Zebrafish need that rhythmic activity to form heart valves. They have unidirectional flow before the valves form, through some magic of resonance and pressure waves reflecting off arterial curves, and the resonance results in (IIRC) a stagnant spot where the valves will form. Do something to interfere with the mechanics and no valves form.
Once more: “fetal cardiac activity” occurs before the fetus has a heart.
The cells that will become the heart start pulsing rhythmically before they are even a hollow space, let alone pumping blood.
You can see this in a petri dish.
Zebrafish need that rhythmic activity to form heart valves. They have unidirectional flow before the valves form, through some magic of resonance and pressure waves reflecting off arterial curves, and the resonance results in (IIRC) a stagnant spot where the valves will form. Do something to interfere with the mechanics and no valves form.
Sounds like we need laws in place protecting the life of zebra fish