The evolution of 'thunniform' body shapes in several different groups of vertebrates, including whales, ichthyosaurs and several species of large pelagic fishes supports the view that physical and hydromechanical demands provided important selection pressures to optimize body design for locomotion during vertebrate evolution. Recognition of morphological similarities between lamnid sharks (the most well known being the great white and the mako) and tunas has led to a general expectation that they also have converged in their functional design; however, no quantitative data exist on the mechanical performance of the locomotor system in lamnid sharks. Here we examine the swimming kinematics, in vivo muscle dynamics and functional morphology of the force-transmission system in a lamnid shark, and show that the evolutionary convergence in body shape and mechanical design between the distantly related lamnids and tunas is much more than skin deep; it extends to the depths of the myotendinous architecture and the mechanical basis for propulsive movements. We demonstrate that not only have lamnids and tunas converged to a much greater extent than previously known, but they have also developed morphological and functional adaptations in their locomotor systems that are unlike virtually all other fishes.

Axial undulations in fishes are powered by a series of three-dimensionally folded myomeres separated by sheets of connective tissue, the myosepta. Myosepta have been hypothesized to function as transmitters of muscular forces to axial structures during swimming, but the difficulty of studying these delicate complex structures has precluded a more complete understanding of myoseptal mechanics. We have developed a new combination of techniques for visualizing the three-dimensional morphology of myosepta, and here we present their collagen-fibre architecture based on examination of 62 species representing all of the major clades of notochordates. In all gnathostome fishes, each myoseptum bears a set of six specifically arranged tendons. Because these tendons are not present outside the gnathostomes (i.e. they are absent from lampreys, hagfishes and lancelets), they represent evolutionary novelties of the gnathostome ancestor. This arrangement has remained unchanged throughout 400 Myr of gnathostome evolution, changing only on the transition to land. The high uniformity of myoseptal architecture in gnathostome fishes indicates functional significance and may be a key to understanding general principles of fish swimming mechanics. In the design of future experiments or biomechanical models, myosepta have to be regarded as tendons that can distribute forces in specific directions.

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Burtsch. P.. Gembd/a. S. & Piotrowski. T. 1997. The embryonic and larval development of Polyptrrus senegalus Cuvier, 1829: its staging with reference to external and skeletal features, behaviour and locomotory habits.-Actci Zuologica (Stockholm) 78: 309-328.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. senegcilus 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$ 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. 0 1997 The Royal Swedish Academy of Sciences.Published by