Aquarium Critters in Science - Diapause
By Sally Boggs
From the Monthly Bulletin of the Hamilton Aquarium Society
Aquarticles
Our recent program about killifish was very enjoyable, but it did not include one of
the most scientifically interesting aspects of killifish breeding; diapause. It is the
pause in diapause that is so fascinating. It was the apparently miraculous appearance of
killifish after refilling of previously dry ponds that originally attracted scientists to
study killifish. Careful observation led to the discovery of the wonderful adaptation
these fish have made, which allowed them to survive as a species (if not as individuals,)
after seasonal drying up of their habitat. It also led to extension of the fish's habitat
so they were able to inhabit temporary ponds where many potential predators could not
live. Thus they were able to establish a niche for their kind in nature.
The so-called annual killifish have to put a lot of living into a single wet season. In
this period of time they must progress from egg to adult and produce new eggs. They are
deposited sufficiently deep into the mud that they don't entirely dry up. The eggs develop
to a certain point that is predetermined by the mother's environment and then they enter a
state of suspended animation or diapause. When the rain comes and the ponds fill up some
of the embryos progress and hatch. Usually, however, some remain in diapause until they
are again dried and wet. This prevents loss of the whole batch in the case of a false or
temporary wetting.
Depending on where the killifish species evolved, the eggs can last months, even years
out of water in damp mud or peat moss. There is, however a minimal time needed in
diapause. Furthermore, diapause can occur at three progressively later stages of
development. The female's environment seems to govern which.
The scientific question is how do the eggs know when to grow and when to stop growing
and when to resume growth? Studies so far indicate that the female's egg carries the
message (probably in a chemical form) as to when the embryo should stop developing and,
after a soaking, when to resume development. You can imagine how useful it might be to
know what chemical or other signals would stop the biological clock. Maybe a baby with a
lethal condition could be put into suspended animation until a cure was found. Maybe a
cancer growth could be arrested until the cancer killing treatment becomes effective.
Maybe aging could be slowed if the mechanism of diapause were known.
Diapause is not the only advantage the killifish embryo has. The egg shell (Chorion)
also preserves it in the damp mud. It consists of many protective layers, which prevent
loss of water. The eggs are so tough that they can be picked up and rolled between the
thumb and forefinger without harm. In fact, since I have trouble seeing the eggs with my
trifocals, I sometimes use the "Braille" system to find and collect killifish
eggs from mops or peat.
Diapause is not unique to killifish, arrest or delay of embryonic development can occur
in many animals if environmental conditions are not favourable. For example, when some
bird eggs become cold, they stop growing until they are re-warmed. The killifish have made
a lifestyle around diapause and have perfected it. For the killifish, many factors affect
diapause including light, temperature, salt and probably pH.
Some other aquarium critters that can produce eggs that are able to survive drying and
even freezing are Daphnia, Triops, and Fairy Shrimp. You have already heard from me on the
subject of Daphnia, but have you ever heard about Triops? The name means three eyes in
Greek and they have three eyes on top of their headshield. They are often called dinosaur
shrimp or tadpole shrimp. They have the general shape of a tadpole. They are crustaceans
and really look like miniature horseshoe crabs. When they first appeared on earth there
was only one landmass and the rest was ocean. Now they are found in many parts of the
world. Their embryos enter a diapause to die for. In this state they survive extremes of
temperature and years of complete dryness. They live in very temporary ponds or even
puddles of water and live three weeks to three months. As adults they are 1 to 3 inches.
Depending on food and light they can double their size daily. They have bright colours due
to red haemoglobin and blue-green pigment in their connective tissue. They are brachiopods
because they obtain oxygen by moving water through their feet, so they are in almost
constant, often acrobatic motion. They go through several larval stages. I was able to
obtain some Triops eggs and plan to watch the appearance and rapid growth of these ancient
critters in my own tank. I can't wait!
The information summarized here came in part from the April 1999 "Live Pet
News" a free newsletter sent to me by Triops Inc., P.O. Box 10852, Pensacola, FL
32524. The information was well worth the free plug. Their neurophysiologist, Dr. Eugene
Hull is studying diapause.
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