Stationary jellyfish (image) Howard Hughes Medical Institute Share Print E-Mail Caption A Cassiopea jellyfish rests upside-down on black sand and pulses, rhythmically contracting and relaxing its bell. At night, Cassiopea jellies pulse less frequently -- a clue that they're sleeping, researchers report. Credit Caltech Usage Restrictions Use with credit. Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.