Page 4 - TBAS-January-2021

-4- TBAS January 2021 ....................

Annualism is the process by which some killifish species have survived for
millennia in what appear to be impossible life-lethal environments. Annual killifish
reside exclusively in temporary bodies of water in regions that experience annual
or semiannual draughts, a “dry season”, typically followed by periods of rain, a
“wet season”. The timing of these events is determined by an enormous array of
environmental factors and variances in local conditions that are unique to each
region.
During the wet season, dry shallow depressions in the ground will fill with
water, forming pools, and life seems to explode around them. As the wet season
ends, rain events become erratic, then cease entirely. Water levels decline until the
pools dry out completely again, extinguishing all water-dependent life. The surface
that once served as the bottom of the pool becomes desiccated, and fractures into
cracks, large and small, shallow and deep. This dried out depression remains so
until the next wet season fills it with water again. Yet despite this apparently lethal
environment, there arose a number of killifish species that developed a unique
ability to survive. These include species of the African genera Nothobranchius and
Callopanchax, and more than 30 genera from South America.
The developmental cycle is known as annualism: It is enormously complex,
and remains a subject of intense scientific inquiry. Central to the process is the
unique ability of annual killifish embryos to both survive desiccation, and to suspend
growth at various points in their development. Both these capabilities result in eggs
that are incredibly durable and able to survive the most demanding environments. The
net effect is that there are eggs that are ready to hatch at any time of the year as
soon as they are wetted.
Although the life of these temporary pools is a continuum, let’s arbitrarily
start with a pool filled with water at a time near the end of the wet season. The
killifish living therein have been continually breeding since reaching sexual maturity,
typically within some weeks after they hatch. The mating process involves a complex
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