How Fish are Different from us.

[Paul B]

I would like to start a thread about fish and how they differ from Humans. How they are better or worse than us and how their systems function differently.

I know when we keep, medicate or just think of fish, we feel they are a lot like us. They are not. Most of us are very different than fish in almost every way. Every organ, including skin, brain, kidney eyes etc. is different than ours.

Lets take skin. Most of us are not covered in scales. Some fish don't have scales but those are covered in thick slime that discourages pathogens and parasites and probably tastes bad so larger fish don't want to eat them. Mandarins come to mind.



I once added a small mandarin to my fully stocked reef and before it made it to the bottom it was immediately eaten by a larger fish. I think it was a Watchman Gobi. The Watchman looked at me with a horrified look and couldn't open it's mouth fast enough to spit the unfortunate, but lucky fish out. That mandarin was fine and maybe even enjoyed the experience as it went on to live many happy years in my tank.

The scales are there to protect the fish from a variety of things and are like a Samauri suit of armor. It is very hard for a parasite to get through these scales and the "spots" we see on the scales of a fish are not harming it.
Those parasites are just resting and maybe thinking of how to get inside this huge "all you can eat buffet".
If the parasite doesn't get into the gills it will fail in it's mission and depending on where it is in it's reproductive cycle it will just die a slow and maybe peaceful death.

Even if the parasite gets into the gills or down the fish's throat, if the fish is healthy, it's immunity will repel or even kill the nasty parasite because fish are covered in slime all over it and in the gills, throat and stomach.

Under the scales, a fish skin is semi porous almost like the membrane of a reverse osmosis filter. Pheromones, and some salts can penetrate the skin. Our skin is waterproof and is dead on the outside. A fish's skin is alive all the way through and if a fish's skin gets cut, it will heal, with the help of the living slime and the fish will heal without a scar. We are not so lucky.

Slime is exuded in many places on the skin and its water soluble so it must be renewed constantly. Just like many female Humans re-apply make up every day.

The slime, in a healthy, immune fish has no problem dealing with parasites, bacteria, viruses and funguses as that is what it is there for. Long quarantine, long periods of stress and many medications will impede the slime's role in disease management.

But where does the slime get the information on what pathogens it should repel? Simple. It gets it from it's gut bacteria, the same place we get information to our immune system to protect us.

We have two kidneys but fish have only one. It is huge and one of the largest organs in a fish except for the swim bladder which most, but not all fish have. In fish it's called a "Head Kidney" and extends from it's head most of the way to the rear of the fish and that kidney works with the gut bacteria to design the exact type of protection the fish needs to add to it's slime.

Another thing on a fish's skin is its lateral line and it is probably the most important thing on a fish. It is the faint line you can see on most fish that goes from its eye to its tail. It is a series of nerves or tiny open pits that allows the fish to "feel" everything in its surroundings. Even behind it. That is the reason fish don't crash into the glass even at night and why we can't catch a fish with a net in the open sea unless we cheat and corner it against a rock or another net. Fish have no problems evading objects and most predators. Of course, fish do get eaten, but that's because predators also have a lateral line.

You can clearly see the lateral line on this fluke.



I feel the reason some fish are flattened laterally like a Copperband Butterfly, Lookdown or tangs is to allow the lateral line to be longer as it goes up almost to its dorsal fin then down to it's tail. I also feel that's why those types of fish are more susceptible to HLLE which always starts at the head where the lateral line enters the brain of the fish. Those nerves get "hyper excited" from all the input into the line from the glass sides, bottom and surface of the water. The fish were not designed to live in a tank in 14" of water so eventually, with all the inputs, that part of the fish just rots away. HLLE doesn't happen in the sea, only captive fish get it.

We still have eyes, reproduction, stress, longevity and a few other things to discuss and I'm sure there are many people that know more about fish than I do.

I will get back to this later.

 

[Paul B]

Next is eyes. We have a pupil that can enlarge or close to allow the proper amount of light in to our retina but a fishes eyes are different.

In us this happens almost instantly so if we are in the dark and a light goes on, almost instantly we can see clearly so we can find our phone to go on Facebook or Instagram (although I have no idea what that is or why anyone would need it)

A fishes pupil doesn't constrict to modify the light, so how do their eyes work?
Both we and fish have rods and cones at the back of our eye that allow us to see by transmitting data to the visual cortex of our brain. These also allow us to see colors. Fish also have rods and cones but these are able to retract back into the retina to lessen the light that reaches it's brain.

(Actually, I think it's only the rods or cones that retract, I forget which one)

It serves the same function of our pupil. The big difference is that unlike how our pupil works, like a camera lens, a fishes rods and cones work their way back into the retina very slow, like 20-30 minutes slow so if a fish is in the dark, like in our bedroom and someone turns on the light, fish can't see right away which is why it takes them a while to start moving around. It's like if we look directly into the sun. Fish evolved like this because in the sea, a fish is never plunged into darkness or bright light instantly. The sun goes up and down slowly allowing the fishes eyes to adjust to the changing lighting. Remember, although we put fish in a tank with artificial light that goes on and off instantly, fish hate that.

When we look far or near, the lens in our eyes change shape allowing us to see clearly, but a fishes eye lens is solid and doesn't change shape, instead, a fish can actually move it's lens to focus the image on their retina.

Fish have lateral vision which means that fish have their eyes on the side of their head so it would seem that they can only see whats beside them. But that is not true, fish actually see much better than us and can see almost 360 degrees.

A fishes color vision is not as acute as ours because we have three types of cones in our eyes and "most" fish have only two. (Lionfish, goldfish and some other predators also have three like us or even four) Most fish can not distinguish between red, yellow and orange but shallow water fish have an extra cone (that we do not have) that allows the fish to see ultraviolet light and many sea creatures exhibit UV colors that we can't see. This allows those fish to see clearly in darker areas or at night.

Many fish can also see polarized light so they can see reflected light much better than we can. Many fish have a reflective film at the back of their eye called a Tapedum that reflects light back through the retina a second time further improving their low light vision.
Other animals like cats also have a tapetum which is why cats eyes often glow in the dark and those animals can see much better in the dark than we can.