Those free-swimming jellyfish in the sea don't start out in that familiar medusa form, but rather start as sessile and asexual polyps. Now, researchers reporting in the Cell Press journal Current Biology on January 16 have discovered what triggers that transformation in the moon jellyfish (Aurelia aurita). The key is a novel metamorphosis hormone that accumulates during the cold winter to induce a synchronized emergence of jellyfish in the spring.
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