Feeding

Unfed Juvenile
The juvenile is brown with red pigment spots prior to feeding.  At this point, the animal has no chloroplasts.  When it feeds on the alga Vaucheria litorea, it will turn green, by putting the algal chloroplasts into its own gut cells.

A Juvenile’s First Meal
When the animal feeds on the alga Vaucheria litorea, the animal punctures the algal cell wall with its radula.  It then holds the algal strand firmly in its mouth, and sucks out the contents, as if the algal filament were a straw.  The animal keeps only the chloroplasts, which it incorporates into the cells of its gut.

First Day After Feeding
Once the animal has fed on Vaucheria, the chloroplasts are distributed throughout the extensive branched gut.  At this point, the animal is green from the chloroplasts.  Initially, the animal needs to continue to feed to retain the chloroplasts.  But later, the chloroplasts become stably incorporated into the gut cells, and the animal remains green even without further feeding.