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Issue: July 2008

Danionella translucida: A Micro Glassfish

Author: Ted Judy

JUDY 0708
Photographer: Ted Judy
Calling them tiny would be an understatement! Learn the origins and breeding habits of one of the smallest vertebrate organisms on the planet.

Danionella translucida: A Micro Glassfish

Ted Judy

The country of Burma (also known as Myanmar) has become one of the hotbeds of new aquarium fish discovery. Recent discoveries include the celestial pearl danio (previously known as the galaxy rasbora) Celestichthys margaritatus Roberts 2007 and the rose danio Danio roseus Fang & Kottelat 2000. This country has also produced many of the very small fish of interest to nano-tank hobbyists, including Microrasbora erythromicron Annadale 1918 and Indostomus paradoxus Prashad & Mukerji 1929.

In the mid 1980s, a very small fish was described from the Irrawaddy (Ayeyarwady) River Basin, so small that it is considered to be one of the tiniest vertebrate organisms on the planet. This fish—Danionella translucida Roberts 1986—grows to just over a centimeter in length, and is less than a millimeter thick. There are currently two identified species in the genus Danionella, along with a few undescribed types. D. translucida is almost completely transparent (hence the common name micro glassfish) except for its eye and some distinctive black spots on the lower half of its body. Those dots are so small, however, that seeing them without the aid of macro photography is difficult. The fish also lack scales, barbels, and a lateral line.

In the Wild and in the Aquarium

D. translucida is found among the roots of floating plants and is a micro-predator that targets very small invertebrate organisms for food; about the largest food that an adult can handle is live Artemia nauplii. The water in their natural habitat is usually muddy, making these fish even harder to see. There is a hypothesis that the species produces a sound with a specific set of bones to help mates find one another in the turbid water. The water chemistry in the river is moderately soft at 100 to 200 ppm total dissolved solids, with a KH less than 3, and a neutral to slightly alkaline pH.

The species is not hard to accommodate in the aquarium—the only important thing is to avoid housing them with fish that might consider them an easy meal, which could be just about any other fish. Ideally, they are best kept in a species-only tank. Water chemistry in the aquarium is not critical, so long as extremes are avoided. Very small tanks are appropriate, and a breeding group of 10 to 20 adults can be easily maintained in a 2½-gallon tank. Filtration should be gentle, so a small air-driven sponge filter will suffice. Power filters are a poor choice, since the fish can be sucked into the intake tube. A temperature between 72° and 80°F is acceptable.


Breeding occurs daily. Females will produce three to eight relatively large eggs that can be seen in the abdomen as they develop. The addition of food equals eggs: a morning meal of baby brine shrimp or microworms, which are visible in the stomach of the fish, will create the day’s load of eggs that evening. They probably breed at night, because the females will be thin again by morning, waiting for another feeding to start making new eggs.

The eggs are non-adhesive and will scatter into mats of plants such as Java moss, or even into a yarn spawning mop placed on the bottom of the tank. Incubation is short and the remarkably large fry will appear three or four days after the eggs are laid. The adults are very proficient fry predators, however, so steps will have to be taken to save the fry. They are too small to be netted, so either collect and move the eggs to a hatching tank, or alternatively, move the adults to a different tank after a few days worth of eggs have been laid. Once the adults are out of the tank, the fry will be able to survive.

The fry are much larger than one would expect—they can eat microworms or vinegar eels immediately, and will also pick at infusoria in the tank. After a week they can be given baby brine shrimp, at which point they will start to grow quickly. Once they are half-grown (about ¼ inch) they can be safely housed with the adults. The development rate is rather impressive: the fry reach adult size within two months, and they start producing eggs and fry of their own within three months.

Getting Your Hands on the Micro Glassfish

The availability of D. translucida is sporadic at best. They are rarely exported for the aquarium trade, and when they are, the losses can be huge. A few specialty importers have been getting them occasionally, so interested aquarists should obtain them when they can, because there is no predicting when they will show up again. They are not difficult to breed however, so tank-raised fish should be more common in the future.

An important consideration is the susceptibility of this species to environmental toxins. They have a low tolerance for poor water quality and harsh medications. The combination of being extremely small and scaleless means that normal concentrations of medications can be toxic. If medicating is necessary, avoid any chemicals not recommended for scaleless fish and use half dosages of everything else. Increase dosage of medication slowly if the problem does not get better, so that there is less risk of toxic shock to the fish. Luckily this species is otherwise very hardy and has not proven to be disease prone.

D. translucida is a species that needs to become established in the hobby. It is interesting, easy to maintain, and is not difficult to reproduce. They are certainly conversation starters.

See the full article on TFH Digital

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