Thermal Shock and its effect on fish development
by John Favorito (Reprinted from the Journal of the American Killifish Assoc. Volume 30, No. 4)

It has been shown that the ectathermal animals of the world are dependent upon temperature for the rate of aging and the life cycle. Oxygen consumption by fish will increase with a lowering of temperature and in turn will slow down metabolism and aging. In contrast, higher temperatures will decrease the oxygen content of the environment and the animal will have less oxygen to consume which will speed up the metabolism and the aging process.

This is a gross oversimplification of the process, of course. In actuality there are more complicated reactions to the higher than normal temperatures (thermal Shock) taking place. The life supporting functions of proteins, nucleic acids, and membranes are disrupted and the organism dies an untimely death from cellular damage. Enzymes exposed to high temperatures for a long time lose their ability to activate permanently, while proteins, deactivated by high temperatures, can be reactivated in the cooling process.

Put simply, short term exposure to high temperatures shock may do little damage to the organism after being returned to a cooler environment. On the other hand, long tern exposure will not only damage life supporting functions, but will result in premature death.

Embryo development depends upon a complex chain of events synchronized with precision. Any deviation in the chain can result in an abnormal development pattern in the embryo. Certain species of fish are particularly prone to abnormal development when exposed to extreme temperatures. An example of this would be guppies raised at temperatures above 30o C. resulting in crooked spines. Other abnormalities in fish include unusual fin and muscle development as well as the famous “two-headed monsters”.

One interesting and useful aspect of thermal shock is the effect of temperature on sex ratios of organisms. For instance, in ants maintained at low temperatures, the female spermotheca are deactivated, and the stored spermatozoa are unavailable to fertilize the eggs, resulting in unfertilized eggs which develop only into male ants.

In fish, such species as Rivulus marmoratus manifest similar results. At low temperatures, the ovarian part of the ovotestis is destroyed, producing only male offspring. On the other hand, the embryos of Menida menida produce only females from low temperatures, while at higher temperatures only males are produced.

In insects, the effect of temperatures have proven to be very fascinating. Thermal shock has been shown to alter genes in certain cases and an individual might only produce one sex or the other because of these alterations. This has been demonstrated clearly with Drosophila (fruit flies).

Certainly much research has yet to be done with fish regarding thermal shock. However, its use is already being applied with the sex ratio aspect in breeding stock of feed fish. You as a hobbyist can do your own experiments, and by keeping careful records, may discover something of importance to the scientific world.