SKELETAL ANOMALIES IN WILD COMMON DAB Word count: 17 911 Christian von den Driesch Student number: 01710623 Supervisor: Prof. Dr. Koen Chiers Supervisor: Dr. Maaike Vercauteren A dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master of Veterinary Medicine Academic year: 2019 - 2020 Ghent University, its employees and/or students, give no warranty that the information provided in this thesis is accurate or exhaustive, nor that the content of this thesis will not constitute or result in any infringement of third-party rights. Ghent University, its employees and/or students do not accept any liability or responsibility for any use which may be made of the content or information given in the thesis, nor for any reliance which may be placed on any advice or information provided in this thesis. IMPACT OF THE SARS-COV-2 PANDEMIC In March 2020, when the SARS-Corona-Virus-2 reached the European Union and Belgium, most of the practical work for this thesis was already done. We had plans to take five more radiographs, to have an equal amount of affected and control fish, and to let the otoliths of affected fish analyze to get their estimated age. Furthermore, we even wanted to do a computer tomography scan of one of the fish with skeletal anomalies to get a better understanding of the three-dimensional configuration of combined vertebral column deviations. Although none of these plans could be carried out, the crisis did not significantly affect the outcome of this work. The communication was switched to electronical forms without problems and without implications for the quality of mentoring and reporting. PREFACE About 20 months ago, when I had to choose a topic for my thesis, I was strongly motivated to write about skeletal anomalies in flatfish, although, or rather because, I did know nothing about flatfish then. I was very much drawn to broaden my horizon, by writing about something only slightly related to my studies of veterinary medicine. “I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.” Sir Isaac Newton Reading his quote, Newton’s pebbles and shells appear to be like cows and sheep for me, only a small fraction of the animal kingdom, which can be seen as the “great ocean of truth”. My studies have equipped me with a broad base of knowledge and veterinarians are taught to think comparatively, figurately taking me to the seashore, ready to – literally – take a look into the “ocean”. Isaac Newton, without any doubts one of the most important scientists of the past centuries, seems to have been quite aware of the fact, that even he himself could never fully study and understand everything, but only small pieces at a time. For my thesis this means that, although we only focused on one certain kind of anomaly, in only one species, in a very small part of the Atlantic Ocean, many small pieces can contribute to understand the greater picture and are therefore necessary and important. Writing this thesis, as well as planning and performing the practical parts of it, were great experiences for me. I got the chance to decide what exactly I wanted to focus on, learned a lot about all elements of scientific research, and had the opportunity to work together with very experienced scientists. Therefore I want to thank my supervisors Prof. Dr. Koen Chiers and Dr. Maaike Vercauteren. Maaike helped me a lot with designing the study, performing it, and putting it all on paper. We could discuss every part of the work during regular meetings, either in person or online and I was able to ask for help with all difficulties or uncertainties I found myself confronted with. Also, I want to thank Dr. Annemie Van Caelenberg (Department of Veterinary medical imaging and small animal orthopedics) for helping us with taking the radiographical images and thoroughly analyzing them afterwards. It was very pleasant for me to work in such a professional and enthusiastic environment. In November 2019 we were able to arrange a meeting with my supervisors, Dr. Van Caelenberg, and Dr. Ana de Azevedo (University of Santiago de Compostela) to discuss the results we had found by then. Dr. de Azevedo did research on skeletal anomalies in Senegalese sole and her articles had a strong influence on how our own study was designed. Therefore, I am very grateful that she was willing to share her expertise with us. I was not the only student this year who wrote his thesis about the common dab. Since the beginning of our work I have been in regular contact with Roxanne Billiet and Sebastian Martelli and we could often help each other with finding suitable literature or discussing problems we all faced. For that reason I want to also thank both of them. Last but not least, I want to express my gratitude towards my girlfriend Jana. She motivated me when I was not, reassured me when I was stressing out, and proof-read my whole thesis. CONTENTS SUMMARY | SAMENVATTING ............................................................................................................................................ 7 1 LITERATURE REVIEW ................................................................................................................................................. 8 1.1 TELEOSTEI ................................................................................................................................................................... 8 1.1.1 Introduction ........................................................................................................................................................ 8 1.1.2 General morphological and physiological differences with mammals ............................................................... 8 1.1.3 The fish skeleton and its differences with mammalian bone ............................................................................. 9 1.2 SKELETAL ANOMALIES IN FISH ........................................................................................................................................ 14 1.2.1 Historical overview ........................................................................................................................................... 14 1.2.2 Wild versus aquaculture ................................................................................................................................... 14 1.2.3 Importance of studying skeletal anomalies in fish ........................................................................................... 15 1.2.4 Diagnostic techniques ...................................................................................................................................... 15 1.2.5 Terminology ..................................................................................................................................................... 15 1.2.6 Types of skeletal anomalies.............................................................................................................................. 16 1.2.7 Tissues and cells ............................................................................................................................................... 18 1.2.8 Causes .............................................................................................................................................................. 18 1.3 TELEOSTEI: FLATFISHES................................................................................................................................................. 20 1.3.1 Systematics ...................................................................................................................................................... 20 1.3.2 History of flatfish research ............................................................................................................................... 20 1.3.3 Phyletic history ................................................................................................................................................. 21 1.3.4 Distribution....................................................................................................................................................... 22 1.3.5 Morphology ...................................................................................................................................................... 22 1.3.6 Physiology ........................................................................................................................................................ 23 1.3.7 Development .................................................................................................................................................... 23 1.3.8 Exploitation ...................................................................................................................................................... 25 1.3.9 Threats ............................................................................................................................................................. 25 1.3.10 Flatfishes in the North Sea ............................................................................................................................... 27 1.3.11 Common dab (Dutch name: Schar) ..................................................................................................................