Annualism—An Adaptation for Survival
© Anthony C. Terceira

The area of East Africa is certainly a study in contrasts. The area is divided into two very different seasons. The Dry season, characterized by the increasing temperature reaching 38 degrees centigrade in the plains areas causing the area to become baked out and a great majority of the water to evaporate causing the death of many smaller animals. Some of the larger animals such as the elephant begin to migrate out of the hot dry baked-out region in search of cooler area and more importantly seek out water which has long since evaporated from what were watering holes. The area becomes so dry and the heat so intense that some animals go into the ground for the duration of the season and pass into a sort of "summer's sleep" or aestivation. One of the better known of the animals is the African lungfish which buries itself in the mud, curls up and secretes a mucus which dries to form a cocoon in which the fish's body stays moist.

The spring and early summer months signal the beginning of the Wet seasons. Once the rain begins to fall it does not fall as a slight drizzle, but ather inundates the baked out regions often in late afternoon or just prior to the onset of darkness. With the down pouring of rain, plants burst into bloom and the baked out plain quickly becomes lush and green. Seasonal holes, some as small as the footprint of migrating animals such as Elephants are filled and water is once again available.

In the warmer parts of Eastern Africa in areas know as temporary pans or swamps we find small hostile niches are the habitat for some of the most beautiful fish In the world--The "annual fish" of the genus Nothobranchius. These small fish, seldom exceeding 60 mm in total length, are some of the most brightly colored of all freshwater fishes. The natural range of these fish is difficult to plot. Many areas, such as pans along the eastern boundary of Mozambique, have been colonized by fish brought about by eggs clinging to the skin of mud-wallowing animals such as elephants, buffalo and warthogs or to the feet of wading birds. These small pans are isolated. They are situated on a watershed and not interconnected by waterways. They are temporary in nature and dry up completely.

The adaptation and reproductive behavior of these small fish is quite remarkable. The eggs of these annual killifish are able to withstand the drying out periods, and the onset of the rains cause these eggs to hatch. The emerging young begin to feed immediately on the small crustacean which also hatch from drought resistant eggs. The young grow at a rapid rate, reaching sexual maturity at 6 to 8 weeks of age. They begin to spawn daily so that new eggs will be safely stored for the next rainy season. Nothobranchius species along with all other annual fishes, do not spawn during a particular season but rather each spawning embrace releases a single egg which is buried in the soft mud bottom. This spawning pattern takes place continuously from the age of sexual maturity until the last bit of water has evaporated from their small pool signaling the end of their life. Each egg contains its own special piece of genetic information presumably to insure that all eggs do not batch at the same time. Some eggs will develop to different stages and then suddenly enter a period of diapause in which development will be arrested. The eggs will remain in this arrested development until the beginning of the rainy season. If for some reason there is a rainstorm followed by a sudden return to hot and dry weather, the species is still insured survival because not all eggs will hatch at the same time. Such is this amazing adaptive pattern for these amazing fish of the "mud".

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