By Donna M. Recktenwalt
How long to incubate killifish eggs is a matter determined by the species, the incubation media and its wetness, and the temperature at which the eggs are stored. Most non-annual killi eggs incubate in water or on wet peat for 14-21 days at average household temperatures of 72-75°F, although a few take considerably longer. At higher temperatures, development is accelerated, with hatching sometimes occurring in as little as ten days.
For annual eggs stored in peat, normal incubation times are generally a consensus of experience and convenience. Some may successfully hatch after a month of water incubation; some may peat may be wet in only a few weeks; but most need to be incubated for several months, a few for as much as 8 or 10 months.
Published tables provide suggested incubation times for various annual species, but these should only be taken as guidelines. It is, however, wise to establish that the embryos are fully developed before wetting stored peat, since individual conditions may result in longer or shorter incubation times.
Temperature has a major influence on development rates, with warmer usually resulting in shorter incubation times. However, warmer and drier may slow development.
Are They Ready to Hatch?
Most non-annual killifish eggs are incubated in water or on wet peat. Development in water is easy to monitor, removal of bad eggs is simple, and when the eggs are ready, they usually hatch without complications. Eggs incubated on wet peat have a reduced risk of fungal contamination, but are more difficult to observe due to the dark coloration of the storage medium.
Especially with annual eggs, knowing whether the eggs are ready is important. Wet peat too early and you may have bellysliders or no hatch at all; wet too late and the embryos may already have died.
The best indication of an embryo’s readiness to hatch is the appearance of a dark, shiny iris in the eye. Or shine a bright light, such as a flashlight, on the embryo. If you can see the heart begin to beat, or a fiftul “swimming” activity within the egg, the embryo is ready. Both these techniques are better accomplished using an inexpensive low-power microscope, or a bright light and a strong hand-held or free-standing lens.
Knowing when annual eggs are ready is of major importance to the aquarist, but eggs in peat are notoriously difficultr to see, especially after they have been in storage for a time. Some squarists can “fluff and spread” the peat over a light colored background and spot eggs fairly easily, although most agree that the longer the eggs have been in storage the more difficult the task, since the eggs take on the peat color.
Some recommend setting a small clear-sided container of water on a sunny windowsill and sprinkling a bit of peat on the surface. The eggs sink quickly (some say “plummet”), allowing observation of the developing fry. However, some annual embryos are pale or nearly transparent, making them doubly difficult to see.
But The Eggs Won’t Hatch!
Sometimes knowing that the embryos are ready isn’t quite enough. What to do when they won’t hatch?
The outside of a fish egg consists of a tough protein coating known as chorion. This is exceptionally thick in killifish and one of the reasons annual eggs are so durable. When the embryo is ready to hatch it produces an enzyme (chorionase) that breaks down this protein, softening the outer shell so the fry can break through. The production of chorionase is triggered by rising levels of Carbon Dioxide (CO2) around the egg. In nature this occurs when soil containing fish spawn is wet by rains. Mud holds less oxygen than moist earth, signaling the developing fry that conditions are right for hatching.
This adapotation can be directly utilized by the aquarist to “force hatch” eggs, by placing them in a container and exhaling into it before carrying it around in a pocket for a time, or by adding a measure of microworms to the hatch water.
— G.C.K.A. Newsletter, Sept/Oct 1998