Polypterus senegalus

Abstract: The embryonic and larval development of the Polypteriformes, the presumed sister group of all other living actinopterygians, is poorly known The main reason is the scarcity of successful breedings in captivity and therefore the lack of developmental series of any one polypterid species A series of five successful breedings of P senegalusnow makes it possible to define developmental stages of this species based on numerous closely timed specimens The staging given here focuses on external embryonic and larval features: epidermal surface structures documented by SEM, colour pattern, development of fins and squamation, larval feeding and locomotory behaviour The development of P senegalusis characterized by a long free embryonic phase Suction feeding is performed from the beginning of larval life (apterolarval phase) The pectoral fins start to become employed for slow locomotion and as supportive structures at around the same time Olfactorily guided prey capture, however, is observed later in the pterolarval phase Quantitative kinematic data also demonstrate a change in the mode of undulatory locomotion during this phase Sustained axial undulation becomes confined to the posterior abdominal and caudal region of the body At about the same time the paraxial high frequency undulation of the pectoral fin fold is replaced by the characteristic propeller-like movement of much greater amplitude and wavelength Surfacing for aerial breathing is not seen before a marked change in colouration has taken place at the beginning of the juvenile period The external gills slowly become reduced during this period The definitions of larval and juvenile stages given here may advance understanding of developmental processes in the ontogeny of these primitive actinopterygians, and may serve as a tool for comparison with the ontogeny of Tetrapoda and Dipnoi, as well as to that of some “primitive” groups of Actinopterygii Judging from its distribution among extant taxa, embryonic and larval ciliation is a character that most probably belongs to the grundplan‡ of Osteognathostomata Phylogenetic evaluation is not so clear for the two other prominent embryonic and larval specializations found in Polypterus: upper labial attachment glands and opercular external gills © 1997 The Royal Swedish Academy of Sciences Published by Elsevier Science Ltd

Abstract: 1. The respiratory behaviour and patterns of oxygen consumption of three Nile species have been investigated. 2. Tilapia nilotica showed a typical pattern of oxygen consumption with an ambient region, adaptive plateau and lethal region (Fig. 2). 3. Specimens of Polypterus senegalus and Clarias lazera (body weights 20–30 and 30–45 g respectively) showed patterns of consumption comparable to that of Tilapia (Fig. 3a and 4a). In larger specimens of the two species the adaptive plateau was either insignificant or completely absent. 4. Specimens of Polypterus and Clarias (20–30 g and 30–45 g respectively) could survive in waters saturated with oxygen (7.4 mg/l) but their tolerance to lower oxygen concentrations was limited. Larger specimens of Polypterus and Clarias failed to survive in oxygen saturated waters. 5. The tolerance of Tilapia nilotica to extremely low oxygen concentration is an adaptation of a tropical and completely aquatic species. Polypterus and Clarias resort to their compensatory mechanisms only when the aquatic respiratory surface fails to satisfy their oxygen requirements.

Abstract: A component of the basal plate which has a plywood-like organization similar to that of the elasmoid scales of teleosts is described in the scales of Polypterus senegalus for the first time. The origin and development of this structure is studied in young (50-117 mm, standard length) and adult (225 and 240 mm) specimens using light and electron microscopy. In 50 mm fish, the scales are imbricated and composed mainly of a succession of orthogonal collagen layers forming a plywood-like structure, the isopedin. The outer surface of the scale is ornamented locally by irregular patches of collagenous material. The layers are not mineralized, whereas the superficial patches are well calcified. The isopedin thickens until it has 12-15 layers and then stops growing (88 mm fish). It mineralizes irregularly from its upper part, and two vascular regions, surrounded by woven-fibered osseous material, form on the outer and deeper surfaces of the isopedin. These regions thicken while the vascular canals close by centripetal deposition of parallel-fibered osseous tissue. The outer region is the superficial part of the mature scale (called here osteodentin), which is covered by the ganoine deposited by the epidermal cells. The deeper part constitutes the definitive basal plate, composed of parallel-fibered osseous tissue. The results show 1) that the young ganoid scales of Polypterus senegalus have a structure similar to that of typical elasmoid scales; and 2) that the isopedin structure does not change during ontogeny and so represents a permanent record of the first ontogenetic stages. The phylogenetic implication of these results is that the elasmoid scales of teleosts arose by a process of paedomorphosis.

Abstract: Significant intraindividual variation in the sequence of the 18S rRNA gene is unusual in animal genomes. In a previous study, multiple 18S rRNA gene sequences were observed within individuals of eight species of sturgeon from North America but not in the North American paddlefish, Polyodon spathula, in two species of Polypterus (Polypterus delhezi and Polypterus senegalus), in other primitive fishes (Erpetoichthys calabaricus, Lepisosteus osseus, Amia calva) or in a lungfish (Protopterus sp.). These observations led to the hypothesis that this unusual genetic characteristic arose within the Acipenseriformes after the presumed divergence of the sturgeon and paddlefish families. In the present study, a survey of nearly all Eurasian acipenseriform species was conducted to examine 18S rDNA variation. Intraindividual variation was not found in the polyodontid species, the Chinese paddlefish, Psephurus gladius, but variation was detected in all Eurasian acipenserid species. The comparison of sequences from two major segments of the 18S rRNA gene and identification of sites where insertion/deletion events have occurred are placed in the context of evolutionary relationships within the Acipenseriformes and the evolution of rDNA variation in this group.