Jellyfish, or Siphonophore Colony?

This colony of Rosacea may look like a single jellyfish, but it is actually a large group of smaller siphonophores clustered and living together. In fact, the zooids (individual siphonophores living in the colony) cannot survive on their own. This specimen was photographed by the Census of Marine Zooplankton, a project of the Census of Marine Life, in the Sargasso Sea in April 2006.

A Rosacea colony's long tentacles extend a meter away from the main body and contract when disturbed by potential food items. The bead-like dots are stinging cells (nematocysts) that immobilize and kill their prey.

See more photos of cool zooplankton collected by the researchers in this zooplankton biodiversity slideshow, and learn more about jellyfish, comb jellies and other gelatinous ocean animals.

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Jellyfish