Biological Description

Physical Description

Members of the genus Aplysia are gastropod mollusks in the subclass Opisthobranchia, meaning “gills behind” (from Greek opisthen for behind, plus branchia for gills) and referring to the location of the gill of Aplysia behind the heart. Aplysia is often called the “sea hare” because the sensory tentacles on the top of its head, called rhinophores, are somewhat reminiscent of rabbit's ears. Unlike other gastropods, members of the genus Aplysia don’t have a large external shell in which to retreat. Adults have a small, flat, vestigial shell, the consistency of cardboard, covering the viscera and nominally protecting the heart and other internal organs. The mantle, or main muscular region on the back, is modified into two sections that look a little like wings, called parapodia, which cover the gill and help to channel water over it so that the animal can breathe. Water that has passed over the gills to extract oxygen is directed out from between the parapodial flaps via a tube, also modified from the mantle, called the siphon.

Due to the absence of a heavily calcified shell, the overall appearance of an Aplysia is somewhat slug-like; a large (sometimes larger than 5 kilograms), soft-bodied animal with a muscular foot covering nearly the entire underside of the animal, crawling along marine bottoms in an inchworm-like fashion. A few members of the genus Aplysia have the parapodia modified into such large wings that they can flap them like fins to swim gracefully for moderate distances. Aplysia have small, pinpoint black eyes in front of the rhinophores, on the top of the head. Even though it has eyes, an Aplysia doesn’t see so much as chemically and tactilely sense its environment with its head, using the rhinophores and a pair of large anterior tentacles at the front of its head. Using the rhinophores and anterior tentacles, Aplysia can quite easily locate food and other Aplysia, and detect danger.


Aplysia californica is a herbivorous marine mollusk that inhabits the marine coastal community in the Pacific Ocean from northern California to Baja California. Individual Aplysia can often be found on their algal food and aggregate in large numbers near food in the summer months (June – August) when they breed.


Aplysia eat marine macroalgae, principally red algae, from which it derives pigments that tint its skin a mottled reddish-brown, and also gives its ink a purplish color.



Aplysia are simultaneous hermaphrodites, meaning each individual has both male and female sex organs at the same time, and each individual can both donate and receive sperm. The eggs are fertilized from a sperm storage chamber called the seminal receptacle just before they are laid in long strings containing millions of embryos. Mating in this hermaphrodite sometimes consists of several animals copulating together in a “daisy chain” in which the animal at the beginning of a chain serves as a sperm recipient for the animal behind, which is simultaneously delivering sperm to the animal in front while receiving sperm from the animal behind, and so on. Mating goes on for several hours, followed by an hour or more of egg laying.


Aplysia californica releases purple ink from the ink gland just under the shell when disturbed. The animal extracts pigments from the red algal food it prefers, and this pigment tints the ink a purple color. Other extracts from the food make the flesh of the sea hare distasteful to potential predators.


An important behavior of Aplysia, often exploited for neurophysiological studies, is siphon withdrawal. This behavior consists of the animal pulling its siphon back into the mantle cavity when disturbed. Siphon withdrawal is a reflex behavior that can be modified by experience and electric shock, making Aplysia an important animal model of three different types of simple learning:

  • Habituation
  • Sensitization
  • Classical conditioning

The most simple of the three forms of learning is habituation, which is a decrease in a response with repeated presentations of the stimulus. Sensitization is a form of learning in which a response is enhanced by a single, noxious stimulus. Classical conditioning is the third type of learning studied in Aplysia, in which a tail shock (the unconditioned stimulus) is preceded by a touch to the siphon (the conditioned stimulus). The animal learns to associate the shock with the siphon touch, and retracts its gill, siphon and tail when the siphon is touched. All three types of learning lead to specific neural changes that constitute learning and memory on a cellular and molecular level, and which can be quantified easily in Aplysia due to the simplicity of its nervous system.