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Jellyfish were a hot topic at the 2018 Caltech Alumni Day Seminar.
Lea Goentoro, who was just promoted to Full Professor of Biology and Biological Engineering, gave an intriguing lecture on two remarkable discoveries about jellyfish.
First, a little background for those who know as little about jellyfish as I do did.
Jellyfish evolved between 500 and 700 million years ago, and were the first multi-organ creatures and the first to have muscles and neurons. Jellies have no brain, but their neurons network, enabling coordinated action. They are among the most populous of all sea animals, and are found in every ocean from the surface to great depths.
The first discovery is how jellies recover from injury. As Professor Goentoro said, all animals have ways of repairing injuries. If our skin is cut, our blood coagulates, a scab forms, and our skin regenerates. Some lizards and salamanders can regenerate amputated tails or legs. Jellies can also recover from an amputated arm, but in a unique way.
A juvenile moon jelly (Aurelia aurita), called an ephyra, ranges from 1 to 3 mm across, and normally has eight arms emanating symmetrically from a central hub, as shown below.
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As an ephrya grows, its body fills in the gaps between its arms, forming a circular bell. An adult moon jelly, called a medusa, can be 30 cm (1 foot) across. (shown below)
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Goentoro’s experiment involved amputating various numbers of ephyra arms and observing the consequences. In the upper row below, five arms are removed, leaving three. In the lower row, three arms are removed, leaving five.
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These ephrya re-symmetrized — not by growing new tissue, but by relocating existing tissue — they are made of jelly, after all.
Goentoro’s team studied amputated ephrya in various conditions, and identified their swimming motion as the key factor in re-symmetrization. Sedated jellies swim slower and also re-symmetrize slower. A mathematical analysis of muscle-contraction forces acting on jellies’ elastic tissue confirms that swimming pushes their arms apart.
Jellies employ a two-stroke motion that both propels and feeds them. Muscles contract in the power stroke (1->2->3 in the image below), pushing water downward and the jelly upward. Muscles relax in the recovery stroke (4->5->6), sucking water and food toward the central “mouth”. This simple motion is extremely efficient, requiring an estimated 48% less energy than other sea creatures.
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The second discovery was that jellies sleep, providing new insight into sleep in all animals.
One might well ask how we can know if a jelly is awake or asleep if it has no brain. Goentoro defined sleep as a state of:
- Reduced activity
- Reduced sensor response
- Rebound tendency: an animal with diminished sleep one night, sleeps longer the next night.
Goentoro’s team studied sleep in cassiopea (shown below), an upside-down jelly (they always swim downward). Goentoro found that their activity normally follows a 24-hour cycle, even in total darkness. and that they do exhibit sleep-rebound. Furthermore, cassiopeas’ sensory response time nearly doubles during “sleep”.
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Some scientists believe animals need sleep to consolidate memories. But brainless jellies have no memories. Goentoro said it seems more likely that animals sleep to restore metabolic balance.
It was a great pleasure listening to a gifted scientist expounding on her favorite topic. Even at the end of a long Seminar Day, none of the jellies in the audience slept.
Best Regards,
Robert
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July 24, 2018
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