The embryonic and larval development of Polypterus senegalus Cuvier, 1829: its staging with reference to external and skeletal features, behaviour and locomotory habits

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. senegalus now 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. senegalus is 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 double dagger 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. (C) 1997 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd.