The Celestial Pearl Danio: A Cautionary Tale

Author: Mike Hellweg

An exclusive in-depth look at the celestial pearl danio Celestichthys margaritatus, separating fact from fiction about this beautiful new cyprinid, with an illustrated spawning account.

A Recent Discovery

In late September of 2006, a photograph began circulating among tropical fish hobbyists on the Internet. It was a shot of a beautiful freshwater fish, almost too beautiful to be real. In fact, many of the first people to see it assumed it had been doctored, figuring that no fish that stunning could have been overlooked by the aquarium hobby for all of these years.

In truth, the fish had only been discovered a few weeks earlier in a small plant-laden spring-fed pool in the Southeast Asian nation of Burma. The area where it was collected had been off-limits to westerners for many years, so it has become the source of several new fish in the past few years. But none of the other newly discovered fish from the area compared to this one—this was definitely something special.

If you haven’t guessed it by now, I’m talking about the beautiful miniature fish known in the trade as the ‘‘galaxy rasbora.” It has also been distributed under the trade names of “fireworks rasbora” and “Microrasbora sp. galaxy.” The reason all of these names are in quotes is that exporters were only guessing as to which genus of cyprinids it belonged, because it really didn’t seem to fit exactly within any of the known genera.

Why Not Galaxy Rasbora?

Scientists agreed that this fish didn’t fit into any known genera, and on February 28, 2007, Tyson R. Roberts of the Smithsonian Tropical Research Institute released his formal description of the species, erecting a new genus for it. He also suggested a new common name, since his discovery of a familial relationship between this fish and the danios meant the trade name galaxy rasbora was misleading.

Naming

Dr. Roberts coined the common name of celestial pearl danio (CPD) to reflect that the fish are actually danionins, related more closely to the danios than to the rasboras. According to Roberts he coined the generic name Celestichthys from the Latin and Greek words meaning “heavenly fish.” Since the specific name margaritatus means “adorned with pearls,” Celestichthys margaritatus translates to “heavenly fish adorned with pearls.” It’s a great descriptive name, but a bit awkward to use in conversation, hence the name “celestial pearl danio.”

Physical Appearance

A description doesn’t do the fish justice, but I’ll try. Males of the original fish are a deep midnight blue overall, with a series of pearlescent spots over their flanks, sometimes arranged in rows. Their unpaired fins have bright red stripes outlined in midnight blue. The ventral fins are solid red, or are filled with red splotches or spots. The males also show a bright red belly, and dominant males, reminiscent of male rainbowfish, are reported to flash a red stripe on their backs that runs from the top of the head to just before the dorsal fin. I have not personally witnessed this, but several others have reported it, and I have seen photos of males with this bright red coloration on their backs. I have witnessed courting males adapting a head-down posture that would show this flash to its best, so it seems a likely explanation for this behavior.

Females are a bit more washed out, with more of an overall golden blue sheen. The pearlescent spots are not as bright, and the red in the fins is more of a faded orange. The ventral fins are clear. Some females develop an orange color in the belly, but most retain the golden color here, too. Females develop a dark spot just in front of the anal fin when they are ready to spawn, and the area swells up a bit. I can’t tell if this is the extension of an egg tube or just swelling around the urogenital pore.

The Habitat

In their homeland, celestial pearl danios are found in a series of groundwater or spring-fed ponds in the Shan Plateau where the Salween River cuts a deep swath. It is important to note that the fish occurs in several ponds, not just one as has been widely reported. These ponds are found in the mountainous area around the town of Hopong in the outskirts of the Shan state capitol of Taunggyi, at an elevation from just over a half mile to nearly a mile above sea level. The area is mostly grassland and rice paddies, so the ponds are open to the sun and filled with a rich variety of aquatic plants. The celestial pearl danio has evolved to take maximum advantage of this, and hobbyists should strive to remember their native habitat when setting up a tank at home.

Information on the water parameters of the fish’s native habitat is sketchy. It appears from various reports that most of the waters are just around or above neutral, with low carbonate content and low conductivity. Aquarists around the world have reported success, including successful spawns, in water ranging from soft acid water in Central Europe and Southeast Asia, to hard basic water in the United Kingdom and the United States. So the exact parameters appear to be unimportant, as long as the water is kept clean, meaning that water changes are done regularly and the dissolved organics and nitrates are kept low.

As you can imagine from the altitude of these habitats, the water temperatures are not exactly tropical. Air temperatures around the area vary from near freezing in the winter to almost 95°F in the summer. Travel agencies report that the weather in the area is “mild and pleasant” for the travel season, but it can be “cold, damp, and miserable” in the rainy season. Many hobbyists have reported success keeping celestial pearl danios in temperatures in the low 70s. Few have had luck keeping them in warm water at 80°F or above, so keep that in mind as well. It might be best to keep them in unheated tanks, similar to the way you would keep their cousins, White Cloud Mountain minnows Tanichthys albonubes.

The Aquarium

At an adult size of about ¾ to 7/8 of an inch, including the tail, the CPD seems perfect for the popular desktop tanks or nano tanks, but only one male can be kept in such a small space. A tank of 10 gallons or more would be perfect for a group of them. In a larger tank you really get to see some of their interesting group dynamics, and subdominant males have a place to get away. Referring back to their local habitat, the tank should be heavily planted. If you skimp on the plants the fish will be skittish, and they will display their discomfort by spending much of their time hiding. If you are planning to get them to reproduce it would be best to keep them in a species tank with no other fish, shrimp, or snails. For display purposes they can be kept successfully in a community tank of small fishes like Microrasbora and Boraras species.

As I mentioned above, water parameters appear to be unimportant as long as extremes are avoided. A pH around 7 or slightly above, with medium hardness, would be perfect, but don’t worry too much about the exact numbers. Instead, spend your time doing large regular water changes. For filtration, a slowly bubbling sponge filter seems to work best, though with a lot of plants you might want to forgo even this type of filter and just use a micro power head-type filter for water circulation. Since they are from open sunlit ponds, the bright lighting required by the plants will be just fine for the fish, too.

Feeding

Most danios are omnivores, and CPDs appear to be as well. My fish spend most of their time chasing and displaying for one another, and I have not seen them spend any time grazing. They have tiny mouths, and like all other cyprinids they have pharyngeal teeth. They also have tiny conical teeth in their jaws, indicating that they likely do prey on smaller critters.

I feed my fish finely crushed premium-quality flakes, finely ground freeze-dried krill (this helps enhance the red coloration), newly hatched brine shrimp, and live foods like daphnia, moina, grindal worms, and small white worms. Several times a week I also feed with a high-quality micro pellet food. They eat everything with gusto!

Behavior

There have been several excellent pictures of large groups of schooling CPD males in dealer’s display tanks on the Internet, and I think that may have fueled the initial charge for obtaining this species. Unfortunately, it appears this schooling behavior is not common once the group settles in. It likely will not be replicated in your tank, so don’t expect to see it. Large groups will congregate, but I wouldn’t really call it schooling in the classical sense. Males spend much of their time courting females and sparring with rival males. The fights between males are a ritualized circular dance, and usually little harm is done if the weaker male can swim away. The dominant male can be surprisingly brutal for such a tiny creature, though, and its tiny teeth can cause some damage if the weaker male cannot get away.

In small tanks, losing males are often harassed to the point of death. Several hobbyists keeping them in small tanks have reported that within a few days a single male may kill all of the other males until he is the only one remaining. In larger tanks with a lot of plants it’s very common to see torn fins on all of the males but one; he’s the dominant fish. That’s why I would recommend at least a well-planted 10 gallon (a 20 long would be even better) for these little guys.

Breeding

Hobbyists all around the world are now reporting success with breeding these fish, though first credit appears to go to aquarists Pete Liptrot and Paul Dixon of the Bolton Museum Aquarium in the United Kingdom. They reported their first spawn in late October or early November of 2006, just a few weeks after the fish had been discovered in the wild. Locally, my good friend and master breeder Charles Harrison was able to beat me to the punch, getting his first spawn in late December of 2006 when I was still trying to find healthy specimens of both sexes. Most of the fish available to me at that time were males, and they were a bit thin. Fortunately that has changed rapidly, and finding healthy fish of both sexes is not difficult.

Spawning

It appears that these fish spawn daily, or almost daily, in both the aquarium and in the wild. Females usually lay less than a dozen eggs, though if the sexes are separated and the fish conditioned for a week or so, some breeders have reported as many as 30 eggs from one large female in a single spawning event.

Eggs

As with most egg scatterers, the adults are ravenous egg eaters, so some sort of spawning grate to separate the eggs from the adults is in order. A setup similar to one for spawning zebra danios works well. Plastic needle-point canvas makes an excellent spawning grate. You can set them up with clumps of plants, or even with acrylic yarn spawning mops. The small clear eggs are only lightly adhesive and fall to the bottom quickly if the mop or plant is disturbed, either by the aquarist checking for eggs or by the fish as they move through while spawning.

Chases

The males will chase the females as other danios do, but these chases don’t often result in a spawning session. Instead, the actual spawning takes place when the male hovers over a clump of plants or a mop, with his body at a slight head-down angle to the bottom. I guess this is to display his red flash, though I have yet to see this particular part of the display. If the female is ready to spawn, she will swim over to him and initiate the spawning by what appears to be a headbutt to the area around his anal fin. They then dive into the spawning medium and quiver side by side, releasing the eggs and milt. The pair then breaks apart, the male and female each going separate ways.

This spawning process may be repeated several times if the fish have been conditioned, or only once or twice if they are allowed to spawn daily. No pair bond is formed, and both sexes will spawn with multiple mates if partners are available. If the breeder wants a decent-size spawn, however, it is best to use just a single pair in a separate spawning tank and to remove them as soon as spawning is complete. Fish that are not spawning will eat any eggs they can find—this includes mom and dad as soon as spawning is over.

Raising the Fry

Depending on temperature, the eggs will hatch in as little as three days at about 76°F, or as long as five days at 70°F. The newly hatched fry are dark colored and spend their time lying on the bottom under cover (if it’s available). They don’t move much, and many breeders have mistakenly thought they were dead. The fry will be up and swimming in two to four days, or even as long as a week, again depending on temperature. Interestingly, they have lost their dark coloration, and take on a lighter silver color.

At this time it is necessary to start feeding them. For the first several days, feeding micro-foods is necessary. I usually start them on things like paramecia, but others have reported success with commercial liquid and dry fry foods. If feeding a commercial diet it’s a good idea to add a few small snails to help clean up uneaten food. I also add in Walter worms, which are a type of microworm. These tiny nematodes are supposedly a bit smaller than the traditional microworm and are cultured in the same way. They spend more time in the water column before sinking to the bottom, so it allows the fry to feed in the open water for a longer period of time. After feeding micro-foods for about a week, I start adding newly hatched brine shrimp. Once all of the fry are taking the brine shrimp, as evidenced by their swollen orange to pink colored bellies after feeding, I discontinue the smaller foods. I continue to add the Walter worms for a few more weeks.

From this point, growth is rapid. They begin taking on adult coloration at about 9 to 10 weeks of age and reach adult size at about 12 to 14 weeks of age. In my experience, this next generation will start reproducing as early as the age of 11 to 12 weeks. I’ve found fertile eggs in with fry that were just over ½ inch in size! This doesn’t appear to be a fluke, as I’ve had tiny newly hatched fry turn up in planted grow-out tanks with no adults in them.

The Cautionary Tale, and Some Good News

The Internet can be a wonderful thing. It is a great way of disseminating information rapidly. Unfortunately, there are no ways of ensuring the information is accurate, even when reported by venerable news organizations. Word of the CPDs discovery spread like wildfire around the world, and their popularity was enhanced by the Internet. Early in 2007, just a couple of months after word of their discovery took the hobby by storm, rumors of their demise also spread around the Internet. Rumor had it that the pool where all of the CPDs were being collected had been completely emptied of fish. International environmental organizations were lobbying governments around the world to have all trading in the CPD halted immediately. All of this happened on the Internet.

You could just hear the enemies of the aquarium hobby salivating and getting ready to jump on us. (Yes, there are enemies of the aquarium hobby out there, and they are just waiting for one screw-up on our part to attack us and prove all of their twisted innuendos as correct.) News organizations were already jumping on this rumor, blaming hobbyists for the demise of this newly described species within just a few months of its discovery.

False Rumor

This was the first time that the hobby would have been directly linked to the demise of a species, although it is often falsely blamed for such things. In fact, research has found that all of the fish taken for the hobby don’t make a measurable dent in wild populations. Collecting benefits local people, the habitat, and the fish being collected. Locals want to keep the fish populations strong so they have a source of income for years to come. But a lot of folks never let the facts get in the way of a good rumor, and the demise of the CPD was a good rumor. I even fell for it myself.

However, there was something very wrong if this rumor were in fact true. Normally when something becomes scarce the price of the ones remaining goes up. However, in this case the wholesale price of the CPD did exactly the opposite; it went down considerably over a very short period of time not long after the stories of the demise of the fish began circulating.

To paraphrase Mark Twain, the rumors of the CPD’s death has been greatly exaggerated. Instead just being found in the one pond reported as drying up, it appears these fish are actually in a fairly widespread series of pools and ponds. Currently, scientists and even collectors are unsure of the extent of their range. What is clear is that the CPD is not “endangered” or “extinct.” Those words mean specific things, and should not be bandied about without a lot of research to back them up. Don’t believe everything you read on the Internet! Using standard collecting techniques and returning the plants to the ponds after collecting, along with rotating the ponds where they are harvested and farm breeding, should ensure the viability of these beauties for generations to come.

Fecundity

Those reporting the fish’s demise also did not take into account the incredible fecundity of these diminutive beauties. Since the adults spawn almost every day, and since the newly hatched fry head to the bottom for several days, a pond could be completely cleaned out of adults and be restocked with a breeding population in just a couple of months. Plus, the species has proven extremely easy to breed and prolific in the tanks of aquarists. If they haven’t already done so, it won’t be long before the fish farmers begin to work with this species. Since the CPD is so easy to spawn, fish farms should be able to produce them by the tens of thousands.

More Variants Coming

In closing, I should mention one other neat little tidbit about the celestial pearl danio. It seems there is more than one color variant coming in now. At least right now, it seems that dealers will either get a shipment of all one type or all of the other. (This backs up the idea that each is collected in different ponds, though I have been unable to verify to this point). The coloration on males of this new variant is more orange in areas that are red on the normal CPD males. They display a different distribution of this coloration on the body as well. Most noticeably, instead of tiny round pearlescent spots, they are covered with long oval spots. The midnight blue coloration of males is less intense, almost an olive-blue.

I don’t know if there is an official name for this new variant yet, or if even anyone else has noticed them. I have been calling them longspot CPDs. Maybe it will catch on, and if it does, remember you read it here first! It is possible this is a distinct variant coming from different ponds, or it might even represent a second species. Who knows?

What I do know is that the beautiful celestial pearl danio is just beginning its time as a magnificent addition to the variety of small fish perfect for aquarium life. It is beautiful, has interesting behavior that we are only beginning to learn about, is easy to care for, and easy to breed. What more could a hobbyist want?