Fish Mucus or Slime Composition, Functions, and Potential Uses
An Important Substance
The surface of living fish is covered by mucus, or slime. Some fish have a thin coating of mucus. Others produce so much slime that it's difficult for a predator or a human to grasp them. Mucus is a very important substance for fish. It protects them in multiple ways and also has some surprising functions beyond protection.
Although the thought may sound disgusting, fish mucus could be useful for humans. It may be possible to use the protein fibres in hagfish slime to make new fabrics and materials. A recent discovery suggests that the mucus produced by some coral reef fish could be used to make a new sunscreen.
In this article I discuss the general functions of fish mucus as well as the specialized ways in which discus fish, parrotfish, and African lungfish use their slime. I also look at the ways in which the substance may help us.
Mucus in Fish and Humans
Mucus is made by many animals and by humans as well. It's useful stuff. Fish mucus is made by goblet cells in the animal's skin. Our goblet cells secrete mucus, too. In humans, the cells are found in the mucous membranes that line the respiratory, intestinal, urinary, and reproductive passages. The mucus in these locations protects the lining of the passage, provides lubrication to allow for the transport of materials, and keeps the area moist. In the respiratory tract, it also traps inhaled dirt and bacteria.
Mucus contains substances called mucins, which are a type of glycoprotein (protein with attached carbohydrate). The protein molecule in a mucin is attached to many carbohydrate molecules. Mucins rapidly form a gel when they leave goblet cells and contact water. They are responsible for both the viscous and the elastic properties of mucus.
Fish slime contains other substances besides mucin and water, including enzymes, antibodies, and salts. Fish that live around coral reefs have been found to have chemicals called mycosporine-like amino acids in their mucus. These chemicals block ultraviolet light.
Mucus in different species of fish is not completely identical in composition. Researchers are finding some novel chemicals in some samples of the mucus.
Protective Slime: Preventing a Pathogen Attack
Aquarists know that their fish can become sick if their protective mucus layer is damaged. Even as a child I was taught not to handle my goldfish because I might remove their mucus and hurt them. Since the mucus has multiple functions, removing it can hurt a fish in several ways. One way is by making the fish more susceptible to infections.
The mucus of a fish provides physical protection by trapping pathogens (microorganisms that cause disease). When the old mucus layer containing the pathogens is shed and replaced by new mucus, the pathogens are lost. Antibodies, antimicrobial peptides, and enzymes in the mucus actively attack pathogens.
The Importance of Osmoregulation in Fish
Fish living in both salt and fresh water have a potential problem with osmoregulation, or the maintenance of the correct water and salt concentration within their body. In science, the word "salt" refers to any ionic compound, including but not limited to sodium chloride. Salts in the body—or the ions that they become when they break down in water—are sometimes referred to as electrolytes or minerals. They are essential for life but are dangerous if they become too concentrated.
There are two trends that a fish needs to fight during osmoregulation.
- Water molecules move from a less salty area to a more salty area.
- Salt ions move from where they are more concentrated to where they are less concentrated.
In the ocean, too much water may leave a fish's body and too much salt may enter. In fresh water, the opposite situation may occur. Too much water may enter the fish and too many salts may leave. These processes can both be deadly. Activities in the gills and kidneys of a fish fight these tendencies, however.
Mucus and Osmoregulation in Fish
Mucus is helpful for a fish because in conjunction with the scales it partially blocks the movement of water into and out of the fish's body. This helps to maintain constant conditions inside the fish.
Other parts of the body also influence the salt and water concentration in the fish. The urine contains more or less water and salt, as necessary. In addition, the gills excrete or absorb salts, depending on a fish's needs.
In addition to its other functions, mucus reduces drag when a fish swims. It covers the scales, filling in any gaps or irregularities and reducing friction.
Discus fish are a type of cichlid. The cichlid family is very large and consists of freshwater fish with a wide variety of characteristics. Some members of the family, including discus fish, have a flattened, laterally compressed body. Unlike most other fish, cichlids demonstrate some form of parental care for their young.
Discus fish are classified in the genus Symphysodon. They have a range of beautiful colours and patterns. An especially interesting feature of the fish is that the fry (young fish) feed on the skin mucus of their parents. The mucus is enriched with nutrients such as protein and amino acids to support the growing youngsters. Like mammalian milk, the mucus changes in composition as the youngsters develop and continues to fulfil their needs.
Fish eggs hatch into larvae. The larvae feed on the yolk in a yolk sac attached to their body. When the yolk sac has disappeared and the youngsters have begun eating food, the developing fish are known as fry. The fry stage lasts for several to many months.
Mucus Feeding in Discus Fish
Some fascinating information about the rearing of discus fish has been discovered by some British and Brazilian scientists. The scientists brought some discus fish into captivity and tried to keep their environment as natural as possible. The fish reproduced successfully, allowing the researchers to study the behaviour of the youngsters.
The scientists noted that the fry travelled to a parent as a group. They bit the side of the adult fish for up to ten minutes, feeding on the mucus. The adult then "expertly" flicked the fry towards the other parent, where they started to feed again. For two weeks, the parents continued to feed the fry in this way.
The discus fish also exhibited behaviour that resembles weaning in mammals. After two weeks of mucus feeding, the researchers noted that the parents sometimes tried to swim away from the fry, who chased them in order to feed. After three weeks, the adults successfully swam away from the fry for short periods of time and the youngsters began looking for other food. After about four weeks, the young fish were finding almost all of their food for themselves and rarely fed on mucus.
Parrotfish live around the coral reefs of tropical water. Their teeth are fused together, forming plates. These plates make the mouth look like a bird's beak and give the fish its name.
Parrotfish are known for their interesting development. Many species change their gender during their lifetime. They start their life as a female (the initial stage) and later change into a male (the terminal phase). The initial phase is often dull in colour while the terminal phase is brightly coloured.
Parrotfish feed on the algae that grows on coral. In order to do this, they scrape the coral with their teeth and bite off pieces in the process. Teeth in their throat grind the coral, producing grit. The grit travels through the digestive tract of the fish and is eventually released into the environment, forming coral sand.
Mucus Cocoons in Parrotfish
Like the skin of other fish, parrotfish skin makes mucus. In addition, parrotfish have mucus glands in their gill chambers. At night, they make a mucus cocoon and enclose themselves within it for protection. The mucus for the cocoon is secreted by the gill glands and released from the mouth of the fish.
The function of the mucus cocoon isn't completely certain. A common theory is that it hides the scent of the parrotfish, preventing attack by predators while it's asleep. Another theory is that the cocoon prevents the attack of little blood-sucking parasites called gnathiid isopods. Cleaner fish remove these creatures from reef fish during the day, but the cleaners aren't available at night.
African lungfish belong to the genus Protopterus and live in fresh water. The four species are all long and eel-like fish. The pair of side fins near their head (pectoral fins) and near their tail (pelvic fins) are long and narrow, unlike those of most other fish. The fins sometimes look like pieces of spaghetti or string. African lungfish are carnivores and feed on small fish and amphibians.
Lungfish got their name because they have a pouch extending from their digestive tract that acts as a lung. African lungfish have two lungs. The fish live in shallow water or in water that is low in oxygen. Like other fish they have gills, which extract oxygen from water. The gills alone don't provide them with enough oxygen, however. African lungfish are said to be obligate air breathers because they can't survive unless they breathe air.
Lungfish periodically come to the surface to take a gulp of air. The air passes along their digestive tract and into their lung (or lungs). The lung contains subdivisions and is richly supplied by blood vessels. Oxygen leaves the air in the lung and enters the lungfish's blood, while carbon dioxide moves in the opposite direction.
The lung of a lungfish is a modified swim bladder. The swim bladder is a gas-filled sac that provides buoyancy in other fish.
Mucus Cocoons in African Lungfish
As the water in their habitat starts to disappear during the dry season, African lungfish bury themselves in the mud on the bottom of their stream, river, or lake and become dormant. They dig a burrow by taking mud into their mouth and then pushing it out of their body through the openings of their gill chambers. Their skin secretes a mucus cocoon to prevent them from becoming dehydrated during dormancy. The cocoon gradually hardens. The heart rate, blood pressure, and metabolic rate of the fish decrease. This state of dormancy during hot and dry weather is known as estivation.
A lungfish continues to breathe air during estivation, but at a greatly reduced rate. The gills are inactive. A small tube leading into the burrow allows air to enter it. A small hole in the mucus cocoon allows the fish to take in oxygen.
The fish slowly breaks down its own muscles for nourishment during estivation. It's therefore in a weakened condition when it emerges from the burrow. African lungfish normally estivate only until the next rainy season, but they have been successfully revived after several years of dormancy.
Although hagfish are commonly referred to as "fish", their structure is very different from that of other fish. They are a strange animal with a slender, elongated body. There is a ring of tentacles around their mouth and a tail fin at the end of their body. They have a partial skull made of cartilage but have no backbone. They also lack jaws and scales. They do have gills, however, and their skin produces mucus.
Hagfish live on the ocean floor. They are sometimes found feeding inside the bodies of dead fish and were once classified as parasites and scavengers. Current research indicates that the main item in their diet is marine worms. As shown in the video below, they also eat other prey. Their rasping tongue enables them to pull flesh off their prey.
Hagfish rapidly increase their mucus production when they feel threatened. The mucus is produced almost immediately after a hagfish is attacked and forms a sheet when it contacts the water. The slime enters the mouth and gill chambers of a predator and suffocates it. Scientists are very interested in the nature of this slime.
Clothing From Hagfish Slime
Hagfish mucus contains many small protein threads that are both strong and elastic. Researchers suspect that these threads could be used to make a fabric with desirable properties. We may one day be able to buy clothing made from the protein found in hagfish slime.
It's unlikely that we will have hagfish farms in the future in order to harvest slime. As is done with many useful substances discovered in nature, the plan is to eventually add the hagfish genes for slime or protein thread production to bacteria. The bacteria will then be "farmed" in fermenters and the resulting protein extracted.
A Natural Sunscreen From Fish Mucus
A research team consisting of Swedish and Spanish scientists has made another interesting discovery about fish mucus. The team has found that when they attach chemicals from fish mucus to one found in crustacean shells, the resulting substance blocks both ultraviolet A and ultraviolet B rays from the sun. These are the rays that cause sunburn and skin cancer. The combined chemicals could be useful as a natural, environmentally friendly sunscreen for humans.
The light-blocking chemicals in fish mucus are known as mycosporine-like amino acids (MAAs). The chemicals have been found in certain fungi, algae, and cyanobacteria as well as in reef-dwelling fish.
The researchers added the MAAs to a lattice made of chitosan. Chitosan is a chemical obtained from crustacean shells. It's an interesting substance in its own right because it seems to have the ability to heal wounds. Chitosan exists as long molecules known as polymers and can be easily applied to the skin when formulated correctly. It acts as a carrier for the MAAs.
Potential Benefits of the Sunscreen
The researchers found that the MAA/chitosan mixture maintained its resistance to UV light for twelve hours and at temperatures up to 80°C. It might provide protection for outdoor furniture as well as people. More research is required before the sunscreen is sold to the public, assuming it eventually becomes available to us.
Finding new human sunscreens that don't harm coral reefs when they enter the water is very important. Oxybenzone is a common chemical in present sunscreens. Evidence suggests that this chemical is damaging coral. An MAA/chitosan mixture should be biodegradable and safer for the environment.
The Importance of Maintaining Biodiversity
Biodiversity is the variety or differences in the characteristics of living things. The ways in which different fish use mucus and the different compositions of fish mucus are examples of biodiversity.
Maintaining biodiversity is important not only for the sake of the other living things on the planet but also for us. We've found many helpful chemicals and materials in nature in addition to hagfish slime, MAAs, and chitosan. There are probably many more beneficial substances to be discovered. The disappearance of animals and plants before we discover these new substances would be sad in more ways than one.
- Discus fish parent young like mammalian mothers from the phys.org news service
- Facts about parrotfish from National Geographic
- Fish mucous cocoons: the "mosquito nets” of the sea from The Royal Society Publishing
- Information about the African lungfish from the Oregon Zoo
- Hagfish slime for clothing from the BBC
- Fish mucus sunscreen from the NIH (National Institutes of Health)
- Mixing a fish secretion with shrimp shells to make a sunscreen from New Scientist
© 2015 Linda Crampton