After mating, lay long tangled yellowish green, spaghetti like egg strings containing up to a million eggs. These eggs hatch in about 12 days and have been found in August through December. These are annual creatures, seldom living more than 1 year. The ink gets its purple color from a derivative of phycoerthryin, the red pigment in the red algae composing part of the sea hare's diet. Animals that are milked of their purple secretion and then are fed only green and brown algae produce no purple dye.
Individuals move about more by day than night.
They move like an inch worm, moving the front of its foot and then reattaching it to the substrate before releasing and moving the rear part of its foot. Few animals are known to prey on the Sea Hare. As mentioned above the giant green anemone Anthopleura xanthogrammica is reported to eat substantial numbers of Aplysia. The digestive gland of the sea hare contains toxic materials and when digestion by the anemone reaches a point where the gland is exposed the anemone regurgitates this part of the meal.