Polypterus senegalus is a basal ray-finned fish that breathes both air and water, and can tolerate life on land for extended periods. Previous research reported that P. senegalus held on land for several months exhibited lower growth rates than counterparts fed equal amounts of food but held under aquatic conditions. Because chronic stress can impair growth, the present research aimed to determine whether P. senegalus experience stress when held under terrestrial conditions. P. senegalus were held for 1, 2, 4, or 8 weeks in aquatic or terrestrial conditions, following which tissues were sampled for analysis. Growth rates were significantly lower in fish held under terrestrial conditions, but neither plasma cortisol levels nor transcript abundances of genes of the stress axis differed between aquatic and terrestrial fish. Terrestrial fish exhibited lower transcript abundance in liver of the growth regulator insulin-like growth factor 1, and higher transcript abundances in muscle of myostatin, an inhibitor of muscle growth, and muscle RING-finger protein-1, an indicator of muscle breakdown. With knowledge from anecdotal observations that P. senegalus tend to be aggressive when housed with a conspecific, P. senegalus were held in a second experiment in pairs for 6 d to investigate stress axis function. Clear differences in territorial and aggressive behaviours between the individuals in a pair were consistent with one fish being dominant over the other, subordinate fish. Dominant fish tended to have higher cortisol levels than subordinate fish, although the difference was not significant, and cortisol levels were correlated with the frequency of aggressive and territorial behaviours, regardless of social status. However, transcript abundances of stress axis genes generally did not differ between dominant and subordinate fish. Collectively, these results suggest that P. senegalus is unusually tolerant of conditions that serve as stressors in other fish species, including emersion and social interactions.

Adhesive organs are widespread structures among vertebrate larvae. They allow the larvae to attach to a substrate, so that the time for the development of mouth or motoric apparatus could be prolonged. Similar structures in ascidians, larvaceans and lancelets are known too. Thus, it might be hypothesized that the presence of some type of adhesive gland could indeed represent the ancestral state for chordate larvae. Interestingly, however, whilst in most species these glands take their developmental origin in ectodermal layer, in bichir, a member of a primitive actinopterygian lineage, their origin was suggested to be endodermal already at the beginning of 20th century. Since then, however, the former study has become almost forgotten and even recent analyses do not come with new findings on this topic. Because of the essential importance of study of bichir cement glands for understanding the relationship between these structures among chordates, I have decided to focus on this subject. To obtain appropriate comparative data Xenopus, Weather loach and Ribbed newt embryos were included in this study as well. By using combination of immunohistochemical and histological techniques the endodermal origin of cement glands in bichir was proven and their morphogenesis was described into considerable details. The...

Aquatic larvae of many vertebrate lineages develop specialized, cranially located cement or attachment glands which allow them to remain attached to a substrate by means of polysaccharide secretion. The larvae can thus remain still and safe in well-oxygenated water out of reach of any predators until the digestive and locomotory apparatus fully develops. Xenopus cement gland is the most thoroughly studied example of this type of glands, since it was used as a model for the anteriormost patterning of the developing head. Based on shared expression patterns of key transcription factors and a similar ectodermal origin it has been repeatedly suggested that Xenopus cement gland is homologous to adhesive organs of teleosts and adhesive papillae of ascidians. The lack of comprehensive knowledge on this type of glands in other lineages however rendered any considerations of homology among such a distant lineages rather inconclusive. In the present work I have focused on a detailed study of the cement glands and other corresponding structures in three representatives of basal actinopterygian lineages: Senegal bichir (Polypterus senegalus), sterlet (Acipenser ruthenus), and tropical gar (Atractosteus tropicus). Using a combination of in vivo fate-mapping approaches with a Micro-CT imaging of cranial endoderm...

Animals control locomotion through unpredictable and complex habitats using a single locomotor control system. Because of the disparate physical mechanics of different environments, behavioural plasticity, based on the complex interplay of sensory feedback and environmental constraints, is likely essential for animals moving across environments. However, few studies have investigated neuromuscular control across different environments. To fill this gap, I make use of Polypterus senegalus to address four primary objectives: (1) to explore the extent of neuromuscular plasticity across environmental gradients (viscosity and water depth), (2) to generate and test hypotheses about paramount signals for this neuromuscular plasticity, (3) to determine the neuromuscular underpinnings of locomotor transitions, and (4) to determine the neuromuscular control of developmental behavioural plasticity in novel environments. I measured the kinematic and muscle activity response of P. senegalus to gradual changes in environment forces using gradients of water viscosity and water depth. I then used a semi-intact preparation to investigate the existence and role of the mesencephalic locomotor region, a brain region that controls locomotor speed and mode in other species, for neuromuscular control in P. senegalus. Finally, I used chronic terrestrial acclimation and exercise to determine the neuromuscular underpinnings of behavioural and morphological plasticity previously seen in P. senegalus reared in a terrestrial environment. I found that in high viscosity environments, P. senegalus maintain routine swimming speed using a swimming-like muscle activity pattern with increased effort in the posterior body and the pectoral fin to generate exaggerated swimming kinematics. These results suggest that sensory feedback is essential to accommodating this novel environment. I then demonstrated that axial red muscle always carried an anterior-to-posterior wave of muscle activity in a series of discrete water depths across the aquatic-terrestrial transition. Thus, discrete changes in axial kinematics and pectoral fin coordination across this transtion are likely the result of sensory feedback and mechanical constraints of the environment. I then performed the first experiments searching for the mesencephalic locomotor region in P. senegalus and demonstrated the presence of a putative mesencephalic locomotor region that controls the frequency of swimming-like movements but does not appear to control pectoral fin movements or the transition to walking. Finally, I exposed P. senegalus to chronic terrestrial acclimation and exercise. My results suggested that while both terrestrial acclimation and exercise generate behavioural plasticity, the former results in a larger plastic repsonse. Subtle changes in the duration and timing of pectoral fin muscle activity helped reduce friction between the body and pectoral fin and the substrate below, potentially resulting in the more “effective” walking gait developed by terrestrial acclimated fish. My thesis therefore sheds light on the essential interplay of sensory feedback and mechanical constraint for generating behavioural plasticity on acute and chronic timescales, highlights the potential value of such plasticity for organismal performance and evolution, and develops study systems and experimental frameworks for further investigating the nature of plastic locomotor control in amphibious fish.

Polypterus is an extant fish that is used as a model to understand the fin-to-limb evolutionary transition. Polypterus exhibits muscle phenotypes relevant to this transition. In particular, plastic changes in bone and muscle in Polypterus have been shown in response to spending time in a terrestrial environment. Muscle fiber changes are usually associated with changes in the performance demand placed on those muscles. We hypothesize that muscle fibers are recruited differently between aquatic and terrestrial environments to explain the change in fiber type. How pectoral fin muscle activity changes between swimming and walking is mostly unknown. Hence, this study utilizes electromyography (EMG) and high-speed videography to understand how the muscle activity pattern and function of all four pectoral fin muscle groups change during swimming and walking in aquatically raised fish. In this experiment, aquatically raised fish were placed in water and on land to observe changes in fin muscle function between behaviours. This study aims to understand how the instantaneous changes in the behaviour of the fish, particularly in the pectoral fin, could explain the muscle plasticity found in previous research. This study showed that fish adduct their pectoral fins much faster with increased muscle effort during walking compared to swimming. The adductor muscle also had the biggest change in function, activating for the majority of the fin-stroke cycle and therefore undergoing eccentric contraction. The increase in muscle effort seen in this study is consistent with the muscle fiber transition seen in fish that spend long periods on land, and the dramatic change of EMG magnitudes found in the adductor muscle may explain muscle damage previously found following acute walking.

Polypterus senegalus, an extant member of the ray-finned fishes, can both swim in water and walk overland. Both environments impose different locomotor requirements on Polypterus fins. In an aquatic environment, forward propulsion is largely generated through oscillations of the pectoral fins working in sync with each other. On land, the pectoral fins are engaged in a contralateral gait, and are involved in lifting the body off the ground while simultaneously balancing the body. Polypterus have been shown to undergo behavioural, anatomical, and physiological changes during both short- and long-term exposure to land. Differences in force environments and locomotor behaviour between aquatic and terrestrial environments are hypothesized to be the cause of these plastic changes observed in the musculoskeletal tissues of Polypterus. Despite these observable changes, it is unclear exactly how the pectoral fins are experiencing ground reaction forces (GRF) during terrestrial locomotion. By measuring and quantifying force production during walking in Polypterus, this thesis provides a first look at the relationship between GRFs produced and experienced during walking and the pectoral fins of the amphibious fish, Polypterus. The kinematics of the pectoral fins and fore body were analyzed during terrestrial locomotion, and strategic points across both pectoral fins and body were digitized. Kinematics were compared with GRFs in the thrust (X), stabilizing (Y) and lifting (Z) planes to understand how impact forces travel through the fin tissues. Further analysis, using inverse dynamics, is required to determine how these impact forces travel through the musculature of the pectoral fins, perhaps providing potential hypotheses as to the effects of GRFs and their role in not only how terrestrial locomotion affects the behavioural, anatomical, and physiological plasticity observed in Polypterus, but also the limbs of tetrapods during the evolutionary transition from aquatic to terrestrial environments.