How do jawless fish breathe?

Jawless fish, such as lampreys and hagfish, have a unique respiratory system that allows them to breathe in water. They lack jaws and fully developed paired fins, which are characteristic features of jawed fish. Instead, jawless fish have a series of gill slits or pouches on the sides of their head or throat region, which are used for respiration. Here's how jawless fish breathe:

1. Water Intake: Jawless fish actively pump water into their respiratory system through their mouth or specialized openings called spiracles.

2. Gill Slits: The water then passes over the gill slits, which are lined with numerous gill filaments. These filaments increase the surface area for efficient gas exchange.

3. Gas Exchange: As water flows over the gill filaments, oxygen from the water diffuses into the bloodstream of the fish, while carbon dioxide diffuses out of the blood and into the water. The oxygen-rich blood is then transported throughout the body.

4. Water Expulsion: After the water has passed over the gill slits, it is expelled through separate external openings called branchial pores. This maintains a one-way flow of water over the gills.

Unlike jawed fish with opercula (gill covers) that can open and close to regulate water flow, jawless fish rely on a muscular pharynx to create the necessary pressure changes for water intake and expulsion.

Additionally, some jawless fish, like lampreys, have specialized structures called velar tentacles or buccal funnels that aid in water circulation during respiration. Hagfish, on the other hand, have unique respiratory adaptations involving mucus production, which helps them breathe in oxygen-poor environments.

It's important to note that jawless fish are a diverse group, and there may be variations in their respiratory systems among different species. Nonetheless, the presence of gill slits and the absence of jaws remain defining features of this group of primitive fish.