of biological investiga• tion. It is the aim of the present book to make this new information on hagfish biology available in a single volume for readers interested in a group of animals that is fascinating not only because of the available knowledge, but also because of the fact that major aspects of their life - such as reproduction and development -largely remain closely guarded secrets. An important step in the production of this book was a symposium organized for the contributors in July 1996. At the symposium speakers addressed an audience from all walks of biology but with a shared interest in hagfishes. It is our hope that this has resulted in a book that will be of use to readers with a general interest in hagfishes, while at the same time reflect• ing recent progress in hagfish research. The symposium was held at the Kristineberg Marine Biological Station near Goteborg, Sweden - a classic locality for hagfish research. We express our gratitude for the hospitality shown by the Director, Professor Jarl-Ove Stromberg and the staff at Kristineberg. The sympo• sium was supported by the Danish Natural Science Research Council and the Carlsberg Foundation. JliJrgen MliJrup JliJrgensen, fens Peter Lornholt, Roy E. Weber, Hans Malte INTRODUCTION: EARLY HAGFISH RESEARCH

R.Fange

SUMMARY The Atlantic hagfish was briefly described by Kalm (1753). Linnaeus (1754, 1758) gave it the name Myxine glutinosa and classified it as a worm, but anatomical studies by Abildgaard (1792) and others proved that it was related to fishes. Research during the nineteenth century by J. Miiller, A. Retzius, F. Nansen, G. Retzius and others centred on anatomy and histology, but during the twentieth century physiological, biochemical and ecological aspects, etc., have become important. Several of the early investigators were Scandinavians. Innumerable ques• tions remain to be solved, for instance concerning the phylogenetic relationships and the reproduction. Some features unique to hagfishes are listed.

EARLIEST OBSERVATIONS Our knowledge of the Atlantic hagfish goes back to Pehr Kalm, a disciple of Linnaeus. In 1747 Kalm started a journey to North America. He left Goteborg in November on board a ship bound for England, but he had to wait many weeks at Grimstad in Norway for the ship to be repaired after a storm. As a naturalist of the Enlightenment he used the compulsory stay to study plants, animals, agriculture, fishery and other things in the surrounding areas. One day in January a local fisherman, 'Pehr i Haven', brought him a fish which he had never seen before. It was called 'Pihrc'U' by the local people and seemed to be a kind of Petromyzon, or 'neijnogon' (Swedish for lamprey, literally meaning 'nine-eyes'). It secreted enormous amounts of slime and was disliked for devouring and destroying fishes caught by hook or net. Linnaeus knew the 'Pihral' from Kalm's travel report (1753) and pictured and described it in a magnificent catalogue of the private Natural History Museum at UIriksdal near Stockholm which was owned by the king, Adolf Fredrik. Linnaeus (1754) introduced the name Myxina glutinosa (Gr. myxa, slime; Lat. gluten, glue). He thought that the hagfish was recognized by contemporary ichthyologists as a blind lamprey (Lampetra caeca, Enneophthalmos caecus), but this was probably wrong (Bloch, 1793). However, disregarding any similarity to lampreys, Linnaeus classified the hagfish among the worms (Vermes intestina) in the tenth edition of the Systema Naturae (1758) and changed the genus name to Myxine. The Norwegian bishop and naturalist Gunnerus (1763) dissected Myxine and observed teeth, jaws, oesophagus, intestine, a two-lobed liver, gall bladder, heart and large yolk-containing eggs, etc. However, the longitudinal lingual muscle he mistook for a cartilaginous trachea connected with 12 so-called lungs. In spite of the anatomical results, Gunnerus followed Linnaeus in calling the animal 'Sleep-Mark', i.e. slime worm. In 1790 A.J. Retzius, professor of xiv Introduction natural history at Lund, Sweden, in a paper entitled 'Anmarkningar vid slagtet Myxine' (Comments on the genus Myxine), wondered what had induced the sharp-sighted Linnaeus to classify Myxine glutinosa among the order of worms (Vermes). Both the literature and inspection of preserved specimens had convinced Retzius that Myxine was related to lampreys or snakes. Two years later Abildgaard (1792), zoologist and father of veterinary medicine in Denmark, on the basis of an anatomical investigation concluded that Myxine was not a worm but a fish. This was confirmed by Bloch (1793), who named the hagfish Gastrobranchus coecus (Gr. gaster, belly; branch us, gill; Lat. caeals, blind; the genus name alludes to ventral gill openings).

RESEARCH DURING THE NINETEENTH CENTURY In 1822 and 1824 A.A. Retzius, the son of A.J. Retzius, carried out detailed anatomical studies on the hagfish. The animal was difficult to get hold of, but Retzius had obtained a few speci• mens preserved in alcohol from the amateur ichthyologist, baron Nils Gyllenstierna at Krapperup, Scania. He investigated the circulatory system by injecting mercury and observed the subcutaneous blood sinus and other vascular structures, the pronephros, the ureters, the muscles of the tongue, different kinds of cartilage, etc. For several years the two brief commu• nications by A.A. Retzius were the main source of knowledge on the anatomy of the hagfish, but during the period 1836-45 the German anatomist, Johannes Muller, published a series of well-illustrated articles on the comparative anatomy of myxinoids. To Muller, as to Retzius, the hagfish was a good vertebrate, more specifically a fish. Muller classified it among the Cyclostomata, which were divided into Hyperoartia (lampreys) and Hyperotreta (hagfishes) (Gr. hypero, palate; artios, whole; tretos, perforated; Hyperoartia have a whole palate whereas, in the Hyperotreta, the palate is penetrated by the nasopharyngeal duct). Muller described with expertise the macroscopic anatomy of the main organ systems including a few micro• scopic details such as blood cells. However, when first investigating the kidney of Myxine, Muller (1836) failed to recognize the true structure of the renal corpuscles. The English physi• cian Bowman (1842) showed in a comparative anatomical study that the capsules of the Malpighian corpuscles of the vertebrate kidney are blind endings of urinary canaliculi and concluded that the glomeruli function by separating water from blood. Muller (1845) in contin• ued work, almost embarrassed, could not but confirm Bowman's anatomical results. He marvelled at the extreme simplicity of the hagfish renal system but abstained from speculating about the function. In 1841 Muller and A.A. Retzius, who were friends, worked together at Kristineberg in Sweden on the circulation of blood in tiny living lancelets (Amphioxus). On that occasion Muller also studied Myxine glutinosa and discovered that a part of the portal vein, previously recognized as a special anatomic structure by Retzius (1822), performed heart-like rhythmic contractions (Muller, 1845). One of several researchers, who worked on the hagfish during the late part of the nineteenth century, was Gustaf Retzius. He was the son of A.A. Retzius and grandson of A.J. Retzius, and among other activities he was for some years professor of histol• ogy at the Karolinska Institute in Stockholm. For many years he intensively investigated the morphology of Myxine glutinosa, following up a more than 100-year-old family tradition. Most of his results appeared in the Biologische Untersuchungen, a magnificent journal which was owned, edited, distributed and mainly written by himself. A younger contemporary of his, the Norwegian zoologist, arctic explorer and diplomat Fridtjof Nansen, also studied the hagfish. To a large extent the achievements of these two scientists concerned the nervous system. Nansen (1887) observed that the nerve fibres of the hagfish spinal cord lacked myelin sheaths, Introduction xv and by the newly invented Golgi technique he discovered that the posterior spinal root axons branched dichotomously, a structural arrangement later detected in all vertebrates. Retzius made careful anatomical observations of nervous and sensory structures. Before dissecting he macerated the hagfishes in 20% nitric acid, which enabled preparation of delicate nerve branches (Figure 1, upper left), and for microscopic work he used mainly the Ehrlich methyl• ene blue vital staining method (Figure 1, lower left) but also the Golgi method (Figure 1, lower right). He studied many different tissues in addition to the nervous system and discovered and described the function of the caudal heart (Retzius, 1890a). Nansen and Retzius were talented artists. Nansen's illustrations were 'drawn under the camera lucida, from the micro• scope directly upon the stone' (Nansen, 1887), (Figure 1, upper right). Retzius made drawings and watercolours himself and also sought assistance by professional artists and engravers. The two researchers corresponded with each other and were friends. Nansen (1888) presented the much debated hypothesis that Myxine is a protandric hermaphrodite. He was visionary and speculative, but he soon gave up his zoological studies for arctic research, and most of his theories on nerve structure are now obsolete or forgotten. Retzius was an accurate recorder of facts who avoided theorizing, and many of his scientific illustrations are still appreciated. An extensive review of investigations on hagfishes up to approximately 1900 is found in Lonnberg (1924).

THE BEGINNING OF THE TWENTIETH CENTURY Since the latter half of the nineteenth century biology has been strongly influenced by the theory of evolution. Myxine was often regarded as a sort of intermediate link between the lancelet (Amphioxus or Branchiostoma) and true vertebrates. By investigating the hagfish anatomists hoped to get clues to understanding morphologic details in higher vertebrates. In Scandinavia the first-year medical students dissected the hagfish as a model of a primitive vertebrate (Muller, 1922), and research on Myxine was performed at both zoological and medical institutes. Schreiner, professor of human anatomy in Oslo, Norway, published results from extensive studies on Myxine carried out over almost 60 years (e.g. Schreiner, 1898, 1955 and 1957), and colleagues at the same university institute followed his choice of research object. When Cole (1925) finished a series of monographs on the morphology of Myxine by publishing a survey of the vascular system, the anatomy and histology of Myxine had become relatively well known, but progress in other sectors long lagged behind. However, since around 1950-60 problems and techniques from physiology, biochemistry, genetics, molecular biology, immunology, pathology, ecology and palaeontology have become increasingly impor• tant. New and more complete and nuanced informations on the myxinoid vertebrates are continuously accumulating.

THE UNIQUENESS OF HAG FISHES Although in their organization hagfishes follow a vertebrate pattern, they are in many respects unique. This is even more evident if other than morphological properties are considered. Some of these points of uniqueness of the hagfishes are listed below.

Liver and biliary system The liver is remarkably large and has a well-developed gall bladder, but unlike the liver of other vertebrates it is built as a tubular gland. The metabolism of the hagfish liver has not xvi Introduction

!I' I .) t~ ~I ,~I !I, V' 'I \1

Figure 1.1 Examples of illustrations of nerve structures in Myxine in works by F. Nansen and G. Retzius. Upper left: dorsal view of the brain (probably prepared by the nitric acid method; see the text), drawn by Ingrid Andersson on behalf of Retzius. Upper right: part of a transverse section through the spinal cord showing neurones (nf), glia cells (gc) and a dorsal nerve root (dnr), stained by the Golgi method, drawn by Nansen 'under the camera lucida'. Lower left: bipolar and unipolar neurons in a spinal nerve ganglion. Methylene blue staining, drawn by G. Retzius. Lower right: Y-shaped branching of dorsal root nerve fibres at their entrance into the spinal cord. Golgi staining, drawn by G. Retzius. Similar pictures are found in Nansen (1887). (After Nansen, 1887, Retzius, 1890b, and Retzius, 1893; the sizes are slightly changed from the originals.) Introduction xvii been much investigated but probably to a considerable extent it resembles that of other vertebrates. For instance it produces a cytochrome P-450 (Andersson and Nilsson, 1989). The gall bladder is controlled by cholinergic nerves, not by the hormone cholecystokinin, and the bile contains a bile alcohol (myxinol disulphate) instead of bile acids (Haslewood, 1966).

Pancreas A specific variety of insulin is produced by cells around the bile duct (Cutfield et al., 1979) and exocrine pancreatic substances, including co lipase (Sternby et al., 1983), are produced by intestinal cells.

Blood and immune system The haemoglobin of the blood is monomeric with unusual composition and function. The defence system produces no immunoglobulins but a complement-like factor may be formed in response to antigenes.

Circulatory system Hagfishes have a low pressure circulation, and the arterial walls contain an unusual type of elastic fibres (Welsch and Potter, 1994). Pulsatory activities and vascular tone are influenced by hormones rather than nerves. The heart, and other tissues, have a remarkable capacity of anaerobic metabolism.

Nervous and sensory systems Eyes, lateral line system, inner ear and pineal complex of the brain are poorly developed, but the skin, especially in the tentacles of the head, has a rich sensory innervation. The physiologic implications of the lack of myelin in the nervous system (Peters, 1964) are not known.

Excretory system Several authors assume that hagfishes have always been living in salt water. However, pecu• liarities of the kidney of hagfishes may indicate that their ancestors were once adapted to fresh or brackish water. The renal corpuscles are relatively enormous, resembling in size those of freshwater fishes (Nash, 1931) or amphibians. Reabsorption of sodium from the urine, a capac• ity which is important for freshwater life, occurs at a low rate in the hagfish kidney (McInerney, 1974). Moreover, hagfishes possess a prominent pronephros and in certain verte• brates (larval Urodela) the pronephros is an osmoregulatory organ eliminating water from the body (Fox, 1963). In myxinoids the pronephros has the appearance of a ciliated apparatus pumping fluid from the body cavity (peritoneal fluid) into the venous blood, and the pronephric duct has regressed. The myxinoid pronephros possibly represents a vestigial water excretory structure. H.W. Smith and A.S. Romer both defended the theory that vertebrates in general originated in fresh water (Smith, 1959a; Romer, 1955). Griffith (1994) assumes that vertebrates once were anadromous, migrating from salt water into fresh water to spawn. The ancestral hagfishes might have returned to a purely marine life very early during evolution, whereas the lampreys, and to a large extent the gnathostomes, remained anadromous.

Reproduction The reproduction biology remains a mystery. The relatively small size of sperm-producing organs may indicate internal fertilization, but no copulation organs have been observed and xviii Introduction Fernholm (1975) supports the hypothesis that hagfishes of both sexes deposit the gonadal products in burrows in the mud. The number of myxinoid embryos which have until now been discovered and examined is extremely limited (Wicht and Northcutt, 1995; Wicht and Tusch, this volume). It is remarkable that so little is known on the embryology and reproduc• tion in spite of the extensive commercial fishery of hagfishes now going on (Gorbman et al., 1990; Honma, this volume). Some features of the hag fishes may be originally primitive, inherited from extinct verte• brates or protovertebrates, but others, such as the poor development of certain sensory organs, could be secondary adaptations to a burrowing habit. Only few features, if any, indicate affin• ity to worms (annelids) or other invertebrates contrary to the imaginations of some of the early investigators. Thus the tooth apparatus has sometimes been compared with a molluscan radula. Liljeqvist et al. (1982) noticed a similarity in the primary structure of haemoglobin from Myxine and from an insect, Chironomus. Future sequence analyses of homeotic genes, etc., may reveal similarities in gene structure not yet anticipated.

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