Embryos Survive Without Oxygen

Biological Strategy

Annual killifish

AskNature Team

Maintain Homeostasis

When a living system is in homeostasis, it means that internal conditions are stable and relatively constant. For example, a human’s internal temperature is approximately 37 degrees Celsius (98.6 degrees Fahrenheit) unless there’s an illness. The human body maintains this temperature despite external ambient temperature. However, as with all physiological processes, maintaining homeostasis requires communication and coordination. So living systems have ways to detect changes from the norm, mechanisms to cause an adjustment, and negative feedback connections between the two. A desert lizard called the Gila monster offers a good example of maintaining homeostasis. The lizard goes from eating large meals to fasting for extended time periods. To maintain its blood sugar levels at a steady level, when food is scarce, its endocrine system releases a hormone that raises its blood sugar levels.

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Protect From Loss of Gases

Gases are part of most living systems’ environment, especially those exposed to the earth’s atmosphere. Many gases are crucial to survival, with the two most important being oxygen and carbon dioxide. In some environments, oxygen or other essential gases may be in low supply, such as underwater where there’s been a decrease in the amount of dissolved gases. Living systems must have strategies to survive in that depleted environment. An example is a shark that reduces its respiration rate to survive when oxygen levels are low.

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Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

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The embryos of killifish survive seasonal droughts buried in mud under anoxic conditions because they can slow their metabolic rate to an extreme degree.

“Annual killifish, Austrofundulus limnaeus, live in temporary ponds in arid regions of Venezuela. Their embryos ride out seasonal droughts buried in mud, where microbial action often uses up all the oxygen. Jason Podrabsky, a comparative physiologist at Portland State University in Oregon, and his colleagues tested killifish embryos by sealing them in oxygen-free vials. After 62 days, half the embryos recovered when given oxygen (The Journal of Experimental Biology, vol 210, p 2253). The next best vertebrates – turtles and a species of goldfish – can survive for only a few days. Podrabsky found that longer-lived killifish embryos accumulated lactate – the end product of anaerobic metabolism – very slowly, suggesting that their anaerobic ability comes from being able to cut their metabolic rate to extremely low levels. Podrabsky is now studying which genes are responsible for the metabolic slowing. Learning how the fish do this may help explain how human tissues respond to anoxia during, say, a heart attack, Podrabsky says.” (New Scientist 2007:15)