1. The Slippery Coating on Fish Is Mucus
The outer surface of a fish is covered by a layer called mucus. The slippery coating on an eel belongs to this category. It has been reported that fish skin mucus is secreted by three types of cells: goblet cells, sac-like cells, and club cells [1]. However, we could not confirm through primary ichthyology sources whether eel mucus is actually produced by all three of these cell types.
The main component of this mucus is a high-molecular-weight glycoprotein called mucin. Mucin is considered to give mucus its viscoelastic (sticky and stretchy) properties [1][2].
2. How Mucus Protects Fish from Disease-Causing Microorganisms
Water where fish live contains microorganisms that can cause disease. Fish mucus is said to serve as the first physical and chemical barrier against these pathogens [1][2]. Since no original source was found that directly demonstrated the defensive effect of eel mucus specifically, we should read this as an example applicable to related fish species.
This wall is not finished once it is made. Mucus is constantly produced and then peeled off. It is explained that this replacement process prevents pathogens from attaching and staying put. Mucus also contains components with antibacterial properties [1].
You can imagine it like a conveyor belt, where whatever lands on it flows away with the belt itself. However, actual mucus differs because it not only flushes things away but also acts on pathogens using antibacterial components.
3. How Mucus Helps Eels Breathe Through Their Skin
Eels breathe using gills, but it has been well known since the work of the researcher August Krogh in 1904 that they can also breathe through their skin [3]. However, skin breathing is not always the main method. Experimental results show that in water, gill breathing is sufficient, and skin breathing is not very important (at around 17°C). When eels are exposed to air, skin breathing becomes important [3]. Krogh pointed out that at low temperatures of around 15°C or lower, life can be sustained by skin breathing alone [3].
It is thought that having wet skin helps with skin breathing. Fish encyclopedias explain that mucus acts as a membrane that prevents drying out. On the other hand, the same source raises the question of whether excessive mucus production might actually interfere with skin breathing in the air [3].
When transporting farmed eels, it is said that if they are kept cool and not allowed to dry out, no harm occurs even during long transport [3].
4. How Hagfish Mucus Clogs Gills
Although the name is similar, the hagfish is a different group. Hagfish are jawless creatures belonging to the Agnatha group, while eels are bony fish with jaws, so they are not close relatives [4].
Along the sides of a hagfish’s body, there are about 70 to 200 pores that release mucus, arranged in a row. When attacked, it is said that they release mucus from these pores to clog the gills of the attacker and drive it away. Whether this leads to suffocation has not been confirmed by observation [4][5].
In underwater videos off the coast of New Zealand, 14 cases of hagfish being attacked were recorded. It is reported that they used mucus to clog the gills of sharks and bony fish, driving them off [5]. However, because the original report has not been verified, we cannot say that "all attempts were successful."
Even though it is the same type of substance, the usage seems very different. For hagfish, mucus is described as something "thrown at the attacker when attacked," while general fish mucus is described as a "membrane that wraps around and protects the body."
5. How to Observe Fish Mucus Yourself
If you buy fish, look at the sliminess of the surface with a family member and touch it gently. After touching, wash your hands with soap.
In fish encyclopedias at school or home, compare photos or drawings of eels and hagfish. Look for differences in body shape and mouth structure.