Zebra Danios

[shatanka]

A cool interesting bit of info I got in my weekly mailing from Wet Spot ..I didn't know they had the ability to regenerate their skin, fins, and even heart and brain! It is so interesting too about how their eyes and how they are now being used for studying a way to utilize these stem cells for human eyes! -



Wet Spot Newsletter 2013

May 3rd 2013



Hello, my friend! I have something a little special this week to discuss. This week's topic is about a fish that has been around the hobby since the early 1820's. It's a fish that beginners are often pointed towards, and even experienced hobbyists use them to cycle their aquarium due to its hardiness. Yes, Danio rerio "Zebra Danio" has found its way into the hearts of us all, but just what is it about this well-known community fish that keeps its place in this hobby?



To begin, Zebra Danios are known to be extremely hardy when it comes time to start cycling your aquarium, but why is this? What makes them able to withstand high levels of ammonia, or when the pH crashes in your tank? The answer may astonish you. The Zebra Danio has the ability to regenerate not only its skin and fins, but their heart can heal itself as well! Even more fascinating is when this fish is in its larval stage, it can regenerate parts of its brain if it becomes damaged. The retinas in the eye are so advanced that they can see a broad spectrum of colors because of ultraviolet-sensitive cells. In 2007, a study done by the University College London used the stem cells they grew from these fish to treat infected eyes of rodents. The team received positive results, which has lead to studying a way to utilize these stem cells for human eyes. 



As I previously mentioned, the Zebra Danio has been around since the 1820's. They were originally collected in the Kosi River of northern India. Collectors had thought for many years that the natural range was throughout the country of India following all the way into Myanmar. Unfortunately, these misidentifications have been occurring for a very long time. Today studies have been done to find that many of these localities are ending with similar looking fish, being completely different species. It's believed that the full range is limited to the country of India. The fish is incredibly easy to breed; because of this wild caught specimens are almost unheard of in the trade.



Something that I found interesting about the Zebra Danio when I was researching these fish is that the natural habitat is actually slow-moving water, versus the swift streams that I had thought they would be found in. The substrate usually consists of silt or rocky habitats with overhanging vegetation where the fish are usually found in more abundant numbers and have a natural behavior to school.  Wild caught specimens were observed as micro-predators, feeding on crustaceans and insect larvae. They'll grow to be about an inch and a half, which makes them another ideal fish for people who wish to keep them in smaller tanks.



Zebra Danios truly are an amazing little animal, and I think it's time we give them a little more recognition. I'm sure most of you weren't expecting to be reading about them this week. However, I've gone through so many rare and beautiful fish with this newsletter, that sometimes I feel you just need to get back to the root of things. These fish certainly have earned their place in the hobby. I hope you have learned at least one thing new about them, and can appreciate them a little more. For now you can view our current stock list on our website, www.wetspottropicalfish.com. If you have any questions, or are looking for a particular items please feel free to contact me.



Oh, and one last thing. These would make great dither fish for the Etroplus canarensis "Pearl Chromide" you can find on our list! See you all next week!



Anthony Perry

Sales Manager

The Wet Spot Tropical Fish

4310 NE Hancock St.

Portland, OR. 97213

PH: 503.719.7003

[PaulineMi]

Thanks for sharing that very interesting info. 

I'm thinking Bill and possibly Dennis will know all about this.

Edit: After thinking about the amazing regeneration abilities of these fish I started wondering about the pain factor.  I've read articles based on studies of whether or not fish experience pain the same way "higher" life forms do.  Is the pain a momentary experience that creates an almost instinctive avoidance type memory?  Losing skin, organ damage....I can't help but think "ouch".

[BillT]

OK Pauline, here's some random research info:

The zebrafish is now one of the major "model" animals in biomedical research (meaning modeling humans biology). This was mostly started in the 1970's by George Streisinger at the U of Oregon (Eugene, OR). Other big research animals are mice, rats, fruitflies, and C.elegans worms (very similar to mircoworms).

I think the regeneration stuff was figured out about 15-20 years ago. Its impressive, but a lot of lower animals can regenerate pretty well. I had not heard about the eye transplant stuff before. That is pretty cool. It always surprises me when a cross species transplant work well.
In the 1980's I got a PhD studying the development of the embryonic zebrafish hindbrain which is surprisingly similar to higher vertebrates.

There are more than 900 labs working with zebrafish since I checked about a year ago. There is so much research going on with these guys now no one person can know all of it.
There are about 30,000 genes in the zebrafish genome (which has been completely sequenced), compared to about 20,000 (as I understand it) in humans). There are several competing/cooperating efforts to isolate and store mutations in each of the zebrafish genes for future study. It is now possible to change sequence in a particular gene and generate a new animal out of it. This has been possible for a long time with mice, but now it has been worked out with zebrafish which is very handy for doing experiments.

Research done with vertebrates (including fish) in the US is usually very well regulated with an emphasis on not causing un-necessary pain to the animal. This usually involves knocking them out with MS-222 (tricaine or mesab) before doing a fin clip. I think fin clips are done a lot these days to get DNA to determine which genes the fish is carrying without having to breed them.

[PaulineMi]

That is really fascinating information Bill.  What does changing a gene sequence to generate a new animal mean? Would you elaborate on that? Would it be the same type of animal but perhaps a different size or color?

Also...my comment regarding pain/regeneration was referring to animals in a natural environment escaping the clutches of a predator by leaving a tail or leg behind.

[BillT]

What does changing a gene sequence to generate a new animal mean?

A genetic mutation is any change in a sequence of a gene. The sequence of the gene refers to the sequence of DNA bases (components) that make up the inherited information of the gene. It is copied and passed onto any cells coming from that cell including the sperm and egg cells that join to for the next generation. I would consider any offspring would be a new animal. But in another sense, the animal with a changed gene sequence would be new since it would have a different gene. It may or may not be something that could be noticed but on the inside it would be different and therefore in that way new.

Most mutations will not show up in the first generation because most mutations are recessive and require getting a mutated gene from both parents. This usually requires several generations of breeding if a large breeding population is involved.

Not all mutations will make any difference to the animal. Some can be "silent" meaning that although the gene sequence is changed the protein coded for by the gene sequence is not changed. These will not show any effect even if they have tow copies of the mutant gene.

Would it be the same type of animal but perhaps a different size or color?

If the mutation showed an effect, it could affect body size, shape or color. The most common thing would be to cause some problem that would end up making the fish sickly or dead. Most mutation are considered deleterious, meaning bad for the animal. Most of them will die off without being noticed much.
With zebrafish research the trick is to
1) find the mutations, make lines of them, maintain them (usually as heterozygous carriers (one copy of the mutant gene) since the homozygotes (tow copies of the mutant gene) would be lethal to the animal), and breed them
2) study the effects of lethal mutations before they kill the embryo.

An example of this would be the cyclops mutation which is a recessive lethal where the embryo forms only one eye in the middle of the forehead. The fish does not move much and dies probably because it can not eat.
It was found in a genetic screen of parents that were mutated by breeding a mutated parent and then breeding together its offspring (the F1's). If a mutation is present in the mutated parent (1 of the 2 gene copies), then a random grab of offspring would yield two parents carrying the mutated gene in 1/4 (1/2 x 1/2 = 1/4) of the cases. When a bunch of pairs like this are bred those with both parents carrying one copy of the same mutant gene will produce 1/4 (25%) mutant offspring). This identifies the parents as carriers of the gene and they are used to make the next generation.

To study this two carriers are bred together and the mutants are then studied before they die.
There are occasional cases of sheep eating some plant that causes them to have cyclops babies which are born an die. This mutation has helped to understand how this happens.
There are also occasional humans born with this condition, which I think usually results from mutations.

Zebrafish embryology has shown that the cyclops condition occurs because the developing eye field (those cells that will eventually become the eye) starts out as a single area in front of the developing brain. Normally the brain would grow forward and push through the eye field separating it into an eye field on either side of the brain. For some reason this does not happen in the cyclops mutant. Besides the weird eye situation, the brain is messed up also. This alone will drastically affect any behavior.

Link to a picture of a cyclops zebrafish (top row is normal, bottom two rows are cyclopian to different degrees):
http://ars.els-cdn.com/content/image/1-s2.0-S0165380604001063-gr1.jpg

Link to a picture of a cyclops baby:
http://archive.ispub.com/journal/the-internet-journal-of-neurology/volume-10-number-1/cyclops-deformity-in-benin-city-nigeria-a-case-report.article-g01.fs.jpg

Also...my comment regarding pain/regeneration was referring to animals in a natural environment escaping the clutches of a predator by leaving a tail or leg behind.

Yeah probably smarts. I used to work on regeneration in crayfish. They have special joints on their legs and claws (modified legs) where they can break them off while minimizing the damage to the crayfish, in case a predator grabs them. Some reptiles can do this with their tails. Don't know but I would guess it doesn't hurt some much if you have evolved a mechanism to make this happen.

I do think fish feel pain generally, but they probably don't have the mental capacity to think very deeply about it.

[PaulineMi]

Thank you for taking the time to explain all of that. The brain/eye development is very interesting as was the rest of the information. Good reading.

[Mugwump]

Nice post, Bill....thanks

[BillT]

Here a link to another short article on the research use of zebrafish.

http://medicalxpress.com/news/2013-05-animals-zebrafish.html#jCp

This one has an excellant short film of early zebrafish development (about 24 hours) made by a former office mate of mine, Don Kane.

[PaulineMi]

More fascinating information. The photos and videos are amazing...especially the first full picture of the zebrafish embryo.

[Mugwump]

Yes it is.....I wish that I could get some shoys like that....

[BillT]

Those shots were made through a research microscope, at low magnification.
With the camera and lenses it probably cost >$50,000.00.

[PaulineMi]

Those shots were made through a research microscope, at low magnification.
With the camera and lenses it probably cost >$50,000.00.

Well Jon....not quite a garage sale item. 

[Mugwump]

Those shots were made through a research microscope, at low magnification.
With the camera and lenses it probably cost >$50,000.00.

Well Jon....not quite a garage sale item. 

Darn it... ..and Iwas going to look for one too...LOL

[BillT]

Sometimes there are decent student (med school or vet school) compound microscopes on e-bay for $300-500.
I have a friend who got one. if he had a good camera attachment it could take some nice pictures.

[BallAquatics]

I picked up a very nice Zeiss microscope a couple years back for less than $300.  It needed a bit of clean-up and a new cord for the light, but works as good as new now.



Dennis