Gently bobbing along on ocean currents, jellyfish have a penchant for turning up on eutrophied shores, where they feast on flourishing zooplankton populations. Most other predators sensibly avoid these oxygen-depleted waters. Since jellyfish don't have circulatory systems to deliver oxygen to their tissues, Erik Thuesen wondered how they cope where most other animals can't(p. 2475).

Jellyfish bodies largely consist of a gel called mesoglea, which provides hydrostatic support. Since the creatures' metabolically active tissues are embedded in this gel, Thuesen figured that it might also have a physiological function. He decided to examine whether the gel stores oxygen to supply the animal's active tissues when external oxygen levels plummet. If it does, he reasoned that jellyfish should be able to cope with a sharp drop in oxygen levels without changing their oxygen uptake.

To test this, Thuesen collected Aurelia labiata, `the lab rat of jellyfish research,' from Puget Sound in Washington. Back in the lab, he and Ladd Rutherford placed them in closed tanks at different oxygen levels. To monitor their oxygen consumption, Thuesen and Rutherford stuck a fibre optic oxygen sensor into each tank. They saw that jellyfish happily swam around and didn't change their oxygen uptake, no matter how little oxygen the water contained. When Thuesen dropped the tanks' oxygen levels to 10%, jellyfish still took up the same amount of oxygen, despite the fact that most other animals faced with this situation would decrease their oxygen uptake. Thuesen concluded that jellyfish use internal oxygen stores to regulate their oxygen uptake, so they're not affected when oxygen levels fall.

To see if jellyfish use their gel as an oxygen reservoir, Thuesen and Rutherford teamed up with Patricia Brommer to measure oxygen levels inside the animals' bodies. The team crafted fine mesh harnesses to support the fragile creatures and used a micromanipulator to carefully push an oxygen sensor through each animal's gel. They saw that oxygen levels in the gel dropped as the sensor got closer to the layer of metabolically active tissue; the tissue was extracting oxygen from the surrounding gel.

But does this stored oxygen allow the animals to keep on swimming when surrounding oxygen levels crash? To find out, Kurt Garrison, Magdalena Gutowska and Trisha Towanda watched jellyfish swim in tanks with normal, low and no oxygen. To their surprise, the creatures behaved normally at low oxygen levels. They only noticed a change in behaviour in zero oxygen; jellyfish pulsated more slowly and swam less far. And when they measured gel oxygen levels afterwards, they found that the animals hadn't even depleted all their oxygen stores. `The oxygen stored in the gel keeps the animals going, even when there's no oxygen around,' Thuesen concludes.

When the first jellyfish roamed the planet's early seas, oxygen levels were drastically lower than they are today. Thuesen views oxygen diffusion pathways in jellyfish gel as a prehistoric step in the evolution of more sophisticated circulatory systems. It may be primitive, but their gel has certainly helped jellyfish stick around for a very long time.

Thuesen, E. V., Rutherford, L. D. Jr, Brommer, P. L., Garrison,K., Gutowska, M. A. and Towanda, T. (
). Intragel oxygen promotes hypoxia tolerance of scyphomedusae.
J. Exp. Biol.