<p>Janthina globosa Swainson, 1823 - purple sea snail shell from Aruba (apical view; modern, latest Holocene; 2.3 cm across at its widest).
</p><p>The Janthina purple sea snail is a remarkable organism.  The snail makes a thin calcareous shell that is dark purple-colored in the lower parts and pale to light purplish-colored in the upper parts.  This is a good example of reverse countershading.  Ordinary countershading involves organisms, such as halibut fish, having dark-colored uppersides and light-colored lowersides.  This makes it difficult for predators to see them from above or from below.  Janthina is reverse countershaded.  One would surmise that the purple sea snail's body is therefore upside-down while it is the water.  This is exactly the situation.  Janthina makes a bubble raft and the shell is oriented spire-downward (or close to it).  The result is an organism that is dark on top and light on bottom.
</p><p>Janthina is a part of the macrozooplankton in the world's oceans.  It's bubble raft keeps it from sinking in the water column.  It preys on jellyfish-like medusiform organisms such as Physalia, Velella, and Porpita (Phylum Cnidaria, Class Hydrozoa).
</p><p>In ancient times, one source of purple dye for clothing was Janthina purple sea snails.
</p><p>Classification: Animalia, Mollusca, Gastropoda, Janthinidae
</p><p>Locality: Bare Ass Beach, northeastern shore of Aruba, southwestern-most Lesser Antilles, southern Caribbean Sea
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The gastropods (snails &amp; slugs) are a group of molluscs that occupy marine, freshwater, and terrestrial environments.  Most gastropods have a calcareous external shell (the snails).  Some lack a shell completely, or have reduced internal shells (the slugs &amp; sea slugs &amp; pteropods).  Most members of the Gastropoda are marine.  Most marine snails are herbivores (algae grazers) or predators/carnivores.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
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<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/15598628180/">Janthina globosa (purple sea snail) 2</a>