Sea hares (Aplysia species and related genera) are very large sea slugs with a soft internal shell made of protein. These are marine gastropod molluscs in the family Aplysiidae and superfamily Aplysioidea.
The original author P. Fischer described this taxon at unspecified rank above family . In 1925 Thiele established this taxon as a suborder. Since the taxon Anaspidea was not based on an existing genus, this name is no longer available. It has been replaced in the new Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005) by the clade Aplysiomorpha .
The scientific name for the order in which they are classified, the (Anaspidea), is derived from the Greek for "without a shield" and refers to the lack of the characteristic head shield found in the cephalaspidean opisthobranchs. Many anaspideans have only a thin, internal and much-reduced shell with a small mantle cavity; some have no shell at all. All species have a radula and gizzard plates.
Sea hares have soft bodies with an internal shell, and like all opistobranch mollusks they are hermaphroditic.
Sea hares are herbivore/herbivorous, and are typically found on seaweed in shallow water. It seems to be the case that some young sea hares are capable of burrowing in soft sediment leaving only their rhinophores and mantle opening showing. Sea hares have an extremely good sense of smell. They can follow even the faintest scent using their rhinophores, which are extremely sensitive chemoreceptors.
Their color corresponds with the color of the seaweed they eat: red sea hares have been feeding on red seaweed. This camouflages them from predators.
When disturbed, a sea hare can release ink from its ink glands, providing a potent deterrent to predators. This release acts as a smoke screen, while at the same time, adversely affecting the smell sensors of their predators. In a small environment, this ink could be toxic to the inhabitants. The color of the ink is white, purple or reddish, depending on the color of the pigments in their seaweed food source. Their skin contains a similar toxin that renders sea hares largely inedible to many predators.