How does the archer fish breathe?

Archer fishes can take in oxygen from both water and air. While they primarily use their gills to extract oxygen when submerged in water, they also have a unique organ called the 'suprabranchial organ', located above their gills. This specialised structure allows them to breathe atmospheric air periodically, even when they're in water.

Here's how the archer fish utilises its respiratory systems:

1. Gills:

The archer fish possesses conventional fish gills, which are designed to absorb oxygen from the surrounding water. When submerged, the fish pumps water through its mouth and the gills, extracting dissolved oxygen from the water before expelling it through gill slits. This is the fish's primary method of respiration while underwater.

2. Suprabranchial Organ:

The suprabranchial organ, also known as the accessory respiratory organ, is an air-breathing structure found above the gills of archer fishes. It consists of a maze of blood vessels and specialised tissue that form an air-filled cavity.

3. Gas Exchange:

When the archer fish needs additional oxygen, it surfaces and takes in air through its mouth, which then enters the suprabranchial organ. Inside the organ, oxygen from the air diffuses into the blood vessels, while carbon dioxide passes out from the blood into the cavity. This allows the fish to acquire atmospheric oxygen directly.

4. Regulation:

The air exchange process is controlled by muscular movements. When the archer fish rises to the surface, its mouth opens, and the suprabranchial organ expands. This allows air to enter the organ. When the fish dives back into the water, its mouth closes, and the suprabranchial organ compresses, preventing water from entering.

By utilizing both gills and the suprabranchial organ, archer fishes have adapted to exploit the interface between air and water, enabling them to survive in low oxygen environments and even breathe while out of the water for brief periods. These exceptional respiratory capabilities are crucial to their remarkable hunting behaviour, where they accurately spit water jets to shoot down insects from overhanging vegetation.