Organisms that spend part of their life cycle as plankton are called meroplankton. These are usually the larval or early stages of reef creatures such as sea urchins, sea stars, crustaceans, worms, gastropods, and fish. Meroplankton either feed on the nutrients from the egg they hatched from, or they feed on other plankton. Meroplankton also look vastly different from their adult forms, and in the past the larvae were believed to be completely different species from the adults.

Source: http://en.wikipedia.org/wiki/Meroplankton

Species featured:

Common starfish (Asterias rubens) 

Green crab (Carcinus maenas)

Moon jellyfish (Aurelia aurita)

Peacock mantis shrimp (Odontodactylus scyllarus)

Common octopus (Octopus vulgaris)

Purple sea urchin (Strongylocentrotus purpuratus)

Nemertean worm (Micrura alaskensis)

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Inspired by Rosemary Mosco's "When I Grow Up..."