Mucous membrane in gills keeps fish healthy

Fish poop in the very water from which they obtain their oxygen. How do they stay healthy? Through an ingenious and sensitive system in the gills.

Gills are possibly even more interesting than brains, says Geert Wiegertjes, Wageningen Professor of Aquaculture and Fisheries. ‘Gills function as lungs, kidneys and immune system. Brains are no match in complexity.’ In collaboration with international colleagues, he studied which tissues in the gills are crucial to the fish’s health and how these tissues have developed through evolution.

Water

Gills are used by fish to extract oxygen and minerals from the water. Fish take in oxygen and expel carbon dioxide and ammonia, all through the water. However, that same water also contains infectious micro-organisms that threaten the fish’s health. How do they keep these at bay?

Mucous membrane

The gills are made up of unique structures of cartilage and thin layers of skin, which together form a large area. This large area makes fish susceptible to pathogens, poor water quality and high concentrations of faeces. Since a few years, researchers know that lymphoid tissue in the gills plays a crucial role in the defence against micro-organisms in the water. Much in the same way the lymph glands around our lungs help protect us against airborne pathogens. Recently, a new tissue structure, the interbranchial lymphoid tissue (ILT) was discovered in salmon. This tissue structure plays an important role in this respect.

Zebrafish

In a new publication in Biology, Wiegertjes and his international colleagues show how these ILT’s have developed through evolution, not just in salmon, but in other types of bony fish as well. The study shows that both large carps and small zebrafish have this tissue structure. Through evolution, however, major differences occurred in the way these lymphoid structures are organised.

Additives

Wiegertjes now aims to find out whether fish, like humans, have an immune system comprised of a total of the different mucous membranes throughout the body. In the case of fish, this mucous defence system is located both in the intestines and in the gills. They hope this knowledge will allow the development of food additives that can fortify the fish’s natural immunity, increasing its resistance to disease. They focus on the intensive cultivation of salmon, but also on the less intensive farming of, for example, tilapia.

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