Data from: Parasitic fish embryos do a 'front-flip' on the yolk to resist expulsion from the host

<p><span>Bitterlings are brood parasitic fish which complete their early development in the internal gill spaces of freshwater mussels. Bitterling embryos have wing-like yolk sac extensions that help prevent them from being expelled from the gills by the water flow</span><span>. The ability to resist expulsion may be helped by the consistent 'head-down' position that all embryos adopt in the gills</span><span>. The mechanism behind this positioning is unknown. We hypothesise here that it might lie in a process of unknown function, specific to bitterlings. That process is <em>blastokinesis</em> — the rotation of the embryo on the yolk ball before hatching</span><span>. </span>We used time-lapse imaging, histology, X-ray tomography, and expression profiling of the genes <em>fgf8a</em>,<em> krt8</em>,<em> msx3 </em>and <em>ctslb</em> by <em><span>in situ</span></em><span> hybridization in the </span>pre-hatching and hatching stages of the rosy bitterling (<em>Rhodeus ocellatus</em>). We find <span>that blastokinesis is a gastrulation process that has been ventralized by the shape of the yolk mass. Furthermore, we show that bitterlings, unlike other teleosts, hatch mechanically without hatching enzymes, and we provide evidence that this is mediated instead by the apical tubercles on the yolk sac extension. Finally, our data suggest that blastokinesis is functional, because it represents the mechanism behind the optimal, 'head-down' positioning of the embryo. Our study provides an example of how selection pressures can lead to a suite of dramatic and coordinated modifications of early development.</span></p>