Raising the annual killifish species has always had a touch of magic about it. Where else can you spawn fish, throw the spawning medium into a plastic bag and forget about it for six months, then add water and get fry?
But–surprise, surprise–not all annual eggs must be stored in peat. Some may be quite successfully incubated in water.
All “annual” killie eggs undergo several periods of diapause (cessation of development) before hatching. However, it isn’t always necessary that development of the egg stop during these diapause periods, as usually occurs when the eggs are stored in peat. Given sufficient oxygen, some eggs will continue to develop, although they will go through all the steps of eggs that pause.
To better understand diapause, we need to consider the original habitat of the annual fishes. In most cases, annuals inhabit “temporary” pools of water–seasonal ponds, rain-filled puddles, or small wetland areas resulting from the overflow of larger watercourses. Over time, the species have adapted to their specialized environment, with extremely rapid growth and reproduction rates. The eggs are well protected against dessication, and development of the embryos can cease for long periods of time until conditions are favorable for successful hatching and growth. In some species, under certain anaerobic conditions, annual eggs have been known to remain viable for three years. (Scheel, 1962).
There are usually three distinct stages of diapause.For all killifish eggs development begins a few hours after spawning, when the fertilized egg forms a small rounded body, a blastodisk or blastule. The cells then migrate to the edge of the egg, becoming a multidisk which extends into the yolk membrane. A deep groove then usually appears around the egg’s equator (gastrulation); in time this disappears, the embryo covers the yolk, and development stops. At this point the eggs appear clear to the naked eye.Diapause I
In the wild eggs laid in mud will be in an environment low in oxygen–even anaerobic. If the egg was spawned by one of the annual species, development ceases at this point; the egg may remain unchanged for months. If the egg was spawned by non-true annuals, development may be forcibly halted by reduction of the oxygen supply, to resume when the oxygen levels rise, usually due to drying of the pool. As water evaporates,
anaerobic mud becomes moist soil with a higher oxygen content. If kept anaerobic for too long, the non-annual eggs may die.
In the aquarium, development from Diapause I is affected by the spawning media, among other factors. A deep container of fine peat will become anaerobic over time, while eggs incubated in water can be kept well oxygenated.Diapause II
When development continues, a short, thin-bodied embryo appears on the yolk membrane, usually in the yolk groove. In another day or two, the embryo half encircles the yolk. At this point, embryos from both African annual and semi-annual species may again rest, sometimes for months. These are known as “resting embryos,” or Diapause II eggs. In American annuals, this stage does not necessarily occur, with egg development apparently controlled by available oxygen. When development resumes, the blood and circulatory system appears. After this has occurred, arresting the development of the egg will kill the embryo.Diapause III
The third phase of diapause occurs after the embryo has consumed its yolk. In annuals, these “resting fry” are characterized by the cessation of circulation, with no heart movement seen. This period of diapause may last for weeks or months. Sudden strong stimuli, such as light, may trigger a heartbeat and circulation, followed by embryo movement, but if the egg is returned to the dark, all movement stops.
Eggs from non-annuals which don’t show these characteristics of “resting fry” don’t hatch when their development is complete, however forced hatching can often be accomplished by adding additional CO2. If there are plants in the container with the eggs, the eggs will often hatch at night.However, It Doesn’t Always Work Out That Way …
Experience in the fishroom has shown that diapause can be affected by a number of factors, and that even annual eggs don’t always have to go through the usual resting stages.
Among other factors that can affect diapause: the presence of adult fish in the tank, higher temperatures, and oxygen levels. Presumably adult fish, especially females, may produce hormones that keep eggs from leaving Diapause I. Killikeepers that specialize in the annual species have observed that warmer and wetter correlates directly with shorter incubation times, and that higher oxygen levels often result in better hatch rates.
But annual eggs can also sometimes develop continuously in water and then hatch successfully. Dr. John Wourms called these “escape eggs,” since they would be the first of any spawn that hatched. He also observed that different eggs in the same spawn often develop at different rates; this is why we can sometimes get repeat hatches from the same (redried) bag of peat.
For those who are interested, successful water incubation has been reported in the following, although this is undoubtedly only a partial list.Austrofundulus myersi
Austrofundulus transiliis
Cynolebias belottii
Cynolebias constanciae
Cynolebias melanotaenia
Cynolebias nigripinnis
Cynolebias whitei
Fundulopanchax filamentosum
Nothobranchius guentheri
Nothobranchius korthause Red
Nothobranchius melanospilus
Nothobranchius palmqvisti
Racovia brevis
Racovia hummelincki
Pterolebias longipinnis
Terranotos dolichopterus
GCKA Newsletter, May 1999