How does a fish embryo feed?

Fish embryos have a unique way of feeding, as they don't have mouths or digestive systems developed enough to take in external food. Instead, they rely on a yolk sac for nourishment.

Here's how it works:

1. Yolk Sac: The yolk sac is a sac-like structure attached to the embryo that contains a rich source of nutrients called yolk. This yolk is essentially a pre-packaged food supply for the developing fish.

2. Absorption: As the embryo grows, it absorbs the yolk from the sac through specialized blood vessels. This process delivers the essential nutrients, like proteins, fats, and vitamins, directly to the embryo.

3. Shrinkage: As the embryo uses the yolk, the sac gradually shrinks in size. By the time the fish hatches, the yolk sac is usually completely absorbed.

4. Transition to External Feeding: Once the yolk is depleted, the fish hatches and is ready to begin feeding externally. It will then develop its mouth and digestive system to hunt and process food from its environment.

Note: Some fish species, like sharks, have a specialized structure called a "placenta" that allows for nutrient transfer from the mother to the developing embryo. However, this is not the case for most fish species.