Fish Health Through Slime

Copperband Butterflyfish (Chelmon rostratus) and Yellow Multibanded Pipefish (Doryrhamphus pessuliferus)

Copperband Butterflyfish (C. rostratus) and Yellow Multibanded Pipefish (D. pessuliferus)

Fish diseases—they are the meat of fish forums and the subject that takes up the most ink (or whatever causes words to form on a computer screen), so for today’s post, I am going to discuss fish immunity in relation to fish slime.

Fish, like every organism, have an immune system that is specifically designed to function in the environment they live in, using as its source of energy the food that the creature is able to acquire through its intake devices, or mouth parts. The immune system is one of three parts of a fish that needs to be fueled to keep the fish operating at peak efficiency. The other two parts are growth and reproduction. If fish don’t take in enough food or get the wrong types of food, there will be less energy to fuel those systems properly and one or all will suffer.

Reproduction goes first…

With improper food, the first thing to go is reproduction. That is the reason very few fish in captivity reproduce. All healthy fish spawn all the time, and it is a huge burden on a fish to produce eggs or sperm. If you have ever seen a pregnant fish, you may have observed that the eggs can be almost the weight of the entire fish, and they lay eggs constantly.

Healthy fish are always pregnant, and those eggs are made almost entirely of oil. Most fish do not make this oil, so it has to come from the food they eat. That’s why tuna, mackerel, sardines, and menhaden are very oily fish. It comes from their diet of oily prey. Whales (okay, I know they are not fish) are full of oil and because of that were harvested for hundreds of years. That oil comes from the krill they eat.

…followed by immunity

The second system to become compromised is the immune system, which is the reason there are so many disease threads online. So what can we get from this? If our fish are not spawning or showing spawning behavior, they are not getting the nutrition they need and their immune system is not operating at peak efficiency. I won’t talk about growth because in a captive environment, that may be a good thing, as many captive fish are stunted. The fish, however, may not agree with that statement.

High time to consider slime!

As I said, this is going to be about fish slime, and fish slime is a fantastic substance. We rarely think about or discuss fish slime, as we are more concerned about color, price, carpet surfing, water parameters, iPhones, bio-pellet reactors, calcium dosing, and super models, but fish slime is more important than all of those things (with the possible exception of supermodels). It is fish slime that keeps most of our fish free from disease, including parasites, and fish slime is the reason fish can regenerate body parts, such as fins and gill covers.

We tend to think of fish slime as a lubricant that allows fish to cram themselves into tight places, and that is true, but it is only an accidental benefit of the slime. We as humans evolved from fish, depending on what you believe. I myself used to date a girl who resembled a flounder, so maybe she was still evolving, but most humans don’t exude slime, although we still do have oil glands. Our ancient slime glands became sweat glands. The reason fish don’t sweat is because if they did, the oceans would overflow. But I digress.

Green Clown Goby (Gobiodon atrangulatus)

Green Clown Goby (Gobiodon atrangulatus)

Fish exude slime from many places, and it is the most important part of their immune system. It protects fish in multiple ways. Fish skin is alive, unlike our skin, which is dead on the outside. A fish’s skin is very thin for a couple of reasons. It allows intake of some nutrition, water, gases, odors, hormones, etc. The slime has to allow those processes to occur while protecting the fish at the same time.

Now I am not the god of fish slime, but I can go into the processes that occur in slime, I can also discuss the lipids, T Cells, macrophages, etc., that come into play in the immune system, but most of us are not doctors and won’t understand or care, so we are going to keep this simple. I will leave that to people who have more degrees than thermometers, but not necessarily fish tanks. By the way, I have had fish for every day of my life for over sixty years, so if you find someone who has had fish longer than I have, don’t ask him anything or he is liable to drool or spit up on you.

The many roles of fish slime

The first way slime protects the fish is mechanical. Slime traps bacteria, viruses, and parasites and just makes it difficult for those pathogens to get to the skin of the fish. Slime is constantly washed away just from the process of the fish swimming or, in some cases, burrowing into the substrate. When the slime washes away, so do the pathogens. But unfortunately for the fish, pathogens evolved right alongside the fish and some of those irritants manage to hold on. Luckily for the fish, the slime is also a “living” part of the immune system with all the anti-pathogen devices that are also inside the fish, so the slime, besides slowing down the pathogen, has the ability to kill most of the offending organisms. (Many supermodels have bodyguards for that.)

Fish slime covers the entire outside of the fish, even the fins and eyes, extending into the gills. That is also the reason fish can quickly regenerate fins. If a fin is damaged, the living slime covers the damaged area, allowing the fish to send cells to the site to repair the damage. If we get an injury, it is easily infected and takes a long time to heal because the outer layer of our skin is dead, so we have to wait for a scab to form to protect the area, then we have to wait for the scab to eventually fall off before the area is healed. During that time, bacteria can enter the wound, causing infection or worse. There is a study going on that uses fish slime to treat human skin wounds.

Slime production takes energy!

Fish continually produce slime and it, like developing eggs, is a huge burden and energy drain on the fish, requiring many calories every day to form efficiently. If a fish is injured or under stress, it will produce more slime. Slime production will also increase if an irritant, such as copper, is used in an attempt to cure a parasite. Extra slime will also make it harder for a parasite to get hold of the skin of a fish and make it easier for the fish to slough off extra slime to help eliminate parasites.

Fish slime is the first—and best—line of defense a fish has. As I said, the slime is alive and just like the insides of the fish, its immune system works with anti-biological, anti-viral, and anti-parasitic substances.

Imparting immunity

These substances, just like in us, depend on an earlier infection so the immune system can recognize the offending pathogen. Some of this immunity comes from the mother and is transferred to the fry as it is developing in the womb. If this were not so, baby fish would have a very hard time surviving in the sea, as their slime coat is very thin.

Also, like us, some of these pathogen fighting substances depend on occasional meetings of the fish and these offending pathogens, which is the reason we humans need occasional injections of anti-viral or bacterial serums to protect us. Before we discovered anti-tetanus shots, many people died from minor injuries. Fish in the sea are constantly exposed to these pathogens, but in a tank, they are not, which is the reason for all the posts in disease forums. Unfortunately, there is much more energy going into curing fish than protecting fish from disease in the first place.

Slime varies

Before I end this long rant, I would like to say that different species of fish exhibit different benefits and detriments from slime. Not all fish are the same, and virtually all fish studies are done on food fish. Fish such as sharks were designed very differently from bony fish. Rays, which are in the shark family, have highly designed slime, and some have toxins in their slime for disease prevention and to avoid being eaten.

Seahorses and pipefishes don’t have smooth skin or scales. Instead their skin covering is composed of calcium-based denticles, and they were once considered insects instead of fish. Some fish are scale-less, and for some reason, freshwater fish produce more slime than saltwater fish. But fish are very good at protecting themselves with little “help” from us.

The correct diet will protect fish from almost everything—except a frying pan.

Photo credits: Paul Baldassano


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About Paul Baldassano

Paul Baldassano has been in the hobby since the 50s and holds two aquarium-related patents. His current reef aquarium was set up in 1971. He is also an avid SCUBA diver and Vietnam veteran.


  1. Wade Lehmann says

    You should look up Ed Noga’s research into immune peptides and other small molecules that occur in slime. It is a whole topic area that you missed. It is not specific to pre-contact immunity at all, but appears to be critical for health.

    • Chris Aldrich says

      This is interesting, Wade. I’ll have to take a closer look at Noga’s research. Thanks for stopping by to join in on the conversation.

  2. Paul Baldassano says

    Ed Noga is one of the sources I used to research that article. I know some of it sounds like I made it up. But I didn’t. I just try to write in every day language.

  3. Besides sponge, what would be your top 10 feeds for a very picky Moorish Idol? Anyone have any thoughts?

  4. Paul Baldassano says

    I kept my last Idol almost five years and killed it in an accident and I did give it sponge as well as clams, live blackworms and sinking pellets that I first soaked in fish oil. I built a feeder that would dispense those pellets into a feeder for the Idle four or five times a day

  5. Mike Johnson says

    Very good article, Paul B. I wholeheartedly agree that aquarists would spend a lot less time trying to cure sick fish if they were properly feeding the fish in the first place.

  6. Matt Carroll says

    There’s never any shortage of surprise when you’re learning! Great article.

    Just to pile on…

    Paraphrasing from Wikipedia’s entry for “dendritic cell”:
    All mammal “tissues in contact with the external environment” (we call them skin/mucus membranes/etc) have dendritic cells. The skin in contact with the air has specialized dendritic cells called langerhans cells which tie our skin into our innate and adaptive immune systems. In essence, I think they are the front-line communications cells for the immune system. If I recall correctly from reading elsewhere, these cells depend on a healthy microbial flora to get “correct” communications into the body.

    Quoting from W’s entry on langerhans cells:
    “Langerhans cells are dendritic cells (antigen-presenting immune cells) of the skin and mucosa, and contain large organelles called Birbeck granules. They are present in all layers of the epidermis…”

    I love the parallels in this research (I’ve been reading up on human gut-health) and Paul’s. Considering how dramatically different from fish we really are, we’re surprisingly similar. We, and the rest of the critters who use one, inherit the pattern for our fermenting guts to herbivorous rayed-fin fishes, who were the first to implement such a thing.

    I’m willing to bet that sooner or later we’ll learn that microbially-healthy skin is important for us very much like healthy slime is important for fish.

    Loved everything I’ve read so far – keep up the good work!

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