Janthina janthina Linnaeus, 1758 - purple sea snail shell from Texas, USA (apical view; modern, latest Holocene; ~2.35 cm across at its widest).

The purple sea snail is a remarkable organism. There are several valid, named species, but the most common and widespread form is this - Janthina janthina. The snail makes a thin calcareous shell that is dark purple-colored in the lower parts and whitish to pale 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.

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).

In ancient times, one source of purple dye for clothing was Janthina purple sea snails.

Classification: Animalia, Mollusca, Gastropoda, Janthinidae

Locality: unrecorded post-storm Gulf of Mexico beach, coastal Texas, USA