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Fish Mucus or Slime Composition, Functions and Sunscreen

Discus fish feed their young with mucus produced by the adult's skin.
Discus fish feed their young with mucus produced by the adult's skin. | Source

Fish Mucus - 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 life of the hagfish and the possibility of using a fish mucus sunscreen.

Another type of discus fish
Another type of discus fish | Source

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.

Fish Mucus Composition

Mucus in different species of fish is not completely identical in composition. Researchers are finding some novel chemicals in some mucus.

Captive Discus Fish Feeding on Beef

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.

This is another variety of discus fish. The fish have a wide range of colours and patterns but all belong to the genus Symphysodon.
This is another variety of discus fish. The fish have a wide range of colours and patterns but all belong to the genus Symphysodon. | Source

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.

Movement of water and ions in a saltwater fish; the arrows into and out of the skin are short because the scales and mucus layer reduce the transport of materials
Movement of water and ions in a saltwater fish; the arrows into and out of the skin are short because the scales and mucus layer reduce the transport of materials | Source

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.

Movement of water and ions in a freshwater fish; once again, the arrows into and out of the skin are short due to the presence of scales and mucus
Movement of water and ions in a freshwater fish; once again, the arrows into and out of the skin are short due to the presence of scales and mucus | Source

Another Function of Fish Slime

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

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 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.

A blue discus fish, or Symphysodon aequifasciatus
A blue discus fish, or Symphysodon aequifasciatus | Source

Fish Larvae and Fry

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.

The daisy parrotfish covers itself with a mucus cocoon at night.
The daisy parrotfish covers itself with a mucus cocoon at night. | Source

Parrotfish

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.

A Parrotfish in its Mucus Cocoon

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.

The marbled or leopard African lungfish
The marbled or leopard African lungfish | Source

African Lungfish

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 of a lungfish 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.

Swim Bladders and Lungs

The lung of a lungfish is a modified swim bladder. The swim bladder is a gas-filled sac that provides buoyancy in other fish.

Lungfish Estivation

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 back out 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 sesson, but they have been successfully revived after several years of dormancy.

Gathering Hagfish Slime for Research

Hagfish

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.

Hagfish Defence and Predation

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 hagfish emerging from a sponge
A hagfish emerging from a sponge | Source

A Natural Sunscreen from Fish Mucus

Researchers have discovered that if they attach a chemical found in 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 moleciules known as polymers. MAAs on their own diffuse through the carrier part of a sunscreen instead of staying where they're applied. Attaching them to a chitosan lattice prevents this problem.

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 safer for the environment.

The male or terminal phase rainbow parrotfish (Scarus guacamaia) is found around coral reefs.
The male or terminal phase rainbow parrotfish (Scarus guacamaia) is found around coral reefs. | Source

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.

© 2015 Linda Crampton

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Comments 59 comments

Buildreps profile image

Buildreps 12 months ago from Europe

I enjoyed reading this beautiful article very much, Alicia. The valuable information you provide is wonderfully presented in carefully composed sentences of well chosen words. Thank you for this great reader!


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you very much, Buildreps. I appreciate your visit and kind comment!


billybuc profile image

billybuc 12 months ago from Olympia, WA

I swear you have the most interesting articles, and they are usually about topics I've never seen another article about....no exception to that rule here. Great article.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you so much, Bill! I appreciate your lovely comment a great deal.


Jackie Lynnley profile image

Jackie Lynnley 12 months ago from The Beautiful South

So disgustingly interesting! Just kidding about the slime. The greatest invention of green I have heard of yet! Sunscreen for us with no danger to the water or fish that live there. You wonder how they come up with these ideas don't you? But sure glad they do!

Outstanding hub; sharing!


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you so much for the great comment and the share, Jackie! I hope the new sunscreen becomes a reality. As you say, it could be ideal - effective for us and safe for the ocean.


Larry Rankin profile image

Larry Rankin 12 months ago from Oklahoma

Very important that they develop a more environmentally friendly sunscreen.

Wonderfully written and compelling.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you very much, Larry! I agree - it is important that researchers develop new sunscreens that are safe for us and the environment, especially when we're being advised to wear sunscreen frequently.


PNWtravels profile image

PNWtravels 12 months ago from Wandering the Pacific Northwest USA

I kept an aquarium for many years, so I knew that fish slime was important, but I learned a lot more from this very interesting article.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Hi, PNWtravels. Thank you very much for the comment. I appreciate your visit.


DDE profile image

DDE 12 months ago from Dubrovnik, Croatia

Interesting about Fish Mucus. I often felt that through my fingers from fish and did not think much of it. I don't touch fish to avoid that slime.


Nadine May profile image

Nadine May 12 months ago from Cape Town, Western Cape, South Africa

I always learn so much from your posts. When I take up finishing my last novel in the awakening to our ascension series next year, your well written researched posts will be an amazing source of information for my novel. ( If so I will add your posts into my sources page )


drbj profile image

drbj 12 months ago from south Florida

I will admit, Alicia, that I did have a few reservations about reading an article about mucus and fish slime but I forged ahead anyway since you were the author.

I'm glad I did. This was an enthralling exposition about those topics and I learned more than expected about those topics and the various fish you chose as illustrations. Thank you for this educational and interesting experience.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you for the visit and comment, Devika.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you very much for the comment, Nadine. Good luck with your novel!


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thanks for reading my hub, drbj. I appreciate your comment very much!


Eva Hard profile image

Eva Hard 12 months ago from Midwest USA

Very interesting article with beautiful images. What a diversity of life forms and biomolecules. Thanks Alicia. I expect we should also thank people who can work with some strange substances, and especially with some horrific-looking creatures as the hagfish.

Eva


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

That's a good point, Eva. We should thank the people who have the courage to work with hagfish slime! Thank you for the comment.


MartieCoetser profile image

MartieCoetser 12 months ago from South Africa

Alicia, this is so-so interesting, I am totally amazed. I hope when people decide to use the mucus of fish for the manufacturing of sunscreens, they will do it with great care.

Thank you for this very interesting and informative hub about fish mucus and its functions.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Hi, Martie. Thanks for the visit and the comment! I hope the fish are treated with care, too. Scientists sometimes discover how to genetically engineer bacteria to make a useful substance made by another living thing. This means that they don't need the living animal or plant to obtain the substance, which is good.


FlourishAnyway profile image

FlourishAnyway 12 months ago from USA

Unique topic that teaches and entertains too! Fascinating!


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you very much for the comment, Flourish!


aviannovice profile image

aviannovice 12 months ago from Stillwater, OK

More fascinating material. You always find the most interesting things to write about.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you, Deb. I appreciate your visit and comment.


Vellur profile image

Vellur 12 months ago from Dubai

Great hub, interesting and informative. Did not know all this about mucus. Clearly explained with images. Thank you for sharing.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Hi, Vellur. Thank you very much for the visit. I appreciate your comment .


bdegiulio profile image

bdegiulio 12 months ago from Massachusetts

How interesting Linda. I was not aware that fish use mucus and slime for protection. The story of that Africa Lungfish surviving during the dry season is fascinating. As always thank you for the education.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Hi, Bill. Thank you very much for the comment.


sgbrown profile image

sgbrown 12 months ago from Southern Oklahoma

This was so interesting! I learned so much reading this hub. It always amazes me as to what we can learn from nature. Great hub and wonderful pictures! :)


poetryman6969 profile image

poetryman6969 12 months ago

I think it's always pretty cool to see something that we might otherwise consider useless or even disgusting can be of use to us after all. Nature provides.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Hi, Sheila. Yes, we can certainly learn a lot from nature! Thank you very much for the comment.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

I agree, poetryman. Nature is useful as well as interesting. Thanks for the visit.


aesta1 profile image

aesta1 12 months ago from Ontario, Canada

Not only is this article very informative, it is also full of colour. Reminds me of my snorkelling days. It is good to see my old friends again especially the parrot fish.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you for the lovely comment, aesta1. I love the thought of fish being your friends!


truthfornow profile image

truthfornow 12 months ago from New Orleans, LA

Your beautiful pictures attracted me towards reading an article about fish mucus and slime. Very informative article.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you, truthfornow. I appreciate your comment.


Genna East profile image

Genna East 12 months ago from Massachusetts, USA

Fish sunscreen for humans. Who knew? This is fascinating. And your photos are stunning. I always enjoy reading your unusual articles, Linda, and learn something new, each and every time.


AliciaC profile image

AliciaC 12 months ago from British Columbia, Canada Author

Thank you very much for such a nice comment, Genna. I appreciate your visit a great deal!


Kristen Howe profile image

Kristen Howe 11 months ago from Northeast Ohio

Alicia, this was really interesting and wonderful to read about--fascinating to know about, too. Thanks for sharing this lens. Lovely pics too!


AliciaC profile image

AliciaC 11 months ago from British Columbia, Canada Author

I appreciate your comment, Kristen. Thanks for visiting!


Kristen Howe profile image

Kristen Howe 11 months ago from Northeast Ohio

You're very welcome!


ChitrangadaSharan profile image

ChitrangadaSharan 10 months ago from New Delhi, India

Congratulations for the well deserved HOTD!

Your articles are full of useful information and this one is no different. Fish is another wonderful and beautiful creature of Nature and is useful to us in so many different ways. While going through your hub I was wondering how little we know about them.

Lovely pictures and a very informative hub. Thanks for sharing!


AliciaC profile image

AliciaC 10 months ago from British Columbia, Canada Author

Thank you so much for the congratulations and the comment, ChitrangadaSharan! I think that fish are very interesting animals. They are often underappreciated as living things.


Kristen Howe profile image

Kristen Howe 10 months ago from Northeast Ohio

Linda, you're on a roll. Since I stopped by here not too long ago, kudos and congrats on HOTD!


AliciaC profile image

AliciaC 10 months ago from British Columbia, Canada Author

Thank you very much, Kristen. It is amazing! I appreciate your kindness.


heidithorne profile image

heidithorne 10 months ago from Chicago Area

Wow, what an interesting and very unusual topic hub! Great pics and such detail info... well deserving of Hub of the Day. Congrats!


AliciaC profile image

AliciaC 10 months ago from British Columbia, Canada Author

Hi, Heidi. Thank you very much for the visit, comment and congrats!


Andriana 10 months ago

I thought this article is a supreme example of a well-written hub. Thank you!


bdegiulio profile image

bdegiulio 10 months ago from Massachusetts

Hi Linda. Congratulations on the HOTD, a very deserving and interesting hub.


AliciaC profile image

AliciaC 10 months ago from British Columbia, Canada Author

Thank you so much for such a kind comment, Andriana!


AliciaC profile image

AliciaC 10 months ago from British Columbia, Canada Author

Thank you, Bill. I appreciate your second visit and the congratulations a great deal.


Taranwanderer profile image

Taranwanderer 10 months ago

At first glance, I probably wouldn't wear a hat made with fish mucus hahaha. No but yet another seriously informative hub. Kudos!


AliciaC profile image

AliciaC 10 months ago from British Columbia, Canada Author

Thanks, Taranwanderer. I'm not sure how I'd feel about wearing a hat made of fish mucus. It's an interesting idea!


Rachel L Alba profile image

Rachel L Alba 10 months ago from Every Day Cooking and Baking

Hi Alicia, It's hard to think of mucus when looking at those beautiful and colorful fish. You have a really interesting hub. Thanks for sharing.

Blessings to you.


AliciaC profile image

AliciaC 10 months ago from British Columbia, Canada Author

Hi, Rachel. Thanks for the visit and comment. Blessings to you as well!


Peggy W profile image

Peggy W 8 months ago from Houston, Texas

Hi Alicia,

Your articles never fail to amaze me. I always look forward to learning things of which I know little and in this article I also learned a couple of new words. Estivation is one of which I was unfamiliar. I never realized that mucus of some fish actually served to feed their young. Interesting how the fry are weaned. Happy to share, pin and tweet.


AliciaC profile image

AliciaC 8 months ago from British Columbia, Canada Author

Thank you so much for the lovely comment and the kind shares, Peggy. I appreciate your visit a great deal!


DrMark1961 7 weeks ago

A species of Panamanian frog went extinct last week; the father fed the young from the slime on his back. Interesting reading about the fish. Shared on Flipboard.


AliciaC profile image

AliciaC 7 weeks ago from British Columbia, Canada Author

Hi, DrMark. I read about the death of the last frog in the species, but I didn't know that the young fed on slime. Thanks for sharing the information. Thank you very much for sharing the article on Flipboard as well!

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