Physical constraints on body size in teleost embryos

All members of the subphylum "Vertebrata" display the characteristics of th e vertebrate body plan. These characteristics become apparent during the ph ylotypic period, in which all vertebrate embryos have a similar body shape and internal organization. Phylogenetic constraints probably limit the morp hological variation during the phylotypic period. Physical laws, however, a lso limit growth and morphogenesis in embryos. We investigated to what exte nt oxygen availability-as a physical constraint-might limit morphological v ariation during embryonic development. This paper gives an analysis of time -dependent diffusion into spherical embryos without a circulatory system. E quilibrium appeared to settle in about 1.5 min in running water and in abou t 10 min in stagnant water. Hence, steady-state conditions were assumed and expressions for maximum body size were obtained for spherical, cylindrical and sheet-like embryos in running water and spherical embyros in stagnant water. Predictions of the model based on literature data suggest that in ru nning water-both for spherical, cylindrical and sheet-like embryos-diffusio n alone suffices to cover the oxygen needs of a teleost embryo in its phylo typic period. The size of carp (Cyprinus carpio) and African catfish (Clari as gariepinus) embryos is very close to the predicted maximum. This suggest s that in these species the development of a functional circulatory system is correlated with the onset of oxygen shortage. Oxygen availability is the refore a potentially important physical constraint on embryonic morphology, though in most species the circulatory system becomes functional well in a dvance of the onset of oxygen shortage and other demands than oxygen delive ry (e.g. nutrient distribution, waste disposal, osmoregulation) might requi re the development of a circulatory system. (C) 2000 Academic Press.