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

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. The ability to resist expulsion may be helped by the consistent ‘head-down’ position that all embryos adopt in the gills. 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 blastokinesis — the rotation of the embryo on the yolk ball before hatching. We used time-lapse imaging, histology, X-ray tomography, and expression profiling of the genes fgf8a, krt8, msx3 and ctslb by in situ hybridization in the pre-hatching and hatching stages of the rosy bitterling (Rhodeus ocellatus). We find that blastokinesis is a gastrulation process that has been ventralized by the shape of the yolk mass. Furthermore, we show that bitterlings, ..., Rhodeus ocellatus embryos
Rhodeus ocellatus embryos of synchronized developmental age were collected at the Institute of Vertebrate Biology in Brno, Czech Republic, following in vitro fertilization (IVF). Embryos of various developmental stages (SI Appendix, Table S1) were fixed in 4% paraformaldehyde, dehydrated step-wise in methanol and stored in 100% methanol at -20°C. The developmental staging of the embryos was based on Yi et al. Two or three replicates were used for each stage.
Danio rerio embryos
Embryos of the zebrafish Danio rerio (AB/TL line) were collected in the fish facility of the Institute of Biology Leiden. The eggs were fertilized by 1:1 spawning (single crossing) at the beginning of the light period (14 h light, 10 h dark). The fertilized eggs were collected and incubated in egg water (containing 60 μg/ml “Instant Ocean” sea salts) at 28.5 °C. After collection, the embryos were immediately fixated in 4% paraformaldehyde. Fixed embryos were dehydrated step-wise in metha..., Files can be accessed in Image J., # Data For Parasitic fish embryos do a 'front-flip' on the yolk to resist expulsion from the host

Raw data for time-lapse recording of embryo hatching process, the outing from chorion.

Raw data for 3D reconstruction data from CT scans.

## Description of the data and file structure

The data consists of two parts: raw data for time-lapse recording and 3D reconstruction data from CT scans. The time-lapse recording files are in JPG format, with filenames numbered sequentially according to time sequence. The 3D reconstruction data from CT scans includes four files named \"host-mussel_dissected \_tomo-A\", as well as a Header.txt file describing the scanning parameters. Among the four files named host-mussel_dissected \_tomo-A, one is an MPG video file, one is a compressed package of raw data, and two are preview images in JPG format.

For time-lapse recording, *R. ocellatus* embryos at the desired developmental stage were mounted in glass view chamber. Stereo microscope (Nikon SMZ1500) was...