 |
|
|
|
|
Killifishes, Part 2: The Nothobranchius Family By Robert J. Goldstein, Ph.D. The most gorgeous and challenging of the killies (in my opinion) are the Nothos (genus Nothobranchius). These East Africa annual fishes live in open, sunny savannahs in flooded fields, pools, rapidly expanding and contracting lakes, river floodplains, and even cattle and elephant footprints filled with rainwater and groundwater. The Nothos are fishes of ephemeral or seasonal waters. Although some species get into permanent waters, the eggs require a protracted drought where high oxygen tension from exposure to air triggers sequential stages in larval development.
All fishes start out as fertilized eggs or zygotes. The zygotes divide once, twice, and so forth until the larva develops into a blastula, then a gastrula, and finally a prolarva. The egg shell or chorion splits to release either the prolarva (with undeveloped eyes, fins that are only buds, and no jaws or gut) or a later stage larva that can see, swim and feed. That’s the general condition. In annual killifishes such as Nothos, several stages are interrupted by resting periods or diapauses, when nothing at all happens until either time passes or an environmental trigger starts the next stage. Nothos go through these diapauses. The most important trigger for getting from stage to stage in development is high oxygen tension. The most oxygenated water in a cold, babbling brook might reach a dissolved oxygen level of 12 parts per million (ppm), but most aquarium water is about 3-5 ppm. Air, on the other hand, has an oxygen concentration of 21 parts per hundred. It’s the high oxygen concentration of dried sandy mud permeated with air that triggers development from diapauses in Nothos.
The final stage, hatching, is triggered by sharply lowered oxygen tension, as when the eggs are submerged in water, combined with an increase in the carbon dioxide concentration, as when organic materials (leaves, twigs, dead worms and bugs) soaking in water rapidly decay. With increased carbon dioxide, the hatching glands in the throat are stimulated to release the enzyme chorionase. This enzyme dissolves the inner layers of the chorion or shell, and the shell then swells with incoming water. Finally, the thrashing of the stimulated larva breaks the outer remaining layer of chorion, and the larva breaks free. The Notho larvae are active at once, searching for protozoa, crustaceans, and newly hatched mosquito and other aquatic insect larvae. They grow rapidly, often reaching sexual maturity in just 30 days. That’s important, since the ephemeral pool may be a temporary winter rainy season pool that will dry out again in two or three months. Nothos have very little time to grow up, breed, and die, leaving their eggs behind for next year’s generation. They breed against the bottom, in materials that are usually a mix of sand and silt and mud that doesn’t pack down so tightly that it smothers everything below. The mud in many lakes and river backwaters tends to pack down and become anoxic just a few centimeters below the surface. In Notho habitat, however, the dried pools crack and split, and the sand and twigs assist in letting air permeate the bottom, so the surrounding mileau doesn’t become anoxic, which would kill the developing young. And that brings us to propagating Nothos in captivity. Some Nothos are territorial and aggressive to other males, some are social and get along fine in breeding colonies, and still others can go either way, depending on space and how they are raised. Nothos, like Bettas and Paradise Fish, can be trained to fight by being raised singly in cramped quarters, and that’s why most of us like to use large grow-out tanks. It’s not that we worry about males killing other males, but about them killing the (usually smaller) females. Notho breeding tanks can be as small as a gallon jar, but a ten gallon tank is better. Use what you have, including plastic shoeboxes. Nothos are not jumpers, but tank covers keep dust-borne oils out of the water. Most of us use ordinary tap water for breeding, as Nothos are tolerant of considerable hardness with no ill effects on the adults or eggs. In fact, the addition of a level teaspoon of marine salt per ten gallons hardens the water and increases salinity, both protective against skin infections with velvet disease (Piscinoodinium).
To substitute for the natural pond bottom, use a thin (perhaps quarter inch) layer of peat moss. Deeper layers can become anoxic. Aeration is desirable but not mandatory, and a hiding place for the female is a good idea, in case the male becomes aggressive. Hiding places can be Java Moss, other vegetation, or synthetic yarn spawning mops. Even PVC tubes can serve as refuges. Snails can help keep the container clean, but they might eat some of the eggs. Freshwater herbivorous shrimp are being tried by some aquarists as scavengers in breeding tanks. Because of the danger of decay, feed live foods only, preferably Daphnia, Brine Shrimp, White worms, Grindal Worms, Tubifex Worms, Black Worms, Fruit Flies, or Microworms. Many aquarists also feed frozen foods, and a few even use dry foods, but I consider these risky. One option is to place the peat moss in a glass goldfish globe and insert that into the breeding tank. Then it doesn’t matter where the food lands or decays, and the peat moss remains fresh, although you still want it to be shallow. The male will find the peat moss inside the bowl, and the female will find the male. In addition, the male’s protection of his breeding site gives the female a needed break from pursuit as the male bangs his jaws against the glass. You can harvest the peat moss with the presumed eggs once a week or once a month. Some people keep males and females in separate tanks, and just put a pair together over clean peat moss, with no feeding, for a day, then separate them again and harvest the peat moss. In my fish room, I keep a pair or group together for a month, then completely change the tank, thereby combining harvest with a complete water change. Other people change large amounts of water twice a week or more. As you can see, Nothos can be handled in many ways.
When the peat moss is removed, it’s poured through a fish net that retains peat and eggs. You can also clean the peat moss by swirling the mass in clean tank water, but don’t clean it with chlorinated tap water or you might kill the eggs. Then squeeze the peat moss with your fist to remove as much water as possible, just as though you were squeezing a sponge. Don’t worry about hurting the eggs. The chorion is hard as a pebble. Then lay the peat moss on paper towels until the peat moss changes from wet brown to damp tan. Don’t let it dry out too long, for once it becomes brittle dry, the eggs collapse and die. With slight dampness, the eggs will begin normal development; with wet peat moss, they won’t even start to develop. Most of this is trial and error, green thumb, or dumb luck for most of us hoping for a nice hatch. Other people consistently get high hatches. Once the peat moss is the proper dryness, pack it in a plastic bag and store it in the dark. Be sure to label the bag with what’s inside, the date of drying, and the probable date for hatching. Quite important is an adequate incubation period or three to six months at high temperature (78°F) that mimics summer in the dried pools of the African veldt. When the eggs are due to hatch, pour the peat moss into a shoe box with an inch or two of soft water, such as distilled (DI) or reverse osmosis (RO) water. You can get DI at the supermarket and RO at any pet store that sells corals. The eggs will begin hatching within hours, and should be all hatched at 24 hours (all those that are going to hatch). Remove the fry with a food baster or a cup or a teaspoon to clean water, and start feeding Brine Shrimp nauplii right away. Put snails or live daphnia in the fry tank to eat dead food and keep the water clean, and do partial water changes at least once a week. When you’ve removed the fry from the hatching container, re-dry the peat moss, as there may be eggs that did not hatch. Many annual fishes produce eggs with different diapauses. This is a survival mechanism that leaves some eggs for next year or for a late winter. It spreads out the hatch so there are always some possible survivors, even if most of the young are killed by a short rainy season and sudden drought. Think of it as money in the bank that you can’t touch until you reach a certain age. See, Nothos are good parents. And that brings us to the types of Nothos. There are many species, some large, others minute. One small subgenus is called Aphyobranchius, with two or three species to date. These one inch fish produce myriad minute eggs that result in enormous hatches. Unfortunately the fry are minute prolarvae that require space, oxygen, and infusoria (usually protozoa) as their first food. This young stage lasts a long time, during which 90 percent of the fry die of starvation (in captivity). These fish are suitable for beginners who keep green water or infusoria cultures around, but that’s not everyone.
Most nothos are in the genus Nothobranchius, but that may change. Some of the larger, predatory types might some day be moved into a separate group, but for now you can call them all by the generic Nothobranchius.
For more information and sources of nothos see:
|
|
|
